Old teleprinters printed each letter with a clutch that turned exactly once
Every character arriving at a teleprinter tripped a special clutch that engaged the motor for precisely one revolution, drove the print mechanism through a single cycle, and then let go. Nineteenth-century factories used the same trick so a shear or press would strike once per pull of a lever.
A clutch lets a driven shaft be disconnected from a spinning input shaft. In a car it sits between engine and gearbox, so disengaging it frees the engine's speed from the wheels. In a power drill, a clutch lets the bit turn at full motor speed, slip at a lower speed, or stop while the motor keeps running. Most manual-transmission cars use dry clutches, where springs press a friction disc against the engine's flywheel and a lever pulls it away.
Wet clutches run their friction surfaces in oil for smoother engagement and longer life, at some cost in efficiency; motorcycles stack several discs to make up for the slippery surfaces. Drag racers use multi-plate clutches to control how fast power reaches the wheels off the line. Centrifugal clutches engage automatically as speed rises, which suits scooters, chainsaws and some early cars. Cone clutches wedge a conical surface into place and serve as synchronizer rings in manual gearboxes, while dog clutches lock without slipping at all.
Single-revolution clutches reached far beyond factories. In tabulating machines the operate key tripped one to process the last number entered, and typesetting machines cycled once per key. Frederick Creed developed a wrap-spring version for teleprinters in 1928: a coil spring held open by a trip lever snaps tight around the power shaft when released. Two Pitney Bowes employees improved the design in 1942, and later cascaded-pawl clutches, whose floating pawls spring outward like drum brake shoes, drove the Teletype Model 28 and IBM Selectric typewriters.
Clock makers found another use. Some synchronous electric clocks built before the 1940s could start backwards, so they carried a kickback clutch-brake that locked in reverse, wound a spring, and released it to restart the motor the right way. Lawnmowers and snow blowers often use the humblest form: slack drive belts that an idler pulley tightens to transmit power.
Source: Clutch
