Coupling Fuse Theory - Blown
With so many different types of couplings to choose from it is perhaps not surprising that, over the years, a little bit of mythology has grown up around certain types of couplings and the task they perform. One such myth, which has established itself a bit like an old wives' tale , is known as fuse theory, and, according to Paul Shuffleton, Director - Renold Gears & Couplings Division, it is costing industry a king's ransom.
According to fuse theory, rubber-in-shear type flexible couplings will protect diesel driven generator sets by failing, or acting like a fuse, when there is a potentially damaging short circuit. And that's the myth, because the majority of rubber-in-shear couplings fail through fatigue rather than because of a short circuit. More specifically, the rubber block around which the coupling is designed fails through fatigue , not because of a short circuit.
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 provides protection 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 that there has been a short circuit and the relatively inexpensive coupling has acted like a fuse and protected the expensive diesel engine. In reality all that has happened is that the coupling has failed due to fatigue. In other words, it has worn out. And here's the crunch, it doesn't take long for a rubber-in-shear coupling to fail on an application like this.
Premature fatigue failure of rubber-in-shear couplings is a virtual certainty when used in conjunction with diesel engines. They are not acting as fuses, they are failing.
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, and when this happens the long chain molecules quickly orientate themselves 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 that will protect the system from shock and vibration and will operate for up to ten years without maintenance or problems with premature fatigue failure.
