Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Lubrication - Case Study No. 57: Incompatible Polyurea and Lithium Motor Greases


Inspection results of a failed motor indicated that the failure was most likely caused by a lack of lubrication on the outboard bearing due to the mixing of incompatible grease thickeners.
Motor Bearing - Inner RacewayInboard Bearing Inspection - No evidence of bearing defects and/or lubrication problems were noted by visual inspection of the inboard bearing.

Outboard Bearing Inspection - Clearly, the outboard bearing failed dramatically as shown in Fig. 1. It is suspected that a lack of lubrication caused the inner race to heat up, expand in size and eventually lock the rolling elements in place against the outer race. Once the bearing seized, the motor shaft started spinning inside the inner race which generated even more heat - enough to lower the yield strength of the bearing steel and allow the rolling elements to deform the raceway simply from the weight of the rotor.

Grease Samples - Samples of grease were taken from the inboard and outboard bearing and sent to the manufacturer for analysis. Lab results showed that a polyurea-thickened grease was mixed with the lithium based complex grease normally used on both bearings although contamination was more significant on the outboard bearing. Mixing thickeners softened the grease mixture from an initial NLGI grade 2 to an NLGI grade 00 consistency. Wear metals such as manganese, copper, and iron were present in both grease samples although at much higher levels on the outboard bearing. The lab report attributed the loss of lubrication observed from the bearing failure to the mixing of incompatible grease thickeners.

The motors were equipped with deep groove ball bearings without shields and were originally greased at the factory with a No. 2 grade polyurea-thickened grease. Polyurea greases are often used by motor manufacturers for their ability to perform at relatively high temperatures, however, polyurea is incompatible with nearly all commonly used thickeners with the exception of calcium complex. Consequently, the motor instruction manual warns, "The mixing of different types of thickeners may destroy the composition and physical properties of the grease."

By Eric Adams

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