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.
Our capabilities range from very thin .002“ thick to heavy .080” material and widths from fractions of an inch to multiple inches. Power springs can provide rotational energy or linear motion if used with a pulley or cable.
Depending on your design requirements, pre-stressing the strip is an option. Most spring motors operate through maximum of about 15 useful revolutions. It is possible to pre-stressed the strip by forming, and then back winding it into the retainer. This causes higher stresses, however, it also increases the number of useful turns to as high as 35. A more constant torque load throughout the entire spring deflection will result.
We invite you to consult with our engineering department to optimize your power spring requirements and design.