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. 24: Grid Flex Coupling Lack of Grease

Remains of Coupling GridGrid type flexible couplings are the backbone of heavy industry, until you forget to lubricate them.

When you see something like the picture to the left and the mechanic says they just lubed it, that's not the case. When a properly lubricated grid-flex coupling does fail, it will have lots of grease in it and the grid will be broken in several places or mangled. Most of the time the vibration and noise level increase before it actually gives out. The problem is detected by either the vibration inspector or somebody else's ear.

This coupling failed because it either ran out of lubricant, or was never recharged when the piece of equipment it was on was replaced.

Grid Type Coupling - Hub

The teeth on the coupling hub, in the picture to the right, is mangled due to the disintegration of the grid. When a coupling hub wears out due to old age the teeth will take on a smooth tapered triangular look. The point of the triangle will be on the side of the tooth that is next to the other hub. The torsional friction forces cause sliding wear between the grid and the hub teeth.

Grid Type Coupling - Cover Grease ZerkThis grease zerk that is on the coupling cover hasn't seen grease in a very long time. The biggest problem with these type of couplings is that they are so reliable that people, such as lubrication mechanics, forget about them.

One tell tale way of knowing whether your lubrication mechanic is serious about lubricating couplings is to inspect the coupling guard. All couplings have to be covered by an OSHA guard. Of course the guard prevents the lubrication mechanic from gaining access to the grease zerk. However, consciencious mechanics will cut a hole in the guard and put an access door on it. Then, when the equipment is down during a scheduled outage he can access the zerk through the guard door, and lubricate the coupling. Believe me, that lubrication mechanic doesn't have enough time in his day to unscrew the guard and remove it so he can lubricate the coupling.

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