Subject :- Machine Design -I

Topic :- Spring

Difficulty :- high

Spring index: It is defined as the ratio of mean coil diameter to wire diameter.

Let

    d = wire diameter of spring.

   Dc = inside or core diameter of spring coil.

   Do = outside diameter of spring coil.  

    D = mean coil diameter.

Significance of spring index:

1. The spring index indicates the relative sharpness of the curvature of the coil.
2. A low spring index means high sharpness of curvature.
3. When the spring index is low (C < 3), the actual stresses in the wire are excessive due to curvature effect. Such a spring is difficult to manufacture and special care is required in coiling to avoid cracking.
4. When the spring index is high (C > 15), it results in large variation in the coil diameter. Such a spring is prone to buckling.
5. A spring index from 4 to 12 is considered best from manufacturing considerations. Therefore, in practical applications, the spring index usually varies from 4 to 12.
6. A spring index in the range of 6 to 9 is still preferred particularly for close tolerance springs and those subjected to cyclic loading.