Renold Couplings highlights folly of fuse theory
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Renold Hi-Tec Couplings product story
Edited by the Engineeringtalk editorial team
Mar 20, 2009
Fuse theory, a myth related to couplings, is costing industry a fortune, according to Paul Shuffleton, managing director of Renold Hi-Tec Couplings.
According to fuse theory, rubber-in-shear type flexible couplings protect diesel-driven generator sets by failing or acting like a fuse when there is a potentially damaging short circuit.
This is a myth, because the majority of rubber-in-shear couplings fail through fatigue rather than because of a short circuit.
This is also the main reason for failing rubber blocks.
Rubber-in-shear couplings consist of a rubber element fused in-between two metal hubs.
One hub drives the other through the rubber, the flexing of which protects against shock and vibration.
The coupling is only as strong as the rubber element, so if the rubber fails then the coupling fails too.
When this happens on a diesel-driven generator set, some engineers believe there has been a short circuit and that the relatively inexpensive coupling has acted like a fuse and protected the expensive diesel engine.
In reality all that has happened is that fatigue has caused the coupling to fail.
Rubber-in-shear couplings can quickly fail on applications like these ones.
Premature fatigue failure of rubber-in-shear couplings is a virtual certainty when used in conjunction with diesel engines.
They do not act as fuses, they just fail.
The rubber element in a rubber-in-shear coupling is made up of lots of long-chain molecules that are distributed randomly in the rubber when the coupling is new.
When the drive-hub turns, the rubber element twists slightly and turns the driven hub, which causes the long-chain molecules to quickly orientate to the direction of twist.
When the coupling is subjected to the diesel engine's torque fluctuations, or worse, torque reversals, the long chains become tangled in their attempt to re-orientate themselves in the opposite direction.
This causes a high degree of stress that will eventually result in rapid fatigue failure.
The solution is to fit a different type of flexible coupling, such as a rubber-in-compression coupling, which protects the system from shock and vibration.
A rubber-in-compression coupling will operate for up to ten years without maintenance or problems with premature fatigue failure.
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