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 and the tasks they perform. One such myth is ‘fuse theory’, and, according to Paul Shuffleton, managing director of Renold Hi-Tec Couplings, it is costing industry a king’s ransom.
According to the 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 that the relatively inexpensive coupling has acted like a fuse and protected the engine. In reality all that has happened is that the coupling has failed, and experience tells us that it doesn’t take long for a rubber-in-shear coupling to fail on an application like this.
When the coupling is subjected to the diesel engine’s torque fluctuations, or worse, torque reversals, the long chain molecules of the rubber elements become tangled in their attempt to re-orientate themselves in the opposite direction. This causes a high degree of stress that will 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.