Why Concentric Springs Are Fitted For Auxiliary Engine Head?

 Concentric springs are helps to open and close the suction and exhaust valve Following are the reasons for Concentric springs in the exhaust and suction valves

1. If one spring fails valve will be held up by the other spring.
2. The thickness of springs can be reduced.
3. Reduce the space required for one large spring.
4. When frequency due to natural vibration of spring is in harmonic with cam shaft speed, then spring may vibrate axially and surge. This can be avoided by using different size spring one inside the other. 
5. To avoid the sided thrust exerted on the spring. one spring is left-handed other is right handed so the resultant thrust is zero.  

If you know more reasons for these concentric springs please comment below