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. 69: Eliminating Pressure Lubricated Systems and Living with High Heat

View of Split Bearing DesignPressure lubricated systems are usually installed for a reason. Before eliminating one, make sure that all of the reasons it was installed in the first place have been accommodated in the replacement design, especially where temperature is concerned.

One popularly overlooked aspect of pressure lubricated systems is heat removal. When eliminating pressure lubricated systems, the first directive given is to size the replacement bearing for the load. That usually equates to simply getting a bearing that has the same inner diameter as the pressure lubricated bearing. For the most part it seems to work, which gives one a false sense of success and thus, security. The trouble is that if one steps back for a moment and ponders the situation it begins to look precarious. The equation just doesn't balance.

Ask yourself for a moment how the bearing shown to the left is going to remove the same amount of heat as a 200 gallon reservoir with a cooling fan on it. It just doesn't make sense, but it happened on three different systems.

Close Up View of Disassembled Split Roller BearingThis split bearing design really has to be given a lot of credit, we punish this design beyond logic and reason. This bearing runs hot to begin with because of all of the sliding motion going on within it. The sides of the roller bearings actually contact the locking collars, and the roller cage also contacts the locking collars. This sliding friction generates a lot of heat at 1800 RPM, the top end for this particular bearing. Add on top of that the fact that the heat generated from the equipment loading has nowhere to go anymore. So guess what? It runs hot. During the winter, there's no problem; but when summer time comes, look out. The early morning low end temperature runs around 140 F and typically it runs at 180 F by mid-day. The equipment knockdown on these bearings is set at 210 F, that's right.

We've used these bearings now for over six years on three different applications that formerly had pressure lubricated systems. It wasn't until we threw away about eight perfectly good bearings that I decided to push the acceptable operating temperatures to what they are now. It was hard to overcome the paradigm that a hot running bearing (i.e. can't hold your hand on it) is a bad bearing. The failure analyses on all eight bearings showed no deterioration whatsoever. Information meetings were required with all Production crews and Maintenance crews. At those meetings, samples of "failed" bearings were displayed to show them that there wasn't anything wrong.

See related articles on this style bearing in the "Machine Design - Case Study No. 70: Pros and Cons of Using Split Bearings" article and the "Vibration - Case Study No. 72: Diagnosing Split Roller Bearing Problems" article.

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