Using Vibration Analysis to Grease Motors?

Using Vibration Analysis to Grease Motors?

By John C. Robertson, maintenance reliability specialist

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 our case-study plant was 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 hp, 3600 rpm motor that had been fitted with open-face ball bearings to provide an easy path for the grease to pass through the bearings to demonstrate a grease over-fill. When the motor was overhauled and the bearings were cleaned, they were hand-packed with sufficient grease to prevent seizure during run-up of the motor. Before the motor was run, the drain plugs were removed from the bearing cap wells to vent off any excess grease that might have accumulated during the greasing process. These drain vents were kept open for approximately 30 minutes after the greasing had been completed. The motor was started and allowed to reach its operational temperature before any attempt was made 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 was set in the upper one-third of scale on the amplitude meter. The analyzer was started and the trace was allowed to stabilize before greasing began.

As the grease was injected into the motor's inboard bearing, a count was started to determine how many strokes of the grease gun it would take to correctly fill the bearing and cavity. As the grease took effect in the bearing, the trace took a downward plunge. (See Figure 1) When sufficient grease was injected, the trace curved and began to track upwards. At this point, the greasing was stopped and the trace was allowed to stabilize. The motor was stopped, and the end bell was removed to inspect the effects of the greasing. The bearing showed that the correct amount of grease had been added without bleeding through into the motor's windings. The appropriate amount of grease added per the grease 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 were noted. (See Figure 2 ) No grease was observed at the drain vents, but the bearing's vibration began to increase noticeably during the latter of stages of the greasing. This is a typical response of a bearing to over-greasing. The motor was stopped, and the end bell was removed. Grease had 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. They saved $65,000 in grease purchased and saw a sharp decline in motor failures due to over-greasing. This also showed positively in overall plant production and lower 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.

This article is provided courtesy of Strategic Work Systems, Inc.

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