Understanding the mechanics of dynamic rope brakes

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.