Flexible motor couplings – torsional rigid or torsional elastic?
The question is not a new one: should elastic couplings (with a rubber element inside) or rigid couplings (typically Gear- Couplings) be applied in cranes? Elastic couplings have been required formerly to absorb and compensate the high peaking starting torque developed by the motors. But in today’s world, there are almost all motors (specifically hoist motors) controlled by frequency converters, providing smooth and mostly torque constant acceleration without any peak starting torques. Also, electric insulation between drive components is not a reason any more to go for elastic couplings, since Gear-Couplings can be provided with electric insulation as well. So the main reason today to go for elastic couplings is that those couplings do not need grease and they are almost maintenance-free.
Maintenance of couplings
Elastic couplings do not need lubr ication, and thus have no problems with grease leaks at all – but the elastic (rubber) part of those couplings is subject to wear and frequently needs replacement. The lifetime of a gear coupling is in fact much longer than the lifetime of any elastic part. Some customers would like to change their elastic couplings for this reason, but hesitate to convert the elastic couplings to gear couplings for two main reasons. On the one hand, a conversion usually means replacing the complete coupling, including the hubs and brake disc, with the necessity to move at least the motor backwards and re-align the drive after the conversion is done. On the other hand, customers are afraid that after the conversion they may not be happy with the gear coupling, and re-converting back to an elastic coupling results further expenditure of time and money.
Solution
The solution for all this provided by Malmedie is now on the market and is called the S-NX gear-coupling. It is available as a complete coupling or as a conversion kit. The ‘inner’ (geared) parts of the S-NX coupling are of the same dimensions as the ‘inner’ (elastic + claw ring) parts of the SDDL and equivalent series, which are the most common elastic couplings.
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