Capstan effect

To properly understand the capstan effect, it is necessary to take account of another force. That is the cable tension. When cable passes through a bend, the cable tension interacts with the reaction between the cable and the duct wall. Ignoring cable stiffness, the reaction is proportional to the tension. This then produces a backward acting friction force per unit length of the cable that is approximately proportional to the tension. In order to balance this, the backward acting tension on a unit length has to be less than the forward acting tension on that unit length. Hence the tension must increase as the cable passes through the bend. The tension is greater immediately downstream of the bend than it is immediately upstream of the bend. The relationship is exponential.