Fluid Coupling Fault Finding

The most common complaint is a driven machine that will not come up to speed. The quickest way to narrow it down is to see what the motor is doing:

• The motor gets to speed but the coupling output does not: the coupling is not transmitting enough torque. This is generally a low oil level or the wrong type of oil.
• The motor cannot get to speed: this points to an overload condition. The driven machine is demanding more power than the drive can supply, for example because it is jammed, overfed or has a mechanical fault.

If you can, check the current the motor is drawing. Motor amps show how much power the system is demanding, which tells you whether the problem is in the coupling or in the load.

With the coupling filled and set correctly, the fusible plug protects the drive: in a sustained overload or stall the oil heats up, the plug melts and the oil is released before the motor, coupling or driven machine is damaged. A plug that keeps melting is telling you something about the load or the fill, not that the plug is wrong. If you send us the motor current and voltage, the coupling type and size, and the driven machine, we can generally advise on the cause.

Multi disc units and driving plates are the flexible elements at the output and input of the coupling. They are designed to take small misalignments, not to correct large ones, so when one fails the cause is almost always alignment. Fitting a new part without finding the cause means it will fail again.

Before fitting a new multi disc unit:

• Check and correct the alignment between the coupling and the driven machine.
• Check the bolt holes in both half couplings. Any elongation of the holes means the half couplings need replacing as well.

Before fitting a new driving plate:

• Check and correct the alignment between the coupling and the motor.
• Check the condition of the motor boss location ring.
• Check the condition of the coupling centre spigot.

FCU. The FCU traction coupling was the forerunner of all later Fluidrive designs, built in sizes from 6 to 41, and early examples are still running today. Over its life it changed from BSF fasteners to UNF, and then to metric threads and metric bearings from around contract C34000 in the early 1970s, with parallel runner shafts replacing tapers. Fluidrive kept the complete unit interchangeable through these changes, but the parts often are not:

• Metric and imperial bearings cannot simply be swapped, because the shaft journals and bearing housings changed with them. Imperial bearings for the smaller sizes are getting hard to find.
• Gland designs are not compatible with one another (see above).
• On small couplings, below size 12.75, repair is rarely economic, and replacing the unit with a current-specification coupling is usually the better choice.

SFU and STC. These steel-cased traction couplings were developed for the British mining industry after aluminium was banned underground. They were the first range with metric size designations, in eight sizes from 300 to 600, and come in two types:

• SFU: mounted on a drive head with a motor boss and resilient driving plate at the input and a multi disc assembly at the output, so the alignment checks above apply. SFUs can be found mounted horizontally or vertically, output shaft up or down.
• STC: fitted with a hollow splined shaft directly onto the gearbox input, with a pin and buffer input drive, inside a bell housing spigoted to the gearbox and motor, which removes the need for conventional alignment.

Both types were later produced in a strengthened version for arduous duties such as coal and mineral breakers, where sudden lock-ups happen, and in a softer-starting "K" version with a different impeller. Many parts are interchangeable between the variants of the same size, so it is important to identify exactly which variant you have before ordering parts.

Use the section drawings on our Fluidrive coupling spares page to identify parts, and see the scoop coupling page for dating a coupling from its contract number.

Fill in what you know. Fields marked * are needed. The nameplate with the contract number, a photograph, and the motor current and voltage help most: attach photos once your email or WhatsApp opens.