MRO Today - Using vibration analysis to grease motors

Using vibration analysis to grease motors?

by John C. Robertson

The thought of lubricating electric motors is controversial because the method used does not provide clear-cut instructions for lubricating bearings without over-greasing. In fact, 86 percent of motor failures in a Strategic Work Systems case study plant were attributed to over-greasing. With this in mind, we performed a study to simplify motor bearing lubrication by using vibration analysis to determine the point of grease cut-off.

The study was conducted on an 85-horsepower, 3,600-rpm motor fitted with open-face ball bearings to provide an easy path for the grease to pass through the bearings to demonstrate a grease overfill. Upon overhauling the motor and cleaning the bearings, they were hand-packed with sufficient grease to prevent seizure during run-up of the motor.

Before running the motor, the drain plugs were removed from the bearing cap wells to vent off any excess grease that might accumulate during the greasing process. These drain vents were kept open for around 30 minutes after completion of the greasing.

The motor was started and allowed to reach its operational temperature before making any attempt to further grease the bearing.

The analyzer was set to record pressure impulse readings with the filter in the "filter out" mode, and the time tracing mode was selected. The pickup stem was attached to the accelerometer and the amplitude range set in the upper one-third of scale on the amplitude meter. The analyzer was started and the trace allowed to stabilize before greasing began.

As the grease was injected into the motor's inboard bearing, a count was started to determine the number of grease gun strokes needed to correctly fill the bearing and cavity.

As the grease took effect in the bearing, the trace took a downward plunge. When sufficient grease was injected, the trace curved and began to track upward. At this point, greasing was stopped and the trace was allowed to stabilize. The motor was stopped, and the end bell removed to inspect the effects of the greasing. The bearing showed the correct amount of grease had been added without bleeding into the motor's windings. The appropriate amount of grease added via the gun was 15 strokes (20 strokes = one ounce of grease).

The motor was reassembled and another test was conducted to observe the effects of over-greasing. Forty-four strokes were admitted to the outboard bearing. The tracing was observed, and the potential cut-off points noted. No grease was observed at the drain vents, but the bearing's vibration began to increase noticeably during the greasing's latter stages. This is a typical response of a bearing to over-greasing.

The motor was stopped, and the end bell removed. Grease overfilled the end cap cavity, forced its way through the bearing and was in the process of entering the rotor/stator air space.

A complete set of vibration spectrum data was taken to verify bearing integrity before and after greasing. A very low 8x rpm amplitude was noted before greasing the inboard bearing. This was indicative of the eight rolling elements that comprised the bearing. After greasing, it showed no evidence of further degradation.

A petrochemical plant in Alberta, Canada, used this method to grease motor bearings over a one-year trial period. The plant saved $65,000 in grease costs and saw a sharp decline in motor failures due to over-greasing. This also led to increased plant productivity and reduced maintenance costs.

Based on the results of our initial tests and subsequent applications, we recommend using vibration analysis for determining proper amounts of grease in critical electrical motors over 1 horsepower.

John C. Robertson is the maintenance reliability specialist
for Strategic Work Systems, a consulting firm based in Greenville, S.C.

This article appeared in the December 1999/January 2000 issue of MRO Today magazine. Copyright, 2000.