For force, and therefore for torque - a fluid acting on a piston area produces far more clamping force than a practical magnetic circuit.

Magnetic force falls away rapidly with air gap and is limited by saturation of the iron circuit. Pressurised fluid has no equivalent ceiling: increase the piston area or the pressure and the force rises proportionally. That makes fluid-actuated clutches the practical choice once the torque requirement exceeds the electromagnetic range.

The second reason is modulation. Because force tracks pressure, and pressure can be controlled through a valve, engagement can be ramped deliberately. An electromagnetic clutch is essentially on or off.

The costs are infrastructure and response. A compressed air or hydraulic supply must reach the drive, with valving and often a rotary union to get fluid into a rotating member, and response times are longer than a coil's milliseconds. Where the machine already has a fluid supply and the torque is high, none of that is a real objection.

A clutch in which an inflatable elastomeric tube expands to press friction shoes against a drum, rather than a piston pushing plates together.

The tube runs around the inside of a rim carrying friction shoes. Admitting air inflates it, and it forces the shoes outward against a drum - or inward, depending on the arrangement - to transmit torque. Releasing the air lets the tube collapse and the shoes retract.

The construction brings three advantages. It tolerates significant shaft misalignment, because the flexible element accommodates it rather than transmitting it. It dissipates heat well, since the ventilated design lets air circulate around the friction surfaces. And the engagement is inherently cushioned by the compliance of the tube.

They are widely used on marine propulsion, oilfield drawworks and mud pumps, mills, crushers and large industrial drives - heavy applications with substantial torque, meaningful misalignment and high engagement energy.

By controlling how fast pressure builds in the actuator, using flow control valves, regulators or a proportional valve.

Clamping force is proportional to pressure, and torque capacity is proportional to clamping force. So a slow pressure rise produces a clutch that transmits gradually increasing torque, allowing the driven inertia to accelerate over a controlled period rather than being snatched into synchronism.

The simplest implementation is a flow restrictor in the supply line, which stretches the pressure rise over a set time. More sophisticated systems use a proportional or servo valve to follow a defined pressure profile, which allows a soft start followed by full clamping.

The trade-off is thermal. A longer engagement means more slipping, and therefore more heat per engagement. So the engagement rate is tuned between two competing requirements - gentle enough to protect the driven machine, brief enough to keep the friction faces within their thermal budget.

Through a rotary union or a rotating seal assembly, which is a component that needs specifying and maintaining in its own right.

The clutch actuator rotates with the shaft, but the supply pipework does not. A rotary union bridges that: a stationary housing carrying the supply connection, and a rotating element sealed against it, so fluid passes from one to the other while they turn relative to each other.

The seals in that union are wearing parts, and they see continuous relative motion whenever the shaft turns - unlike the clutch friction faces, which only wear during engagement. On many installations the rotary union is the component that needs attention first.

Some designs avoid it by mounting the actuator stationary and acting on the rotating clutch through a thrust bearing.

When specifying, confirm what supply arrangement the clutch requires, whether the union is included, and what its service requirement is - it is easily overlooked at the quotation stage.

Air where the supply exists and the force required is moderate; hydraulic where the force is high or the actuator must be compact.

Air is convenient. Most plants have a compressed air supply, valving is cheap, leaks are harmless, and the compressibility of air gives a naturally cushioned engagement. The limit is pressure - typical plant air is modest, so a large clamping force needs a large piston or tube area, making the clutch physically big.

Hydraulic pressure is an order higher, so the same force comes from a much smaller actuator, and the clutch is more compact for a given torque. Oil is also incompressible, which gives precise, repeatable engagement control. The costs are a power pack, more expensive valving, and leaks that are a genuine nuisance and a hazard.

In practice the decision is often made by what the machine already has. A press with a hydraulic system uses hydraulic; a conveyor drive in a plant with ring main air uses air.

Heat first, then valve and actuator response.

As with any friction clutch, each engagement dissipates the energy of the speed mismatch as heat, and the sustainable cycle rate is whatever keeps the friction faces within their thermal rating. Because these clutches are used on high-inertia machines, the energy per engagement is typically large, so the thermal limit bites at relatively low cycle rates compared with an electromagnetic unit.

Air-tube designs help here, because the ventilated construction sheds heat well.

The second limit is response. Filling and venting an actuator takes time - more for a large volume, more through small valves and long pipe runs, and more for air than for oil. Where fast cycling is needed, valve sizing and pipe runs matter as much as the clutch.

If a machine needs both high torque and high cycle rate, expect the thermal calculation to dominate the selection and consider forced cooling.

Attention to the supply, the rotary union and the elastomeric element, plus the usual friction wear.

On air systems, supply quality matters. Moisture and oil carried into the actuator degrade elastomeric elements and can freeze in exhaust ports, so filtration and drying should be in place and maintained.

The rotary union's seals wear continuously and are frequently the first thing to need service. Watch for leakage and for pressure loss during engagement.

Air tubes have a finite life and perish with age, heat and oil contamination. They are a scheduled replacement item on heavy-duty installations, and a tube that fails in service takes the drive out.

Friction faces wear as on any clutch, and units generally have a means of checking lining thickness or actuator travel. Increasing travel to engage is the usual indicator that relining is due.