With fluid couplings I have always selected them according to the manufacturers charts. ■

Please confirm with manufacturer for the oil specification & oil quantity also.

Sometimes I wonder that all fluid couplings will be replaced by V V V F drive in future.

Regards, ■

Fluid couplings have their own characteristics that are different to variable frequency drive units (if that's what is meant by "V V V F drive"). ■

Hi

Fluid couplings are normally selected by comparing the power generation capability of the coupling at a given motor speed. The diameter of the coupling working circuit is central to these calculations, which is why all couplings of a certain size produce roughly the same power output, irrespective of the coupling manufacturer.

The principle of selection is based on a principle devised by Hermann Fottinger in the 1900's and developed into commercial products by Harold Sinclair (Vulcan Sinclair later Fluidrive Eng) and Voith.

Looking at the manufacturers charts or tables will give you the power operating range of each individual coupling size for a given motor speed.....and that is all you need to do. You do not need to add any service factor.

For a conveyor or fan/pump system the published charts should suffice for a basic selection. For crusher/mill or any 'constant torque' machines, I would always suggest you discuss the application with the fluid coupling manufacturer.

Typically the power range charts will be based on the coupling giving an output shaft speed of between 95% and 98% of the motor shaft speed, so that does need to be factored in when doing your calculations. Any manufacturer will be able to give you that information as a matter of course if required.

A couple of other things to remember about fluid couplings is that the basic operational function allows the machine designer to select a motor that is sized for running the driven machine, not starting it. Therefore the motor does not need to be oversized for the duty. The coupling will take care of the starting.

The second consideration is that the coupling will only ever absorb from the motor, the power that is required to drive the coupling and the driven machine. Therefore an empty conveyor belt will draw much reduced amps, compered to the same conveyor fully loaded, which I guess is to be expected!!!

However this feature as well as being a useful 'energy saving' function is also an invaluable fault finding tool so long as the motor amps can be studied.

As a rule of thumb, in a troubleshoot condition, if the coupling output shaft is not turning ( or not turning at full speed), and the motor is not drawing full amps, the fault generally lies upstream of the driven machine. If the motor is drawing full amps, and the coupling output shaft is not rotating (or not turning at full speed), then the fault is typically downstream of the coupling.

It doesn't cure the problem, but at least it gives you a few pointers where to look.

Hope this helps ■

Another Comment:

Do not oversize the coupling without sufficient knowledge on what the larger size can do. This is sometimes done to achieve larger surface area for cooling, as is also done by delay filling.

The coupling filling must cover the motor torque curve otherwise you may stall the motor before it reaches full speed. This is typical of users that do not know that they have overfilled the coupling and need reduced coupling filling to allow the motor to pass through its torque stall point on the coupling fill level. ■