### Interview Questions (Design of Machine Elements - Springs)

Define pitch.
Pitch of the spring is defined as the axial distance between the adjacent coils in an uncompressed state. Mathematically Pitch-free length/n-

What are the points to be taken into consideration while selecting the pitch of the spring?
The points taken into consideration of selecting the pitch of the springs are
• The pitch of the coil should be such
• that if the spring is accidentally compressed the stress does not increase the yield point stress in torsion.
• The spring should not be close up before the maximum service load is reached.

Define active turns.
Active turns of the spring are defined as the number of turns, which impart spring action while loaded. As load increases the no of active coils decreases.

Define inactive turns.
Inactive turns of the spring are defined as the number of turns which does not contribute to the spring action while loaded. As load increases the number of inactive coils increases from 0.5 to 1 turn.

What are the different kinds of end connections for compression helical springs?
The different kinds of end connection for compression helical springs are
• Plain ends
• Ground ends
• Squared ends
• Ground & square ends

If the load acting on the spring does
not coincide with the axis of the spring, then spring is said to have an eccentric load. In eccentric loading, the safe load of the spring decreases and the stiffness of the spring is also affected.

Explain about the surge in springs?
When one end of the spring is resting on a rigid support and the other end is loaded suddenly, all the coils of spring does not deflect equally, because some time is required for the propagation of stress along the wire. Thus a wave of compression propagates to the fixed end from where it is reflected back to the deflected end this wave passes through the spring indefinitely. If the time interval between the load application and that of the wave to propagate are equal, then resonance will occur. This will result in very high stresses and cause failure. This phenomenon is called a surge.

What are the methods used for eliminating surge in springs?
The methods used for eliminating surge are
• By using dampers on the centre coil so that the wave propagation dies out
• By using springs having a high natural frequency.

What are the disadvantages of using helical spring of non-circular wires?
• The quality of the spring is not good
• The shape of the wire does not remain constant while forming the helix. It reduces the energy absorbing capacity of the spring.
• The stress distribution is not favourable as in circular wires. But this effect is negligible where loading is of static nature.

Why concentric springs are used?
• To get greater spring force within a given space
• To ensure the operation of a mechanism in the event of failure of one of the spring

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