|By Prof S S Chauhan, IEC College of Engineering & Technology, Greater Noida|
|Spring Surge and Critical Frequency
If one end of a compression spring is held against a flat surface and the other end is disturbed, a compression wave is created that travels back and forth from one end to the other exactly like the swimming pool wave. Under certain conditions, a resonance may occur resulting in a very violent motion, with the spring actually jumping out of contact with the end plates, often resulting in damaging stresses. This is quite true if the internal damping of the spring material is quite low. This phenomenon is called spring surge or merely surging. When helical springs are used in applications requiring a rapid reciprocating motion, the designer must be certain that the physical dimensions of the spring are not such as to create a natural vibratory frequency close to the frequency of the applied force.
The final equation for the natural frequency, derived from the governing equation of the wave motion, for a spring placed between two flat parallel plates is given by:
For steels this can be simplified as:
The fundamental critical frequency should be from 15 to 20 times the frequency of the force or motion of the spring in order to avoid resonance with harmonics. If the natural frequency is not high enough, the spring should be redesigned to increase k or decrease the weight W.