11.12  Self-locking: wedges, screws and cam levers

A mechanism is self-locking when no load on its output can drive its input, because friction absorbs the work. The wedge and the screw lock when their angle is below the friction angle, \(\tan\lambda < \mu\), which is the screw jack of Friction and the worm drive of Transmissions; the efficiency of a self-locking drive is then below 50 %. The eccentric cam lever of a bicycle quick release, which locks by friction or over centre depending on its geometry. Rope clamps and cam cleats. Self-locking and over-centre compared: one holds by friction and loosens with vibration and lubrication, the other holds by geometry.