The clutch drum is attached to the prime mover, the hub carrying the flexibly mounted lined shoes forming the driven half coupling. To initiate engagement, the clutch shoes are moved outwards into pressure contact with the rotating drum air thrusters built into the driven half hub.
This starts the driven half rotating, and as it accelerates the centrifugal effect causes the clutch shoe assemblies to press harder against the drum, giving progressively firmer engagement until after a few seconds the starting air pressure can be removed.
When up to operating speed the clutch transmits normal power, being "locked up" by centrifugal force and therefore operates without further slip.
Should an overload occur sufficient to cause clutch slip or to slow the prime mover significantly, the clutch disengages immediately protecting the transmission from the high inertia torque which would otherwise be applied by the rotating masses of the engine, electric motor or turbine rotor.
Clutch disengagement (if required) makes use of the this feature. It is effected by a disc brake operating on the driven hub. Thus, when the driven machine is stationary, it is also secured against rotation by the brake. This is an essential requirement in marine propulsion or wherever the load can drive back, as in centrifugal pumps.
The disc brake is necessary if the clutch has to be disengaged whilst the machinery is running or if the driven shaft must be held stationary whilst the clutch is disengaged. (If the driven shaft can be rotated by the load, it will try to engage the clutch, making a brake essential).
The size of brake is chosen to ensure that clutch disengagement is swift and clean, and this of course depends on the shaft speed because the clutch slip torque is proportional to the square of the speed of rotation of the driven shaft.
Although it is most often convenient to mount the brake disc directly to the clutch driven hub it can be mounted anywhere on the driven shafting. The can be desirable in cardan shaft arrangements with flexibly mounted machinery in order to provide satisfactory alignment between calipers and disc.
- Engagement at any speed.
- Disengagement at any speed.
- Overload protection by disengaging immediately on heavy overload therefore protecting the clutch and rest of transmission equipment.