A helical compression spring is required to provide a minimum force of 80N at in

Question

A helical compression spring is required to provide a minimum force of 80N at installation and have a working deflection of 25mm. The spring rate is 35N/mm.  The coil must fit inside a 50mm diameter hole with 2.5 mm clearance.  The spring has squared and ground ends and is made from ASTM A227 4.0mm wire, without presenting.  Using a 15% clash allowance find

1. The number of coils required

2. The free length of the spring

NOTE:  The text book used for this problem will be Juvinal Marshek - Fundamentals of machine component design - Chapter 12.

Explanation / Answer

F = 80 N

k = 35 N/mm

G = 11.5*10^6

x = 25 mm

d = 4 mm

D = 47.5 mm

1) F = [x*G*d^4] / [8*D^3*i]

     number of ative coil (i) = [25*11.5*10^6*(4^4)] / [80*8*(47.5^3)] = [736 * 10^8] / 68590000 = 1073.04

2) free lenght of spring = d*i = 4*1073.04 = 4292.17 mm

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