User:DirkJandeGeer/Articles/Auto Belay

An auto belay (or autobelay) is device which automatically belays a climber without the need for a second person. As a climber ascends, the device takes in the slack webbing (or less often rope ), and as the climber descends, the device slows their decent to a safe speed.

Types
There are three primary types of belay device: hydraulic, magnetic, and friction braking.

Hydraulic auto belays
Hydraulic auto belays work by using the force of a climbers descent to force oil through a narrow pipe into a chamber of pressurized gas. The combination of pressure from the gas and a small pipe to flow through resists the movement of the oil, thereby resisting the descent of the climber. The pressurised gas also pulls the line towards the hydraulic device, keeping the line taut as the climber ascends.

Friction breaking auto belays
Friction auto belays were developed after hydraulic auto belays. As a climber ascends, their webbing is taken in using a retraction spring. As the climber descends, their descent is slowed by a drum brake. Friction auto belays are subject to wear under heavy use.

Magnetic auto belays
Magnetic auto belays are the most recently developed form of auto belay. Like friction auto belays, a climber's webbing is taken up using a retraction spring as they ascend. Magnetic auto belays have powerful permanent magnets near the edge of the spool drum used to collect the climber's webbing. As the climber descends, the drum spins, and the presence of the magnets generates eddy currents in the drum, which oppose the magnetic field created by the magnets, slowing the descent of the climber.