Spring index: It is defined as the ratio of mean coil diameter to wire diameter.
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:
- The spring index indicates the relative sharpness of the curvature of the coil.
- A low spring index means high sharpness of curvature.
- 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.
- When the spring index is high (C > 15), it results in large variation in the coil diameter. Such a spring is prone to buckling.
- 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.
- A spring index in the range of 6 to 9 is still preferred particularly for close tolerance springs and those subjected to cyclic loading.