It performs both functions in the space a coupling alone would occupy.

In a conventional arrangement, the clutch is a distinct component mounted on one of the shafts, and a coupling then connects it to the other shaft. That needs axial length for both, and the clutch often needs its own bearing support because it is not simply spanning a gap.

A clutch coupling is designed as a single unit that fits between the two shaft ends. It carries a hub on each shaft, transmits torque between them when engaged, and separates them when disengaged.

The practical effect is that a disconnect capability can be added to a drive train without lengthening it or redesigning the layout - frequently the deciding factor when retrofitting one to an existing machine where there is no room to insert two components.

Wherever a driven machine has to come on and off line while the prime mover keeps running.

Common cases are auxiliary drives taken from a main engine or line shaft - pumps, blowers, compressors, generators - which need to be brought in and out of service independently. Marine and engine-driven installations use them heavily for this reason: the engine cannot be stopped to change what it is driving.

They are also used to isolate one machine in a multi-machine drive for maintenance while the rest of the line continues, and to disconnect a high-inertia driven machine so a motor can be started unloaded.

That last case is worth noting: starting a large motor against a large inertia draws heavy current and stresses the supply, and a clutch coupling that lets the motor come up to speed before picking up the load can be cheaper than upsizing the motor and its starter.

It depends on the type, and it is an important thing to establish before ordering.

Friction clutch couplings can engage on the move: the friction faces slip while the driven side accelerates to match, exactly as any friction clutch does, and the same thermal considerations apply - the energy of that speed mismatch becomes heat, and the cycle rate must respect the thermal rating.

Positive or jaw-type clutch couplings engage interlocking teeth and cannot be engaged at any significant speed difference. Attempting it damages the teeth and can be violent. These are engaged at rest, or at very low creeping speed with a synchronising arrangement.

The distinction determines what the machine can do operationally, so state clearly whether on-the-fly engagement is required. Positive types are chosen when the connection must never slip once made; friction types when the load has to be picked up while the drive is running.

Most do, and it is one of the reasons to use one rather than a clutch plus a rigid connection.

A coupling's normal job includes accommodating the residual misalignment between two separately mounted machines - angular, parallel and axial. Clutch couplings are generally designed to retain some of that capability, using flexible elements, gear teeth or a floating member in the connection.

The permitted misalignment is usually less than a dedicated flexible coupling of the same size would allow, because the clutch mechanism occupies space and imposes its own constraints.

So do not assume a clutch coupling will absorb whatever misalignment exists. Check the published figures, align the machines properly, and treat the coupling's capability as tolerance for thermal growth and settlement rather than as a substitute for alignment. A clutch coupling running persistently misaligned wears its mechanism as well as its bearings.

The same range as clutches generally - mechanical lever, pneumatic, hydraulic and electromagnetic - chosen on torque, control and what services reach the drive.

Mechanical lever actuation is simple and needs no services, suiting occasional manual disconnection where an operator is present. Pneumatic and hydraulic actuation give high force for high torque and allow remote operation, but need a supply and usually a rotary union to get fluid into the rotating assembly. Electromagnetic actuation gives fast remote electrical control at more modest torque.

Because the unit sits between two shafts and rotates, getting the actuating medium into it is the design problem, and it is worth asking specifically how a given product does it - a stationary actuator working through a thrust bearing avoids a rotary union and its maintenance.

Match the actuation to what the installation already has rather than introducing a new service for one component.

By torque and service factor first, then by the physical geometry, which is often the binding constraint.

Size on the transmitted torque with a service factor for the duty, and if engagement occurs on the move, check the thermal capacity against the energy per engagement and the cycle rate.

Then deal with geometry: the two shaft diameters, the distance between shaft ends, and the space available radially. Because the unit must fit between existing shafts, that gap frequently decides which products are candidates - and on a retrofit it may be fixed.

Installation follows coupling practice: align the machines within the published limits, fit the hubs correctly on the shafts, and check that the actuator has clearance to operate and that its supply or wiring can reach it.

Check axial float too. Both shafts will move axially with thermal growth, and the unit must accommodate that without loading its bearings or affecting engagement.

Alignment checks, actuator attention, and the friction or tooth wear appropriate to the type - with the actuation system usually needing more attention than the clutch.

Re-check alignment periodically, particularly after any work on either machine. A clutch coupling running misaligned wears its internal mechanism as well as the machine bearings either side, and the symptoms appear as engagement problems rather than obvious misalignment.

On pneumatic and hydraulic units, the rotary union seals wear continuously whenever the shaft turns and are commonly the first service item. Watch for leakage and for slow or weak engagement, which indicates pressure loss.

Friction types need lining thickness checked; positive types need the engaging teeth inspected for wear and chipping, which usually indicates engagement attempts at too high a speed difference.

Keep contamination away from friction faces - a leaking bearing seal on the adjacent machine is a common cause of sudden slipping.