LUBRICATION GUIDE

Ultrasonic Condition-Based
Lubrication

14 Hayes Street, Elmsford NY 10523-2536 USA  Tel: 914-592-1220

Toll Free USA & Canada 24 Hour Fax Email: Internet

800-223-1325 914-347-2181 info@uesystems.com http://www.uesystems.com
 Ultrasound Condition Based Lubrication

Traditionally, lubrication scheduling has been “time-based.” Equipment

suppliers often recommend lubrication schedules based on hours of operation.
In addition, they frequently provide instructions as to the amount of lubricant to
be applied during these scheduled maintenance procedures.

The problem is that not every bearing needs to be lubricated when they
are scheduled for lubrication. Or if they do need to be lubricated, they might not
need as much lubricant to be added as stated in the scheduled work order. The
lubrication level might be fine and adding more might result in an over lubricated
bearing.

The concept of establishing lubrication intervals is based on a simple

premise: to keep equipment running optimally by preventing a bearing from
running dry and causing catastrophic damage. It is a solid “preventive” concept.
However, there is a balance that must be struck between preventing lubrication
starvation and the extreme of over lubrication. In fact, one of the most common
causes of bearing failure is over lubrication, not lubrication starvation.

To accomplish the goal of equipment optimization, it is best to know when

to lubricate and when to stop applying lubricants to a bearing. This can be
accomplished with a condition-based lubrication strategy. Simply put, the
condition of the bearing determines when to lubricate. If a bearing is working
properly and does not demonstrate any changes that warrant lubrication, the
bearing should be left alone. Should conditions change and a bearing
demonstrates a lessening of lubricant, then lubrication should be applied.
Monitoring the lubricant as it is applied will also determine how much lubricant to
add and when to stop the application.

Ultrasound technology is ideally suited for condition-based lubrication methods.

With ultrasonic inspection instruments a program can be established that will
inform inspectors which bearings need to be lubricated and help lubrication
technicians know exactly how much lubrication to apply.
 To understand how these instruments can work effectively in the loud
environments of a typical plant, one must understand the technology of
ultrasound, how ultrasound is produced by bearings, and how ultrasound
monitoring instruments can help maintain optimal lubrication levels in bearings.

The technology is based on the sensing of high-frequency sounds.

Ultrasound is considered to start at 20,000 cycles per second, or 20 kilohertz
(kHz). This is considered the high-frequency threshold at which human hearing
stops. Most ultrasonic instruments employed to monitor equipment will sense
from 20 kHz up to 100 kHz. The range of human hearing covers frequencies of
from 20 cycles per second (20 Hz) up to 20 kHz. The average human will often
hear up to 16.5 kHz and no more.

These frequency comparisons are important to note because there are

differences in the way low-frequency and high-frequency sounds travel, which
help us understand why ultrasound can be effectively instituted in bearing
monitoring and lubrication programs.

Size Differences.

There are substantial differences in the size of low-frequency, or audible

sound waves, and the size of high-frequency/ultrasound waves. The size of
audible or low- frequency sound waves will range from ¾” (1.9 cm) to as large as
56’ (17 m). Ultrasound waves range from 1/8” (.3 cm) to maximally 5/8”
(1.6 cm). These physical differences in wavelength help us understand why
ultrasound has an advantage in condition monitoring. Low-frequency sounds,
being large, tend to maintain a high intensity of sound volume over greater
distances than high-frequency sounds. High-frequency sounds, being
magnitudes smaller than low-frequency sounds, will not travel as far. Therefore,
the amplitude will fall off rapidly as the high-frequency sound waves move away
from the sound source.

Low frequency sound waves will tend to travel large distances and working in
these low frequency environments often makes identification of a sound source
difficult. In addition these gross signals can produce confusing “cross-talk”
effects in which a sound can travel from one part of a machine to another
producing confusing inaccurate test results.

Ultrasound wavelengths range from 1/8” (.32 cm) to 5/8” (1.6 cm)

Low Frequency wavelengths range from ¾” (1.9 cm) to 56’ (17 M)

 Instruments based on the technology of Airborne/Structure-
Borne Ultrasound are referred to as ultrasonic translators. They
receive the inaudible high-frequency sounds and electronically
translate them down into the audible range through a process
called heterodyning. The heterodyning method works in a similar
fashion to an AM radio. While we cannot hear radio waves, this
method helps us easily identify different voices and musical
instruments when we listen to the radio. Similarly this
heterodyning process provides an accurate translation of
ultrasound produced by operating equipment and enables users
to readily identify one sound component from another. Most
ultrasonic translators provide feedback two ways: through
headphones and on a meter where the amplitude of these sounds
can be viewed as intensity increments or as decibels.

Lubrication Procedures

It is imperative to consider two elements of potential

failure: lack of lubrication and over lubrication.

Normal bearing loads cause an elastic deformation of the

elements in the contact area providing a smooth elliptical
distribution. But bearing surfaces are not perfectly smooth. For
this reason, the actual stress distribution in the contact area will
be affected by a random surface roughness. In the presence of a
lubricant film on a bearing surface, there is a dampening effect
on the stress distribution, and the acoustic energy produced will
be low. Should lubrication be reduced to a point where the
stress distribution is no longer present, the normal rough spots
will make contact with the face surfaces and increase the
acoustic energy. These normal microscopic deformities will
begin to produce wear and the possibilities of small fissures may
develop which contributes to the “pre-failure” condition.
Therefore, aside from normal wear, the fatigue or service life of a
bearing is strongly influenced by the relative film thickness
provided by an appropriate lubricant.

Avoiding Over Lubrication

When too much lubricant is put into the bearing housing,

pressure builds up and can lead to an increase of heat, which can
create stress and deformity of the bearing. Or it can break or
“pop” the bearing seal allowing lubricant to spill out into
unwanted areas (such as a motor winding), or allow
contaminants to enter the raceway. All of which can lead to
bearing failure.

The appropriate amount of lubrication is very important. If

a bearing is over lubricated the bearing can be pushed
excessively by the lubricant causing additional wear of the
bearing. On the other hand, if there is not enough lubricant, the
bearing will rub on the solid surface, again causing friction and
wear on the bearings. Either case is detrimental to the life of the
bearing. Using airborne/ structure borne ultrasound takes the
guess out of lubrication.

Ultrasound Monitoring

Ultrasound instruments detect changes related to friction.

A properly lubricated bearing will have very little friction. The
lubricant evens out any stress the bearing encounters as it rolls
around the raceway thereby reducing the potential for
destructive friction. As the bearing rolls, it produces a
recognizable “rushing” sound akin to the sound of air leaking out
of a tire. This rushing sound is referred to as “white noise.” It
includes all sounds, both low and high frequencies. The high-
frequency waves generated by this white noise are more
localized than those of the lower frequencies. Using an
ultrasonic translator, these signals can be detected with little or
no interference from other mechanical noises generated by other
components, such as a shaft or another bearing close by.

As the lubrication level in a bearing falls or deteriorates,

the potential for friction increases. There will be a
corresponding rise in the ultrasound amplitude level that can be
noted and heard. The method to determine when to lubricate and
when to stop applying lubrication with ultrasound instruments is
as simple as: setting a base line, setting inspection schedules
and monitoring as you lubricate.
Setting a Baseline:
A baseline for a bearing reflects in decibels the level at which it
is operating under normal conditions with no observable defects
and with adequate lubrication.

There are three methods for setting a baseline.

1. Comparison: when there is more than one bearing of the
same type, load and rpm, multiple bearings can be
compared one to the other. Each bearing is inspected at
the same test point and angle. The decibel levels and
sound quality are compared. If there are no substantial
differences, (less than 8dB) a baseline dB level is set for
each bearing. This is usually performed with a portable
ultrasonic translator.

2. Set while lubricating. While lubrication is being applied,

listen until the sound level drops down and begins to rise.
At that point no more lubricant is added and the dB value is
used as the baseline.

3. Historical: bearing dB levels are obtained from an initial

survey. Thirty days later the bearing dB levels are taken
and compared. If there is little (less than 8dB) to no change
in dB than the base line levels are set and will be used for
comparison for subsequent inspections.

Baselines are usually set with a portable

ultrasonic translator

Setting Inspection Schedules:

The criticality of equipment as it relates to production,
environmental or operational consequences is the primary factor
as to which equipment to test and how often to test it. After the
baseline inspection has been performed, most often a schedule
of once per month will prove adequate. For bearings that have
had high levels and have been subsequently lubricated, it might
be necessary to test more frequently to note any possible
changes. If a bearing is in a failure mode, the lubricant will
temporarily mask the fault. However the fault will quickly
produce a rise in the dB level. In some instances this will happen
in minutes, in others, days.

Monitoring as you lubricate:

If a bearing exceeds 8 dB over a set baseline it can be presumed
to need lubrication. Once a bearing has been identified for
lubrication, to prevent over lubrication, a technician must know
when to stop applying the lubricant. This is accomplished in one
of two ways:

Lubricate until the dB level drops

to the baseline.

1. The lubrication technician monitors the bearing with an

ultrasonic instrument as the lubricant is being applied. But
be cautious! Some lubricants will need time to uniformly
cover the bearing’s surface. Lubricate a little at a time
until the decibel level drops to the baseline dB level.

2. If it is not possible to use a dB level as a guide, the

lubricant is applied until the sound drops off. At that exact
moment, the technician stops applying the lubricant.

If readings do not go back to original levels and remain

high, consider that the bearing is on the way to the failure mode
and recheck frequently.
 Accessibility issues: there may be situations in which it may
be difficult to gain access to some bearings. For example, there
may be a complex machine where a bearing is embedded in an
area where only a lube tube is extended outside the casing. If
the lube tube is a conductive metal such as copper, the bearing
can still be tested and a lubrication action level set. If the
fitting is of a non-sound conductive material such as plastic, a
separate conductive metallic wave guide can be installed so that
the bearing can be monitored. The wave guide can be isolated
from structure borne noise of the machine (the mounting point)
via rubber isolation material. Should it not be possible to place
a wave guide, there s an alternative solution. A transducer can
be permanently mounted on the bearing housing and a cable run
to an opening. The cable can be attached to a specialized
connector that can be “plugged-in” to the ultrasonic sensor, as
shown below.

Auto Lubrication Devices

There are debates about the use of automatic lubrication
devices. In some instances a permanently installed ultrasonic
sensor could be used to turn an auto greaser on and off instead
of having it operate with a constant flow. When the sound level
goes up 8 dB, the bearing is lubricated automatically. Otherwise
no lubricant is released. This method can assure adequate
lubrication is being applied since no other technology is as
comprehensive for monitoring lubrication and friction changes.

Conclusion
Ultrasound technology is ideally suited for effective condition-
based lubrication programs. The short wave nature of the signal
reduces interference from competing noises and allows
inspectors to accurately monitor bearing condition. By
establishing an alarm level of 8 dB over a given baseline,
 inspectors will know when and when not to lubricate. Over
lubrication can be avoided by applying only enough lubricant to
achieve baseline levels or listen to a drop in the sound level
should no dB reference be available.

2004 UE Systems, all rights reserved

14 Hayes Street, Elmsford NY 10523-2536 USA  Tel: 914-592-1220

Toll Free USA & Canada 24 Hour Fax Email: Internet

800-223-1325 914-347-2181 info@uesystems.com http://www.uesystems.com