To determine the modulus of rigidity (Shear modulus) of the material of the spring.
I. Spring Testing Machine,
2. Dial gauge.
DESCRIPTION OF THE MACHINE
The spring testing machine has two uprights fixed to a rigid base and three cross heads. The upper and lower cross heads are rigidly attached to the uprights while the middle one is made to move on finely finished gunmetal bearings over the uprights. To the middle cross head is attached the loading cradle where tire weights for loading the spring during the test are placed. The spring for tension test is held on hooks between the upper and middle cross heads while that for compression test is placed on collar between the middle and lower cross heads .Clutch mechanisms are provided to help holding the middle cross heads for fixing the spring. Thus also an attachment with a fine adjustment device to fix the dial gauge for measuring extensions of the spring.
I: Close coiled spring
2. Open coil spring.
1. Tension Test. (Test on close coil spring).
Fix the close coiled spring to the hooks between the upper and the middle cross heads with the help of clutch mechanism. Release the clutches and let the spring hang freely between hooks. Fix the dial gauge and adjust it to read zero on the dial. Load the spring by placing one calibrated tire weight, on the cradle and note the corresponding deflection from the dial gauge. Place the tire weights one by one, noting the dial gauge reading every time a weight is placed. Continue the observation while unloading the spring also.
2. Compression Test (Test on open coiled spring).
Fix the spring on the bearing collars between the middle and lower cross heads with the help of the clutch mechanisms. Release the clutch and let the spring rest freely between the collars. Adjust the dial gauge to read zero on the dial. Load the spring and note its deflection in the same way as for the tension spring.
Record the observation taken in table
Plot the variation of deflection with load for both the close coiled and open coiled spring.
Load — Deflection Graph
1. Close coiled spring
R = Mean radius of the spring
N = No. of Coils.
d = diameter of the wire.
N = Shear modulus of the spring materials.
W = Load applied.
2. Open coiled spring
Where N, R, n, d, W & are as above.
E = Modulus of Elasticity of the spring Material
P=L/n = ———————-
Take E= 2×10^5 N/mm2 and complete the calculation.