condition-based lubrication using ultrasonic technology.pdf

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Tags: industrial lubricants

Determining when to lubricate bearings and how much lubrication to apply are two of the perplexingissues facing technicians responsible for maintaining bearings. Underlubrication can cause a bearing to

wear out before its time. Applying too much lubrication often can lead to catastrophic results to the

bearing or long-term damage to motor coils and windings.

Traditionally, lubrication scheduling has been time-based. Equipment suppliers often recommend

lubrication schedules based on hours of operation. In addition, they frequently provide instructions on

the amount of lubricant to be applied during these scheduled maintenance procedures. It is common forclients to be told to lubricate at short-term intervals and to add what appears to be excessive amounts

of grease. In one instance, a client was advised to lubricate motor bearings every two to three weeks

and to add one ounce of grease. The origin of these suggestions leads one to speculate that they are

often based on some unknown factors that have no real-world application.

Lubrication intervals are based on a simple premise: 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 gross

overlubrication.

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

cease 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 adding lubrication, the bearing should be

left alone. Should conditions change and a bearing demonstrate a need for lubrication, then a lubricant

should be applied. Monitoring the bearing as the lubricant is applied will also help determine how much

to add and when to stop the application.

Ultrasound technology is ideally suited for condition-based lubrication methods. To understand why,

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.

ition-Based Lubrication Using Ultrasound Technology http://www.machinerylubrication.com/Articles/

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The technology is based on the sensing of high-frequency sounds. Ultrasound starts 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 will sense from 20 kHz up to 100 kHz. The range of human

hearing covers frequencies 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.

Ultrasound InstrumentationInstruments based on the technology of airborne/structure-borne ultrasound are referred to as

ultrasonic translators. They receive the inaudible high-frequency sounds and electronically translatethem 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 in two ways: through headphones and on a meter where the amplitude of these sounds can be

viewed as intensity increments or as decibels (dB).

Ultrasound MonitoringImagine a properly installed bearing. It has been given the right amount of lubricant and is in perfect

alignment. As it rolls around the raceway, any stress it may have is evened out by the lubricant, and itmoves stress-free. As it does, 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 opposed to vibration meters, which detect vibratory displacements of rolling

elements, the ultrasound translator will detect friction.)

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 assimple as setting a base line, setting inspection schedules and monitoring as during lubrication

applications.

Setting a BaselineA baseline for a bearing reflects the decibel level at normal

operating conditions with no observable defects and with

adequate lubrication. There are three methods for setting a

baseline:

Comparison: When there is more than one bearing of the

same type, load and rpm, multiple bearings can becompared. 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 eight dB) a baseline dB level is set for each bearing.

1.

Set while lubricating: While lubrication is being applied,

listen until the sound level drops and then begins to rise. At that point, no more lubricant is

added and the dB value is used as the baseline.

2.

Historical: Bearing dB levels are obtained from an initial survey and compared 30 days later. If

there is little (less than eight dB) to no change in dB, then the base line levels are set and will be

3.


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used for comparison for subsequent inspections.

Setting Inspection SchedulesEquipment criticality, as it relates to production, environmental and operational consequences, is the

primary factor in selecting and setting frequencies for assessing mechanical systems. After the baseline

inspection has been performed, a monthly interval should be sufficient. For bearings with high decibel

levels that 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 inminutes, in others, days.

Monitoring as You LubricateA bearing that exceeds eight dB over a set baseline can be presumed to need lubrication. Once a

bearing has been identified for lubrication, knowing when to stop applying the lubricant will prevent

overlubrication. This is accomplished in one of three ways:

Calculate the quantity based on the bearing manufacturers guidelines, and apply that quantity of

lubricant and no more. This has little dependence on ultrasonic methods, is subjective, and is

often poorly executed.

1.

The lubrication technician monitors the bearing with an ultrasonic instrument as the lubricant isbeing applied. Lubricant is applied slowly until the decibel level drops to the baseline level.

2.

If it is not possible to use a dB level as a guide, the lubricant is applied until the sound drops off

and begins to rise. At that exact moment, the technician stops applying the lubricant.

3.

Ultrasound Bearing InspectionUltrasonic inspection and monitoring is the most reliable method for detecting incipient (the very

beginning of) bearing failure. The ultrasonic warning appears prior to a rise in temperature or an

increase in low-frequency vibration levels. This method of inspecting bearings is useful in recognizing

the beginning of fatigue failure, brinelling of bearing surfaces, and an excess or lack of lubricant.

Detecting Signs of FailureIn ball bearings, as the metal in the raceway, roller or ball bearing begins to fatigue, a subtle

deformation begins to occur. This condition will produce irregular surfaces, which will cause an increase

in the emission of ultrasonic sound waves. A change in amplitude from the original reading is an

indication of one of two conditions: prefailure or lack of lubrication and incipient bearing failure. An

eight dB increase over baseline accompanied by a constant rushing noise suggests lubricant failure (dry

bearing surfaces). When an ultrasonic reading exceeds any previous reading by 12 dB accompanied by

crackling noises, it can be assumed that the bearing has entered the beginning of the failure mode.

This information was originally discovered through experimentation performed by NASA on ball

bearings.1In tests performed while monitoring bearings at frequencies from 24 through 50 kHz,

researchers found that the changes in amplitude indicated incipient failure before any other indicatorsincluding heat and vibration changes. An ultrasonic system based on detection and analysis of

modulations of bearing resonance frequencies provides subtle detection capabilities, whereas

conventional methods are incapable of detecting slight faults.

As a ball passes over a pit or fault in the race surface, it produces an impact. A structural resonance of

one of the bearing components vibrates or rings by this repetitive impact. The sound produced is heard

as a qualitative change in the headphones, usually as a crackling sound and observed as an increase in

amplitude in the monitored ultrasonic frequencies of the bearing.

Lubrication Procedures


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It is imperative to consider two elements of potential failure: lack of lubrication and overlubrication.

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 normalrough 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,

contributing to the prefailure 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.

The appropriate amount of lubrication is important. If a bearing is overlubricated, the bearing element

or cage can be pushed (pressed) excessively by the lubricant causing additional wear of the bearing.

Too much lubricant causes churning and fluid friction. The heat from the fluid friction, coupled with the

squeezing effect of churning, serves to drive the oil of the thickener prematurely, the oil dissipates

through the seals and into the housing, and is lost.

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 guesswork out of lubrication.

Important ConsiderationsAs with any technology, there is no one-size-fits-all application procedure. For example, there are some

limitations of estimated regrease intervals/volumes using standard equations. One of the problems with

time-interval lubrications is that there are different applications for different bearings, which vary

among facilities. For example, a food-processing plant may use an application in which machinery is

steamed down every day for cleaning. Another facility might not follow the same procedure because it

may operate in a different environment that does not require cleaning as frequently, while anothermight have many airborne particulates that could contaminate the bearing. Because of this kind of

variability, one can only guess what the lubrication cycle should be. It depends on what a facility is

producing and under what situations the machinery is operating.

Additionally, there are some considerations regarding where, when and how an ultrasonic approach can

be used. For example, if a facility approaches lubrication haphazardly, in a disorganized fashion, or

relies solely on sound quality as a determining factor, the effectiveness of the program can be

compromised. One needs to track bearing condition over time, setting baseline levels and determining

dB changes over that baseline to indicate the need to lubricate or not. Such a program helps personnel

understand exactly what they are looking at.

There may be situations in which it is difficult to gain access to some bearings. If the fitting 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 nonsound-conductive material such as plastic, a separate conductive metallic wave guide

can be installed so that the bearing can be monitored.

The waveguide can be isolated from structure-borne noise of the machine (the mounting point) via

rubber isolation material.

Reference

1. NASA Tech Brief. Report B72-10494. August 1972.


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condition-based lubrication using ultrasonic technology.pdf

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