Dynamic Control of Spring-driven Mechanisms

A common method for moving a machine member from one position to another is the use of a spring. Spring-driven devices are simple, inexpensive, and easy to implement; however, the velocity characteristics of such devices leave much to be desired. The velocity increases from the initial position to the final position resulting in a maximum velocity, and therefore maximum energy in the mechanism, at the final position. The simplest method of stopping the device, a rigid stop, results in high impact forces and undesirable noise. Many methods have been developed for limiting the velocity of spring-driven mechanisms, such as the centrifugal friction brakes used in telephone dials and fans used in music boxes. Other approaches such as dashpots or shock absorbers have been used to decelerate devices. This paper discusses a method for both velocity control and deceleration by the use of a single pneumatic cylinder. In addition, a method of reducing velocity variability due to differences among the work functions of the mechanism is described, and the application of such a device to a paper-cutting mechanism is presented. The concepts and theory presented are general and therefore apply to the entire class of spring-driven mechanisms.