Condition monitoring helps manage over-lubrication.
by Allan Rienstra
June 11, 2019

Ultrasound is a guide to precision grease replenishment in motor bearings. It is also known for its versatility for leak detection, valve assessment and electrical fault detection.

Acoustic lubrication is an integral component of ultrasound programs. Fewer than 95 percent of all roller bearings reach their full engineered life span, and lubrication is the culprit in most cases. In fact, poor lubrication practices account for as much as 40 percent of all premature bearing failures. Yet, when ultrasound is used to assess lubrication needs and schedule grease replenishment intervals, that number drops below 10 percent. What would 30 percent fewer bearing related failures mean for an organization?

To understand the role precision lubrication plays in bearing life extension, it helps to understand basics of bearings, their lubrication mechanism and how ultrasound helps.

The insides of a bearing consist of four components. The inner and outer raceways form a path for the rolling elements to glide on a thin film of lubricant. A metal cage separates the rolling elements, keeping them evenly spaced to distribute the load and stop them from crashing into one another. These components move in concert producing frictional forces from rotational inertia, surface load, misalignment, imbalance and defects. Zero friction is impossible, but optimal levels of friction are achievable with correct installation techniques and proper amounts of lubricant.

Ultrasound works on the FIT principle—it responds well to defects that produce friction, impacting and turbulence (FIT). For motor bearings, two of these phenomena apply: friction and impacting. Ultrasound detects high-frequency signals produced when two surfaces slide together or come in contact with any force. Stage 1 bearing failures happen at the micro level. Because ultrasound ignores low-frequency audible signals, it forms the perfect companion for measuring, trending and analyzing defects despite high levels of noisy interference encountered on the factory floor.

Ultrasound detectors detect friction and impacting as acoustic energy from rolling friction and defect impulses. When lubricant levels are optimum, the energy created is at its lowest. As frictional forces increase, so does the acoustic energy. Ultrasound instruments measure friction and impacting as energy using the scaled value dBµV (decibels/microvolt). The results are presented as condition indicators, and there are four of them:

  • root mean square (RMS)—an indicator of friction
  • maxRMS—an indicator of stability
  • peak—an indicator of impacting
  • crest factor—which surmises the relationship between friction and impacts

Condition indicators are most responsible for transforming ultrasound technology from a simplistic, “point the gun and pull the trigger” gadget, to being recognized as analysis and trending technology. Condition indicators add validity to trending by going beyond the single decibel. If a user currently uses an ultrasonic gun that does not have condition indicators, they should question the data.

Benefits of Optimized Lubrication Practices

Optimizing grease replenishment practices will deliver both fast and longer-term benefits. What are some of the indicators that your lubrication program is on the right track?

A Change in Quantity of Grease Consumed
Maintenance departments track their grease consumption to monitor and control costs. A change in consumption is a sure sign that the lubrication program is on the right track.

Most organizations are guilty of over-lubricating. Expect lower grease consumption as the program matures. Bad procedures lead to bearings routinely receiving more grease than they are designed to handle. The excess ends up being pushed into the motor casing or purged onto the floor.

Over lubrication happens when regreasing intervals are scheduled based on time instead of condition. Control lubrication tasks with ultrasound to monitor condition and maintain optimal friction. The time between greasing intervals increases, resulting in less grease used per bearing.

Fewer Lube-Related Failures
Organizations with optimized greasing programs experience fewer lube-related failures. Less fixing and fire-fighting translates to more creative time for maintenance. That time can be used to bring more machines into the greasing program. Additionally, with ultrasound, a user finds many nontrendable defects. For example, broken or blocked grease pipes and incorrectly fitted grease paths prevent grease from reaching the bearing.

Optimized MRO Spares Management
This lubrication program provides better control of grease consumption, fewer failures and more productivity for maintenance. A decrease in bearing-related failures improves spares optimization. Users can share the ultrasonic lubrication data with a maintenance, repair and operations (MRO) manager to create a plan to reduce the number of emergency parts on hand.

Increased Number of Machines Monitored
One benefit of an effective lubrication program is time: time allotted to monitoring machines instead of fixing them; time allotted to correctly assessing the real needs for lubrication and; time to look at the big picture.

Take for instance, criticality assessment. Many lubrication programs begin with small steps. All the “A” critical machines receive priority. But, what about the rest? With more time to plan, organize and schedule, the number of machines acoustically monitored for optimal lubrication increases.

Combine Acoustic Lubrication and Condition Monitoring
Acoustic lubrication can ensure precise bearing lubrication. The newest technologies on the market combine the power of onboard lubrication guidance with four condition indicators for bearing condition assessment.