Use of Concentric Spring in Auxiliary Engine Valve Arrangement.

There is a slight misconception among students regarding this topic. Usually it seems obvious that the use of two springs gives additional safety in case of failure of one. But wait !! Well that may be a one reason for it, the primary reason for use of two springs is different.

1. If one would look carefully, it is noticed that the two springs have their coil in opposite direction.One is right handed and other left handed. This is important since when the springs compress and decompress at high speed , there is a side thrust acting on the spring retainer due to coils of spring. To cancel this side thrust, another spring with opposite coil direction is introduced. Since the side thrust from second spring is opposite to that of first spring, the net side thrust is zero. 

2. There is the effect of resonance at high engine speed. To prevent the matching of natural frequency of the running engine to the natural frequency of the spring, additional spring is added. This addition increases the natural frequency of the spring combination much higher than the engine's natural frequency, thus preventing any damage to the engine by excessive amplitude of vibration. 

3. Lastly, two spring do provides an additional safety in case of failure of one. But remember this is not the primary reason for use of two.