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STP 491-1971

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HYDRAULIC SYSTEM

CLEANLINESS

A symposium
presented at the
Seventy-third Annual Meeting
AMERICAN SOCIETY FOR
TESTING AND MATERIALS
Toronto, Ont., Canada, 21-26 June 1970

ASTM SPECIAL TECHNICAL PUBLICATION 491

List price $4.50

04-491000-12

AMERICAN SOCIETY FOR TESTING AND MATERIALS

1916 Race Street, Philadelphia, Pa. 19103
#

Copyright by ASTM Int'l (all rights reserved); Fri Jan 1 22:59:37 EST 2016
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University of Washington (University of Washington) pursuant to License Agreement. No further reproductions autho
© BY AMERICAN SOCIETY FOR TESTING AND MATERIALS 1971
Library of Congress Catalog Card Number: 70-151113
ISBN O-8031-0125-6

NOTE
The Society is not responsible, as a body,
for the statements and opinions
advanced in this publication.

Printed in Baltimore, Md.

June 1971

The Symposium on Hydraulic System Cleanliness was given at the Seventy-

third Annual Meeting of the American Society for Testing and Materials
held in Toronto, Ont., Canada, 21-26 June 1970. The symposium was
sponsored by Committee D-2 on Petroleum Products and Lubricants.
J. J. Weaver, Shell Oil Company, presided as symposium chairman.

Significance of ASTM Tests for Petroleum Products,

STP 7-B (1957), $3.50

Introduction 1

Hydraulic Oil Cleanliness: A Key to N /C Machine Maintenance—o. H. ARNDT 3

Maintenance of Cleanliness of Hydraulic Fluids and Systems—L. B. BARA-

NOWSKI 18

Fluid Conditioning for Servo Systems—j. N. CORONEOS 29

Improve System Contamination Control and Increase System Efficiency—

J. p . DUNCAN 34

Requirements for an Effective Program in Fluid Contamination Control—

E. C. FITCH, JR. 39

Contribution of Hydraulic Fluids to System Contamination—E. A. KIRNBAUER 50

Status Report on the Tech O/F-7 Contamination Methods Program—D. A.

MARLOW AND A. BEERBOWER 69

Filtering—From the Moon to the Mines—w. j . MARSH 78

Copyright by ASTM Int'l

Downloaded/printed by
University of Washington (Unive
STP491-EB/Jun. 1971

Introduction

Experience in the hydraulic industry indicates that there is a need for

test methods to deal with problems caused by dirt in hydraulic systems.
Representatives of industry have asked Technical Division N to consider
the possibility of developing applicable test methods. To achieve a better
understanding and definition of the problems, we have organized our sym-
posium to discuss the various aspects of industrial hydrauHc system clean-
liness. We planned the symposium to include contributions from a wide
range of interested parties. Our authors represent users of hydraulic systems,
equipment manufacturers, filter manufacturers, technical societies, and educa-
tional institutions.
We beheve that the authors have done an excellent job for us. Their papers
show the benefits and problems associated with hydraulic system cleanliness
and demonstrate the need for test methods to evaluate filters and measure
contamination. It is hoped that Technical Division N will undertake the
development of these test methods. Our symposium and the following list of
D-2 Committee, ASTM Standards are suggested as guides for such an under-
taking.

ASTM Standards Relating to System Cleanliness

Sampling
D 2388 Field Sampling of Aerospace Fluids in Containers
D 2407 Sampling Airborne Particulate Contamination in Clean Rooms
for Handling Aerospace Fluids
D 2429 Sampling Aerospace Fluids from Components
D 2437 Open-Bottle Tap Sampling of Noncryogenic Fluid Systems
D 2535 Samphng for Particulates from Aerospace Components with
Convolutes
D 2536 Sampling Particulates from Reservoir Type Pressure-Sensing
Instruments by Fluid Flushing
D 2537 Sampling Particulates from Storage Vessels for Aerospace Fluids
by Vacuum Entrainment Techniques (General Method)
D 2542 Liquid Sampling of Noncryogenic Aerospace Propellants

Sample Processing
D 2391 Processing Aerospace Liquid Samples for Particulate Contamina-
tion Analysis Using Membrane Filters
1

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2 HYDRAULIC SYSTEM CLEANLINESS

Tests for Contamination

D 893
Test for Insolubles in Used Lubricating Oils
D 1404
Estimation of Deleterious Particles in Lubricating Grease
D 2273
Test for Trace Sediment in Lubricating Oils
D 2276
Test for Particulate Contaminant in Aviation Turbine Fuels
D 2387
Test for Insoluble Contamination of Hydraulic Fluids by Gravi-
metric Analysis
D 2390 Microscopic Sizing and Counting Particles from Aerospace
Fluids on Membrane Filters
D 2430 Tests for Identification of Metallic and Fibrous Contaminants in
Aerospace Fluids
D 2546 Test for Identification of Solder and Solder Contaminants in
Aerospace Fluids
Filters
D 2499 Method of Test for Pore Size Characteristics of Membrane
Filters for Use with Aerospace Fluids
D 2767 Test for Liquid Flow Rate of Membrane Filters
A cknowledgmen t
I wish to thank K. G. Henrikson, Mobile Oil Corp.; R. C. Givens, Texaco
Inc; P. K. Schacht, Rex Chainbelt Inc.; R. H. Schmitt, Mobile Research
and Development Co.; and W. M. Schrey, U.S. Steel Research, for their
work and guidance in organizing the symposium.

J. J. Weaver
Shell Oil Company;
symposium chairman.

Hydraulic Oil Cleanliness: A Key to

N/C Machine Maintenance

REFERENCE: Arndt, O. H., "Hydraulic Oil Cleanliness: A Key to N/C Machine

Maintenance," Hydraulic System Cleanliness, ASTM STP 491, American Society
for Testing and Materials, 1971, pp. 3-17.

ABSTRACT: High uptime on numerical control (N/C) machining centers is the

result of correcting any condition that may ultimately lead to some component
failure. This requires an intimate knowledge of the machine hydraulic system
and components, and that rare sense of what is right (or wrong) in the entire
operation of a machine. A measurement of oil cleanliness can be most helpful.
A remarkable increase of N/C machines in manufacturing industries presents
a challenge to develop personnel for proper care of these installations. Develop-
ment of this skill in machine maintenance is the users' achievement.
Present day machines, with improved components and hydraulic systems, are
less sensitive to hydraulic problems. Several years of experience has provided
technical knowledge in the art. This information can be passed on by a machine
builder to a user, or from one user to another, in symposiums such as this.

KEY WORDS: failure, clean oil, contaminants, filters, lubrication, manuals, oil
tests, oil coolers, particle size, numerical control, machine tools, hydraulic sys-
tems, servomechanisms, maintenance, cleaning, preventive maintenance, temp-
erature control, education, varnishes, evaluation

All our problems have a beginning some place. When high pressure hydrau-
lic servo systems and numerical controls were married to machine tools, a
new family of problems appeared. But thanks to the progressive nature of
our industries and the ingenuity of people associated with them, many new
fields of adventure and accomplishment developed.
I like to think that the pioneer in this story was a revolutionary new machine
with an automatic tool changer and numerical control of the late fifties,
Kearney and Trecker's Milwaukee-Matic Model II Machining Center,
Fig. 1. In a few years, many of these machining centers were serving industry
throughout the country.
Various other models of the Milwaukee-Matic Machining Center followed,
along with a variety of profilers, and an assortment of special numerical
1
Senior mechanical engineer, Kearney and Trecker Corporation, Milwaukee, Wis.
53214.

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HYDRAULIC SYSTEM CLEANLINESS

FIG. 1—Early Milwaukee Matic Model U Machining Center.

control (N/C) machines developed by Kearney and Trecker, Figs. 2 to 5, as

well as similar machines of many other machine tool manufacturers.
The very fact that industry absorbed all this new equipment is a monu-
mental credit to the men who accepted such a challenge. No doubt, the
pioneers in programming, operating, and maintaining these earher machines

FIG. 2—Milwaukee Matic Model III 5-Axis Machining Center.

FIG. 3—Milwaukee Mafic Model V 5-Axis Machining Center.

were rewarded with more responsible positions and may even have directed
more N / C acquisitions.
So now we have more of these sophisticated machines in our shops and
relatively less help qualified to service them. Fortunately, development
progressed on a broad front, providing solutions as quickly as complications
appeared. Pump and servo valve manufacturers developed products for N / C
machines that were more reliable and less vulnerable to contaminants.
Hydraulic oils were adapted specifically to servo systems and are provided
by suppliers to meet certain standards of cleanliness. A variety of filters are
available to meet the needs of N / C servo systems.
New technology, developed and disseminated by the oil companies and the
many component suppliers, has enabled machine tool builders to manufac-
ture machines that are less sensitive to contaminants and capable of giving
longer trouble free service. Helpful as these improvements were, industry
still had a new line of machines to become familiar with. This brings us to
the problem of N / C machine maintenance. The general principles of plant
maintenance apply to machine tool maintenance, and it would be well to
mention them here.
The primary objectives of maintenance are:
1. To maintain machines in an adequate operating condition.
2. To keep down time to a minimum.
3. To keep break down costs to a minimum.
To attain these objectives, it is first essential to have an adequate staff" and
appropriate supervision, an effective preventive maintenance program, and a
thorough knowledge of the equipment.

FIG. 4—3 spindle profiler.

FIG. 5—3 spindle, 5 axis N/C machine with tool changer.

Cooperation between production and maintenance is of prime importance.

Each must have a realistic appreciation of the other's role in the profit
picture. Preventive maintenance must be provided for in the production
schedule. Preventive maintenance work must be conscientiously completed
on time. Good service records must be kept-available-and used! These
records establish the characteristics of a machine and how it is used (or mis-
used). A review of the records can show where more or less attention is
required or what special work may be necessary. This can then be scheduled
for the convenience of both production and maintenance. Even the com-
petence of the operator may be reflected in such records, especially where a
number of similar machines permit comparison.
Follow-up on breakdowns is valuable, too. Could this break down have
been prevented? Was it forecast in the pm check? Analysis of a failure may
show the cause and thus prevent a recurrence. Preventive maintenance work
must be organized around the skills of the personnel and facilities existing
in the plant as well as the requirements of the machines to be serviced.
Skills of personnel can be developed [1]} Training programs at the machine
builder's plant are conducted under ideal conditions with good facihties
and qualified instructors available. Component suppliers offer similar
opportunities or may even bring their "school" to your plant. Technical
schools often provide special courses if requested when the required courses
are not already part of the regular curriculum. Home study courses and
seminars, like this, provide other sources of upgrading the individual. The
will to learn can be stimulated and is contagious once begun in a plant.
The added knowledge also brings with it a new pride and satisfaction for the
workman in his job. He has become more involved through his own personal
efforts.
Now a word for the man who services the machine [2]. He is as important
as the operator or the millwright. He must know the importance of guarding
against contamination and following the lubrication schedule. The cost of
the oil or coolant he handles may be small, but if he does not handle it
properly, a quarter million dollar machine tool may be out of production.
That can run into real money. The machine serviceman must be properly
oriented for these responsibilities.
This is where the maintenance manual comes into the picture. A machine
tool manufacturer provides instructions and recommendations for proper
care and servicing of the equipment he builds. We should like to center our
discussion from here on, on the "Contaminants in the Hydraulic System"
section of a Milwaukee-Matic Machining Center Maintenance Manual [i].
In preparing this section, we learned:
1. Well written material on contamination for a maintenance manual
was hard to find.
2 The italic numbers in braclcets refer to the list of references appended to this paper.

2. Few people had a full appreciation of what was involved in a servo

hydraulic system [4].
3. Among knowledgeable persons, there is general concurrence on most
principles involved in the design of a good servo hydraulic system.
4. There was a real need for some simple explanation of the hydraulic
system for the maintenance man.
5. The big question was: What is clean oil?
It was decided to very briefly cover the main points concerned with the
hydraulic system, so that everyone would be more hkely to read this section.
A skilled technician would readily get the picture. It is hoped one less knowl-
edgeable would be alerted to things requiring special consideration and would
realize when additional information or help should be obtained.
Perhaps the manual itself can best illustrate our approach.

Contaminants in the Hydraulic System

The hydraulic system (see Fig. 6) is designed for accurate performance and
long troublefree service of a fine precision machine tool. Certain special
features play important roles to this end. Understanding their function will
help the maintenance personnel minimize service work and machine down
time.
Coolers and temperature controls maintain hydraulic oil operating temper-
atures between 110 and 135 F. This temperature range gives ideal servo
performance, is high enough to free water in the system, and yet not high
enough for rapid varnish formation. A separate circuit with a low volume
pump continuously circulates the oil through a large capacity filter and the
coohng system. This filter is not subjected to the surges in the operating
circuit of the hydraulic system. Each axis servo valve is preceded by a filter
in the hydraulic circuit.

Effects of Contamination
Contamination in a hydraulic system causes corrosion, erosion, and ac-
celerates wear, reducing the useful life of all components. Pump slippage
and system leakage gradually increase until the system is no longer able to
perform the job it was designed to do. Components must be overhauled or
replaced.
The sharp metering edges in servo valves are especially vulnerable to
erosion that reduces accuracy and impairs performance. Silting decreases
system resolution and accuracy, and can result in system "hunting." Clogging
of servo valve orifices can result in a "hardover" valve condition. Tests in-
dicate that an absolutely clean system would almost eliminate all component
wear.

Systems completely free of contaminants never can be attained; however,

a good testing program can help keep contaminants to a practical minimum.
To help the reader visualize the microscopic size of contaminants, the follow-
ing chart is presented:
Particle Size
1 micrometer = 0.001 millimeter
25 micrometers = 0.001 inch

Microns Inches

25 to 50 0.001 to 0.002
Table salt 100 0.004
Human hair 75 0.003

Contaminants in a hydraulic system are generally smaller than a grain

of salt. Visualize these particles traveling at 200 to 300 ft/s, as the fluid flows
through restricted passages of pumps, motors, and valves. The ballistic
impact of microscopic particles becomes measureable and the frequency of
impact is high, even at acceptable levels of contamination. The picture of
erosion becomes quite realistic.

Sources of Contamination and Steps of Prevention

1. Contaminants in new oil and equipment used in adding oil.
(a) Store oil drums on sides.
(b) Clean top of drum before opening for pump or suction lines.
(c) Use only clean transporting containers.
(d) Filter all oil added through a 5-ixm filter.
(e) Keep reservoir filled at proper level to prevent formation of moisture
and rust.
2. Leakage of water, coolants, lubricants, etc., into the system.
(a) Keep all line connections tight.
(b) Keep all seals in good condition.
(c) Avoid improper use of coolants and lubricants that may cause system
contamination.
3. Infiltration of gases, fumes, and dust through breathers.
(a) Clean air breathers regularly.
(b) Provide special filters or duct in a clean air supply for severe conditions.
4. Contaminants introduced while replacing filters, valves, pumps, etc.
(a) Break fines only when absolutely necessary.
(b) Cap all line openings.
(c) Instafi replacement parts immediately or protect opening from air-
borne particles.
(d) Use only a solvent and lint-free wipers for cleaning.

5. Varnish or scale forming from deterioration of oil.

(a) Check operation of temperature controls.
(b) Check effectiveness of cooling system.
6. Particles resulting from wear and erosion.
(a) Check filters and change as required.
(b) Sample oil for analysis regularly. Particle identification can pin point
an impending failure, if wear or erosion becomes abnormal.

FIG. 6—Hydraulic power supply unit.

Filters
Filters are provided throughout the hydrauhc system to remove solid
contaminants from the system. The hydraulic power supply unit has a filter
that will light the "oil filter clogged" light on the power distribution panel
when the cartridge becomes clogged. Each axis drive hydraulic circuit has a
filter with a replaceable cartridge.
The spindle drive hydraulic circuit has a filter with a pop-up indicator to
show the condition of the filter cartridge. When this filter cartridge is re-
placed, each axis drive filter is also to be replaced, since the spindle hydraulic
circuit is set up as the "control circuit" for axis filter replacement.
The following observations on filters are general in nature and are applica-
ble to all filters:
(a) The best available filter will pass many particles larger than its rated
pore size.
(b) The quantity and size of particles passed by a given filter is dependent
on how the filter is used. The cause of unusual virbration, excessive pressure
changes, and flow surges should be checked and promptly remedied.
(c) Filter elements must be replaced, as required, to maintain their effec-
tiveness.

Fluid Analysis
Samples of oil taken from the hydraulic system and analyzed according to
standard ASTM procedures adequately reveal the condition of the oil and
its suitability for servo machine tool service. The degree and nature of con-
tamination indicate what corrective steps may be necessary. A sample of
oil may be used to determine the specific gravity, viscosity, water content,
and neutralization number. A change in any of these from normal indicates
dilution or contamination by water, coolant, lubricants, or other fluids. The
neutralization number also indicates any increase in acidity of the fluid due to
a breakdown of the oil. These contaminants are more of a "chemical"
nature, and cannot be easily removed, as particles are by filtration. If tests
show a high degree of such contaminates, the oil must be changed.
Foreign particles in the oil can usually be removed by filtration. This
is a continuous process as long as the filters in the system are functioning
properly. External filtration may be necessary to clear up an excess condition
of particle contamination and bring the system back to a safe operating level.
A Particle Count—to show the extent of such contamination, is obtained
by passing a known quantity (100 ml) of this sample through a filter. The
particles left on the filter are counted by size groups and also may be identi-
fied as to their origin. Such particle identification can help pin pointing
trouble, with filters, seals, motors, pumps, etc., that may be averted by prompt
attention.

From the particle count, the class of contamination is determined. Class

0 through 4 oil is considered to be satisfactory for further use; sampling
again at the regularly scheduled time. For Class 5 (or dirtier) oil, all filters
must be checked, and changed, if necessary. Careful examination of each
filter should be made to identify the type of contamination. External filtration
also is recommended, especially if contamination is worse than Class 5.
Anything that appears to contribute to the contamination should be corrected,
and after 16 h of operation another sample should be taken for analysis.
A particle count is practical on oil samples of Class 6 or cleaner. Gravi-
metric analysis test provides a useful measure of contamination for Class 6
and worse, but particles are not identified easily. Silt index shows up an
excess of particles below 5 fim and also varnish formation in the oil. For a
high count of particles smaller than 5 ij.m, or a high silt index, external filter-
ing with 2 ^ni, or finer, filters is recommended.
When the benefits of clean oil and the potential problems of contaminants
are realized, the value of a regular oil sampUng program becomes quite
obvious. It is recommended that samples of oil be taken, see Fig. 7, and ana-
lyzed as follows:
First—Upon completion of installation.
SAMPLING PROCEDURE

Take samples of fluid from the hydraulic system as follows:

a. Take fluid samples while the machine is in operation and has been running 15 minutes or more.

b. Always take sample at same place, specified by manufacturer of machine.

c. Clean the area surrounding the sampling valve on the hydraulic system with a filtered solvent, such as
Freon TF.

NOTE: Rags should not be used for cleaning, because the lint they leave contaminates the oil. When
wiping is necessary only lint free wipers, such as KIMWIPES, should be used.

Keep the sampling bottle absolutely clean and free of any additional contaminants at all times.

d. Open the sampling valve and flush a quart or two of fluid into a pail, fill the 1000 ML shipping bottle
with fluid, remove the bottle and then close the valve. (Discard the fluid in the pail; do not return it to
the hydraulic system).

e. Place a 3 " square of "Saran" or similar material over the shipping bottle to seal the opening and screw
the cap on tightly over this seal for shipping.

f. Identify each sample with date taken and machine serial number. Give name and address of person who
is to receive the reports and the returned bottle.

g. Place the bottle with the sample of fluid in the plastic bag and seal it. Prepare it for shipping by wrapping
with corrugated paper and placing in corrugated paper container and seal. (Do not pack in saw-dust.)

h. Label package "Hydraulic Oil Sample".

FIG. 7—Recommended sampling procedure.

ROSAEN FILTER DIVISION

Phone: 313-566-4778
@) PARKER HANNIFIN
]77« E.NmtM.lfRoa

FLUID ANALYSIS REPORT

Company ,

Address _

City & State ,

Attention

From Machine No.

Sample Received _ . Analysi s Made .

(Dot.) (Dole)

'CONDITION
TYPE OF TEST READING
A B c
Specific Gravity
Viscosity
Silt Index
Water Content
Gravimetric
Neutralization No.
Particle Count
SIZE SAMPLE RECEIVED 0 3 4 5 6
1 2
RANGE
2,700 4,600 9,700 24,000 32,000 87,000 128,000
10-25 f i m 670 1,340 2,680 5,360 10,700 21,400 42,000
25-50 ( i m 93 210 380 780 1,510 3,130 6,500
50-100 |J-m 16 28 56 110 225 430 1,000
>-100p.m 1 3 5 11 21 41 92

Fibers Hydra ulic Contamination Standards -

Porlicl •s Per 10 loss ot C] ,'sfem

Amount of Sample

This sample is: DBe. • Wo. I I Same as the previous sample

analyzed on , _ (See remarks)

* Conditions: A. Oii is satisfactory for further use. Test at regular schedule.

B. Oil condition is poor. Replace all filter elements and submit another sample for analysis
after 16 hours of operation.
C. Change oil and flush system. Readings are not within safe limits.

REMARKS:

FIG. 8—A typical fluid analysis report.

Second—After first 160 h of operation.

Third—After 480 h of operation and every 480 h thereafter.
These are recommendations for normal operating conditions. Where un-
usual conditions exist, the schedule may be modified to suit. Should it be
necessary to change the oil, or some major component where the cleanliness
of the oil may be affected, a sample analysis should be made.

^ g i ROSAEN FILTER DIVISION /g>^

PARKER H A N N I F I N \J\J
1776 E.Nii,<M>l«Road,Haz<l Pork.Michiggn 48030

DO NOT OPEN BAG UNTIL

READY TO SAMPLE

RECOMMEND NEXT SAMPLING

PLEASE COMPLETE AND FORWARD WITH SAMPLE:

MACHINE SERIAL NUMBER

DATE OF SAMPLING

HOUR METER READING

LAST SAMPLE TAKEN

OIL ADDED OR CHANGED (DATE).

RETURN REPORT TO

ATTENTION

REMARKS ON MACHINE OPERATION

^Please note failures and causes, if known.

FIG. 9—Sample data sheet.

Oil sample reports, Fig. 8, should be filed, together with service work
records, for a convenient reference book of the machine. Comparing succes-
sive reports helps to establish the characteristics of the machine, and varia-
tions from normal then can be readily detected.

Measuring Contamination
Measuring contamination by fluid analysis is a laboratory procedure with
standards established by American Society for Testing and Materials (ASTM)
and Society of Automotive Engineers (SAE) in general use in industry.
Plant laboratories with competent personnel can carry on an excellent oil
sampling and analysis program. When there are no such facilities, the sam-
pling can be assigned to a responsible person or made part of the preventive
maintenance program. Oil samples are then sent to a laboratory providing an
analysis service, see Fig. 9.
Companies supplying hydraulic oils also provide testing services; however,
the important particle count is generally not included. The economics of
component replacement costs and lost production time that may be avoided,
not only justify a good hydraulic fluid testing program, but indicate that the
best is required.

External Filtration
When the particle count shows contamination of the hydraulic oil to be
up to Class 6, it is advisable to use an external filtration unit, Fig. 10, in
addition to changing the spindle and axis filters. Various portable units are
available with excellent filtering capabilities, 1 ^m range, that can clean a
hydraulic system up overnight.

Conclusion
All our discussion up to this point, still leaves us with a dire need for a
simple, fast, and rehable means of determining the cleanhness of hydrauhc
oils. This remains the key to effective N / C machine tool maintenance.

Acknowledgments
The author wishes to thank Kearney and Trecker Corporation for making
this presentation possible. The help of many co-workers, especially William
Bartz and Robert Sedgwick gratefully is appreciated. The technical assistance
of F. S. Stilwagner of Rosaen Filter Division and M. O. Thommesen of
Mobil Oil Corporation in preparing the Maintenance Manual is also
appreciated.

FIG. 10—A portablefiltrationunit.

References
[1] Bachmann, R. W., "Personnel Selection and Training for Numerical Control."
[2] Maintenance Engineering Handbook, Morrow, L. C. ed., McGraw Hill, New York, 1966.
[3] "Maintenance Manual Milwaukee-Matic Model III," Kearney and Trecker Corpora-
tion.
[4] Sedgwick, R. K., "Hardware for Applications of Hydraulic Servo Systems to Machine
Tools."

Maintenance of Cleanliness of
Hydraulic Fluids and Systems

REFERENCE: Baranowski, L. B., "Maintenance of Cleanliness of Hydraulic

Fluids and Systems," Hydraulic System Cleanliness, ASTM STP 491, American
Society for Testing and Materials, 1971, pp. 18-28.

ABSTRACT: The cleanliness of a hydraulic system and its fluid is an important

factor in the proper operation of such a system and its components. The sources
of contamination and prevention of it are discussed. Types of contaminants are
classified as particulate matter or liquids and gases in both free and soluble phases.
Their effect on systems and components are discussed briefly. The methods for
measurement of contaminants are reviewed with special attention to water and
gas content. Techniques and practices for contamination prevention and control
are described. Applications of filtration, dehydration, and degasification of
hydraulic fluids ,ri hvHmniiV cvctpmc arp cnoopQtpH inrlnHina totally enclosed
circuits Copyright" 1971 by ASTM International www.astm.org

KEY WORDS: hydraulic equipment, hydraulic fluids, filtration, oilfilters,vacuum,

degassing, dehydration, contamination, purification, maintenance, cleaning,
diluents, exhaust gases, particle size, grit, evaluation, tests

There is a growing awareness of the importance of cleanliness in the

hydraulic fluids and systems.
A survey of the articles published in Hydraulics-Pneumatics in the two
nine-year periods clearly indicate the trend [I]2 as shown in Table 1. The
rate of innovations dealing with contamination control and related subjects
increased from 10 in 1948-1957 to 48 in the 1958-1967 period.
This trend originated in aerospace industries, due to the critical nature of
hydraulic applications, and later spread into earth-bound industries, which,
prior to that time, were reluctant to accept higher standards for contamination
control. The reasons for adaptation of this philosophy are mostly economical:
to reduce maintenance costs and to increase reliability and accuracy.
During a recent 25th Annual Meeting of the American Society of Lubrica-
tion Engineers (ASLE), the Machine Tool Committee held a panel discussion
"Solving the Problem of Clean Fluids for Machine Tool Hydraulics." Also,
1
Manager, system application engineering, Fluid Handling Division, Keene Corpora-
tion, Cookeville, Tenn. 38501. Member ASTM.
2
The italic numbers in brackets refer to the list of references appended to this paper.
18

Copyright by ASTM Int'l (all rights reserved); Fri Jan 1

Copyright" 1971 by ASTM International www.astm.org
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BARANOWSKI ON HYDRAULIC FLUIDS AND SYSTEMS 19

TABLE 1—Filtration innovations."

Feb. 1948 through Jan. 1958 through

Dec. 1957 Oct. 1967

Number of Articles

Subject of Articles Total Aerospace Total Aerospace

Contamination control, filtration, particle

counting, dehydration and deaeration, etc. 10 1 48 29
Leakage control 42 7 32 7

" From: Hydraulic and Pneumatics, Nov. 1967.

a special ASLE/FPS Symposium was held on Hydraulic Fluids and Fluid

Conditioning [2].
The panel members reported that by proper filtration, the life in mobile
hydraulics increased as high as 100:1 ratio. A numerical control machine
builder reported improvement in cleanliness level from 4 to 8 down to 0
through introduction of a 3-Min filtering system.

Sources of Contamination

There is always some residual contamination left in the system after as-
sembly or overhaul. Even by application of the most careful flushing proce-
dures, it is difficult to attain a low level of contamination in the flushing fluid.
Contamination may enter the system with hydraulic fluid when filHng the
system or when adding make-up oil. Checks of oil at the refineries frequently
indicate a very high level of contamination in 5 to 10 )j,m range prior to
filtration. Additional contamination is picked up during transfer and from
the shipping containers.
During the operation of the system, contamination can enter with atmos-
pheric air or be generated within the system. Air breathing reservoirs collect
atmospheric dust and moisture which condenses as the temperature changes.
System generated contamination consists of rust and products of corrosion
due to the presence of water and oxygen. Decomposition of hydrauhc fluids
in the presence of water, oxygen, and catalytic eff"ect of metals results in the
formation of sludge. High temperatures accelerate decomposition, resulting
in the generation of gums and varnishes.
Seals and gaskets also may contribute to contamination due to wear or
incompatibility with the fluids.
Last but not least are the accumulating particles of wear of the system,
which in turn breed more wear.

Particulate Matter
A great deal of attention is being paid to the particulate matter contamina-
tion and the cleanhness levels, which are, in a way, the reciprocals of con-
tamination.
I would like you to refer to a paper by J. A. Briggs on "Determining
Contamination Levels in Hydraulic Systems" [3]. Table 2 shows a tentative
SAE standard based on particle count. It illustrates a particle population
explosion. There are several other competitive systems of classification; for
example, Cincinnati Milling Machine Company with classes from 00 to 10
overlapping the SAE standard.

TABLE 2—SAE-A-6D tentative.

System Class MIL

Size Range, H
Mm 0 1 2 3 4 5 6 5606B

5 to 10 2 700 4 600 9 700 24 000 32 000 87 000 128,000 2 500

10 to 25 670 1 3 4 0 2 680 5 360 10 700 21400 42 000 1000
25 to 50 93 210 380 780 1510 3 130 6 500 250
50 to 100 16 28 56 110 225 430 1000 25
>I00 1 3 5 11 21 41 92 0

The paper by J. A. Briggs contains ample bibliography on contamination

level. A very commendable effort is conducted by the National Fluid Power
Association to establish a uniform method of reporting data on contamina-
tion levels by the Hydrauhc Filter and Separator Group.
For this reason, and to avoid being repetitious, I would hke to deal in more
detail with contaminants other than particulate matter, which are frequently
overlooked and underestimated.

Water as a Contaminant
Water is always present in hydraulic fluids. This water may be present in
its free form or as water dissolved in the fluid, mostly in both forms. Free
water may appear in a precipitated form, separated from the fluid by dif-
ference in specific gravity. Thus in phosphate esters and chlorinated hydro-
carbons free water will appear on top of fluid. In some silicone type
fluids, free water will remain in oil in a state of weightlessness as their specific
gravity is close to 1.00.
The other form of free water in oil is emulsion. When thoroughly dispersed
by the mechanical action of pumps, or by passage of oil, the emulsions may be
more or less permanent. Less permanent emulsions, given time, will separate
by gravity or by a centrifugal force, provided a difference in specific gravity
exists.

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BARANOWSKI O N HYDRAULIC FLUIDS AND SYSTEMS 21

In general, oils containing surface acting additives or contaminants tend to

form tighter emulsion with water. Tighter emulsions are characterized by
smaller water particles and a slower rate of separation. Some emulsions do
not separate at all.
While most additives and inhibitors are put in oil for a specific purpose,
their presence frequently has an adverse effect on the water separating proper-
ties of the oil. The presence of soluble oxidation products and particles of
rust tend to stabilize emulsions still further.
In addition to free water, oils contain water in solution. Figure 1 shows
maximum water content in its soluble phase for petroleum oils. Curves A
and B show the relationship between oil temperature and soluble water con-
tent for a typical light viscosity oil. Curve A shows a transformer oil, Curve B
oil with high aromatic content.

Air as a Contaminant
Hydraulic systems are highly sensitive to the presence of free air or gas.
Gases are highly compressible and dissolve easily in fluids; therefore, they
affect the operation of a hydrauhc system in many ways. Cavitation of the
pumps, erosion of the valves and orifices, and mushy operation are credited to
the presence of gas in hydraulic systems. Rapid compression of free gas
generates heat, which in the presence of oxygen can scorch or oxidize the oil.
Foaming is another ill effect of gas in a hydraulic system.

PPM.

I
/
1000- «_
V
/
4
I
I
I
I
I
//
800-
/
//
t
//
600-

~ ^ /
//
400- / A
^^ ^

2 0 0-

20 40 iO 100 Deg. C.

FIG. 1—Maximum water solubility in petroleum type oils.

Gases are soluble in oils in quantities depending on the type of oil and
gas. Air is soluble in mineral oil to approximately 12 percent by volume at
atmospheric pressure. A silicone type oil will dissolve approximately 20 per-
cent of air under the same conditions.
Solubility of gases in fluids changes only little with the temperature, but is
affected greatly by pressure. At twice atmospheric pressure, the total possible
soluble gas content is double that at atmospheric pressure, while at full
vacuum, the theoretical gas content is nil. This shows almost a linear rela-
tionship between pressure and soluble gas content.
The reverse process to solubility is evolution of gas from the fluid, caused
mostly by the reduction of pressure. There are additional factors involved,
including time, for complete solution or evolution of gas from liquid [4].
The measurements of the dissolved gas can be accomplished by one of the
following methods: ASTM Test for Gas Content of Insulating Oils (D 831-63)
or ASTM Test for Gas Content (nonacidic) of Insulating Liquids by Dis-
placement with Carbon Dioxide (D 1827-64). First, the gas is evolved in a
vacuum chamber and its contents calculated from the increase in pressure.
The second is by displacement of air by carbon dioxide and is suitable for
nonacidic oils or nonreactive with strong caustic solution.

Other Contaminants
The other intangible contaminants are some things that sometimes are
lumped under the heading of surfactants, or sometimes suspected as bacterial
contamination. It interferes with super-fine filtration and the coalescing
action of filters. They plate out as a slimy coating on the filter fibers causing
premature plugging of the filter. We have observed this phenomenon on some
new rust and oxidation inhibited turbine and hydraulic oils. One of the
plausible explanations is that in the presence of water, some rust inhibitors
plate out on the porous filter media.

Effects of Contaminants
While effects of the contamination by particulate matter are publicized
widely, the effects of water and gas, present in hydraulic oils, are not fully
appreciated.
With the reduction of pressure, dissolved gas evolves from fluid in the form
of bubbles. This gas in a free state may cause cavitation of the pump, erosion
of the components, oxidation and deterioration of oil, overheating, foaming,
and resulting loss of lubricity.
The other detrimental effects of air in oil were described in the June 1968
[5] and November 1969 [6] issues of Hydraulics and Pneumatics magazine.
While the effects of gas or water contamination on a hydraulic system are
considerable, the combined effect of both is still more damaging. Oxygen
from air and water causes corrosion of both liquid and gas space of the

hydraulic system, thus generating contaminant right within a system and

even the best micronic filters cannot prevent it.
Rust inhibition of oils is one of the preventive measures designed to form
a barrier to water and oxygen on ferrous surfaces of the system. The presence
of water causes this inhibitor to plate out the unprotected surfaces. Too much
water, however, causes inhibitors to precipitate from oil, clog fine filters,
and to deplete the inhibitor. Too much inhibitor reduces the ability of water
to separate from oil.
In active hydrauhc systems, water alone does not affect system operation
as it forms a water-in-oil emulsion, and has no opportunity to wet the ferrous
surfaces. In stagnant or intermittently operated systems even a trace of water
may cause a great deal of trouble. A small water particle resting in a fine
clearance of a spool valve may start a rusty spot. Larger water drops may
shorten the life of a pump due to its poor lubricity.
Rust inhibitors and oxidation inhibitors are added to the hydraulic oils
to counteract or prevent effects of water and oxygen. In absence of water and
oxygen hydraulic oils can be run at higher temperatures without the varnish-
ing effect. It follows that the ideal operating conditions of a hydraulic system
are a complete absence of gas and water contamination in their free and
soluble forms.

Synthetic Fluids
Perhaps we should mention briefly other types of hydraulic fluids such as
synthetic fluids which for our purpose may be defined as nonmineral fluids.
For some of the synthetic fluids, water could be a component, not a con-
taminant.
The popular fire resistant phosphate esters become frequently corrosive
due to development of acid in the presence of water and exposure to high
temperature [7]. Dehydration and corrective adsorptive filters are used to
maintain condition of the fluid and stop erosion occurring in the system.
Water is a contaminant for automotive brake fluids. These are mostly
a mixture of glycols, polyglycols, polyglycol esters, lubricating additives,
inhibitors, and some castor oil. In modern brake systems, particularly disk
brakes, higher temperatures are encountered and the boihng point must be in
the 460 to 550 F range. Water lowers the boiling point considerably. The
brake fluid is highly hydroscopic and mixes with water in any proportion.
Only one half percent of water in fluid can reduce boiUng point by approxi-
mately 50 F.
Mineral oils are also contaminants for brake fluids, as they affect the rub-
ber parts of the system.
Solvents, fuels, and other types of fluids also may contribute to contamina-
tion. Proper separation of the fluids in handling and maintenance will prevent
accidental mixing.

Detection and Measurement of Contamination

Although this is not in the scope of this paper, the measurements and
monitoring of the system fluid contamination is very important for the proper
maintenance of hydrauHc systems.
It is done mostly on a periodic basis by sampUng. It is possible that in the
future a continuous monitoring method will be applied to some larger or
more critical hydraulic appHcations.
The prevailing methods of contamination detection and measurement are
as follows:

1. Particulate Matter
1.1 Gravimetric—by weight in milligrams of contaminant per 100 ml of
fluid
1.2 By particle count—using microscope or automatic optical or capaci-
tance counters.
1.3 By changes in turbidity of fluid.

2. Water Content
2.1 Centrifuge for free water.
2.2 Karl Fischer for total water content.
2.3 Boiling point for brake fluids.
2.4 Optical—by turbidity change.
2.5 Water in oil meters, conductivity type.

3. Gas Content
3.1 Vacuum decay in vacuum chamber.
3.2 Direct reading method—gas content meters.
3.3 Oxygen content monitor.

4. Other Contaminants
4.1 Acidity by neutrahzation number.
4.2 Surfactants by interfacial tension.
4.3 Dilution by distillation.
4.4 Chemical by infrared spectrometry.

Purification Practices
Contamination control can be exercised as preventive or corrective meas-
ures.
Table 3 shows purification methods. Table 4 shows most common practices
in maintenance of cleanliness of fluids and systems.
Figure 2 shows the usual locations of the filters and other purification
devices in a typical hydraulic system. Each location has some advantages and
some Hmitations.

TABLE 3—Purification methods.

Removes

Solids Water Gas Other

Method h 1 Free Sol Free Sol Dil Add

Settling X X X
Centrifuge X X
Filtration X X
Coalescence . . . . X X X
Absorption X" X°
Adsorption X" X X
Vacuum X X X X X
Air drying X X X
Vacuum filter... X X X X X X X

' Limited.

TABLE 4—Purification practices.

1. Prior to operation:
1.1 System flushing
1.2 Filtration of fluid at the source
1.3 Filtration and purification while filling the system
2. During the operation of the system:
2.1 Suction filters
2.2 Pressure filters
2.3 Bleed filters
2.4 Discharge filters
Mode of operation is intermittent
3. Continuous operation on bypass:
3.1 Fine and superfine filtration
3.2 Dehydration
(a) By dry air or gas
(b) By blotter media
(c) By vacuum
3.3 Degasification
3.4 Corrective treatment

The changing filtration requirements of the last 15 years indicate a trend

towards better degree of filtration or fine filtration in the micrometer range
1 to 5 ^m-
This in turn imposes some limitations on the flow rates through the filter
and may lead to a relatively short life of the filter element.
With a fine filter located in high pressure part of the system, Location 2,
the cartridge changes become very inconvenient and cause introduction of
hard to displace air into the system. This location, however, is ideally suited
for protection of the servo valve from the effects of pump erosion.

@
Hydraulic Cylinder

Rttura Line By-Pass

=HP>
Low Pressure Pump

FIG. 2—Location of filters and purifiers in hydraulic systems.

Further limitations are imposed on the size of the fiUer housing due to
high pressures.
The discharge filter, Location 4, is not the best location. It is subjected to
sudden return flow, and hydraulic shocks operate only part time of the work
cycle.
The suction filter on the pump intake. Location 1, is advantageous for
the protection of the hydraulic pump. There are, however, severe limitations.
Fine filtration media cannot be used, as the restrictions of the flow cause
cavitation of the pump. A coarse inlet filter will protect the pump only from
larger particles, which, any way, should not be present in a clean system.
Orifice or bleed type filters. Location 3, can be of a low pressure case
design; however, they operate only when the system is running and take only a
small percentage of the pump flow.
These limitations do not apply when the filter is located on the bypass
loop. Location 5. The filter housing on the separate low pressure circulating
pump 25 to 50 psig can be considerably larger and carry more filter cartridges.
Filter elements can be changes at any time without interference with the
operation of the hydraulic high pressure system. Also, air introduction into
the system is ehminated.
Furthermore, the purification devices for water removal, degasification
of fluid, and a corrective filter also should be installed on a separate loop on
the fluid reservoir.
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BARANOWSKI ON HYDRAULIC FLUIDS AND SYSTEMS 27

The ultimate results can be obtained by continuous filtration or purification

or both on a bypass loop at a flow rate slightly greater than that of the main
hydraulic pump or pumps.
The discharge from the purification loop into a hydraulic pump suction
manifold would assure 100 percent purified and filtered fluid supply to the
pump and eliminate the need for the suction filter.
In some instances a central purification system may serve the reservoirs
of several hydraulic systems, using the same fluid.
The use of a continuous purification device on the bypass from the reservoir
and the protection of the air space above the fluid in the reservoir will result
in cleanliness of both.
To enlarge the scope of this paper, I would like to show a system used for
dehydration and degasification of the brake fluid used in the automotive
industries. The brake lines are evacuated by vacuum and filled with degassed
and dehydrated fluid under vacuum in order to eliminate the possibility of air
pockets and mushy operation of the brakes (Fig. 3).
Perhaps vacuum bleeding and filling could be a good practice to follow
in other types of hydraulic systems.
Recently a central single point evacuation and fiU system is considered
for the automotive assembly lines. High vacuums applied at the master
cylinder will remove air. The void will be filled with dehydrated and degasi-
fied brake fluid in the upright stages of car assembly in a 1 to 2 min automated
cycle.

FIG. 3—System used for dehydration and degasification of the brake fluid used in the
automotive industries.
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28 HYDRAULIC SYSTEM CLEANLINESS

Conclusion
Hydraulic systems and fluids can be kept clean, dry, and free of dissolved
and entrained gases.
Proper maintenance of hydraulic fluids should start at the refinery and
continue during the storage and transfer to its destination.
Once in the system, the hydraulic fluids should be maintained at the lowest
possible contamination level by fine filtration.
Additional means of purification should be provided if water and entrained
air or chemical deterioration are present.
The advantages of a continuous bypass filtration and purification should
be considered as means to assure a constant level of cleanHness, independent
of the operating cycle and frequency of the system activity.
Closed systems, vacuum fill, and degasification of hydraulic fluids are
predicted as an ultimate target in hydraulic system design and maintenance
[8].

References
[1] Jacobs, Edwin, assistant editor, "Is Contamination Control Possible?" Hydraulics and
Pneumatics, Nov. 1967, pp. 128-129.
[2] Twenty-Fifth Annual ASLE 1970 Meeting Program, 4-8 May 1970, American Society
of Lubrication Engineers, Chicago, 111.
[3] Briggs, J. A., "Determining Contamination Levels in Hydraulic Systems," Annual
Meeting, American Society of Lubrication Engineers, Philadelphia, 5-9 May 1969.
[4] Bucher, L. T. and Beerbower, A., "State of Art of Gas-Liquid Solubility Experiments,"
Annual Meeting, American Society for Testing and Materials, 21-26 June 1964,
Chicago, 111.
[5] Magorien, V. C , "How Hydraulic Fluids Generate Air," Hydraulic and Pneumatics,
June 1968.
[6] Magorien, V. G., "What Is Bulk Modulus and When Is It Important?" Hydraulic and
Pneumatics, Nov. 1969, Chicago, 111.
[7] Wolfe, G. F., Cohen, M., and DimitrotT, V. T., "Ten Years Experience with Fire
Resistant Fluids in Steam Turbine, Electrohydraulic Controls," Lubrication Engi-
neering, Jan. 1970.
[8] Baranowski, L. B., "Degasification and Dehydration of Hydraulic Fluids and Systems,"
National Conference on Fluid Power, Chicago, 111., 20 Oct. 1967.

Fluid Conditioning for Servo Systems

REFERENCE: Coroneos, J. N., "Fluid Conditioning for Servo Systems,"

Hydraulic System Cleanliness, ASTM STP 491, American Society for Testing and
Material, 1971, pp. 29-33.
ABSTRACT: In the design of hydraulic servo system filtration systems, the de-
signer must be aware of the effects of contamination in a servo system. The
tolerance level of a system will be affected by the physical characteristics of the
servo components. The designer must take into consideration the critical clearances,
type of force motor, and whether valve dither is used. When all the component
facts are known, the proper filtration system can be designed to provide the re-
quired reliability and component life.
KEY WORDS: servovalves, servomechanisms, filtration, contamination, parti-
cles, hydraulic equipment, hydraulic fluids, clearances, tolerances (mechanics),
silts, agitation, evaluation, tests

In the design of electrohydraulic systems, a major factor in ensuring its

success will be a properly designed and maintained filtration system. With-
out proper attention given to the fluid conditioning, all system components
will experience shortened service life. The servo valve itself, in addition to
shortened life, will more than likely cause lowered system response and
reduced system accuracy. The filtration system must eliminate to an accep-
table level, the foreign materials such as metal particles and airborne dirt
which may enter or are generated by the hydraulic system itself.
A starting point in the process of designing the filtration system is t o
determine the critical clearances of the selected servo valve. Since 1 /im is
equal t o 1 X 10~6 of a meter or approximately 0.00004 in., the critical
clearances, therefore, can be related to the micronic filtration rating required
for the system.
Typical critical clearances of servo valves follows:
Servo Valve Type Typical Critical Clearance

Medium pressure sliding plate 0.0008 to 0.001 in. = 20 to 25 Mm

High pressure sliding plate 0.0002 to 0.0004 in. = 5 to 10 »im
Flapper nozzle between flapper and nozzle 0.0008 to 0.0020 in. = 20 to 50 Mm
Typical servo valve orifice size 0.004 to 0.30 in. = 100 to 7,500 M™
Spool clearance 0.00004 to 0.00016 in. = 1 to 6 jim
1
Chief systems engineer, Racine & Vickers-Armstrongs, Inc., Racine, Wis. 53404.
29
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30 HYDRAULIC SYSTEM CLEANLINESS

The critical clearances normally decrease as the system requirements progress

from single stage medium pressure sliding plate to multistage high pressure
spool valves. In order to ensure proper attention to the filtration requirements,
an appreciation of the effects of contaminants within a servo system should
be understood by the designer.
When contaminants become lodged between the torque motor clearances
in the flat plate valve or between the flapper and nozzle in the first of a two
stage valve, the total servo system response will be lowered. In the single
stage valve the maximum valve flow rate will be decreased due to restricted
valve opening. The response time of a two stage servo valve will be reduced
since the flow from the first stage determines the speed of response of the
second stage spool. Because most servo systems are designed for the normal
valve flows and response rates, this type of contamination usually results in
lowered system accuracies. This contamination, if allowed to proceed, will
result in the single stage valve becoming inoperative at one flow rate and the
two stage valve failing at its maximum flow rate in one direction or the other.
Sudden and complete failure can occur when a particle large enough to
completely block spool or plate motion enters the valve. This happens when
the particle is just the proper size to enter the critical clearance between the
moving parts. Small particles will pass through the valve, whereas the particles
larger than the critical sliding clearance will be prevented from entering
between the moving parts.
When servo valves contain internal filters these filters may become con-
taminated at different rates. This is especially noticeable in valves with
parallel filters in spool end cavities. When filter differential contamination
rate occurs, an increase in the pressure differential between spool ends shifts
the spool gradually according to supply pressure changes. This type of valve
is pressure sensitive, and the condition will gradually worsen as the filters
plug until complete loss of control occurs.
Silting is another condition which can occur when the servo valve is
maintained near its null position for a length of time. It is most common in
the second stage of spool type servo valves or high pressure sliding plate
valves where close tolerances are held to minimize valve leakage. Minute
particles are introduced by the fluid leakage between the mating parts, and
these close tolerances act as a very fine filter increasing the total valve friction
characteristics. Silting increases the valve hysteresis which can greatly reduce
system response especially at low amphtude input signals. If for example,
the servo valve gain normally is set at 500 psi/mA, a 1-mA signal could
produce a net change of 500 psi. The effect of silting can be seen if the friction
level on the spool is increased by a force equal to 10 lb. Assuming a }/^-\n.-
diameter spool, a differential pressure equal to 200 psi would be required to
overcome this increased frictional force. This would be equal to an input
signal in the magnitude of 0.2 mA before any valve response would be seen.

Contaminants within any electrohydraulic servo system can cause increased

wear in which degradation of the system occurs over a relatively long term
period but at a rate above the normal design life of the components. Particles
larger than the critical clearances normally do not cause wear problems since
they are too large to pass through the components. Particles substantially
smaller than the critical clearances, however, will accelerate the wearing rate
of the valve as they strike orifices, nozzles, and flapper surfaces, causing the
edges and surfaces to become galled and misshaped. Particles of the approxi-
mate size as the critical clearances cause the greatest and fastest wear rate.
As they move through the moving members, they act as a lapping compound
wearing away the two mating parts. This action increases clearances between
the spool or sliding plate and the valve body which increases the leakage rate
and wears the metering edges causing lower system response.
On many servo valves dither is required to maintain the system resolution.
Resolution is the maximum increment of input current required to produce a
change in the valve output flow. Dither is an a-c input current superimposed
on the servo valve input. This small amplitude signal maintains spool motion
at all times considerably reducing the effects of slip-stick friction. This con-
tinuous motion also minimizes the effects of silting which occurs when the
system is such that the valve remains near its null position for any length of
time. In many servo systems proper fluid filtration can reduce or eliminate
dither requirements and still provide the required response rates.
Metal contamination normally is generated within the hydraulic system by
moving parts such as hydraulic motors, pumps, valves, and actuators. These
particles are normally ferrous and have been shown to be one of the greatest
contributors to servo system failures since they are normally heavy, jagged
particles that tend to retain their shape. Also many of the single stage flat
plate type servo valves are designed to operate with the force or torque motor
immersed within the operating fluid. Since these devices normally contain a
very strong magnet it is the natural place for these ferrous materials to accu-
mulate, preventing full movement of the torque motor reducing maximum
valve opening. System performance is reduced as the amount of contaminants
increases until eventually complete failure takes place.
One way to reduce this problem, other than removing the motor from the
fluid, is to provide a shield around the torque motor. Normally this is not
designed to seal the force motor push rod which would decrease the resolution
of the valve but to provide an enclosure to prevent circulating fluid from pass-
ing across the torque motor. Because the material is normally ferrous a high
efficiency magnet filter ahead of the valve inlet can be used to remove most of
the detrimental portion of the contaminants smaller than the designed filtra-
tion.
Once the overall system requirements are known, the properly sized servo
valve with the required characteristics can be selected. It is then possible to

determine the critical clearances and to proceed with the filtration system
design.
Starting with the system supply pump, the most common placement of the
first system filter would be in the pump suction Hne. These are normally rated
from 10 to 144 MHI with the most common being from 74 to 144 to prevent
cavitation. The inlet to any pump must not be restricted; therefore, most
filters of this type are provided with bypass arrangements to allow fluid to
enter the pump in the event the filters become clogged before they are changed.
In order to prevent dissolved air from being released from the fluid due to the
vacuum in the filter housing, the submerged type units which are serviceable
from the exterior of the reservoir are preferred.
Another common placement of filters is in the system return lines. In this
location it is possible to remove contaminants that are generated by the
system components. This arrangement also protects the pumping equipment
once the system is in operation, assuming the pump case drain lines also are
interconnected through the filters. It is important, however, to make sure all
components can tolerate the increased back pressure as the filter differential
pressure increases. For this reason, filters used in the return lines normally are
provided with a bypass to prevent damage in the event they become plugged.
Since it is usually possible to maintain some nominal back pressure on a sys-
tem, the filter ratings are normally in the 3 to 25 fim range.
Separate pump return line filters also are commonly used to prevent con-
taminants from entering the system from the pumping mechanism itself.
Again, however, it is normal to use bypass filters to prevent damage to the
pumping equipment in case of a clogged element. Because normal return
flow from pumping equipment does not exceed 10 percent of the rated out-
put, it is possible to install filters with 3 to 10 Mm in these fines at reasonable
cost.
Main hne pressure filters directly before the servo valve are the surest
protection that can be provided. It should be selected with its micronic rating
compatible with the smallest critical clearance of the component it is to
protect. The system should be designed to locate this filter with minimum
fine length between it and the servo valve to decrease the possibility of con-
taminants being introduced into the servo valve during the initial startup.
These contaminants take the form of loose dirt, chips, and scale which may
be present in the original installation. Maintenance of long pipe runs also can
cause problems if reasonable care is not used during repair or replacement.
In the case of the servo valves with small critical clearances, it is advisable to
use filters without bypass relief valves to prevent contaminants from entering
the system if the filter is allowed to reach its clogged condition, and to avoid
filter bypass under cold starting conditions due to the larger pressure drop
caused by increased fluid viscosity.
In many servo systems where down time is critical, it is advisable to use
dual filter arrangements in the pressure fines. Each filter should be capable of

full system flow with manual or automatic valving arrangements to allow

changeover and element replacement during operation. Mostfilterstoday can
be provided with both visual and electrical indication for use in determining
filter change requirements. In all nonbypass filters a relief valve arrangement
or filter elements designed to withstand full system pressure without ruptur-
ing should be used to prevent contaminants from entering the system. Many
filters also can be provided with magnetic rods to help collect ferrous materials
smaller than the micronic rating.
The hydraulic reservoir should be designed in a manner to prevent the
introduction of unfiltered air during operation. All air entering the reservoir
should pass through a minimum of lO-^m air breather. By preventing un-
necessary foreign material from entering the reservoir filter element life can
be greatly increased and failure rate of the servo equipment reduced.
Selection of the filters with properly selected micronic ratings, dirt holding
capacities, and differential pressure drop indicators together with their proper
placement will prevent many of the hydraulic failures seen in today's servo
valve system. The fluid conditioning system should be a major consideration
in designing any hydraulic system to ensure the life expectancy required by
the industrial user.

Improve System Contamination Control

and Increase System Efficiency

REFERENCE: Duncan, J. P., "Improve System Contamination Control and

Increase System Efficiency," Hydraulic System Cleanliness, ASTM STP 491,
American Society for Testing and Materials, 1971, pp. 34-38.
ABSTRACT: This paper presents a manufacturer's view of the critical areas
regarding contamination control based on our experience with both hydraulic
components and systems. Fluid contamination is a leading contributor to reduced
system efficiency, and its control should be given consideration during system
design. Contamination sources and the acceptable contamination levels are pre-
sented in addition to the types of contamination normally encountered. The ratings
and location of filters in the system are discussed, and a brief outline for a main-
tenance program is included at the end of the paper.
KEY WORDS: hydraulic equipment, maintenance, decontamination, hydraulic
fluids, oil filters, contaminants, cleaning, efficiency, evaluation, particle size

This paper is presented from the viewpoint of a manufacturer of hydraulic

components. It is intended to point up some of the areas that are critical
regarding contamination control. These comments are based on our experi-
ence with both individual components and total systems. Most manufacturers
and users will agree that contamination in hydraulic systems should be
avoided, but our problem is what degree of contamination can be tolerated
(no system will ever be or need t o be free of all contamination) and how t o
effectively control contamination. As we have striven to gain system efficiency
we have learned that it is necessary to employ a sound program for system
contamination control.

Importance of System Contamination Control

Hydraulics are being put to a wide range of uses, varying from applications
requiring sophisticated and complex controls to systems that are very basic in
function. Demands are increasing for components with improved reliability,
efficiency, and lower maintenance costs. The efficiency of a component, in
particular a pump, is critical to the overall system performance whether it
is used on a Lunar module or your neighborhood garbage truck. Degradation
1
Section chief, systems, Denison Division, Abex Corporation, Columbus, Ohio 43216.
34

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DUNCAN ON SYSTEM CONTAMINATION CONTROL 35

of performance results in time and money lost and in certain applications

threat to human life.
Hydraulic pumps and controls are devices requiring close tolerances,
controlled wear surfaces, accurate finishes, and an adequate supply of clean
hydraulic fluid with the proper characteristics. The lubrication of moving
parts in a hydraulic pump or motor is a prime function of the hydraulic fluid,
and fluids that are contaminated will not provide satisfactory lubrication
with the result that critical parts in the system will deteriorate causing an
inefficient and malfunctioning system. Fluid contamination is a leading con-
tributor to reduced efficiency, excessive down time, and increased mainte-
nance costs in hydraulic systems. The dependence of the system on the
hydraulic fluid has increased greatly in the last few years as demands for
small, high horsepower packages, at low cost, have resulted in operating at
higher speeds and pressures, in turn producing more friction requiring better
lubrication.

Sources of Contamination
System contamination control should be given the same considerations as
component selection and maintenance to help ensure that a hydrauhc system
will provide the desired performance and reliability. System contamination
control means very simply, to be sure the system is clean when assembled and
to be sure it stays clean. There are three major sources of contamination
that must be considered.
1. Contamination at assembly due to the condition of parts and compo-
nents and contamination introduced from sources such as rags, welding,
threading, etc.
While it is not necessary to maintain a "clean room" assembly operation,
some basic precautions will help avoid premature system malfunctions.
(a) Keep all openings into the hydraulic system adequately covered.
(b) Take steps to ensure that pipe and fittings are free from rust, scale,
and dirt.
(c) Remove burrs from parts prior to assembly.
2. Contamination which is introduced into the system after assembly from
the environment or human sources.
The human element can be a source of many contamination problems due
to a lack of education regarding the system (not usually a lack of formal
education). The following are common findings.
(a) Fluid added to a system which is of unknown origin and condition.
(i) Filter elements removed from filters entirely to reduce the frequency
of maintenance.
(c) Filter elements "cleaned" in solvent which is heavily contaminated.
3. Contamination generated internally due to wear of a component or a
malfunction generating metal particles or fluid breakdown due to excessive
temperature or improper maintenance.

Acceptable Levels of Contamination

The level of contamination control required varies with the application,
and each system should be evaluated using its specific requirements. There
is some correlation between operating pressure and the allowable contamina-
tion level; however, each application should be reviewed from an overall
standpoint. It is generally true that a 2000-psi system can tolerate more con-
tamination, therefore wear, than a 5000-psi system before appreciable effi-
ciency losses are noted; however, if a servo control is used in this 2000-psi
system, a high level of cleanliness is necessary to ensure proper valve opera-
tion. Servo components (valves, motors, etc.) require 3 to 5-fim nominal
filtration with an overall system contamination level not more than a Class 6
[National Aerospace Standard (NAS) 1638, see Fig. II]. Other components
generally should have lO-^m nominal filtration with a system contamination
level not over a class 8 (NAS 1638). Filtration systems larger than lO-^m
nominal ratings can be used with discretion but do not provide the optimum
in system cleanliness for most hydraulic components.

Filter Ratings
It is recommended that the micronic rating of the system filter elements
be selected based on the finest filtration required. If there is a servo valve
in the circuit then the filters should be sized at 3 to 5 fixn. Even though only
a small percentage of the total system flow may pass through this valve
during a given time (flow rate), the entire volume of system fluid (capacity)
will eventually pass through it during operation. If a small capacity 3-Mm
filter were installed for this valve and a IO-MHI filter used for the remainder
of the system, the 3-tim filter would actually become the prime system filter
which would result in excessive maintenance costs for the system as the small
filter could not handle the large volume of contaminants from 10 to 3 ixm.
Critical Period
The hydraulic system sees a critical period regarding contamination during
the first few hours of operation. There has been a relatively large amount of
contamination put into the system as it was assembled due to scale on metal,
small burrs being chipped off, residue from assembly functions such as weld-
ing and pipe threading, dirty materials, and numerable other sources. These
contaminants will normally pass through the filtration system during the
first few hours of operation and be trapped by the filter elements; therefore,
it is necessary to replace all filter elements after the first few hours of system
operation.
Types of Contamination
There are many different types and classes of contamination that can
seriously damage hydraulic components. We normally visualize the large
particle type when discussing contamination as we can see most of these

pieces either with the naked eye or low power magnification. These particles
are too large to pass through the clearances in the components and can be-
come wedged between parts resulting in galling, excessive heat, and usually
a basket case failure. A more subtle problem is caused by the contaminants
under 25 nm which require high magnification to detect. These particles tend
to embed themselves into soft or porous materials forming an abrasive sur-
face which steadily wears away moving parts. The end result of this wear
will be a steady loss of performance until an unacceptable level is reached and
premature repair or replacement of parts or components is necessary.
Chemical contamination of the system always should be considered. This
can range from an accidental mixture of a foreign substance (such as water)
to the hydrauUc fluid to improper maintenance of the hydraulic fluid itself.
Certain chemical combinations that have been formed in hydraulic systems
which attacked the base material of critical parts. Some hydraulic fluids
should not be mixed even though their functional characteristics are similar
because of possible acid formation. A source of contamination that is often
given too little consideration is the operating environment of the equipment.
An environment involving heavily polluted air may require a sealed system.
The selection of air breathers for open type systems is extremely important
in a contaminated environment. It would be of little value to install a 10-;um
fluid filtration system but use a 100-;;im air breather in a coal mine where
abrasive dust is present.

Placement of Filters
The placement of filters and their design is, of course, up to the discretion
of the equipment manufacturer; however, there are certain areas that always
should be given careful consideration to ensure contamination control and
prevent component damage. Suction strainers are a risk and should be used
with caution. These devices must be installed in the inlet line causing an
increase in the work load of the pump at a critical point. It is generally not
possible to place strainers where they can be conveniently serviced so that
the strainers plug up causing an excessive pressure drop in the inlet fine
resulting in cavitation and probable pump damage due to a lack of fluid.
It is more desirable to establish a system to control the reservoir cleanliness
and to monitor the fluid cleanliness before it is put into the system. Reservoirs
should be designed with contamination control in mind so that the accidental
entrance of foreign material is less likely to occur. Locating access covers
somewhere other than the top of the reservoir is recommended. The selection
of filter flow capacity should be done with the knowledge that as the filter
element becomes contaminated, its surface openings are reduced resulting in
increased pressure drop. Premature bypassing of the filter or excessive pres-
sure drop through the filter negates an otherwise sound filtration system.
It is desirable to filter the drain oil from all pumps and motors in a system
to protect the system from contamination generated from the pump or motor

due to heavy wear or a malfunction. In most instances this contamination

washes out with the drain oil rather than with the main system fluid and can
be readily removed with drain line filtration. This can be usually accom-
plished with low capacity filters or by using an existing filter, providing the
resulting case pressure does not exceed the pump or motor capability.

Maintenance Program
System contamination control programs are never complete unless ade-
quate maintenance is performed during the life of the equipment. Filter
elements are made to deteriorate (become contaminated); therefore, they
must be replaced on a regular basis. Fluids deteriorate due to thermal and
stress cycles, and need replacement on a scheduled basis. The items are con-
sidered necessary for establishing a sound maintenance program.
1. Monitor the hydraulic fluid before it enters the system as well as during
the life of the system on a regulated basis.
(a) Check contamination level by millipore patch and particle count
(Reference NAS 1638).
(b) Check chemical composition.
(c) Check viscosity index.
2. Regularly replace all filter elements including air breathers, if applicable.
3. Keep adequate records of the maintenance performed and the results.

Requirements for an Effective Program

in Fluid Contamination Control

REFERENCE: Fitch, E. C , Jr., "Requirements for an Effective Program in Fluid

Contamination Control," Hydraulic System Cleanliness, ASTMSTP491, American
Society for Testing and Materials, 1971, pp. 39-49.
ABSTRACT: Contamination control in a hydraulic system means that the filtra-
tion equipment of a system has established a fluid contamination level which is
within the contaminant tolerance specifications of the hydraulic components.
Such a control condition is not beyond the realm of practicality and must be
achieved if hydraulic systems are to function in an optimal manner. Sufficient
technology has been developed to permit the specification of the requirements for
achieving, maintaining, and appraising contamination control conditions.
This paper reviews the present status of the various factors which are involved
in achieving contamination control conditions. A general discussion is presented
of such details as sample bottle cleaning, fluid sampling methods, fluid contamina-
tion analysis, component tolerance profiles, and filtration performance specifica-
tions.
KEY WORDS: hydraulic equipment, hydraulic fluids, filtration, tolerances
(mechanics), cleaning, maintenance, contaminants, concentration (composition),
particle size distribution, gravimetric analysis, cleaning

The purpose of this paper is to review the requirements for establishing

contamination control in hydraulic systems and to present a method for
achieving a control condition. The basic requirement for establishing con-
tamination control is simple—the contamination level of the fluid must be
less than the contaminant tolerance of the operating components. In practice,
the main problem is knowing what conditions should be specified and how to
physically implement the system to obtain the conditions.
It should be recognized that the contaminant sensitivity of the operating
components must dictate the maximum levels of contamination which can
be tolerated in a given hydraulic system. A subtle fact is that these maximum
levels are not the same for all hydraulic systems even though the same com-
ponents may be used. The contaminant tolerance of components depends
upon such factors as operating speed, temperature, and pressure as well as the
hydraulic fluid. In addition, the requirements regarding component life and
1
Professor, School of Mechanical and Aerospace Engineering, Oklahoma State Univer-
sity, Stillwater, Okla. 74074.
39

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40 HYDRAULIC SYSTEM CLEANLINESS

reliability are important in establishing the maximum contamination level of

a system.
Once the maximum contamination level of a system has been defined,
the selection of a system filter is of paramount importance. The filter not only
must ensure that the contamination level of the system will be maintained
below the tolerance level of the components but also must exhibit sufficient
capacity to provide a reasonable operating life. In a properly designed system,
the hfe of a filter will depend almost entirely upon the rate of contaminant
ingression.
Techniques are currently available for establishing the contamination
tolerance level of components and for appraising the ability of a filter to
maintain a prescribed contamination level in a system. However, if the
established limits of the components and the capabiUty of the filters are not
expressed in terms which can be compared with the contamination level of
the operating system, contamination control is a meaningless concept.
The method presented in this paper was developed at Oklahoma State
University and has been employed successfully to estabhsh contamination
control in operating hydraulic systems. The method correlates all control
factors in a manner which can be interpreted easily and quickly by the hy-
draulics engineer.

Particulate Contamination Chart

The basis of the contamination control method reported in this paper is
the particulate contamination chart shown in Fig. 1. The chart was developed
to give an added dimension (gravimetric level) to cumulative particle size
distributions of hydraulic system fluid. The particulate contamination chart
is a log-log^ graph (a graph having a logarithmic ordinant (log y) and an
abscissa of (log x)^) which contains superimposed lines of constant gravi-
metric levels. The gravimetric lines were derived on the premise that the
ratio of the contaminant specific gravity to the contaminant shape factor is
equal to one. This assumption has been shown to be quite valid, since the
average density and the shape factor of particulate matter found in hydraulic
systems will both range between 2.5 and 3.5. The log-log^ model proposed by
Cole [If in 1966 has been widely adapted throughout the fluid power in-
dustry and provides the best method to date for linearizing cumulative
particle size distributions from hydraulic systems.
The particulate contamination chart was designed in such a manner that a
distribution drawn on the chart will exhibit a gravimetric level corresponding
to the value of the gravimetric line with which it is tangent. The actual distri-
bution is identified uniquely and rigorously by the extrapolated number of
particles per millihter in the distribution greater than 1 nm and its gravimetric
' The italic numbers in brackets refer to the list of references appended to this paper.

30 40 50 60 70 80 90 100 120 140 160 180 200

PARTICLE SIZE, miwons

FIG. 1—Particulate contamination chart.

level. For example, a 9500-2.0 distribution shown in Fig. 2 statistically con-

tains 9500 particles per milliliter greater than 1 /xm and exhibits a gravimetric
level of 2.0 mg/liter.
The accuracy of the chart in predicting the gravimetric level of a system
fluid has been shown to depend almost entirely upon the amount of deviation
of the actual particle size distribution from the extrapolated straight line

30 40 50 60 70 80 90 100 120 140 160 ISO 200

PARTICLE SIZE, microns

FIG. 2—A 9500-2.0 distribution.

distribution. For example, the gravimetric level of a fluid which is based upon
the particle counts greater than 10 ^m will not be accurate if the fluid contains
a preponderance of particles (significantly more than indicated by a straight
line extrapolation) below 10 jum. Such conditions can prevail in hydraulic
systems containing old oil and where the system filter was not capable of
controUing the build-up of fine particles. In such cases, a comparison of the
actual gravimetric level with the gravimetric level from the chart is important
in the detection and quantitative appraisement of fine particle build-up.
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FITCH ON FLUID CONTAMINATION CONTROL 43

The use of automatic particle counters for obtaining distribution data has
been found very acceptable in most contamination control work. Recent
studies have shown remarkable agreement between particle counts obtained
optically and with properly calibrated automatic particle counters. These
same studies also have verified quantitatively the values associated with the
gravimetric levels on the chart.

Contamination Level Requirements

The maximum contamination level which can be permitted to exist in a
hydrauUc system depends entirely upon the contaminant sensitivity of the
individual components. Although control valves having small clearances and
fluid passages can place severe limitations upon the size and number of
particles in the fluid, in many industrial systems it is the pump that usually
estabhshes the limits of the complete distribution. For example, in high
pressure systems, the pump for volumetric efficiency reasons must exhibit
very small clearances yet have elements moving at extremely high velocities.
Such a combination of wear factors creates excruciating conditions which
must be recognized and considered in order to achieve an acceptable pump
performance, life, and reliability.
If a program of contamination control is to be pursued effectively, the
contaminant limitations of the pump must be determined and expressed
in a form which is interpreted and applied easily. The unique characteristics
of the particulate contamination chart offer such a method because it can
display acceptable particle size distribution regions for any operating con-
ditions or component.
To establish acceptable regions for particle size distributions, the pump or
other sensitive components needs to be subjected to special contaminant
tolerance level tests. The purpose of these tests is to determine what exposure
in terms of particle numbers and sizes will cause a specific performance
deterioration in a given period of time. Such information can be extrapolated
to reflect a maximum particle size distribution necessary to ensure a given
service hfe for the component.
The tolerance level tests for pumps are conducted under full operating
conditions of speed, pressure, and temperature. The oil to be used in the
actual system also is used as the test fluid. Except for the duty cycle of the
machine, an attempt should be made to simulate the various conditions
associated with the appUcation.
The contaminant tolerance level test for a pump is divided into two parts:
(1) the size sensitivity test and (2) the gravimetric sensitivity test. The size
sensitivity is defined as the particle size above which would promote fast,
positive deterioration in the performance of the component. The quantity
or gravimetric sensitivity of a component is defined as the maximum con-
centration of cut-off contaminant which does not cause degradation of
performance. Cut-off" contaminant is a particular test dust in which all
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44 HYDRAULIC SYSTEM CLEANLINESS

particle sizes above the size sensitivity value have been removed. Such con-
taminant characterizes the influence of a specific filter in the system.
A fluid component's sensitivity to the size of contaminant can be estab-
hshed by subjecting the component to various S-nm size ranges of contaminant
under normal operating conditions of speed, pressure, flow, and temperature.
A component such as a pump normally is subjected to a gravimetric level of
each 5-Mni size range contaminant to produce an accelerated effect upon the
component. When the size sensitivity value is reached, flow degradation
is pronounced and positive.
Once the micrometer cut-off size has been determined, various concentra-
tions of contaminant below the cut-off size are added to the same component
to establish the gravimetric level limitation of the component. The results
of a typical contaminant tolerance level test on a hydraulic pump are pre-
sented in Fig. 3.
The data from a contaminant tolerance level test can be interpreted in terms
of a contaminant tolerance profile which reflects operating time. The profile
is derived by extrapolating the test degradation time to a specified operational
time and adjusting the contamination level accordingly. An extrapolated
1000-h profile line has proved to be acceptable for defining an ideal contamina-
tion level and 333-h profile has been used to establish the minimum require-
ments for a filter to protect the component. The tolerance profile shown in
Fig. 4 represents the operating restrictions for the pump presented in Fig. 3.
Any contamination level having a particle size distribution which is below
the 333-h profile would be considered acceptable but not ideal for maintaining
the desired optimality of the pump.

Filter Performance
In order to implement a contamination control program, the capabihty
of filter elements must be expressed in terms which can be directly related
to the requirements of a system. In other words, the performance of a filter

PARTICLE SIZE INJECTED,/!.

10 20 30 40 50
130 —I 1 r-
GRAVIMETRIC LEVEL == 75mg/Jl
size test
120
o o o o o-

110
gravimetric test
100 >

I 90
-0-40^1-
80,
150 300 450 600 750
GRAVIMETRIC LEVEL, mg/Jl

FIG. 3—Typical results of contaminant level lest.

PARTICULATE CONTAMINATION CHART

A PARTICLE SIZE DISTRIBUTION IS
DESCRIBED BY THE EXTRAPOLATED
NUMBER OF PARTICLES GREATER
THAN ONE MICRON AND ITS
GRAVIMETRIC LEVEL.
EXAMPLE: A 5 5 0 0 - 8 . 0 DISTRIBUTION
INTERCEPTS THE ONE MICRON
ORDINATE AT 5500 PARTICLES
AND IS TANGENT TO THE e.Omg/l
GRAVIMETRIC LINE.

30 40 50 60 70 80 90 100 120 140 160 180 200

PARTICLE SIZE, microns

FIG. 4—Pump contaminant tolerance profile.

must be described in such a way that the user will know what contamination
level will be maintained in his system. This means, of course, that the con-
ventional micrometer ratings used for filters are not adequate for specifying
the performance characteristics of elements. A proper rating should reveal
the capability of an element to maintain a given particle size distribution in a
fluid system.
Several methods have been proposed to establish and rate the performance
of a filter on the basis of what it is capable of accomplishing in a system. The
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46 HYDRAULIC SYSTEM CLEANLINESS

method which approaches the conditions of an actual system closest is called

the multipass test. In this test, fluid is circulated through the test filter at
rated flow while a contaminant slurry (a-c fine test dust) is continuously
introduced into the test system. Fluid samples are extracted periodically to
evaluate filter performance. The contaminant slurry is added until the desired
differential pressure across the element is obtained.
The fluid samples are analyzed to determine their particle size distributions.
The distributions provide a direct means of evaluating the capability of an
element throughout its entire useful life. If the element's medium exhibits
improvement in efficiency when contaminant is added, the distributions will
be lower for each sampling period. If the element is initially stable (distribu-
tions are all the same) and eventually "dumps," the final samples will show
a significant degradation in their particle size distributions.
A filter element is rated on the basis of its worst distribution during the
entire test. There are many filter elements on the market today that are classi-
fied as "dumpers." They are constructed in such a way that a small differential
pressure across the media will cause structural deformation and allow much
of the contaminant collected to be released in the system. The multipass test
exposes these elements, and the rating system will reflect their detrimental
behavior.
The value of a filter element rating is based upon the worst particle size
distribution found in the fluid samples. The distribution is identified by the
extrapolated number of particles per milliliter in the distribution greater than
1 /im and the gravimetric level of the distribution. For example, consider the
filter distributions shown in Fig. 5. The element is a "dumper" and received
a rating of 30,000-30.
In addition to establishing the filtration performance rating of an element,
the multipass test also provides the contaminant capacity of the test element.
Thus, a method is available for appraising what particle size distribution an
element can maintain in a system as well as the useful life of the element
relative to another element.

Conclusions
The contaminant tolerance profile for the pump of a hydraulic system
generally establishes the tolerance profile of the entire system. As shown in
Fig. 6, filter distributions above the system tolerance profile are not satis-
factory. A filter distribution below the profile provides adequate protection
for the components and establishes a control mode.
The contamination control method presented in this paper has general
application. By the use of the particulate contamination chart, a method is
available to rate filters, describe the contamination conditions of an operating
system, and represent the tolerance of system components with respect to
contaminant size and quantity. The need for contamination control in

PARTICULATE CONTAMINATION CHART

A PARTICLE SIZE DISTRIBl/TION IS
DESCRIBED BY THE EXTRAPOLATED
NUMBER or PARTICLES GREATER
THAN ONE MICRON AND ITS
GRAVIMETRIC LEVEL.
EXAMPLE: A 5S00 - 8.0 DISTRIBUTION
INTERCEPTS THE ONE MICRON
ORDINATE AT 5500 PARTICLES
AND IS TANGENT TO THE B.Oirg/l
GRAVIMETRIC LINE.

:io'

SPECIFIC CBAVITY
SHAPE FACTOR

10'

d :o'

20 30 40 50 eo 70 80 90 100 120 140 160 ISO 200

PARTICLE SIZE, mleioni

FIG. 5—Filter performance rating.

hydraulic systems is great, and the details of the method presented in this
paper already are being incorporated in new component specifications.
The work highlighted in this paper represents the results of an intensive
study at Oklahoma State University. Appreciation is extended to the spon-
soring members of the Basic Fluid Power Research Program and the U.S.
Army (MERDC) for providing the funds and establishing the goals for this
work.

30 40 50 60 70 80 90 100 120 140 160 ISO 200

PARTICLE SIZE, micron!

FIG. 6—Contamination control conditions.

References
[/] Cole, F. W., "Particle Count Rationalization," American Association on Contamina-
tion Control, 1966.
[2] Fitch, E. C , "A New and Fundamental Criterion for Specifying the Cleanliness Levels
of Fluids for Industrial and Aerospace Utilization," Liquid-Borne Particle Metrology,
Annals of the New York Academy of Sciences, Vol. 158, Art. 3, 1969, pp. 601-611.
[3] Fitch, E. C , "Correlation of Hydraulic Component Contaminant Tolerances with
Filtration Capabilities," Paper 69-DE-44, American Society of Mechanical Engineers,
May 1969.

[4] Contamination Control, Report 69-1, Basic Fluid Power Research Program, Okla-
homa State University, Stillwater, Okla., July 1969.
[5] Fitch, E. C , "Hydraulic Filtration and Component Life Correlation," Paper 690604
Society of Automatic Engineers Farm, Construction and Industrial Machinery Meet-
ing, Milwaukee, Wis., Sept. 1969.
[6] Fitch, E. C , "Measuring Contaminant Tolerances in Terms Compatible with Filtra-
tion Specifications," National Conference on Fluid Power, Chicago, 111., Oct. 1969.

Contribution of Hydraulic Fluids

to System Contamination

REFERENCE: Kirnbauer, E. A., "Contribution of Hydraulic Fluids to System

Contamination," Hydraulic System Cleanliness, ASTM STP491, American Society
for Testing and Materials, 1971, pp. 50-68.
ABSTRACT: In the continuing investigation of causes of contamination in
hydraulic systems, two types of contaminants attributable to hydraulic oils have
been observed. The first is the reaction of some hydraulic fluids with water and
the forming of precipitates. The second is the inability of some hydraulic oils as
delivered to the customers to befilterable.Since this phenomenon is not consistent
with all the oils meeting general categories of specifications, more needs to be
learned about why some oils generate this type of contamination while others
do not.
The test used to differentiate between the (2) types of fluid contaminants is
described, and typical data is presented for petroleum based fluids, including
single grade and multigrade motor oils, automatic transmission fluids, and
hydraulic fluids. Photographs of some of the unusually shaped contaminants are
included. The author suggests a test method which will allow the oil manufacturer
and oil users to screen oils to eliminate those which may have undesirable charac-
teristics. The author concludes with a request that the oil manufacturers and oil
users work together to learn more about the cause of this phenomenon and its
elimination.
KEY WORDS: hydraulic fluids, contamination, water, precipitates, filtration,
lubricating oils, oil filters, additives, plugging, clearances, tests

With the increased sophistication of industrial and mobile equipment

hydraulic systems, contamination control is becoming of considerable im-
portance as a factor in the proper operation and frequency of maintenance
of hydraulic systems.
As a result of this trend a substantial amount of work has been and pres-
ently is being done in hydraulic system contamination, including detection
of its sources, techniques for analyzing and eliminating contamination, and
methods for determining contamination tolerance levels of hydraulic system
components.
1
Corporate consultant, contamination, Pall Corporation, Glen Cove, L. I., N. Y. 11542.
50
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KIRNBAUER ON SYSTEM CONTAMINATION 51

The sources of contamination in hydraulic systems, in the past, have been

generally classified as follows:
1. System generated.
2. Environmental.
3. Built-in contamination.
This paper covers observations we have made concerning petroleum base
hydraulic fluids as a potential source for "built-in" contamination, and raises
the possibility of a fourth class:
4. Reaction products where contamination is generated by reaction with
the hydraulic fluid.
For the purpose of this discussion, the fluids used in industrial and mobile
hydraulic systems are classified as follows:
1. Hydraulic petroleum base fluids, especially designed for use in hydraulic
systems.
2. Automotive type lube oils used as hydraulic fluids.
3. Automotive type automatic transmission fluids used as hydraulic fluids.
It should be noted that automotive type lube oils and automatic trans-
mission fluids are used frequently in the hydraulic systems of mobile equip-
ment. Water-oil emulsion and synthetic hydraulic fluids were not considered
in this study.
We have seen evidence of hydraulic system problems which appear to be
caused by contaminants in or generated by the hydraulic fluid. These have
occurred in both mobile and industrial systems which in one extreme case
caused all of the filters of the hydraulic systems, including the coarse suction
strainer, to become plugged. It is for this reason that we undertook this study.
Review of the data presented in this paper indicates that the contamination
levels of the oils "as received" (as determined by clogging of a laboratory
membrane when filtering the test sample and microscopic examination of the
contaminant) vary substantially depending on the type of fluid and its source.
Also, when adding small quantities of water, certain oils form emulsions or
precipitates, which in some cases can be separated from the oil by filtering
through a membrane.
The eff"ect of water on the filterability and stability of hydrauhc oils was
considered important because water so often is found as a contaminant in
both industrial and mobile hydraulic systems. Also water has been the sus-
pected cause of certain field problems in hydraulic systems.
Except for some preliminary discussion with fluid suppliers and lubrica-
tion consultants, we have not performed any in-depth investigation regarding
the nature and constituents of these contaminants. We have heard several
opinions regarding the possible cause of this phenomenon including the
following:
(a) Clogging of the filter membrane is caused by partial retention of por-
tions of the additives not in solution such as viscosity improvers and detergent-
dispersant additives.

(b) Interaction between additives resulting in filterable polymers.

(c) Membrane clogging is caused by random contaminants in those oils
which are not manufactured and packaged under clean conditions.
The data presented in this paper are prehminary, relating to semiquantita-
tive estimation of the total amount of contaminant. We feel that sharing our
observations made to date will be beneficial to those who contemplate
further investigation of this type of fluid contamination, and to those who
wish to include fluid cleanliness requirements as part of fluid specifications
when selecting a fluid for a more sophisticated hydraulic system with a known
contamination tolerance (or contamination sensitivity).

Test Method Used for Assessing the Contamination Level of the Hydraulic
Fluid Test Samples

Test Fixture
A test fixture per Fig, 1 was prepared. One liter of the fluid sample to be
tested was placed into the container and pressurized by air to 15 psi at room
temperature. The amount (cm^) of fluid filtered prior to clogging a membrane
with an area of 3.87 cm^ was recorded. In addition to pressure testing, tests
were performed using vacuum (per Fig. I).
The membranes used for test were of two pore sizes and two types of
materials:
(a) Epoxy impregnated inorganic fibers S-jxm absolute (also used as filter
material).
(b) Cellulose acetate nitrate laboratory type filters, 5-ixm grade.
The filterability data are reported in terms of milliliters of test fluid filtered
per square centimeter membrane area at 15-psi differential pressure.

V cm' oil to plug membrane

filterability = - = TW^
A 3.87 cm^
for example: 985 cm' of oil passed through the membrane without plugging,
the factor F = 254+.

Microscopic Examination
Part of the test sample was filtered through a laboratory membrane (0.45-
jLtm pore size), and the particles retained on the membranes were examined
microscopically.

Test Specimens
The hydraulic fluid specimens tested are summarized in Table 1. The
hydraulic fluid samples were tested with and without the addition of 1 percent

distilled water. Sources for the hydraulic fluid samples included a large
storage vessel, 55-gal drums, and 1-qt containers which were procured from
local gas stations.

TABLE 1—Preliminary data obtained.

Filterability Factor" of Oils from Various Manufacturers

(pressure filtration tests)

Filterability Factor

Fluid Type Sample No. Without Water, % With 1 % Water

low-30 motor oils... 0 37.5 0

I 32.4 58.2
II 26.0 107.2
III 22.0 31.0
IV 18.0 47.8
V 23.0 26.0
VI 23.0 2.6
VII 39.0 10.1
l o w motor oils X 254+
Y 32.4 51.8
30 motor oils A 6.45 6.45
B 254+ 254+
Automatic transmission fluids
Suffix "A"—Type '" A " 0 254 +
A 196.0 210.0
B 89.0 120.3
C 47.8 43.8
D 18.0 37.8
E 12.9 3.3
F 10.3 10,3
G 6.45 33.5
Hydraulic fluids A 254+ 254+
B 254+ 254+

"The filterability factor is the volume of oil in cm' which can be filtered through a
3-Mm absolute filter disk with 1-cm^ effective area before reaching 15-psi diff'erential pressure.

Data Obtained on Various Hydraulic Fluid Test Specimens

Filterability Data
The data obtained are summarized in Tables 1, 2, and 3. Table 1 shows the
filterability factors when using a 3-Mm filter disk (inorganic fiber membrane)
and 15-psi pressure.
Table 2 shows variations of test results due to change to cellulose acetate
nitrate membranes and change to vacuum filtration.

Pressure
( 15 psl)

1 Liter Reservoir

Membrane Filter Holder

Vacuum
C2 8.5 inches Hg.)

Graduated Flask

FIG. 1—Test fixture.

TABLE 2—Comparison of the effect of pressure versus vacuum filtration

using two types of membranes.

Pressure Test Vacuum Test

(15 psi) (28.5-in. mercury)
Fluid Type Membrane A Membrane B

10W30 motor oil. 22 7.7

23 20.6
18 15.5
37 43
ATF. 196 206
47.8 35.9
89 15.2
Hydraulic fluids. 254+ 254+

NOTE—Membrane A: S-^m absolute pore size disk inorganic fiber membrane. Mem-
brane B: 5-Mm rated disk cellulose nitrate laboratory membrane.

Table 3 shows data from a particular fluid from one supplier. The samples
were obtained from a large storage vessel, a 55-gal drum and from quart
containers procured at local gas stations.

TABLE 3--low30 oil from one supplier (used as hydraulic fluid)

pressure test with membrane A.

Filterability Factor,
Source without water added

Storage vessel 23
55-gal drum 23
Quart containers 23

Reproducibility of Data
An insufficient amount of tests have been performed for establishing the
reproducibility of the test method. However, most of the data of repeated
runs showed less than ± 1 0 percent variation.

Additional Filterability Data

Several filtration tests were performed with the following variations in
testing:
1. Filtration at elevated temperature.
2. Exposure of the sample to ultrasonic energy prior to testing.
3. Agitation of the sample in a blender prior to testing.
There was no significant change in the filterability factor of the test
samples.
Numerous tests were performed by passing the test samples which showed
poor filterability factors through a 3-/im absolute system filter prior to per-
forming the filterability test.
In all cases the filterability factor after filtration was 254+ (that is, no
clogging of the test membrane occurred).

Visual and Microscopic Examination of the Test Samples

Figures 2-6 show photographs of some of the test samples prior to and
after addition and 24-h exposure to 1 percent water. In addition, some of
the contaminants were photographed and are also depicted in the photo-
micrographs in Figs. 2-6. As can be seen, the contaminant on those photo-
micrographs contains particle sizes up to 200 ixm in size.

Discussion of Data Obtained

The data strongly indicated that certain of the fluids tested appear to be
very clean while others appear to be a significant contributor to hydraulic
system contamination. The latter ones could possibly cause improper opera-
tion of a hydraulic system for the following reasons:
1. Clogging of small orifices and clearances in hydraulic system components.
2. Premature clogging of system filters resulting in filter bypass and subse-
quent loss of efl'ectiveness of the filter as a system protector.
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56 HYDRAULIC SYSTEM CLEANLINESS

These two potential problems can be best illustrated as follows:

Possible Effect on Small System Orifices and Clearances

Table 4 shows a summary of typical critical clearances of fluid system com-
ponents.

TABLE 4—Typical critical clearances fluid system components.

Typical Clearance

Item ^m In.

Gear pump (pressure loaded)

Gear to side plate 3^ to 5 0.000,02 to 0.000,2
Gear tip to case 3^ to 5 0.000,02 to 0.000,2
Vane pump
Tip of vane >^ to 5« 0.000,02 to 0.000,04
Sidesofvane 5 to 13 0.000,2 to 0.000,5
Piston pump
Piston to bore (R)'' 5 to 40 0.000,2 to 0.001,5
Valve plate to cylinder 3^ to 5 0.000,02 to 0.000,2
Servo valve
Orifice 130 to 450 0.005 to 0.018
Flapper wall 18 to 63 0.000,7 to 0.002,5
Spool sleeve ( R ) ' 1 to 4 0.000,05 to 0.000,15
Control valve
Orifice 130 to 10,000 0.005 to 0.40
Spool sleeve ( R ) ' 1 to 23 0.000,05 to 0.000,90
Disk type J^tol" 0.000,5 to 0.010
Poppet type 13 to 40 0.000,5 to 0.001,5
Actuators 50 to 250 0.002 to 0.010
Hydrostatic bearings 1 to 25 0.000,05 to 0.001
Antifriction bearings 3^° 0.000,02
Side bearings }4' 0.000,02

" Estimate for thin lubricant film.

' Radial clearance.
Reference: Machine Design, May 1967.

Comparing the radial spool sleeve clearance of a servo valve (1 to 4 /:im)

or a control valve (1 to 23 fivn) with the particle size obtained in some of the
test samples shown in Figs. 2 and 5, it becomes obvious that these particles
in larger quantities conceivably could effect performance of the valve,
particularly with regards to valve silting and stiction.
In addition, if one examines the emulsions or precipitates generated in
some automotive lube oils by the presence of water (most likely due to the
dispersant and detergent additives), one can foresee problems of setthng in
low velocity areas which could result in partial clogging of the heat exchanger
when using this type oil in a hydraulic system.

Possible Effect of Contaminant on Premature System Filter Clogging

1. We have seen cases of premature filter clogging because of an accumu-
lation of substances on the filter element surface which can be best described
as slimy materials. This, by the way, is not necessarily a function of the filter
pore size although filter clogging may take longer as the filter pore size in-
creases. However, we have seen cases where even 100-Mm suction strainers
clogged because of contaminant generated in the hydraulic fluid in the pres-
ence of water.
2. The data indicated that in general the 10W30 motor oils appeared to
show higher contaminant levels (as indicated by the filterability factor) than
the fluids specifically designed for use in hydraulic systems, while the auto-
matic transmission fluids range from good to poor when using the filter-
abihty factor as a criterion. The limited testing on single viscosity material
indicates similar results.
3. When comparing the data in Table 2, there appears to be a reasonably
good correlation between data obtained by pressure testing with Membrane
A (filter medium) and vacuum testing with Membrane B (laboratory mem-
brane). This would indicate that the vacuum test using a standard membrane
filter would be worthwhile to investigate for use in a proposed standard
filterability test procedure (as will be discussed later).
4. The effect of water appears to be more substantial on some of the
automotive lube oils tested, possibly due to precipitates formed by detergent-
dispersant additives.
5. The data presented in Table 3 indicates that the mode of packaging
did not seem to affect the filterability factor of the particular oil tested.

Conclusions and Recommendations

From the above the following can be concluded:
1. Water contamination will have a particle generating effect in some of
the fluids tested.
2. Some of the fluids tested are poorly filterable in the "as-received" condi-
tions even without adding water.
Therefore, it appears advisable to investigate what is causing clogging of
the filter membranes when testing some of the as-received oils. Also, the
forming of precipitates in the presence of water in oil, in many cases, may
make this oil less desirable for use as a hydraulic system fluid.
In order to test this parameter, development of a standard test method
seems advisable. The method I envision is a filterability test similar to that
outlined in this paper which should be performed with and without 1 percent
water contamination. Once developed and validated, the filterabihty test
should be considered a parameter for the selection of hydrauhc fluids. In
addition, it is recommended that long-term storage of hydraulic fluid should
be evaluated from a particle generation point of view.

#. ^*

FIG. 2a—ATF without water—2, 4, 6; ATF with 1 percent water—1, 3, 5.

FIG. 2b—ATF without water—7, 9; ARF with 1 percent water—8, 10; lOW-30 oil with-
out water—11; lOW-30 with 1 percent water—12.

1 j£sS.
, I*.

FIG. 2C—10W-30 oil without water—1, 3, 5; lOW-30 oil with 1 percent water—2, 4, 6.

FIG. 2d—10W-30 oil without water—8, 10, 12; lOW-30 oil with 1 percent water—7, 9, 11.

FIG. 3a—lOW-30 oil. Photographed at X200 magnification using polarized light, slightly
uncrossed polars (85 deg). Panicle longest dimension 100 itm.

FIG. 36—30 oil. Photographed at X20 magnification using bright field light. Largest
particles approximately 100 txm.

\ \
Cm

FIG. 3c—10lV-30 oil without water. Photographed at XS magnification using bright field
light. Particles longest dimension approximately 350 ^im.

FIG. Id—lOW-SO oil without water. Photographed at X50 magnification using polarized
light, slightly uncrossed polars (85 deg). Largest particle longest dimension approximately
30 itm.

" ^W - « ^ {

I
I'
V, ' " ^ ^ J ;^

**.ct
i'
ff* •* «* ^.^^
''-•A-SJl-
^^^*"tw^,i'^«*Utlf

FIG. 4a—lOW-30 oil with I percent water. Photographed at X5 magnification using

bright field light.

FIG. 46—IOW-30 oil with I percent water. Photographed at X8 magnification using

bright field light. Analysis membrane shows a slimy deposit.

FIG. 4c—lOtV-30 oil with 1 percent water. Photographed at X20 magnification using
bright field light. Average particle diameter approximately 50 nm.

FIG. 4d—lOW-30 oil with I percent water. Photographed at X200 magnification using
polarized light, slightly uncrossed polars (85 deg). Largest particle longest dimension approxi-
mately 50 iim.
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64 HYDRAULIC SYSTEM CLEANLINESS

FIG. 5a—lOW-30 oil with 1 percent water. Photographed at X200 magnification using
polarized light, slightly uncrossed polars (85 deg). Needles approximately 30 to 40 imt long.

FIG. 5b—30 oil with 1 percent water. Photographed at X 200 magnification using polarized
light, slightly uncrossed polars (85 deg). Crystals approximately 50 nm.

•'r-.- • m

^ 4." -"

^1 ('• i ^

FIG. 5c—30 oil with 1 percent water. Photographed at X 200 magnification using polarized
light, slightly uncrossed polars (85 deg). Average particle dimension is smaller than 5 ^Lm.

FIG. 5d—30 oil with 1 percent water. Photographed at X 200 magnification using polarized
light, slightly uncrossed polars (85 deg). Average particle dimension is larger than 5 ixm.

k- ^

FIG. 6a—ATF with 1 percent water. Photographed at XS magnification using bright field
light. Black membrane is covered with white deposit.

FIG. 66—ATF with 1 percent water. Photographed at X200 magnification using polarized
light, slightly uncrossed polars (85 deg). Average particle size approximately 5 tim.

FIG. 6c—ATF with 1 percent water. Photographed at X200 magnification using polarized
light, slightly uncrossed polars (85 deg). Largest particle longest dimension 55 nm.

FIG. 6d—ATF with 1 percent water. Photographed at X200 magnification using polarized
light, slightly uncrossed polars (85 deg). Largest particle longest dimension 10 fim.

Acknowledgment
I would like to thank Robert Pratt (Pall Corporation) for his assistance in
developing the data presented and the representatives of several oil com-
panies for technical review of the data pertaining to their particular oil.
In addition, I would like to thank Dick Leslie (Vickers) who has supplied
me with confidential data on other oils.
I also want to thank Dr. H. Rylander (University of Texas, Austin, Texas)
and Dr. E. Klaus (Pennsylvania State University, University Park, Pennsyl-
vania) for their technical review of the data and helpful suggestions concern-
ing possible causes of the problem.
I want to thank A. Siegelman (Glen Cove, New York) for preparation of
the photomicrographs.
It should be noted that the conclusions presented herein are the author's,
and this acknowledgment in no way implies the agreement of the listed
individuals and companies with the author's conclusion.

Status Report on the Tech O / F - 7

Contamination Methods Program

REFERENCE: Marlow, D. A. and Beerbower, A., "Status Report on the Tech

O/F-7 Contamination Methods Program," Hydraulic System Cleanliness, ASTM
STP 491, American Society for Testing and Materials, 1971, pp. 69-77.
ABSTRACT: Part I, written by D. A. Marlow, concerns the activities of ASTM
Committee F-7-C on Contamination. Part II, written by A. Beerbower, concerns
the methods that were developed by Technical Division O of Committee D-2 and
recently turned over to Division C of Committee F-7. These include ten methods
for sampling, three methods for processing samples, and four for analysis of
particulates. Plans for further work to complete the coverage of sampling methods
for additional situations, and some of the problems related to precision statistics,
are described briefly. The new user is urged to avoid using methods more com-
plicated than his situation actually demands.
KEY WORDS: particles, sampling, counting, size determination, precision,
contamination, evaluation, tests

PART I by D. A. Marlow

Introduction
The subject of Part I is "The Activities of ASTM Committee F-7-01 on
Contamination." There are three key points that in essence are applicable to
ASTM Committee F-7 and thereby to Division C, F-7-01 on Contamination.
These points are:
1. The "scope" of Committee F-7 through which Division C functions.
2. The operating policies and the purposes of Technical Committee F-7.
3. The background of Committee F-7 approach to aerospace industry
methods for contamination.
I will begin with Point 1, the Scope of Technical Committee F-7 Aerospace
Industry Methods (AIM). Point 1—The promotion of knowledge of aero-
space, aircraft, and allied industry materials test methods and techniques
and the provision of standards for use in industry through their adoption by,
1
Research specialist, design, Lockheed Missiles and Space Company, Vandenberg Air
Force Base, Calif. 93437. Member ASTM.
2
Senior research associate, Products Research Division, Esso Research and Engineering
Company, Linden, N. J. 07036. Member ASTM.
69

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70 HYDRAULIC SYSTEM CLEANLINESS

or development within, ASTM Committee F-7 and by recommendations

to other ASTM technical committees or to other technical organizations.
Documentation, including methods of test, recommended practice, nomen-
clature, specifications and related technical information developed by this
committee and by others with their consent, will be co-ordinated, letter
balloted or otherwise promulgated.
Areas of standards developments applicable to this scope, but under the
jurisdiction of other ASTM committees or other organizations are excluded
from development by this committee unless those other technical bodies do
not choose to act directly on the development of specific standards needed
by the aerospace, aircraft, and allied industries. In such cases. Committee
F-7 may elect to develop, letter ballot, co-ordinate, and promulgate the needed
standards.
Before I proceed with Point 2,1 would like to note that our committee as it
exists today is the oldest sole ASTM aerospace activity. We feel it is also the
hardest working, most productive, and efficient activity of its kind.
To date, test methods have been the primary documentation output of
Committee F-7 with some work as to nomenclature and recommended
practice, and no work on materials specifications. It is expected that this
pattern of method promulgation will continue for the projected future. In
Key Point 2 certain operating policies of ASTM Committee F-7 and thereby
F-7-01, Division C are set forth to illustrate our service to the aerospace
industry:
Item (a) Letter Ballot—To provide a valid aerospace letter ballot as the
basis of the industry-wide participation and acceptance of material test
method standards.
Item {b) Coordinate—To provide the coordination services required to
find, develop, and process needful and applicable test methods regarding less
of source, and prepare same for the letter ballot.
Item (c) Plan—To plan and schedule the documentation development
work so that duplication is avoided, the broadest application possible from
each document is reahzed, and that any interdependency of the documentation
is recognized and resolved on a timely basis.
Item {d) Develop—To develop test methods, recommended practice, and
nomenclature. Utilize the work of existing activities where possible and
create new documents only where such are required by aerospace industry.
Item (e) Disseminate—To increase state-of-the-art knowledge through the
conducting of Symposia and Panel discussions, by performing data research,
and by publication.
Item (/) Compile—To compile and list the origin of the industry materials
standards in existence or being developed regardless of source.
Item (g) Report—To periodically report to the aerospace industry the
schedule and schedule compliance of Committee F-7 and of the other par-
ticipating or cooperating activities. The report will list material test method

documentation, its state of development, the responsible activity, and selected

state-of-the-art knowledge.
At this point I would like to make clear, so that there is no misunderstand-
ing, that Committee F-7 has no interest in doing or interfering in what is
being done by others. The cooperation of other existing activities is sought
wherever and whenever their output is usable. For the most part. Com-
mittee F-7 documents will be twofold; test methods that apply to more than
one material, and test methods concerning a family of materials, within a
given technology. Contamination is an example of the former, and propellants
is an example of the latter. A single material will be of interest only if there is
no present activity, and if it is predominantly an Aerospace application.
We note, of course, that an overlap of interest is obvious where the above
twofold activities may interface. The only way to curtail duplication of effort
in committee work and still provide a practical, economical service is to
bring the interdependent activities together. This can be accomplished only
when the work is performed in the same family and where master planning
is possible. In this way, document output is maximized while conflicts in
interest and jurisdiction are minimized.
Now let us examine the purpose of Committee F-7 aerospace industry
methods. One purpose of Committee F-7 as previously stated is to create and
adopt test methods, recommended practice, and nomenclature so as to
provide the industry with a comprehensive family of related and interrefer-
encible document standards. This family of standards are for use as design
disclosure and definition documents, as procurement and receiving inspection
documents, as research and development and manufacturing documents, as
quaUty assurance documents, and as contract and buy-off documents.
A further purpose is to arrange and develop standards into a building block
matrix whereby the required documents can be combined to establish com-
plete control or definition with the absence or minimal need of taking excep-
tions to the standards. Mr. Beerbower will pointedly illustrate the building
block matrix in his presentation. A working example is found in Committee
F-7-C contamination methods where the sample procuring, the sample
processing, and the sample analysis may be chosen to fit the application of a
wide range of materials and end-use requirements by simply stating, for in-
stance, "sample per F 303," "process the sample per F 311, and analyze per
F 312." The standards matrix and recommended practice usage of same is
a time saver and costly error eliminator.
A most needful purpose of this activity is to conserve time, funds, and
manpower through flexibility. This approach brings together interfacing
technologies and develops documents that in themselves are as flexible as
possible in their application and requirements, and that will fit the matrices
or building block concept.
For Key Point 3, the background of our approach to aerospace industry
methods shows Committee F-7 functions to fulfill the needs of the aerospace

industry. A number of these needs are listed here in the form we know best in
F-7-01, Division C; that is, specific directions to produce an answer that will
serve the need.
1. Application of a given document to as many materials as is practical.
2. A family of related documents that can be interreferenced and that
work together.
3. Documents appHcable to a given technology on a timely basis.
4. Documentation applicable to interacting materials.
5. Documentation applicable to interfacing technologies.
6. A documentation matrix system organized to allow quick and accurate
choice of required documents.
7. A means of using documentation now in existence, but currently not
known of or recognized by the aerospace industry.
8. Increasing the state-of-the-art knowledge.
9. Conserving manpower, calendar time, and funds.
10. Providing a vaUd aerospace industry letter ballot that ensures industry
cognizance and that can be of benefit to other ASTM activities.

PART II by A. Beerbower

Early History
By the late 1950's it clearly was established that control of particulate
contamination in the fuel was necessary to prevent rapid and disastrous
failures of jet aircraft engines. This could be easily demonstrated on test
stands, and a realistic level was soon agreed upon, using ASTM Method
D 2276-60 T (published for information in 1959). The level was, and still is,
defined in terms of milligrams per liter with 1 max as the normal shipping
limit. Meanwhile, it became evident that related problems were arising on
missiles, and that both the propellant and hydraulic fluid were involved. The
propellant specifications tended to follow the jet fuel pattern with 1 to 10
mg/liter, but those on hydraulic fluid took a new turn.
It is evident that the problems of a fuel system have inherent differences
from those of a hydrauUc system, and that more is involved than the obvious
fact that many thousands of gallons of fuel are used while the original few
gallons of hydraulic fluid are still circulating. While both systems can fail
by clogging control valves, fuel systems more usually fail by plugging nozzles.
Hydraulic systems sometimes fail by jamming actuators, but it was the great
number and delicacy of the valves in a hydraulic system that led to a new
principle of particle size control as well as quantity control. Reasons for this
change are shown in Fig. 1. Given a diametrical clearance of the valve piston
of 15 fim, the 5-(xm particle is essentially harmless. The 45-Mm particle filters
itself out, but the 15 Mm is just right to silt up the clearance. A 200-/im particle
can jam between spools or under a needle valve tip.
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MARLOW AND BEERBOWER ON F-7 STATUS REPORT 73

I PORT

^ 45 MICRON PARTICLE

PISTON —>• MOTION

15 MICRON PARTICLE
J?
5 MICRON PARTICLE

FIG. 1—Particles in a servo valve.

Unfortunately, due to the classified status of the missile programs, very

little of the data justifying the decision has ever been published, but by 1960
it was customary to specify hydraulic fluids by the laborious particle count
rather than the gravimetric method used for fuels. One "justification" was
the apparently greater sensitivity, but this is an illusion; ovbiously, use of a
larger sample and a better balance builds any degree of sensitivity desired into
the gravimetric method.
As experience developed, it became evident that two other particle char-
acteristics were of great importance. These were shape and chemical composi-
tion. Fibrous particles had to be counted separately as no "diameter" could
be assigned, and they also represented a special problem in that they tend to
mat, forming a larger "particle." On encountering a small clearance, this
stalls and becomes a "filter" which accumulates ordinary particles until
failure is inevitable. Even the 5-/im size participates in this reaction.
The composition of the particles becomes important when one hangs up as
in Fig. 1. A metallic particle may deform and so escape, or even cut its way
out. A siliceous one may crush due to brittleness, or again cut itself loose on
repeated actuation. However, a plastic one will smear and become lodged
permanently. The composition of the particles also serves a very important
purpose in diagnosing the source of contamination.
The importance of the factors of size, shape, and composition reached
back into the propellant technology, and the particle count method has be-
come popular there also.

Tech O Program
Faced with this array of problems, the new Technical Division 0 (formed
from Section I of Tech N) undertook a broad program designed to provide
methods for all contingencies. This was set up by the chairman, J. L. Botkin,
on a "building block" basis so that complicated options such as those in the
jet fuel method D 2276 could be avoided. The basic plan consisted of only
three steps:
1. Sampling.
2. Processing.
3. Analysis.
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74 HYDRAULIC SYSTEM CLEANLINESS

However, a great many possibilities had to be recognized in sampling, and

the original plans (Figs. 2 and 3) are far from complete. At the time of
changeover to Committee F-7, the ten methods shown in Table 1 had been
accompHshed. These cover a wide spectrum of conditions, and many are
applicable to industrial systems. Very few difficulties were encountered in
this part of the program, as these were generally formalizations of existing
aerospace (National Aeronautics and Space Administration NASA) and
U.S. Air Force (USAF) practices.
In the following discussion, it is important to note that all methods were
renumbered when transferred from D-2 to F-7 in mid-1970. The new F num-
ber is cited first, with the old D number in parentheses.

TABLE 1—Sampling methods.

New Old
Number Number Source of Sample

F302 D 2388 Containers in field

F318 D 2407 Air in clean rooms
F 303 D 2429 Components
F301 D 2437 Noncryogenic system taps
F304 D 2535 Components with convolutes
F305 D 2536 Pressure-sensing instruments
F306 D 2537 Storage vessels
F309 D 2542 Noncryogenic propellants
F310 D 2543 Cryogenic fluids
F308 D 2545 Components or systems by blow down

Processing, strictly speaking, is covered entirely by F 311 (D 2391) for

filtering liquid samples. However, as a matter of convenience two methods
for characterizing analytical membrane filters were put in this group. The
basic one is F 316 (D 2499), by which the pore size (maximum, average, and
distribution) may be controlled. The second, F 317 (D 2767), also is related
to porosity but measures the liquid flow rate.
The analysis of particulates includes the two basic methods, F 313 (D 2387)
for gravimetric analysis and F 312 (D 2390) for sizing and counting particles.
In addition, F 314 (D 2430) for identification of contaminants and F 315
(D 2546) for identification of solder and solder flux are used for diagnostic
purposes. Each of the first three ran into problems, but all for different rea-
sons.
F 313 hinges on the use of a microbalance for weighing the particulates
from 100 cm^ of fluid. The first description was protested by some balance
manufacturers as being too restrictive, so an appeal was made to Committee
E-1 for help. After some years of revisions, the description is acceptable, but
Committee E-1 probably will continue to work on a better reference for us to
use.

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MARLOW A N D BEERBOWER O N F-7 STATUS REPORT 75

AEROSPACE APPLICATIONS
SYSTEM SAMPLING

1
LIQUID

1
1
CRYOGENIC NON-CRYOGENIC

1 1
1 1
BOTTLE ON LINE
SAMPLING ANALYSIS

1 1

CLOSED BOTTLE OPEN BOTTLE IN LINE

END LINE ON LINE AUTOMATIC
TAP SAMPLING TAP SAMPLING FILTERING

F 310 F309 F30i>

FIG. 2—Proposed family of liquid sampling methods.

F 312 was carefully set up to permit the use of either the globe and circle
or linear reticule. This proved to be an emotionally charged matter, and is
still not resolved completely.
F 314 provides good methods for the scope as stated. However, there is no
provision for plastic chips (known to be of special importance), paint chips,
or organic materials other than fibers.

AEROSPACE APPLICATIONS
SYSTEM SAMPLING

GAS

BLOW DOWN

IN LINE LINE END PRESSURE

AUTOMATIC
FILTER FILTER VESSEL

F 30a
FIG. 3—Proposed family of gas sampling methods.

Interaction with Other Groups

Considerable friction arose in 1965 on the matter of interactions, and a
Co-ordinating Committee on Contamination was formed by Headquarters
to deal with it. Representatives from Committees D-2, F-1 on Electronic
Parts, D-19 on Industrial Water, D-22 on Atmospheric Sampling and Analy-
sis, the Society of Automotive Engineers (SAE) and the American Association
on Contamination Control (A^C^) met three times. The first item on every
agenda was an apparent overlap of ASTM's F 318 ( D 2407), F 25, and D 2009
methods for air sampling with SAE's Aerospace Recommended Practice 743
(ARP-743). It was agreed eventually that Committee D-22 would attempt to
prepare a unified modification of D 2009 and that the other groups would
continue as they were until then. Another controversy on the similarity of
F 312 (D 2390) and ARP-598 was never resolved, and SAE proceeded to
issue ARP-598 A the same year that F 312 became a standard method, though
the differences are not very evident. The problems proved to be mainly due
to poor communication, which the three meetings helped to correct, and no
further meetings have been held.
The Federal Test Methods Standards (FTMS) Committee had indicated
certain changes desired in D 2390 and D 2391 to permit using these in place
of Method 3009.2. The changes were made in F 312 and F 311, but cancella-
tion of the FTMS method is not yet complete. The Institute of Petroleum
(IP) has expressed interest in these same methods, but negotiations are far
from complete on an ASTM/IP standard.

Precision Statistics
The program is far from complete, as only single laboratory repeatabihties
are available, but it is quite evident that the advantage lies with the "gravi-
metric" over the "sizing and counting particles" philosophy. Table 2 shows
the comparison of F 313 (D 2387) and F 312 (D 2390). The former does not
follow the building-block scheme as it includes its own processing instructions,
so that the difference in precision is probably even greater than that shown.
The results should serve to dispel forever the illusion that "seeing is believing."
Over ten times the error results from the combined effects of misjudgment of
size and failure to count every particle. It is beheved that the former is the
more important, as the error generated by counting all 14-Mm particles as
15 to 25 would be obviously quite significant. The message should be clear:
"If the situation does not demand examination of individual particles, gravi-
metric is best."
Only F 301 (D 2437) has been evaluated for repeatability, and the errors
in sampling even by this sophisticated method were bad enough to mask
almost completely even those due to F 312. The results had to be expressed
in a rather complicated way to be technically correct, but are reduced to
common terminology in Table 2 despite some loss of logic. They do not say

TABLE 2—Repeatability statistics.'

F313 F312 F301

Method (D 2387) (D 2390) (D 2437)

Procedure Gravimetric, % Sizing and Counting, % Sampling, %

2500 particles 26 80
280 particles 34 100
33 particles 40 >100
6 particles 50 >100
Total (weight average) , 39 ~90

" All results are expressed in Committee D-2 format, that "duplicate results by the same
operator should be considered suspect if they differ by more than this amount."

that such results are useless, but merely that "nit-picking" the difference
between (say) 1000 and 2000 particles per 100 cin^ is ridiculous. Counts of
1000 and 3000 are significantly different by F 301 [7]'. However, it is probable
that many of the thousands of man hours spent in recleaning systems, oils,
etc., were wasted because of faith in statistically insignificant results.
Two articles on selecting an allowable contamination level should be cited.
Hocutt [2] describes the procedure used for the YJ93-GE-3 turbojet hydraulic
system, and Huggett [i] shows the effect of contamination control in improv-
ing the performance of a missile hydraulic system.

References
[7] Beerbower, A. and Hadel, J. J., "Evaluation of the Precision of Tap Sampling for
Particulate Contamination," Tech O Symposium, June 1967.
[2] Hocutt, M. G., "Establishing Hydraulic System Operational Contamination Limits,"
SAE Preprint 650333, Aerospace Fluid Power Conference, 18-20 May 1965, pp. 227-
233.
[3] Huggett, H. L., "Servo Valve Internal Leakage as Affected by Contamination," SAE
Preprint 650334, Aerospace Fluid Power Conference, 18-20 May 1965, pp. 238-244.

3 The italic numbers in brackets refer to the list of references appended to this paper.

Filtering —From the Moon to the Mines

REFERENCE: Marsh, W. J., "Filtering—From the Moon to the Mines,"

Hydraulic System Cleanliness, ASTM STP 491, American Society for Testing and
Materials, 1971, pp. 78-83.

ABSTRACT: The rapid advances in the level of hydraulic power utilized in

underground mining equipment has not found necessary filtering and maintenance
techniques keeping pace.
This paper describes methods employed to assure full flow filtration, incorpora-
ing a rather drastic departure from tradition in choice of filter opening to optimize
life of component. Additionally, problems of adverse environment and fluid
handling are described.

KEY WORDS: hydraulic equipment, cleaning, mobile equipment, underground

mining, filtration, maintenance

I feel moved to comment on this title, "Filtering—From the Moon to the

Mines." Obviously, hydraulic cleanliness is not as critical in mining as space
programs. It is, however, at least as demanding in mining as most any other
industry. Frankly, the title is for contrast and attention. We need to some-
what counteract the recent legislative "flurries," and the ever present ten-
dency for the press to report the sensational, and the public tending to be-
lieve the worst. Sure, there are some bad mines, just as we have all seen bad
foundries or bad machine shops. Perhaps this public uplift can be served by a
little better description of conditions as they exist in the modern mining indus-
try. They may be at least partially personified by our equipment sophistica-
tion.
As this is largely a nonmining group, I thought a very brief explanation
of underground procedures would be in order.
If we refer to our general locality, the seams are essentially horizontally
bedded. If the seam outcrops, it is entered by digging straight into the hill-
side establishing a "drift" mine. If the seam does not outcrop, it can be
entered, in relatively shallow instances, by an angled entry, becoming a
"slope" mine. Deeper seams must be approached, serviced, and extracted
through vertical entries, thus, a "shaft" mine.
1
Superintendent, District Maintenance, Frick District, United States Steel Corporation,
Uniontown, Pa.
78
Copyright by ASTM Int'l (all rights reserved); Fri Jan
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MARSH ON f=ILT£RING 79

Seam heights vary considerably, but generally in a range from 30 in. to

about 6 H ft- Also varying considerably are the basic characteristics of the
coals which determine its end use and oftentimes the method of mining.
A mining plan common to the Pittsburgh seam displays a series of tunnels
or "entries," advancing on a predetermined survey course, a predetermined
distance. At planned intervals they are connected by "crosscuts" which serve
to assure a supply of fresh air to the advancing areas or "faces."
Vital and necessary support efforts include the above mentioned ventilation
(fresh air normally is directed up the center entries and "returns" to the
ventilating fan after sweeping the active working places), roof support (which
is instalhng devices to keep the roof rock in place), electrical power distribu-
tion, basic haulage systems, water supply and discharge systems, and often-
times compressed air lines.
People can be best categorized as willing and hard working. Many, how-
ever, served their apprenticeship in an equipment era demanding considerably
less maintenance activity. I think we can best describe attitudes as not par-
ticularly mindful of design hmitations.
Material handhng is beset by long distances and relatively slow travel
speeds. Compounding this is haulage priority of both loaded and empty
mine cars. Fluids and lubricants, of course, are in this category. Let me now
quickly tie an environmental condition with the mention of fluid movement.
The condition to which I refer is just plain dirt. This is not to be confused
with dust in finely divided form, which is controlled by a variety of methods.
But remember, we are not on a paved floor, or under a fabricated ceihng,
nor inside plastered walls. One main point of contaminant entry into hydraulic
systems has been the actual transfer of fluid from a container to the mining
machine. Other potential points in fluid handhng might be from a bulk tank
to oil car, bulk tank to drums, and drums to 5-ga] containers. These latter
points can be corrected by purchasing oil packaged in 5-gal containers—at a
decided premium I might add. The chief off'ender, however, remains the
actual entry of fluid into the machine.
Replacement of hoses and hydraulic components during repair functions
further establishes prominent means of adding dirt in our systems.
The majority of prime producing equipment today are called "continuous
miners." Several early versions [If made their appearance, but these and
subsequent models have never really earned (literally) the title "continuous."
The term "conventional" equipment often is heard. This refers to separate
pieces of equipment which respectively cut a relief area (or kerf) in the coal
face, drill the coal for placement of explosives, and load the coal after shoot-
ing. Shuttle cars are accepted almost universally as the first step of the mine
haulage system.
2 The italic numbers in brackets refer to the list of references appended to this paper.

Today's continuous miner for coal may typically have 500 connected hp.
Weight may vary between 35 and 50 tons. Prime energy source is direct
electrical for the cutting, but electrohydraulics are vital. Moving from front
to rear we find applications in the cutting head elevate and lower, gathering
head elevate and lower, and at times the power for gathering arms and
conveyor chain is hydrauhc. Tramming, or machine movement, is motivated
hydraulically and also all rear conveyor movements. It is this basic design of
machine we will discuss in this paper.
Early last year a recap was made of hydraulic component changeout
activity. It was frankly appalling. Investigation revealed the preponderance
of failures due to fluid contamination (Table 1). These statistics are from
certain steel mill studies, but I doubt seriously that continued analysis of
hydraulic difficulties in the mines will vary a great deal. Also, conversion of
circuitry into standard graphical diagrams revealed eleven areas in the system
that were devoid of fluid filtering.
TABLE 1—Trouble-shooting analysis {10 years).
1. 50%fluidproblems.
2. 25% mechanical failures.
3. 25% suction inlet problems.

The problem was referred to U.S. Steel's Applied Research Laboratory.

The analysis and proposed corrections were presented and accepted. Explana-
tion of a successful filtering revision in a mill application was dramatic
evidence of need. This was the 43-in. hot strip mill at McDonald Mill in
Youngstown, Ohio.
The circumstances were as follows, and I quote [2];
This 43-in. hot strip mill has a 6-stand finishing train following a 4-stand rougher.
Stands of thefinishingtrain are numbered 5 through 10. Stand 9 is equipped with
a prediction-type automatic gage control. In this system, an X-ray gage located
between Stands 8 and 9 measures actual strip thickness. A temperature sensor,
similarly located, measures actual strip temperature. The hydraulic force required
by Stand 9 to reduce the strip to the desired gage is computed in a simple analog
computer. This calculation is constantly changed, as needed, to reflect the devia-
tions monitored by the X-ray and temperature sensors. Controlled screwdown is
accomplished in a closed-loop force system that utilizes a high-response wedge
actuator and a force feed-back signal from the pressductors located in the stand.
This signal, representing actual force in the stand, is compared to the calculated
force required. Any difl^erence between the two will result in a control error signal
to the wedge electrohydraulic servo valve, which then orders wedge motion to
reduce the error to zero
Associated with each wedge actuator is a hydraulic-control enclosure that con-
tains a hydraulic manifold, a servo valve, a filter, shutofF valves, an electrical
junction box, and associated supply and return line piping.
The hydraulic power plant consists of three identical pump-motor combinations,
an accumulator, a reservoir, filters, a heat exchanger, and an integral control panel.
One pump-motor combination is not required for normal operation and is used

for Standby operation. The hydraulic power plant is designed to supply 70 gpm
of clean phosphate-ester hydraulic fluid at 3100 psi at the proper operating tem-
perature to the wedge.
This unit was initially installed with a recommended fire-resistant fluid of a
phosphate-ester type having a viscosity of approximately 120 SUS at 100 F. After
one month of operation, a series of servo valve failures occurred in which the valves
became erratic in operation. Inspection disclosed signs of additive-plating on
the high-pressure lobes of the "slave positions" and definite signs of impingement
corrosion on both the slave pistons and the "pilot pistons." Very little is now known
concerning fluid impingement corrosion or the corrective measures needed to
eliminate the problem. In this case, it was decided to try several types of steel for
the spools. When this failed to stop the problem, it was attacked from a fluid stand-
point. The following measures were taken:
1. A change in fluid was made in order to use a phosphate-ester fluid known to
be a non-additive or "pure" fluid. Also, the new fluid was of a viscosity more
suitable to the pump limitations (namely a minimum of 150 SUS @ 100 F). The
system was flushed and cleaned three times with the new fluid before a full charge
was made.
2. New elements were installed in the original S-^m and 3-^m filters.
3. A bypass "fullers earth" filtering system was installed near the power unit in
order to thoroughly clean the fluid at any time it was deemed necessary.
4. A fluid monitoring program was initiated where daily samples would be taken
during the first week of operation and weekly samples thereafter to guarantee a
clean system. These samples were to be analyzed at the Research Laboratory for
two important conditions; one, the amount of fluid contaminants in the various
micron ranges, and two, the acidity of the fluid. The contamination count was to
be made using the ARP-598 aerospace method of fluid contamination count with
a maximum limit of 15,000 particles in the 5 to 15 /xm range [3].
5. The acidity was to be held at a maximum of 0.08 using the ASTM Method
D-974 [4].
6. If either criteria were not met during these scheduled analyses, the mill was
to be notified and the system was to be shut down and the fluid was to be circulated
through the "fullers earth" bypass system.
In the three years since this system has been operating under these controls,
there has been no further problem with the servo valves.
Table 2 shows the results of typical samples taken recently from this system.
Considering the fact that this is a hot-strip mill, the condition of this fluid in the
aerospace cleanliness range is rather amazing. Bui, it does demonstrate that the
environmental conditions have nothing to do with what happens inside the system—
if proper filtering is adhered to.

TABLE 2—Particulate count method.

Sample Date Neutron ARP Count, Pressure, Temperature,

Number Taken No. 5 to 15 ^m psi deg F
B-233 4- 5-68 0,06 1 320 2 650 125
B-234 4-11-68 0.08 4 200 2 650 138
B-235 4-19-68 0.04 1 320 2 650 132
B-245 7- 5-68 0.06 1 840 2 650 134
B-246 7-12-68 0.06 9 000 2 650 138
B-247 7-19-68 0.08 12 600 2 650 134
B-248 7-23-68 0.04 3 620 2 650 132

Again, the reference to contaminant control is Aerospace Recommended

Practice 598 (ARP-598). This specifically is a "Procedure for the Determina-
tion of Particulate Contamination of HydrauHc Fluids by the Particle Count
Method." This is further defined as classes (Table 3) with respect to maximum
contamination limits. Repeating: work done here is in the 5 to 15 /am size
range. Later reference to particle counts on mining equipment refer to this
standard procedure.

TABLE 3—Aerospace Recommended Practice 598

(5 to IS iim range).

Class Limit, max Class Limit, max

00 125 6 16 000
0 250 7 32 000
1 500 8 64 000
2 1 000 9 128 000
3 2 000 10 256 000
4 4 000 11 512 000
5 8 000 12 1 024 000

This background allowed us to proceed with corrective planning.

1. Fluid samples were drawn from several mining machines and contam-
inant counts were made (Table 4). This was a real "cHncher" to formahze
action.

TABLE 4—Typical 5 to 15 nm count before full-flow fine-filtering.

Miner No. 1 764 000

Miner No. 2 496 000
Miner No. 3 1 497 000
Miner No. 4 1 077 000

2. The original circuitry was studied, revised, and drawn to include all
fluid passing through return filters. Actually, it developed most appropriate
to use five separate filters. I might add that the total maximum flow of oil is
96 gal/min at 3000-psi relief setting. Another point of real importance is that
all make-up oil must pass through a filter prior to entry into the reservoir.
Filter opening size of 3 jam has been chosen. Although initially it was felt to
be a rather optimistic approach, we have been able to obtain good results.
Results to date (Table 5) can not match what our associates have done
in the mill application described (Table 2). Let me hasten to add, however, for
an underground efl"ort, it is not too bad. In fact, I am proud of the eff"orts
of our field personnel. The first miner described was completed in March
1969. Subsequently, we have converted an additional eleven. Filter cartridge
hfe has been surprisingly good.

TABLE 5—Actual 5 to 15 iim count after full-flow fine filtering was installed.

New fluid from bulk storage 133 920

15 minute operation: 10 ^m element 258 960
30 minute operation: 10 ^m element 100 000
15 minute operation: 3 ^m element 49 200
3 days operation: 3 ^m element 116 640
1 week operation: 3 ^m element 52 320
4 weeks operation: 3 ^m element 64 920
5 weeks operation: 3 jum element 39 000
6 weeks operation: 3 ^m element 99 000
13 weeks operation: 3 ^m element 133 920
15 weeks operation: 3 ^m element 72 600

Technical feasibility of fine filtration has been proven in underground

operations. Now the tough problem—changing people's established methods
and building into their work a new awareness. In this instance cartridge
change. Example! Let me add a couple more readings to this table. At four
months the count was 177,000, getting pretty high. At eleven months, soaring
t o 381,000! Here it was decided that control could be best served by local
particle counting. As a result, our own facilities were installed. The counts
have now "come back down from the m o o n . "
The object of all this is reduced component failure. This has happened.
Over the past 14 months, changeouts have been made almost universally
because of mechanical deficiencies. Cleanliness pays!
It is hoped that an easier means of field analysis may be found. A device
currently is being tested. The instrument simply measures light intensity
passing through a sample after first being "zeroed" on a filtered test quantity.
We have not fully decided on its continued usage, but correlations to date
with the ARP-598 method have been fairly good.

References
[1] Lubrication Engineers Manual, United States Steel Corporation, Pittsburgh, Pa.
[2] Schrey, W. M., "The Selection and Maintenance of Reservoirs, Strainers and Filters
for Fluid Power Systems," Applied Research Laboratory, United States Steel Corpora-
tion, Monroeville, Pa., paper presented at the Industrial Hydraulics Seminar of the
Upper Ohio Valley Chapter of ASLE, Wheeling, W. Va., 6-7 Nov. 1968.
[3] "Aerospace Recommended Practice 598; Procedure for the Determination of Particu-
late Contamination of Hydraulic Fluids by the Particle-Count Method," Society of
Automotive Engineers, Inc., New York, 1960.
[4] "Neutralization Number by Color-Indicator Titration," American Society for Testing
Materials, 7965 Book of ASTM Standards, Part 17, p. 334.

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