SURGE IN SPRINGS : INTRODUCTION
SURGE IN SPRINGS:
This condition, mainly seen in various cases of spring motion and oscillations.
When a spring subjected to external periodic force , the spring starts vibrating or oscillating with a certain frequency. When the natural frequency of spring vibrations becomes equal to frequency of External periodic force; resonance occurs.
In this state, the spring experiences a wave of successive compression of coils . And this wave travels from one end of the coil to the other, then back to initial point and so on. This results in a continuous vibrational motion.
This vibratory motion ; now known as the condition of SURGE in a loaded spring.
EX : Surge mainly seen in Valve Springs; which are subjected to periodic forces.
OCCURRENCE OF SURGE IN SPRINGS:
Consider a Helical Compression Spring. One end of the spring connected to a flat rigid surface , whereas other end made to left free. An external periodic force applied at the free end.
The force now transmits from the free end upto the fixed end on the plate. This process takes place in a definite time interval.
Energy transmission done by compression of coils. First , the coil portion at free end gets compressed, then the next coil, compresses ans so on. This creates a wave of Compression of compressed coils moving along the spring length. The wave generates as a result of Energy transfer occurring from one coil to another.
This process continues until the wave hits the rigid surface end, where the wave reflects back and continues the to and fro periodic motion.
CONDITION OF RESONANCE :
When the time taken by the wave to complete motion from one end to other = Time taken between two successive periodic external forces; Resonance observed here .
Resonance causes large deflections in the spring system. Due to which High Stresses induces in the system.
Let this stress be Si.
And , Let Se be the endurance limit
of the spring material.
When ; Si>Se , fatigue failure occurs in the spring . This leads to excessive oscillations and finally reduces the life of the spring.
This Phenomenon came to said as SURGE IN SPRINGS.
CALCULATION AND FORMULA:
PRECAUTIONS TO AVOID SPRING SURGE:
1) The frequency of spring designed in such a way that Frequency of Spring ≈15-16 times (frequency of periodic force) to avoid resonance.
2) Spring provided with friction dampers on central coils in order to avoid further propagation of surge wave and break its continuity.
3)Springs should be made of standard wires in order to avoid surge condition. On coils’ compression, the standard wired coils approaches each other closely. This produces coil friction ;thereby avoiding surge wave propagation.