clutch

A clutch is a coupling device that engages and disengages power flow between a driving shaft (usually the engine crankshaft) and a driven shaft (usually the transmission input shaft). In manual-transmission vehicles, the driver operates it via a foot pedal; in automatic transmissions, a fluid coupling or torque converter replaces the friction clutch entirely. The basic friction clutch consists of a pressure plate, friction disc (or discs), and release mechanism, all housed in a bell housing bolted to the engine.

The friction disc sits between the engine flywheel and pressure plate. When the clutch pedal is released, spring pressure clamps the disc against the flywheel, transmitting engine torque to the transmission. When engaged, the spinning friction surfaces create enough grip to lock the two shafts together; when disengaged, they slip freely, breaking the drive line. Most automotive clutches use organic, ceramic, or sintered metal facing materials bonded to a steel backing plate. Single-disc clutches dominate cars and light trucks; heavy trucks and industrial machinery often use multi-plate clutches (wet or dry) to handle higher torque loads.

Wear and failure modes

The friction disc wears gradually with use. Slipping under load accelerates wear; so does riding the pedal or holding the clutch partly engaged on hills. Worn facings reduce grip and allow the engine to spin faster than the transmission (slippage), generating heat. Complete failure occurs when facing material is gone or the disc becomes damaged. Pressure plate springs fatigue and weaken over time. Hydraulic release mechanisms can fail if seals leak, leaving the clutch stuck engaged or disengaged. Most clutches are replaced as an assembly rather than repaired.

The term originates from the verb "to clutch," meaning to grip or hold. Industrial applications beyond vehicles include conveyor drives, textile machinery, machine tools, and pump couplings, where clutches allow smooth starts under load and protection against shock loads. In these contexts, safety clutches (slip clutches or shear-pin couplings) are often added downstream to limit transmitted torque and prevent damage to downstream equipment if jamming occurs.