The geometry of the sprags or rollers does it - they wedge in one direction of relative rotation and release in the other.

In a sprag clutch, shaped cams sit between a cylindrical inner and outer race, lightly sprung so they always touch both. Relative rotation one way rolls the sprags into a position where their effective height exceeds the gap, so they jam and lock the races together. Rotation the other way rolls them the opposite way, reducing their height, and they release.

Roller ramp clutches work similarly, with rollers running up wedge-shaped ramps machined into one race.

The action is entirely mechanical and responds to relative motion, so engagement is immediate and requires no power, no signal and no operator. Torque capacity comes from the wedging action itself - the harder the load pulls, the tighter the sprags jam, which is why these clutches carry high torque in a small package.

The same mechanism doing three different jobs, and the intended duty changes the sizing and the selection.

Overrunning lets the driven part rotate faster than the driving part. The classic case is a two-speed drive where a high-speed motor takes over from a low-speed starting motor - the clutch simply releases and the starting drive is left behind. Here the clutch spends most of its life freewheeling, so overrunning speed and duration are the key figures.

Indexing converts an oscillating input into intermittent one-way output: the clutch drives on the forward stroke and releases on the return. Cycle rate and engagement count dominate.

Backstopping prevents reverse rotation entirely. The clutch is stationary and locked whenever the drive stops, holding the load. Here static torque capacity is what matters.

State the duty when specifying - a clutch chosen for overrunning may be quite wrong as a backstop.

On the runback torque from the loaded machine, which is not the drive torque and is frequently larger.

The temptation is to size the backstop to the motor or the gearbox rating. That is the wrong number. What the backstop must hold is the torque gravity applies through the loaded conveyor, elevator or column of liquid when the drive stops.

On a fully loaded inclined conveyor that runback torque can substantially exceed the normal running torque, because running torque is partly offset by the drive's efficiency and by friction that no longer helps once the direction reverses.

It must also be calculated at the shaft the backstop is fitted to. A backstop on a high-speed shaft sees the runback torque divided by the gear ratio, which makes it small - but it then depends on every component between it and the load remaining intact.

Use the worst-case load, include a service factor, and state the duty clearly to the supplier.

The machine will not run at all in the intended direction, which makes it an obvious error - and that is fortunate, because the alternative failure is not obvious at all.

A reversed backstop locks against the driving direction, so the drive stalls immediately on starting. It is discovered within seconds.

The dangerous error is the opposite one: a backstop removed during maintenance and not refitted, or refitted but not reconnected. The machine then runs perfectly normally in every respect, and the absence is invisible until the moment the drive stops with a full load - at which point the conveyor runs back.

So the procedural control matters more than the physical one. Backstops should be listed on maintenance records, checked as part of post-maintenance commissioning, and their presence verified rather than assumed.

Direction of rotation should also be marked clearly on the machine, since a drive rewired after a motor change can end up running the other way.

Yes - most need oil or grease, and lubrication failure is a common cause of trouble because the failure mode is unhelpful.

Sprags and rollers must move freely to engage and release. Inadequate or degraded lubricant makes them sluggish or sticky, so they may fail to engage promptly, or fail to release and drag continuously during overrunning.

Dragging during overrunning is the damaging case. The sprags are in contact and skidding at speed, generating heat and wear in a mechanism designed to be either locked or free, and it destroys the clutch relatively quickly.

The lubricant specification matters more than in an ordinary bearing: too heavy an oil in cold conditions slows engagement, and an extreme-pressure additive intended for gears can interfere with the friction the wedging action depends on. Follow the manufacturer's recommendation rather than sharing the gearbox oil by default.

On high-speed continuous overrunning, lift-off designs that mechanically separate the sprags are used instead.

Only with designs made for it - a standard clutch overrunning continuously at speed wears out.

When a sprag clutch overruns, the sprags stay lightly in contact with both races under their springs, skidding against them. At low speed and short duration that is harmless. At high speed and continuously it generates heat and wears the sprags and races steadily.

For continuous high-speed overrunning - as in a two-speed drive where the auxiliary clutch overruns for the whole running period - centrifugal lift-off designs are used. Above a certain speed, centrifugal force swings the sprags clear of the inner race so there is no contact at all, and the clutch overruns with no wear.

When specifying, always state the overrunning speed and how long the clutch spends overrunning, not just the driving torque. It is the most commonly omitted figure and the one most likely to produce an unsuitable selection.

Indexing mechanisms, two-speed and standby drives, and anywhere a load must not reverse.

Backstop duty extends well beyond conveyors: bucket elevators, screw conveyors, pumps and fans that would windmill backwards on shutdown, gearbox output shafts on inclined drives, and cooling tower fan drives.

Overrunning duty appears in two-speed drives, in engine starting arrangements, in standby and dual-drive systems where one prime mover takes over from another, and in turbine auxiliary drives.

Indexing duty is found in feed mechanisms, packaging machinery, and anywhere an oscillating motion must be converted into intermittent one-way rotation.

They are also used as a simple form of emergency protection - allowing a manual drive to take over if a powered one fails, without any changeover mechanism. In every case the mechanism is the same and the selection question is which of the three duties governs.