Spring manufacturing processes
If springs are of very small diameter and the wire diameter is also small then the springs are normally manufactured by a cold drawn process through a mangle.
However, for very large springs having also large coil diameter and wire diameter one has to go for manufacture by hot processes.
First one has to heat the wire and then use a proper mangle to wind the coils.
Two types of springs which are mainly used are, helical springs and leaf springs.
We shall consider in this course the design aspects of two types of springs.
It is made of wire coiled in the form of helix having circular, square or rectangular cross section.
Terminology of helical spring
The main dimensions of a helical spring subjected to compressive force are shown in the figure. They are as follows:
d= wire diameter of spring (mm)
Di= inside diameter of spring coil (mm)
Do=outside diameter of spring coil (mm)
D= mean coil diameter (mm)
There is an important parameter in spring design called spring index. It is denoted by letter C. The spring index is defined as the ratio of mean coil diameter to wire diameter.
C = D/d
In design of helical springs, the designer should use good judgment in assuming the value of the spring index C.
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 in coiling is required to avoid cracking in some wires. When the spring index is high (C >15), it results in large variation in coil diameter.
Such a spring is prone to buckling and also tangles easily during handling. Spring index from 4 to 12 is considered better from manufacturing considerations.