**Introduction:**

Therefore, in practical applications, the spring index in the range of 6 to 9 is still preferred particularly for close tolerance springs and those subjected to cyclic loading.

**Types:**

There are three terms - free length, compressed length and solid length that are illustrated in the figure. These terms are related to helical compression spring. These lengths are determined by following way

**1) ****Solid length:**

solid length is defined as the axial length of the spring which is so compressed, that the adjacent coils touch each other. In this case, the spring is completely compressed and no further compression is possible. The solid length is given by.

**Solid length = N**_{t}**d**

Where N_{t}= total number of coils

**2) ****Compressed length:**

Compressed length is defined as the axial length of the spring that is subjected to maximum compressive force. In this case, the spring is subjected to maximum deflection. When the spring is subjected to maximum force, there should be some gap or clearance between the adjacent coils. The gap is essential to prevent clashing of the coils. The clashing allowance or the total axial gap is usually taken as 15% of the maximum deflection.

Sometimes, an arbitrary decision is taken and it is assumed that there is a gap of 1 or 2 mm between adjacent coils under maximum load condition.

In this case, the total axial gap is given by,

**Total gap = (Nt-1) x gap between adjacent coils**

**3) ****Free length: **

Free length is defined as the axial length of an unloaded helical compression spring. In this case, no external force acts on the spring.

Free length is an important dimension in spring design and manufacture. It is the length of the spring in free condition prior to assembly. Free length is given by,

**Free length = compressed length y = solid length total axial gap y**

The pitch of the coil is defined as the axial distance between adjacent coils in uncompressed state of spring. It is denoted by p. It is given by,

The stiffness of the spring (k) is defined as the force required producing unit deflection Therefore

Where **k**= stiffness of the spring (N/mm)

**F **= axial spring force (N)

**Y **=axial deflection of the spring corresponding to force p (mm)

There are various names for stiffness of spring such as rate of spring, gradient of spring, scale of spring or simply spring constant.

The stiffness of spring represents the slope of load deflection line. There are two terms are related to the spring coils, viz. active coils and inactive coils.

Active coils are the coils in the spring, which contribute to spring action, support the external force and deflect under the action of force.

A portion of the end coils, which is in contact with the seat, does not contribute to spring action and called inactive coils. These coils do not support the load and do not deflect under the action on external force. The number of inactive coils is given by,

**Inactive coils = N**_{t}**– N where N = number of active coils**