JOHN EVANS’ SONS INC.

Power Springs

Power Springs

John Evans’ power springs, also known as clock springs, are constructed by winding a strip of spring-tempered material on an arbor in a circular case (keeper). The spring stores rotational energy by being stressed when wound around the arbor. The spring delivers rotational energy to the arbor when it rotates, by expanding to the maximum curvature allowed by the keeper.

The torque developed as the spring unwinds, is rather difficult to evaluate due to the effect of friction between coils. A typical torque curve obtained during unwinding is shown (below). This illustrates how the torque drops off rather slowly at first as the spring unwinds, and then more rapidly. In general, the length of the strip should be less than 15,000 times the thickness, and to avoid excess frictional effects, the arbor diameter should be greater than 15 to 25 times the thickness, avoiding excessive stresses.

The torque deflection characteristics of the power spring is nonlinear. This condition is caused by the constantly changing amount of active material, the normal hysteresis affect throughout the working deflection, and inter-coil friction.