By the balance between the springs pulling the shoes inward and centrifugal force throwing them outward - so it is fixed by the mass of the shoes and the rate of the springs.

At rest and at low speed the springs hold the shoes clear of the drum. Centrifugal force rises with the square of speed, so as the engine accelerates the outward force grows quickly. At the point where it exceeds the spring preload the shoes begin to move out and touch the drum, and the clutch starts to transmit.

Because the force rises with the square of speed, engagement is progressive but not gradual - once it starts, a modest further increase in speed produces full engagement.

The engagement speed is therefore set at design and build, by choosing shoe mass and spring rate. It is not adjustable in service on most units, and it must be selected to sit comfortably above the engine's idle speed and below its working speed.

Because an internal combustion engine cannot start against a load, and a small one cannot even turn a stationary driven machine over.

An electric motor develops torque from standstill, so it can be coupled directly to its load. An engine develops no useful torque until it is turning, and it must reach a certain speed before it will run at all. Couple it rigidly to a stationary machine and it cannot be started - the starter or pull cord has to turn both the engine and the whole driven train.

A centrifugal clutch disconnects them. The engine starts and idles completely free of the load, then engages automatically as the throttle is opened and speed rises.

It also makes the machine safer and easier to use: a chainsaw's chain is stationary at idle, a mower's blade is not turning while the engine warms up, and the operator has a clear stop state without touching a clutch lever.

Of a kind - it protects the engine, not itself.

If the driven machine jams, a rigidly coupled engine stalls, and repeated stalling is hard on an engine and its starter. With a centrifugal clutch, the clutch simply slips instead: the engine keeps running while the output stays stopped.

That saves the engine and gives the operator a chance to shut down and clear the obstruction.

But it is not protection for the clutch. Slipping converts the entire engine output into heat in the friction faces, and that destroys them very quickly - a matter of seconds to minutes rather than hours. A clutch that has been slipped hard usually needs new shoes.

So treat it as a safety valve for the engine and the drive train, not as a torque limiter. Where genuine repeated overload protection is required, a proper torque limiter should be fitted in addition.

Running at or near the engagement speed for extended periods, which produces continuous partial slip.

A centrifugal clutch is designed to be either disengaged, below engagement speed, or fully locked up, well above it. In between there is a band where the shoes are touching the drum but not clamped hard enough to prevent relative motion, and in that band the clutch slips continuously, dissipating power as heat.

An engine held at part throttle in that band - a machine being worked gently, or an engine that has lost power and cannot get past it - wears the shoes out rapidly and can overheat the drum enough to damage it.

The defences are to select an engagement speed well below the normal working speed, to operate the machine at proper working revs rather than nursing the throttle, and to fix an engine that will not reach its rated speed rather than working it in the slipping band.

No - not while the engine is above engagement speed, which is a genuine design constraint rather than an inconvenience.

The clutch is engaged by speed alone. As long as the input is turning fast enough, the shoes are thrown out and the drive is connected. There is no external control to override that.

So disconnecting the drive means slowing the engine below the engagement speed, which usually means closing the throttle to idle.

Where a machine needs to run its engine at speed with the drive disconnected - to power an auxiliary, or to keep the engine warm while stationary - a centrifugal clutch alone cannot do it, and a manually operated clutch, an additional clutch in the train, or a belt-tensioning arrangement is required.

This also means the machine has no neutral at working revs, which matters for safety design: the stop state depends on the engine returning to idle.

It is far simpler and cheaper than either, and correspondingly less capable.

A manual clutch gives the operator control - engage when you choose, at any speed, and slip deliberately for fine control. It needs a linkage, a lever and an operator, and it can be abused by riding it.

A torque converter uses fluid coupling to transmit torque, multiplies torque at low speed, and never wears because nothing rubs. It is much more expensive, less efficient at speed unless locked up, and larger.

A centrifugal clutch has no controls at all, costs very little, and is compact. It suits equipment where the requirement is simply 'connect when running, disconnect at idle'.

For handheld and small engine-driven equipment that describes the requirement exactly, which is why it dominates that market despite the alternatives being technically superior.

Shoe lining thickness, drum condition, spring integrity and cleanliness - and most units are serviced by replacing the shoes as an assembly.

Check the friction lining thickness against the manufacturer's limit. Worn shoes engage later and less firmly, which pushes the machine towards operating in the slipping band and accelerates the wear.

Inspect the drum for scoring, heat checking and bluing. A drum that has been overheated by sustained slipping will have a hardened, glazed surface that never grips properly again, and replacing the shoes alone will not restore performance.

Check the springs. A broken or weakened spring lowers the engagement speed, so the clutch begins to engage at idle - which on a chainsaw or mower is a safety issue as well as a wear one.

Keep the assembly free of oil and grease. Lining contaminated by a leaking crank seal will slip regardless of its thickness.