Page 168 - Gear Technology Solutions

12 Couplings and Clutches

12.1 The Function of Couplings

Couplings are designed for a variety of duties. Hardy disks, for example, are com-
pliant elements which connect a transmission output in a vehicle with the propeller
shaft. Compliance is required in order to accommodate the misalignments the
propeller shaft encounters when the vehicle is loaded with passengers, or if the
vehicle drives on uneven pavement.

Bellows couplings are designed to transmit precise rotation between two slightly
misaligned shafts. Those couplings are used to couple an encoder to an electric
motor for example. However, all couplings are per definition elements which do
not allow any relative motion in their rotational direction. Couplings are not de-
signed for frequent open and close. The two coupling halves are screwed or bolt-
ed together with the shafts they connect, and they are screwed or bolted together
with each other. Coupling will stay in the connected position unless a repair or dis-
assembly requires separating the two units which are connected by the coupling.

A good example is the Cyclocut coupling, which is mostly used to connect drive
shafts to the wheel hub unit. Although the drive shaft has for example, a constant
velocity joint (CV-Joint) to allow for the vertical movements of a vehicle, the CV-
joint itself has one half of the coupling integrated. The wheel hub unit has the op-
posite coupling member integrated at the end facing the drive shaft. The coupling
is the means to transmit high torque from the drive shaft to the wheel. Thus, the
two coupling halves act as one solid unit after assembly. Any movement or com-
pliance between the two coupling halves could have fatal consequences for vehi-
cle safety.

Clutches on the other hand are designed for frequent connecting and disconnect-
ing. Clutches might even accommodate some misalignment in special cases.
Clutches are not bolted rigidly together like couplings.

In both cases, couplings and clutches do not perform a meshing action between
their two members. In the case of couplings, this would physically not be possible
because they are bolted together. In case of clutches the misalignment leads to
unwanted friction and heat up of the units. The reason is because the tooth geom-
etry of couplings and clutches does not have involute flank surfaces and are there-
fore not designed to perform any meshing action. Meshing would not even be

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