By circulating oil between two bladed wheels facing each other.

The impeller is attached to the input shaft and the runner to the output, mounted face to face inside a sealed housing partly filled with oil. As the impeller turns, its vanes throw oil outward by centrifugal action. That oil crosses to the runner at the outer diameter, is decelerated as it flows inward through the runner's vanes, and returns to the impeller at the centre.

The change in the oil's angular momentum as it passes through the runner is what applies torque to the output shaft.

Because the two wheels never touch, there is no wear surface and no mechanical connection at all. Torque transmission requires a speed difference between them - the slip - because without it there is no circulation. That is why a fluid coupling always runs a little slower on the output than the input, and why it transmits nothing at all when both are at rest.

Because at standstill it transmits almost no torque, so the motor accelerates itself before it has to accelerate the load.

A motor connected directly to a heavy load must produce breakaway torque from zero speed, which draws very high current for an extended period and shocks the whole drive train. On a long loaded conveyor it may not manage it at all, and repeated attempts overheat the motor.

With a fluid coupling, the impeller churns oil past a stationary runner at first, absorbing very little torque. The motor reaches close to full speed almost unloaded - where it develops its full torque efficiently - and only then does the coupling begin to transmit meaningfully, picking the load up progressively.

The results are much lower starting current, a smaller motor and starter for the same duty, no shock loading, and on conveyors, no belt snatch.

Delay-fill designs, which hold most of the oil in a reservoir until the drive is running, extend the effect further for very high inertia loads.

By having a maximum torque it can physically transmit, so it slips instead of transmitting the stall torque.

The torque a fluid coupling transmits depends on the fill, the geometry and the speed. There is a ceiling: beyond it, increasing the load simply increases the slip rather than the transmitted torque.

So if the driven machine jams, the motor does not stall and the drive train does not see a huge torque spike. The coupling slips, the motor continues to run near its normal speed, and the mechanical components downstream are protected.

Unlike a shear pin or a friction torque limiter, this protection cannot be adjusted out, cannot seize, and needs no resetting - it is a property of the physics rather than a device.

The limitation is that all that slip energy becomes heat in the oil very quickly. Protection is against a momentary jam; a sustained stall will overheat the coupling, which is what the fusible plug addresses.

A thermally sensitive plug that releases the oil if the coupling overheats, so it fails safely instead of bursting.

When a coupling slips continuously - a jammed conveyor that nobody stops, or repeated failed starts - the whole motor output is being dissipated as heat in a small volume of oil. Temperature and internal pressure rise quickly.

The fusible plug contains an element that melts at a set temperature. When it does, the oil is discharged, the coupling immediately stops transmitting torque, and the drive is disconnected. That prevents a pressurised housing failure, which would be dangerous.

A blown plug is therefore a protective action, not a fault in itself - but it is a signal. The correct response is to find why the coupling was slipping, not simply to replace the plug and refill.

Plugs are available at different temperature ratings, and fitting a higher-rated plug to stop nuisance operation removes the protection rather than solving the underlying problem.

A few per cent slip at rated load, which is a continuous efficiency loss for the life of the drive.

Because torque transmission requires relative motion between impeller and runner, the output always runs slightly slower than the input. At full load that difference is typically a small percentage, and the corresponding power appears as heat in the oil.

On a large continuously running drive that loss is real money, and it is worth including in the comparison against alternatives such as a variable frequency drive, which can provide soft starting electronically with no permanent slip.

The slip also means the output speed varies slightly with load, which matters where precise speed is required.

Against that, the coupling is simple, robust, needs no electronics or cooling, tolerates hostile environments, and provides overload protection that a VFD does not. On heavy conveyor and crusher duty in dirty locations those advantages frequently outweigh the efficiency.

On high-inertia drives that are hard to start and prone to jamming.

Long belt conveyors are the classic application, especially in mining, quarrying, ports and cement, where the belt is heavy, may be started loaded, and the drive must not snatch. Crushers, mills and shredders use them for the same reasons plus the overload protection.

Other common applications are large fans and blowers with high inertia, centrifugal pumps, mixers and agitators in process plant, and marine and rail traction.

They are also used in multi-motor drives to share load between motors, since a coupling's slip characteristic naturally balances torque between several drives on one shaft - which is difficult to achieve with rigid connections.

They are less appropriate where efficiency dominates, where precise speed is needed, or on light drives that start easily and where the coupling's cost and losses are not justified.

Oil level and condition, seal integrity, and attention to why any fusible plug operates - otherwise very little.

The oil fill determines the torque characteristic, so it must be correct: underfilled, the coupling slips more and runs hotter; overfilled, it transmits more torque than intended and reduces the soft-start benefit. The fill is specified by quantity, not by a level gauge on most units.

Check for leaks. A coupling that has lost oil will slip increasingly, run hot and eventually blow its plug, and the first symptom is often a drive that has become sluggish to reach speed.

Monitor running temperature. Because there is nothing to wear, temperature is the primary condition indicator, and a coupling running hotter than it used to is reporting increased slip - usually from a heavier load or a lost fill.

Investigate every fusible plug operation rather than refilling and restarting.