Effectiveness of mountaineering manual belay/abseil devices

Mountaineers and rock climbers use a belay device to increase tension in the rope that links the belayer to a falling climber—this rope slows and finally stops the fall. With a manual (passive) belay device the belayer can hold a force of several kN although he/she applies a hand force of only 0.2–0.35 kN on the rope; i.e. the device increases the hand force by a factor between 5 and 10. This investigation provides dynamic measurements of force amplification by various manual belay devices when used on a range of both wet and dry climbing ropes and it examines the source of force amplification in these devices. The force amplification is found to be due to a combination of friction and distortion of the rope as it traverses around tight corners within the device. In modern devices, the tension amplification due to distortion exceeds that due to friction.