Failure of Angular Contact Bearing on Newly Overhauled Motor - Failure & Findings
Written by Stephen H Shakeshaft
Monday, 07 June 2010 18:30
Page 2 of 3
When the motor was first run up it sounded fine, low vibration levels, quiet and all seemed well.
However, after a few minutes as the gearbox was warming up, the top bearing started to get noisy, it suddenly got very noisy indeed and smoke began to pour out of the top of the motor.
The motor was stopped and came rapidly to a stand still.
At this point there was considerable panic since the compressor was an integral part of a large single stream plant that was returning to service after a week long outage.
As there is no spare, the motor was removed overnight and taken back to the contractors workshop where it was stripped down.
The lower bearing (single row roller bearing) and the rotor were in generally perfect condition.
However, the top bearing (angular contact) that took the weight of the rotor was in a terrible condition.
The bearing was completely black, the cage was destroyed and the balls were completely misshapen and blackened.
As sometimes happens in extreme bearing failures, some of the balls had gone hollow. Clearly, given the length of time that the bearing had run, something was definitely very wrong.
The bearing had been lubricated before start up so it came down to either bearing fitting or specifying.
On the outer race on the side could clearly be seen the bearing designation, without suffixes.
Having spent sometime earlier in the day studying the motor assembly drawings I was sure that the correct bearing had a suffix to the designation.
The bearings that had been removed from the motor at its first strip down were in fact still in the workshop waiting to be cleared away.
The bearing that had been removed clearly had the suffix "B" after the designation.
Enquiries made with the planning officer who was responsible for procuring the spare parts for motors in the workshop revealed that he did not know what the significance of any of the bearing suffixes meant.
In fact the workshop supervisor and engineer did not know either. The only bearing designation suffix that they were aware of was "C3" for increased clearance.
The purchase requisition, order and the delivery note also showed that the planner had requested the bearing without the "B" suffix.
In fact the absence of the suffix in this angular contact bearing designation meant that the rolling element contact angle was incorrect.
A suffix "B" in the bearing designation means that the contact angle of the bearing is 40 degrees.
The same bearing designation without the suffix has a lower contact angle and so a lower axial thrust capacity.
The entire integrity of the motor depends on the thrust capability of the top bearing. The considerable weight of the rotor and coupling is carried and located by the top bearing.
The bottom bearing is a roller bearing and thus only carries any radial load from the drive.
To add to the problems, the design speed rating of the bearing that was fitted is lower than the suffix "B" bearing. The net result is an overloaded bearing running above its maximum speed capability.
The effect was catastrophic, the bearing had a total failure within minutes of starting up.