5.

LUBRICATING YOUR BEARINGS

A. BASIC LUBRICANT FUNCTIONS
1. Elastohydrodynamic lubrication
2. Film thickness on the raceway
3. Film thickness at rib/roller end contact

B. SPEED CAPABILITIES

1. Measuring speed
2. Speed capability guidelines
3. Bearing material limitations

C. GUIDANCE FOR OIL/GREASE SELECTION

1. Grease lubrication
1.1. Soap type
1.2. Consistency
1.3. Filling practices
1.4. Required grease quantities
1.5. Regreasing cycle
1.6. Typical grease lubrication guidelines
1.6.1. General purpose industrial grease

2. Oil lubrication
2.1. Petroleum oils
2.2. Synthetic Oils
2.3. Selection of oils
2.4. Classification
2.5. Typical oil lubrication guidelines
2.5.1. General purpose rust and oxidation lubricating oil
2.5.2. Industrial extreme pressure (EP) gear oil
2.6. Heat generation and dissipation
2.6.1. Heat generation
2.6.2. Heat dissipation
2.6.3. Heat dissipation by circulating oil
2.6.4. Heat dissipation through housing
2.7. Oil systems
2.7.1. Oil level systems
2.7.2. Forced-feed oil systems
2.7.3. Oil mist systems
3. Lubricant additives

D. CONTAMINATION
1. Abrasive particles
2. Water
3. Filtration

E. CONCLUSION
5. LUBRICATING YOUR BEARINGS
Timken Contents

Contents

B A 307

A. Basic lubricant functions

Proper lubrication is essential to successful performance of any bearing. Making the best
bearing selection includes considering the type of lubricant, the right amount of lubricant and
the correct application of the lubricant on the bearing.
The three fundamental functions of a lubricant are as follows:

To separate mating surfaces and reduce friction

To transfer heat (with oil lubrication)

To protect from corrosion and, with grease lubrication, from dirt ingress.

These functions include consideration of the lubrication and generated film thickness on the
raceway (simulated according to elastohydrodynamic effects) and on rib/roller end contact.

5. LUBRICATING YOUR BEARINGS

Timken Contents

A. Basic lubricant functions

B A 308

1. Elastohydrodynamic lubrication
The formation of the lubricant film between the mating bearing surfaces is called the
elastohydrodynamic (EHD) mechanism of lubrication (fig. 5-1).
The two major considerations in EHD lubrication are:

The elastic deformation of the contacting bodies under load

The hydrodynamic effects forcing the lubricant film to separate the contacting surfaces
while the pressure is deforming them.
Pressure

Speed
h

h min.

Speed

Inlet zone

Hertzian zone

Outlet zone

Fig. 5-1
Elastohydrodynamic (EHD) lubrication.
1. Elastohydrodynamic lubrication
5. LUBRICATING YOUR BEARINGS
Timken Contents

A. BasicA.lubricant
Basic lubricant
functions functions/1. Elastohydrodynamic lubrication

B A 309

2. Film thickness on the raceway

EHD lubrication is important because lubricant film thickness between the two contacts can be
related to the bearing performance.
The thickness of the generated film depends on the operating conditions such as:

Velocity

Loads

Lubricant viscosity

Pressure/viscosity relationship.

2. Film thickness on the raceway

5. LUBRICATING YOUR BEARINGS
Timken Contents

A. BasicA.lubricant
Basic lubricant
functions functions/2. Film thickness on the raceway

B A 310

Analytical relationships for calculating the minimum and the average film thickness
have been developed:
Minimum film thickness (based on Dowson Equation):
0.7

hmin = KD (o V)

0.43

0.13
W
R

0.54

where:
hmin = minimum lubricant film thickness
KD = constant containing moduli of elasticity
o = lubricant viscosity at atmospheric pressure
V = relative surface velocity
= lubricant pressure viscosity coefficient
W = load per unit length
R = equivalent radius

2. Film thickness on the raceway

5. LUBRICATING YOUR BEARINGS
Timken Contents

A. BasicA.lubricant
Basic lubricant
functions functions/2. Film thickness on the raceway

B A 311

Average film thickness (based on Grubin Equation):

h = 0.039 ( V ) 0.728 (P/ l) 0.091 ( 1/ R) 0.364

where:
h = lubricant film thickness (m)
= viscosity of lubricant
V = surface velocity
= lubricant pressure viscosity coefficient
P = load between inner race and rollers
l
= effective length contact between rollers and inner race
1/ R
= sum of inverses of contact radii
The major factors influencing the lubricant film thickness are viscosity and speed. Load has less
importance.
These thin EHD films are often not much larger than the surface roughness height.

2. Film thickness on the raceway

5. LUBRICATING YOUR BEARINGS
Timken Contents

A. BasicA.lubricant
Basic lubricant
functions functions/2. Film thickness on the raceway

B A 312

As shown in Section 3 Calculating the performance of your bearings, the fatigue life of a
bearing is related in a complex way to speed, load, lubrication, temperature, setting and
alignment. The lubricants role in this interaction is determined primarily by speed, viscosity and
temperature. The effects of these factors on bearing fatigue life can be dramatic. In a test
program, table 5-A, two bearing test groups were subjected
to identical conditions of speed and load. Differing film thicknesses were achieved by varying
operating temperature and oil grade, and thereby, oil operating viscosity. Life was dramatically
reduced at higher temperatures with lower viscosity and thinner film thickness.
Test
group

Temperature

Viscosity @ test temperature

EHD Film (hmin)

Life %

mm2/sec (cSt)

in

275

2.0

0.038

1.5

13 - 19

150

19.4

0.264

10.4

100

A-1

135

A-2

66

Table 5-A
Relative bearing fatigue life vs EHD film thickness (Constant speed - variable temperature).

2. Film thickness on the raceway

5. LUBRICATING YOUR BEARINGS
Timken Contents

A. BasicA.lubricant
Basic lubricant
functions functions/2. Film thickness on the raceway

B A 313

In another investigation, table 5-B,

viscosity and load were held constant,
but speed was varied producing results
similar to those in table 5-A - higher
speeds produce thicker film and longer
lives.
The selection of the correct lubricant for
any application requires careful study of
expected operational and environmental
conditions. Equations given in Section 3
permit the evaluation of the lubricant
and these conditions relative
to fatigue life in the form of a life
adjustment factor for lubrication.
The calculated L10 life is multiplied by
this factor to obtain a life adjusted for
lubrication effects.

Test group

Speed

EHD Film (hmin)

rev/min

in

B-1

3600

0.102

4.0

100

B-2

600

0.028

1.1

40

Table 5-B
Relative bearing fatigue life vs EHD film thickness.
(Variable speed - constant temperature)

2. Film thickness on the raceway

5. LUBRICATING YOUR BEARINGS
Timken Contents

Life %

A. BasicA.lubricant
Basic lubricant
functions functions/2. Film thickness on the raceway

B A 314

3. Film thickness at rib/roller end contact

The contact between the large end of the roller and the inner race rib is described as an
elastohydrodynamic contact or a hydrodynamic contact (elastic deformations are negligible). As
the roller/rib loads are much lower than the roller/race loads, the film thickness at the rib/roller
end contact is usually larger than the film thickness on the roller/race contact (approximately 2
times more).
Nevertheless, in severe conditions, scoring and/or welding of the rib/roller asperities can
occur. This is related to speed, oil viscosity, load or inadequate oil supply to the rib/roller end
contact. In these conditions, the use of Extreme Pressure (EP) lubricant additives may help
prevent scoring damage.

3. Film thickness at rib/roller

5. LUBRICATING YOUR BEARINGS
end contact

Timken Contents

A. BasicA.lubricant
Basic lubricant
functions functions/3. Film thickness at rib/roller end contact

B A 315

B. Speed capabilities
The speed capability of a bearing in any application is subject to a number of factors including:

Temperature

Bearing setting

Lubrication

Bearing design

The relative importance of each of these factors depends on the nature of the application.
The effect of each factor is not isolated each contributes in varying degrees, depending on
the application and overall speed capability of the design.
An understanding of how each of these factors affects performance as speeds change is
required to achieve the speed capabilities inherent in tapered roller bearings.

B. Speed capabilities
5. LUBRICATING YOUR BEARINGS
Timken Contents

B. SpeedB.capabilities
Speed capabilities

B A 316

1. Measuring speed
The usual measure of the speed of a tapered
roller bearing is the circumferential velocity at
the midpoint of the inner race large end rib
(fig. 5-2), and this may be calculated as:
Rib speed:

Vr =

Dmn

The rib diameter at the midpoint of the roller

end contact can be scaled from a drawing of
the bearing, if available, or this diameter can
be determined by contacting a Timken sales
engineer or representative. The inner cone rib
diameter can be approximated by taking 99%
of larger rib OD.

(m/s)

60 000
Dmn

(ft/min)

12

where:
Inner race rib diameter

Dm = Inner race rib diameter mm, in

n = Bearing speed rev/min

Fig. 5-2
Cone rib diameter. The inner race rib diameter may be scaled
from a print.

B. Speed capabilities
5. LUBRICATING YOUR BEARINGS
Timken Contents

B. SpeedB.capabilities
Speed capabilities/1. Measuring speed

1. Measuring speed

B A 317

DN values (the product of the inner race bore in mm and the speed in rev/min) are often used as
a measure of bearing speed by other bearing manufacturers.
There is no direct relationship between the rib speed of a tapered roller bearing and DN value
because of the wide variation in bearing cross sectional thickness.
However, for rough approximation, one meter per second rib speed is about equal to 16,000
DN for average section bearings.
One foot per minute is equal to approximately 80 DN.

B. Speed capabilities
5. LUBRICATING YOUR BEARINGS
Timken Contents

B. SpeedB.capabilities
Speed capabilities/1. Measuring speed

1. Measuring speed

B A 318

2. Speed capability guidelines

3. Bearing material limitations

Fig. 5-3 is a summary of guidelines relating to

speed and temperature based on customer
experience, customer tests and research
conducted by The Timken Company.
Contact The Timken Company for questions
regarding high speed capability.

Standard bearing steel can operate continuously

at temperatures up to 135C (250F) for
extended periods of time without altering the
hardness of the steel.
For higher operating temperatures, special high
temperature steels are available.
Contact your Timken sales engineer or
representative.

Special high speed bearings with circulating oil

Oil jets

Speed capability guidelines

Oil mist
Circulating oil

Typical industry experience indicates no

problems under ordinary circumstances.

Oil level

Industry experience indicates testing may

be required to optimize system.
Testing will be needed and special bearings

Grease

0
0

10
2,000

20
4,000

30
6,000

50
10,000

40
8,000

100
20,000

200 m/s
40,000 ft/min

Inner race rib speed

Fig. 5-3
Speed capability guidelines for various types of lubrication systems.
B. Speed capabilities
5. LUBRICATING YOUR BEARINGS

2. Speed capability guidelines

Timken Contents

B. SpeedB.capabilities
Speed capabilities/2. Speed capability guidelines/3.
Bearinglimitations
material limitations
3. Bearing material

B A 319

C. Guidance for oil/grease selection

1. Grease lubrication
The simplest lubrication system for any bearing application is grease. Lubricating grease as
defined by the National Lubricating Grease Institute (NLGI) is a solid to semi-fluid product of
dispersion of a thickening agent in a liquid lubricant.
Conventionally, greases used in Timken bearing applications are petroleum oils of some specific
viscosity that are thickened to the desired consistency by some form of metallic soap.
Greases are available in many soap types such as sodium, calcium, lithium, calcium-complex,
and aluminium-complex. Organic and inorganic type non-soap thickeners are also used in
some products.

1. Grease lubrication
5. LUBRICATING YOUR BEARINGS
Timken Contents

C. Guidance
C. Guidance
for oil/grease
/1. Grease
selectionlubrication

B A 320

1.1. Soap type

Calcium greases have good water resistance. Sodium greases generally have good stability
and will operate at higher temperatures, but they absorb water and cannot be used where
moisture is present. Lithium, calcium-complex and aluminium-complex greases generally
combine the higher temperature properties and stability of sodium grease with the water
resistance of calcium grease. These greases are often referred to as multipurpose greases
since they combine the two most important lubricant advantages into one product.

1. Grease lubrication
5. LUBRICATING YOUR BEARINGS
1.1. Soap type

Timken Contents

C. Guidance
C. Guidance/1.
for oil/greaseGrease
selectionlubrication /1.1. Soap type

B A 321

1.2. Consistency
The consistency (hardness) of a grease is defined as the depth (in tenths of a millimeter) to which a
standard testing cone penetrates a grease sample under prescribed conditions.
The NLGI has classified greases, table 5-C, according to consistency as measured by penetration.
Tapered roller bearings generally use NLGI No.2 or No.1 greases. No.3 and heavier greases
are seldom used in Timken bearings because they tend to channel, causing lubricant starvation.
NLGI No.0 or softer grease will circulate readily within the bearing chamber which accelerates
softening and possible leakage.
NLGI number

Penetration range

000
00
0
1
2
3
4
5
6

445 - 475
400 - 430
355 - 385
310 - 340
265 - 295
220 - 250
175 - 205
130 - 160
85 - 115

Table 5-C
NLGI classification of greases.

1. Grease lubrication
5. LUBRICATING YOUR BEARINGS
1.2. Consistency

Timken Contents

C. Guidance
C. Guidance/1.
for oil/greaseGrease
selectionlubrication /1.2. Consistency

B A 322

1.3. Filling practices

When grease lubrication is chosen, the
grease should be packed into the
bearing making sure that it goes
between the rollers and cage. When
hand packing bearings, force grease
through the bearing under the cage
from the large end to the small end to
ensure proper grease distribution.
A bearing completely filled with grease
will purge itself of the excess when
rotation starts. If provision is not made for
grease exit from the cavity, the churning
of the grease could cause excessive heat
generation if rotational speeds are high.
At initial assembly, it is advisable to
smear grease on to the outer race. The
area between the cage and the inner
race should be filled (fig. 5-4).
Fig. 5-4
Grease application at assembly.

1. Grease lubrication
5. LUBRICATING YOUR BEARINGS
1.3. Filling practices

Timken Contents

C. Guidance
C. Guidance/1.
for oil/greaseGrease
selectionlubrication /1.3. Filling practices

B A 323

It may be advantageous to use internal closures to keep purged grease in the vicinity of the
bearing (fig. 5-5). There is a separate Timken Company publication regarding workshop fitting
practice, describing assembly procedures in more detail and showing various proprietary
equipment to inject correct grease quantities into bearings automatically.

Fig. 5-5
Closures to keep purged grease in the vicinity of the bearing.

1. Grease lubrication
5. LUBRICATING YOUR BEARINGS
1.3. Filling practices

Timken Contents

C. Guidance
C. Guidance/1.
for oil/greaseGrease
selectionlubrication /1.3. Filling practices

B A 324

1.4. Required grease quantities

To ensure optimum lubrication, the required
quantity of grease has to be packed into the
bearing. This quantity is based on the volume
of open space in the bearing calculated as
follows:
2 2
V=
(D d ) T _ M
4
7.89

Depending on the application type, the

quantity to fill the bearing will be
approximately:

2/3 of V for conventional mineral grease

1/3 of V for synthetic grease.

To determine the corresponding weight of

grease, approximate the grease density to
0.9 g/cm3.

(in cm3)

where:
V = volume of open space in the bearing
D = outer race O.D. (cm)
d = inner race bore (cm)
T = overall width (cm)
M = bearing weight (g)

If speeds are very low or there is a dirty

environment, it is suggested that the housing into
which the bearing is fitted is completely filled. In
high speed applications, overgreasing will
generate excessive heat which can lead to
lubricant degradation and bearing damage.

1. Grease lubrication
5. LUBRICATING YOUR BEARINGS

1.4. Required grease quantities

Timken Contents

C. Guidance
C. Guidance/1.
for oil/greaseGrease
selectionlubrication /1.4. Required grease quantities

B A 325

1.5. Regreasing cycle

The two primary considerations that determine the regreasing cycle on any
application are operating temperature and sealing efficiency. Obviously,
seal leakage will dictate frequent relubrication. Every attempt should be
made to maintain seals at peak efficiency. It is generally stated that the
higher the temperature, the more rapidly the grease oxidizes. Grease life is
reduced by approximately half for every 10C rise in temperature.
Therefore, the higher the operating temperature, the more often the grease
must be replenished. In most cases, experience in the specific application
will dictate the frequency of lubrication.

1. Grease lubrication
5. LUBRICATING YOUR BEARINGS

1.5. Regreasing cycle

Timken Contents

C. Guidance
C. Guidance/1.
for oil/greaseGrease
selectionlubrication /1.5. Regreasing cycle

B A 326

1.6. Typical grease lubrication guidelines

1.6.1. General purpose industrial grease
These are typical of greases that can be used to lubricate many Timken bearing applications in
all types of standard equipment. Special consideration should be given to applications where
speed, load, temperature or environmental conditions are extreme.
Suggested general purpose industrial grease properties
Soap type
Consistency
Additives
Base oil
Base oil viscosity at 40 C
Viscosity index
Pour point

Lithium 12-hydroxystearate, or equivalent

NLGI No. 2
Corrosion and oxidation inhibitors
Solvent refined petroleum oil
100 mm2/s (cSt) to 320 mm2/s (cSt)
80 min.
110C max.

1. Grease lubrication
5. LUBRICATING YOUR BEARINGS

1.6. Typical grease lubrication

Timken Contents

C. Guidance
C. Guidance/1.
for oil/greaseGrease
selectionlubrication /1.6. Typical
grease lubrication guidelines
guidelines

B A 327

General purpose industrial grease should be a smooth, homogeneous and uniform premiumquality product composed of petroleum oil, a thickener, and appropriate inhibitors. It should not
contain materials that are corrosive or abrasive to tapered roller bearings. The grease should
have excellent mechanical and chemical stability and should not readily emulsify with water.
The grease should contain inhibitors to provide long-term protection against oxidation in highperformance applications and protect the bearings from corrosion in the presence of moisture.
The suggested base oil viscosity covers a fairly wide range. Lower viscosity products should be
used in high-speed and/or lightly loaded applications to minimize heat generation and torque.
Higher viscosity products should be used in moderate to low-speed applications and under
heavy loads to maximize lubricant film thickness.

1. Grease lubrication
5. LUBRICATING YOUR BEARINGS

1.6. Typical grease lubrication

Timken Contents

C. Guidance
C. Guidance/1.
for oil/greaseGrease
selectionlubrication /1.6. Typical
grease lubrication guidelines
guidelines

B A 328

2. Oil lubrication
Lubricating oils are commercially available in many forms for automotive, industrial, aircraft
and other uses. Oils are classified as either petroleum types (refined from crude oil) or synthetic
types (produced by chemical synthesis).

2.1. Petroleum oils

Petroleum oils are used for nearly all oil-lubricated applications of Timken bearings. These oils
have physical and chemical properties that can help select the correct oil for any bearing
application.

2. Oil lubrication
5. LUBRICATING YOUR BEARINGS

2.1. Petroleum oils

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.1. Petroleum oils

B A 329

2.2. Synthetic Oils

Synthetic Oils cover a broad range of categories, which include polyalphaolefins, silicones,
polyglycols, and various esters. In general, synthetic oils are less prone to oxidation and can
operate at extreme hot or cold temperatures. Physical properties such as pressure-viscosity
coefficients tend to vary between oil types and caution should be used when making oil
selections.
The polyalphaolefins have a hydrocarbon chemistry which parallel petroleum oil both in their
chemical structures and pressure-viscosity coefficients. Therefore, PAO oil is mostly used in the
oil-lubricated applications of Timken bearings when severe temperature environments (hot and
cold) are encountered or when extended lubricant life is required.
The silicone, ester, and polyglycol oils, however, have a oxygen based chemistry which is
structurally quite different from petroleum oils and PAO oils. This difference has a profound
effect on its physical properties where pressure-viscosity coefficents can be lower compared to
mineral and PAO oils. This means that these types of synthetic oils may actually generate a
smaller EHD film thickness than a mineral or PAO oil of equal viscosity at operating
temperature. Reductions in bearing fatigue life and increases in bearing wear could result from
this reduction of lubricant film thickness.

2. Oil lubrication
5. LUBRICATING YOUR BEARINGS

2.2. Synthetic Oils

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.2. Synthetic Oils

B A 330

2.3. Selection of oils

The selection of oil viscosity for any bearing application requires consideration of several
factors: load, speed, bearing setting, type of oil, and environmental factors. Since viscosity
varies inversely with temperature, a viscosity value must always be stated with the temperature
at which it was determined. High viscosity oil is used for low-speed or high-ambient temperature
applications. Low viscosity oil is used for high-speed or low-ambient temperature applications.

2.4. Classification
There are several classifications of oils based on viscosity grades. The most familiar are the SAE
classifications for automotive engine and gear oils. The American Society for Testing and
Materials (ASTM) and the International Organization for Standardization (ISO) have adopted
standard viscosity grades for industrial fluids.
For reference purposes, fig. 5-7 shows a partial listing of the ISO/ASTM grades plotted on a
viscosity-temperature chart.
Fig. 5-8 shows the viscosity comparisons of ISO/ASTM with SAE classification systems at 40C.

2. Oil lubrication
5. LUBRICATING YOUR BEARINGS

2.3. Selection of oils

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.3. Selection of
Classification
2.4.oils/2.4.
Classification

B A 331

500 000
100 000
200 000
50 000

20 000
5 000

10 000
5 000

1 000
500

Kinematic viscosity , mm2/s (Centistokes, cSt)

100
50

100
ISO VG
680
460
320
220
150
100
68
46

20
10

5
4

100
75
50
40

32

35
33

Viscosity, Saybolt Universal Seconds (SUS)

2 000
1 000
500

200

1.5

50

20

60

100

100

140

180

220
210

Fig. 5-7
ISO/ASTM viscosity system
for industrial fluid lubricants
(ISO 3448, ASTM D2442)
assuming a viscosity index of
90 Kinematic viscosity, mm2/s
(Centistokes, cSt).

150C
260

300F

2. Oil lubrication
5. LUBRICATING YOUR BEARINGS

2.4. Classification

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.4. Classification

B A 332

10 000

2 000
1 500

1 000
800

1 000

250 W min.

5 000
4 000

680

600

Kinematic viscosity, mm2/s (Centistokes, cSt) at 40C

7 500

3 000
140

460
400
300

320

200

220

150

150

100
80

100

1 500
90

50

68

40

46

30

32

20

22

15

15

1 000
700

40

60

2 000

85 W min.

500
400

30

300

80 W min.
20

200
150

70 W min.
10 W min.
5 W min.

100
SAE Gear Oil
75

SAE Crankcase Oils

60

10
8

50

40

35
33

Viscosity, Saybolt Universal Seconds (SUS) at 100F

1 500

ISO/ASTM

Fig. 5-8
Viscosity classification comparison
between ISO/ASTM grades (ISO
3448/ASTM D2442)
and SAE grades (SAE J 300-80 for
crankcase oils, SAE J 306-81 for
axle and manual transmission oils).

2. Oil lubrication
5. LUBRICATING YOUR BEARINGS

2.4. Classification

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.4. Classification

B A 333

2.5. Typical oil lubrication guidelines

In this section, the properties and characteristics of lubricants for typical tapered roller bearing
applications are listed. These general characteristics have resulted from long successful
performance in these applications.

2.5.1. General purpose rust and oxidation lubricating oil

General purpose rust and oxidation (R&O) inhibited oils are the most common type of industrial
lubricant. They are used to lubricate Timken bearings in all types of industrial applications
where conditions requiring special considerations do not exist.
Suggested general purpose R&O lubricating oil properties
Base stock
Additives
Viscosity index
Pour point
Viscosity grades

Solvent refined, high viscosity-index petroleum oil

Corrosion and oxidation inhibitors
80 min.
10C max.
ISO/ ASTM 32 through 220

Some low-speed and/ or high-ambient temperature applications require the higher viscosity
grades, and high-speed and/ or low-temperature applications require the lower viscosity
grades.
2. Oil lubrication
5. LUBRICATING YOUR BEARINGS

2.5. Typical oil lubrication guidelines

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.5. Typical oil lubrication guidelines

B A 334

2.5.2. Industrial extreme pressure (EP) gear oil

Extreme pressure (EP) gear oils are used to lubricate Timken bearings in all types of heavily
loaded industrial equipment. They should be capable of withstanding heavy loads including
abnormal shock loads common in heavy-duty equipment.
Suggested industrial EP gear oil properties
Base stock

Solvent refined, high viscosity index petroleum oil

Additives

Corrosion and oxidation inhibitors.

Extreme pressure
(EP) additive* - 15.8 kg(35 lb) min. OK
Timken load rating

Viscosity index

80 min.

Pour point

10C max.

Viscosity grades

ISO/ ASTM 100, 150, 220, 320, 460

* ASTM D 2782

2.5. Typical oil lubrication guidelines

5. LUBRICATING YOUR BEARINGS
2.5.2. Industrial extreme pressure

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.5. Typical oil(EP)
lubrication/2.5.2.
Industrial (EP)
gear oil

B A 335

Industrial EP gear oils should be composed of a highly refined petroleum oil base stock plus
appropriate inhibitors and additives. They should not contain materials that are corrosive or
abrasive to tapered roller bearings. The inhibitors should provide long-term protection from
oxidation and protect the bearing from corrosion in the presence of moisture. The oils should
resist foaming in service and have good water separation properties. An EP additive protects
against scoring under boundary-lubrication conditions. The viscosity grades suggested represent
a wide range. High temperature and/or slow-speed applications generally require the higher
viscosity grades. Low temperatures and/or high speeds require the use of lower viscosity
grades.

2.5. Typical oil lubrication guidelines

5. LUBRICATING YOUR BEARINGS
2.5.2. Industrial extreme pressure

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.5. Typical oil(EP)
lubrication/2.5.2.
Industrial (EP)
gear oil

B A 336

2.6. Heat generation and dissipation

One of the major benefits of oil lubricated systems is that the heat generated by the bearings is
carried away by the circulating oil and dissipated through the system.

2.6.1. Heat generation

Under normal operating conditions, most of the torque and heat generated by the bearing is
due to the elastohydrodynamic losses at the roller/race contacts.
The following equation is used to calculate the heat generated by the bearing:

(1)

Qgen = k4 n M

where:

M = k1 G1 (n)0.62 (Peq) 0.3

Qgen
M
n
G1

Peq
k1

= generated heat (W or Btu/min)

= running torque N.m or lbf - In
= rotational speed (rev/min)
= geometry factor from bearing data tables
= viscosity at operating temperature (cP)
= equivalent dynamic load (N or lbf)
= bearing torque constant
= 2.56 x 10 6 for M in N - m
= 3.54 x 10 5 for M in lbf - In

2.6. Heat generation and dissipation

5. LUBRICATING YOUR BEARINGS
2.6.1. Heat generation

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.6.1. Heat generation

B A 337

2.6.2. Heat dissipation

The heat dissipation rate of a bearing system is affected by many factors. The modes of heat
transfer need to be considered. Major heat transfer modes in most systems are conduction
through the housing walls, convection at the inside and outside surfaces of the housing, and
convection by the circulating lubricant. In many applications, overall heat dissipation can be
divided into two categories: heat removed by circulating oil and heat removed through the
housing.

2.6. Heat generation and dissipation

5. LUBRICATING YOUR BEARINGS
2.6.2. Heat dissipation

Timken Contents

C. Guidance
C. Guidance/2.
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lubrication/2.6.2. Heat dissipation

B A 338

2.6.3. Heat dissipation by circulating oil

Heat dissipated by a circulating oil system is:

Qoil = k5 f

(o i)

(2)

If a circulating lubricant other than petroleum oil is used, the heat carried away by that
lubricant will be:

Qoil = k6 Cpf (o i) (3)

If lubricant flow is unrestricted on the outlet side of a bearing, the flow rate that can freely pass
through the bearing depends on bearing size and internal geometry, direction of oil flow,
bearing speed, and lubricant properties.
A tapered roller bearing has a natural tendency to pump oil from the small to the large end of
the rollers. For maximum oil flow and heat dissipation, the oil inlet should be adjacent to the
small end of the rollers.
In a splash or oil level lubrication system, heat will be carried by convection to the inner walls
of the housing. The heat dissipation rate with this lubrication method can be enhanced by using
cooling coils in the housing sump.

2.6. Heat generation and dissipation

5. LUBRICATING YOUR BEARINGS

2.6.3. Heat dissipation by circulating oil

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.6.3. Heat dissipation by circulating oil

B A 339

k4

k5

k6

Dimensional factor to calculate heat

generation rate in equation (1)
k4 = 0.105 for Qgen in W when M in N-m
= 6.73 x 10 4 for Qgen in Btu/min
when M in lbf.in

Dimensional factor to calculate heat

carried away by a petroleum oil in
equation (2)
k5 = 28 for Qoil in W when f in L/min and in C
= 0.42 for Qoil in Btu/min
when f in U.S. pt/min and in F

Dimensional factor to calculate heat

carried away by a circulating fluid in
equation (3)
k6 = 1.67 x 10 5 for Qoil in W
= 1.67 x 10 2 for Qoil in Btu/min

Qoil

Heat dissipation rate

of circulating oil

W, Btu/min

Oil inlet temperature

C, F

Oil outlet temperature

C, F

Cp

Specific heat of lubricant

J/(kg x C),
Btu/(lb x F)

Lubricant flow rate

/min, U.S.
pt/min

Lubricant density

kg/m3, lb/ft3

2.6. Heat generation and dissipation

5. LUBRICATING YOUR BEARINGS

2.6.3. Heat dissipation by circulating oil

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.6.3. Heat dissipation by circulating oil

B A 340

2.6.4. Heat dissipation through housing

Heat removed through the housing is, in most cases, difficult to determine analytically. If the
steady-state bearing temperature is known for one operating condition, the following method
can be used to estimate the housing heat dissipation rate.
At the steady-state temperature, the total heat dissipation rate from the bearing must equal the
heat generation rate of the bearing. The difference between heat generation rate and heat
dissipation rate of the oil is the heat dissipation rate of the housing at the known temperature.
Heat losses from housings are primarily by conduction and convection and are, therefore,
nearly linearly related to temperature difference. Thus, the housing heat dissipation rate is:

(4)

Qhsg = C ( ambt)

At the operating condition where the steady-state temperature is known, the housing heat
dissipation factor can be estimated as:

C=

Qgen Qoil

(5)

ambt
2.6. Heat generation and dissipation
5. LUBRICATING YOUR BEARINGS

2.6.4. Heat dissipation through housing

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.6.4. Heat dissipation through housing

B A 341

2.7. Oil systems

There are 3 different types of oil lubricating systems commonly used in industry.

2.7.1. Oil level systems

Oil level systems where the
bearings are partially submerged
in a static oil reservoir are the
simplest types of oil lubrication
systems (fig. 5-9). The oil level
system is generally only used for
low and moderate speed
applications because of the limited
ability to transfer heat. Effective
sealing is important to maintain the
required oil level; sight gauges are
often used to monitor the oil level.

Oil level

Fig. 5-9
Oil level system.

2.7. Oil systems

5. LUBRICATING YOUR BEARINGS

2.7.1. Oil level systems

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.7. Oil systems/2.7.1. Oil level systems

B A 342

Heat dissipation can be improved in an oil level system if the oil is splashed on the entire inner
surface of the housing (fig. 5-10). Components such as gears, which rotate through the
reservoir, splash oil onto housing walls, adjacent bearings and/or into catch troughs that
distribute oil by gravity flow through channels to the bearings.

Oil inlet hole

Oil inlet hole

Catch trough

Catch trough

Oil level

Oil dam
Two-row mounting

Single row mounting

Fig. 5-10
Oil splash systems.

2.7. Oil systems

5. LUBRICATING YOUR BEARINGS

2.7.1. Oil level systems

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.7. Oil systems/2.7.1. Oil level systems

B A 343

An example of splash
lubrication is an automotive
differential (fig. 5-11).
Oil level systems with splash
oil can be used at moderately
high speeds if properly
designed with a large oil
reservoir and large cooling
surface.

Catch trough
Oil inlet hole

Housing design can have a

major influence on the
degree of cooling provided.

Oil drain
Fig. 5-11
Oil splash system - automotive differential.

2.7. Oil systems

5. LUBRICATING YOUR BEARINGS

2.7.1. Oil level systems

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.7. Oil systems/2.7.1. Oil level systems

B A 344

2.7.2. Forced-feed oil systems

Forced-feed oil systems are more elaborate than static oil systems. In a typical system (fig. 5-12),
oil is pumped from a central reservoir to each bearing. Oil is introduced at the small end of the
bearing and drained away at the large end to take advantage of the natural pumping action of
tapered roller bearings.
Circulating oil provides a continuous,
regulated oil flow. This provides the
Oil inlet hole
advantages of maximum heat removal
and washing action, which removes
contamination or debris that could
cause bearing wear. Heat exchangers
can be included in a circulating
system to reduce oil temperature and
extend lubricant life. Filters should be
used to remove debris which will
cause bearing wear. Circulating oil
systems are particularly beneficial on
high-performance bearing
applications where heat removal and
Oil drain
long-term oil life are primary
Fig. 5-12
requirements.
Forced-feed oil system.

2.7. Oil systems

5. LUBRICATING YOUR BEARINGS

2.7.2. Forced-feed oil systems

Timken Contents

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C. Guidance/2.
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lubrication/2.7. Oil systems/2.7.2. Forced-feed oil systems

B A 345

Forced-feed oil systems with oil jets are used at higher speeds (fig. 5-13). The jets are
positioned to direct oil to the space between the cage and the inner race at the small end of the
roller. In addition, oil-jet orifices - usually about 2.5 mm (0.1 in) diameter - can be arranged
around the circumference of the bearing to distribute oil at the small and sometimes at the large
end of the rollers for maximum cooling efficiency.

Whenever large quantities of oil are

used, it is important to balance the
quantity of oil drained away with the
oil directed into the bearing area.
Large drain areas are necessary to
prevent a build-up of oil in the
bearing. If oil is not properly drained
away, temperature will elevate
because of excessive heat generated
due to churning of the oil.

Oil jet

Fig. 5-13
Forced-feed oil system with oil jet.

2.7. Oil systems

5. LUBRICATING YOUR BEARINGS

2.7.2. Forced-feed oil systems

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.7. Oil systems/2.7.2. Forced-feed oil systems

B A 346

2.7.3. Oil mist systems

Oil mist systems deliver very fine particles of oil suspended in a low-velocity, low-pressure air
stream (fig. 5-14). Oil particles are wet-out in the bearing by reclassification nozzles and/or
impingement on high-speed rotating bearing parts.
Oil mist provides minimum cooling capacity because the air-flow rate and the specific heat of
air are low. However, mist systems can be used on high-performance bearing applications,
because heat generation in the equipment is minimized.
For additional information on lubrication systems relative to bearing speeds, see The Timken
Company publication Speed Capabilities.

2.7. Oil systems

5. LUBRICATING YOUR BEARINGS

2.7.3. Oil mist systems

Timken Contents

C. Guidance
C. Guidance/2.
for oil/greaseOilselection
lubrication/2.7. Oil systems/2.7.3. Oil mist systems

B A 347

3. Lubricant additives
Additives are materials, usually chemicals, that improve specific properties when added to
lubricants. Additives, when properly formulated into a lubricant, can increase lubricant life,
provide greater resistance to corrosion, increase load-carrying capacity, and enhance other
properties. However, additives are very complex, and therefore, should not be added
indiscriminately in lubricants as a cure-all for all lubrication problems.
The more common lubricant additives include:

Oxidation inhibitors for increasing lubricant service life

Rust or corrosion inhibitors to protect surfaces from rust or corrosion
Demulsifiers to promote oil and water separation
Viscosity-index improvers to decrease viscosity sensitivity to temperature change
Pour-point depressants to lower the pouring point at low temperatures
Lubricity agents to modify friction
Antiwear agents to retard wear
Extreme pressure (EP) additives to prevent scoring under boundary-lubrication conditions
Detergents and dispersants to maintain cleanliness
Antifoam agents to reduce foam
Tackiness agents to improve adhesive properties.

3. Lubricant additives
5. LUBRICATING YOUR BEARINGS
Timken Contents

C. Guidance
C. Guidance/3.
for oil/greaseLubricant
selection additives

B A 348

Inorganic additives such as molybdenum disulphide, graphite, and zinc oxide are sometimes
included in lubricants. In most tapered roller bearing applications, inorganic additives are of no
significant benefit; conversely, as long as the concentration is low and the particle size small,
they are not harmful.
Recently, the effects of lubricant chemistry on bearing life (as opposed to the purely physical
characteristics) have received much emphasis. Rust, oxidation, extreme pressure and antiwear
additive packages are widely used in engine and gear oils. Fatigue testing has shown these
additives may, depending on their chemical formulation, concentration and operating
temperature, have a positive or negative impact on bearing life.
Contact a Timken Company sales engineer or representative for more information regarding
lubricant additives.

3. Lubricant additives
5. LUBRICATING YOUR BEARINGS
Timken Contents

C. Guidance
C. Guidance/3.
for oil/greaseLubricant
selection additives

B A 349

D. Contamination
1. Abrasive particles
When tapered roller bearings operate in a clean environment, the primary causes of damage is
the eventual fatigue of the surfaces where rolling contact occurs. However, when particles
contamination enters in the bearing system, it is likely to cause damage such as bruising which can
shorten bearing life.
When dirt from the environment or metallic wear debris from some component in the application
is allowed to contaminate the lubricant, wear can become the predominant cause of bearing
damage. If, due to particle contamination of the lubricant, bearing wear becomes significant,
changes will occur to critical bearing dimensions that could adversely affect machine operation.
Bearings operating in a contaminated lubricant exhibit a higher initial rate of wear than those not
running in a contaminated lubricant. But, with no further contaminant ingress, this wear rate
quickly diminishes as the contamination particles are reduced in size as they pass through the
bearing contact area during normal operation.

1. Abrasive particles
5. LUBRICATING YOUR BEARINGS
Timken Contents

D. Contamination
D. Contamination/1. Abrasive particles

B A 350

2. Water
Either dissolved or suspended water in lubricating oils can exert a detrimental influence on
bearing fatigue life. Water can cause bearing etching that can also reduce bearing fatigue life.
The exact mechanism by which water lowers fatigue life is not fully understood. But it has been
suggested that water enters microcracks in the bearing races that are caused by repeated stress
cycles. This then leads to corrosion and hydrogen embrittlement in the microcracks which
reduce the time required for these cracks to propagate to an unacceptable size spall.
Water-base fluids such as water glycol and invert emulsions have also shown a reduction in
bearing fatigue life. Although water from these sources is not the same as contamination, the
results support the previous discussion concerning water-contaminated lubricants.
The following chart (fig. 5-15) gives a good idea of the influence of water on bearing life.
Based on Timken Research tests, it was determined that water content of 0.01% (100 parts per
million) or less, had no effect on bearing life. However, greater amounts of water in the oil will
reduce bearing life significantly.

2. Water
5. LUBRICATING YOUR BEARINGS
Timken Contents

D. Contamination
D. Contamination/2. Water

B A 351

Relative (factor) life

3
2

0.5

0.3

0.1
0.001

0.01

0.1

0.5

% water in the lubricant

Fig. 5-15
Life reduction with water contamination.

2. Water
5. LUBRICATING YOUR BEARINGS
Timken Contents

D. Contamination
D. Contamination/2. Water

B A 352

3. Filtration
Many oil lubricated bearing applications are operated satisfactorily without the benefit of filters.
However, where environmental contamination can have a detrimental effect on bearing
performance, filtration equipment is recommended. Experience has shown that nominally rated
40 m (0.0015 in) filters are satisfactory for most Timken bearing industrial applications.

3. Filtration
5. LUBRICATING YOUR BEARINGS
Timken Contents

D. Contamination
D. Contamination/3. Filtration

B A 353

E. Conclusion
The lubrication concepts and guidelines presented in this chapter are, by necessity, general in
nature. These basic suggestions have been developed by the technical staff of The Timken
Company based on knowledge gained from bearing application experience and specific
laboratory work. However, the final selection of a lubricant for a particular application is the
responsibility of the user.

5. LUBRICATING YOUR BEARINGS

Timken Contents

E. Conclusion
E. Conclusion

B A 354