Modulus of rigidity is the measurement of the stiffness of spring material or its elastic ability. The modulus of rigidity of a spring material varies as a function of chemical composition, cold working, and degree of aging.
Apparatus Required
- Spring Testing Machine
- Dial Guage
- Vernier Caliper
Test Principle
A helical spring is formed when a wire of solid circular cross-section is
wound on a circular core in a spiral form. Under the axial load, the spring is
subjected to both bending and twist. At any point in the coil tangent to helical centers line is not perpendicular to axial force.
If 'w' is the force component parallel to the tangent at any point produces the bending moment 'M' and the component perpendicular to the tangent produces a torque 'T', where alpha is the angle of the helix.
Open coiled springs are those for which the effect of angle of the helix cannot be neglected under the action of axial load, both the torsional and bending moments are to be considered. If the effect of helix is negligible, they are known as closed coiled helical spring. Hence, only the effect of torsion is considered.
Description of Machine
The spring testing machine has two uprights fixed to a rigid base and three crossheads. The upper and lower crossheads 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 crosshead 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 a compression test is placed on collar between the middle and lower crossheads.
Clutch mechanisms are provided to help to hold 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.
Test Procedure
Tension Test (Test on close coil spring).
- Fix the close coiled spring to the hooks between the upper and the middle crossheads 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 weight is placed.
- Continue the observation while unloading the spring.
Compression Test (Test on open coiled spring)
- Fix the spring on the bearing collars between the middle and lower crossheads 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.
Observation and Calculation
Plot the variation of deflection with load for both the close coiled and open coiled spring.
Load-Deflection Graph
1. For Closed Coil Spring
2. For Open Coil Spring
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