Introduction/Definition:
The arrangement is called Concentric spring or Composite spring if one spring is placed inside the other spring.
Purpose:
A concentric or composite spring is used for one of the following purposes :
1. To obtain greater spring force within a given space.
2. To insure the operation the event of failure of one of the springs. a mechanism in the event of failure of one of the springs.
To obtain greater spring force within a given space:
The concentric or composite springs for the purposes given above may have two or more springs and also have the same free lengths as shown in figure below
and are compressed equally. Such springs are used in clutches of vehicles, valve springs in aircraft, heavy duty diesel engines and rail-road car suspension systems.
To insure the operation the event of failure of one of the springs. a mechanism in the event of failure of one of the springs:
Sometimes concentric or composite springs are used to obtain a spring force which does not increase in a direct relation to the deflection but increases faster. Such springs are made of different lengths as shown in Figure below
The shorter spring begins to act only after the longer spring is compressed to a certain amount. These springs are used in variable speed engines of governors to take care of the variable centrifugal force.
The adjacent coils of the concentric or composite spring are wound in opposite directions to eliminate any tendency to bind.
If the same material is used, the concentric or composite springs are designed for the same stress. In order to get the same stress factor (K), it is desirable to have the same spring index (C ).