Understanding the mechanics of dynamic rope brakes

Abstract

Dynamic rope brakes are integral elements of the standard belaying equipment. They multiply the hand force of the belayer and convert a fraction of the falling climber's kinetic energy into thermal energy. As the rope is deflected by the brake, the thermal energy is produced by belt friction. This paper introduces a brake model based on static and viscous belt friction. The contact angles of three different brakes (figure-of-eight, ATC and HMS [Munter hitch]) were determined. These data as well as data taken and/or derived from the literature served as input for the model. The static and viscous belt friction coefficients range between 0.2-0.3 and 0.015-0.04, respectively, for the three brakes. Depending on the magnitude of the friction coefficients, three conditions can be distinguished: 1) no rope slip at high friction; 2) rope slip and stop; 3) critical braking; and 4) continuous rope slip (no stop). The window of the friction coefficients between no slip and critical braking is independent of the hand force. However, the smaller the hand force is, the larger are the friction coefficients at no slip and critical braking. Belayers with smaller hand forces should therefore avoid rope brakes with small static and viscous friction coefficients (like the Figure-ofEight) and apply the ATC or HMS.

Experimental Analysis of the Mechanics of Sport Climbing Falls

“…Parallel to outdoor climbing on natural cliffs, indoor climbing gyms offer a sheltered variety of the sport with all routes pre-equipped with bolts. All belay devices until 1991 followed the principle of dynamic belaying, providing a limited braking force and friction-controlled slippage of the rope once that limit is reached [9]. This enabled a soft stopping of the falling climber, thus limiting the peak forces on the anchors.…”

Frictional mechanics of knots

“…and the contact angle Φ between the different rope parts is half of the granny knot (see Ref. [13] for a more accurate estimation of Φ and p Φ ). For the same friction coefficients…”