Condition-Based Lubrication

By Mark A. Goodman, UE Systems

Determining when to lubricate bearings and how much lubrication to apply are two of the perplexing issues facing technicians responsible for maintaining bearings. Under lubrication 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 for clients 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 over lubrication.

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.

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 Instrumentation

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 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 Monitoring

Imagine 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 it moves 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 as simple as setting a base line, setting inspection schedules and monitoring as during lubrication applications.

Setting a Baseline

A 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:

1. Comparison: When there is more than one bearing of the same type, load and rpm, multiple bearings can be compared. 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.
2. Set while lubricating: While lubrication is being applied, listen until the sound level drops. 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 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 used for comparison for subsequent inspections.

Setting Inspection Schedules

Equipment 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 in minutes, in others, days.

Monitoring as You Lubricate

A 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 over lubrication. This is accomplished in one of three ways:

1. 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.
2. The lubrication technician monitors the bearing with an ultrasonic instrument as the lubricant is being applied. Lubricant is applied slowly until the decibel level drops to the baseline level.
3. 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.

Ultrasound Bearing Inspection

Ultrasonic 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 Failure

In 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.

Click here to download the FREE UE Lubrication Inspection Guide (PDF).