A sprag clutch (also called a one-way clutch, overrunning clutch, or freewheel) lets a shaft drive a hub in one rotational direction but slip freely in the other. No electronics, no engagement command — the geometry itself decides when to lock. If the inner race tries to rotate the "drive" direction relative to the outer race, cam-shaped sprags or rollers wedge into a taper and lock the two races together. Reverse the relative rotation and they release instantly.

Three common constructions:

• Roller ramp clutches: cylindrical rollers ride in wedge-shaped ramps machined into one race. Cheap and compact; moderate torque.
• Sprag clutches: asymmetric figure-8 sprags between two smooth cylindrical races. Highest torque density and best for high-speed overrun.
• Wrap spring clutches: a helical spring grips a shaft when torqued one way and unwinds the other. Common in printers and paper feeders.

Real-world example: the starter motor on your car. When the starter pinion spins the engine at 200 RPM to crank it, the sprag clutch inside the Bendix drive locks. The moment the engine fires and spins the ring gear at 1,500 RPM — far faster than the starter — the sprag overruns and disconnects, saving the starter from being spun to destruction. Bicycle rear hubs use the same principle: pedal forward and the freewheel engages; coast, and it clicks (that's the pawls skipping over ratchet teeth on cheaper versions, or sprags releasing on high-end hubs).

Other places you'll find them: helicopter main rotor autorotation (engine failure → rotor keeps spinning freely instead of dragging a dead turbine), backstops on conveyor drives (prevents a loaded belt from rolling backward when power cuts), indexing mechanisms, and two-speed transmissions.

Rule of thumb — sizing: Manufacturers publish two torque ratings: static (holding) and indexing. Indexing torque is typically 40–60% of static because repeated engagement fatigues the wedge contact points. Also watch the overrunning speed limit — sprags in an overrun condition still have some drag, and centrifugal force can lift them off the race above ~15,000 RPM, causing skipping. If you'll overrun at high speed, specify a design with centrifugal sprag lift-off features.

Failure modes: the most common is rollover — applying reverse torque above the rated capacity forces the sprags flat, permanently indenting the races. Once that happens, the clutch slips in both directions and must be replaced. Shock loads (like slamming a backstop when a conveyor belt breaks) are the usual culprit.