The stiffness or elastic ability of a spring material is measured by its modulus of rigidity. This measurement is not constant, but instead varies depending on the chemical composition of the material, how much cold working it has undergone, and the extent to which it has aged.
Apparatus Required
A spring testing machine, a dial gauge, and a vernier caliper are three types of equipment commonly used in mechanical testing and measurement.
The spring testing machine is used to test the strength and elasticity of a spring by applying a controlled force and measuring the amount of deformation. It is an essential tool for manufacturers who need to ensure that their springs meet specific quality standards.
A dial gauge, also known as a dial indicator, is a precision measurement tool that is used to measure small distances or changes in dimensions. It is commonly used in mechanical engineering, manufacturing, and quality control to ensure that parts and products are made to the correct specifications.
A vernier caliper is a precision measuring tool used to measure the internal and external dimensions of objects with high accuracy. It consists of two jaws that can be adjusted to fit around an object, and a vernier scale that allows for very precise measurements to be taken. Vernier calipers are commonly used in engineering, manufacturing, and scientific research.
Test Principle
A helical spring is created by wrapping a solid wire with a circular cross-section around a circular core in a spiral shape. When an axial load is applied to the spring, it experiences both bending and twisting. At any point on the coil, the tangent to the helical center line is not perpendicular to the axial force. If a force component ‘w’ parallel to the tangent is applied at any point, it produces a bending moment ‘M’, while the component perpendicular to the tangent produces a torque ‘T’. The angle of the helix is denoted by alpha.
Open coiled springs are a type of helical spring in which the angle of the helix cannot be ignored under the influence of axial load. This means that when an axial force is applied, the helix angle of the spring comes into play, affecting the spring’s behavior. In addition to the axial load, both the torsional and bending moments also need to be taken into account in order to fully understand the behavior of open coiled springs.
On the other hand, closed coiled helical springs are those for which the effect of the helix angle can be neglected. In such springs, only the effect of torsion is considered when an axial load is applied. This is because the helix angle has a negligible impact on the spring’s behavior under axial loading. Therefore, the design and analysis of closed coiled helical springs are simpler compared to open coiled springs.
Description of Machine
The spring testing machine comprises of a rigid base and two uprights that are fixed to it, along with three crossheads. While the upper and lower crossheads are firmly attached to the uprights, the middle one can move over them on finely finished gunmetal bearings. The loading cradle, where the tire weights for loading the spring during testing are placed, is connected to the middle crosshead.
For a tension test, the spring is held on hooks between the upper and middle crossheads. On the other hand, for a compression test, the spring is placed on a collar between the middle and lower crossheads. Clutch mechanisms are provided to hold the middle crosshead and fix the spring in place during testing. Additionally, an attachment with a fine adjustment device is available to secure the dial gauge, which is used for measuring the extensions of the spring.
Test Procedure
Tension Test (Test on close coil spring).
To conduct a test on a close coiled spring, it is necessary to follow a set of steps. First, the spring should be fixed to the hooks between the upper and middle crossheads using a clutch mechanism. Once the spring is securely attached, the clutches can be released, allowing the spring to hang freely between the hooks.
Next, a dial gauge should be fixed to the spring, and adjusted to read zero on the dial. This step ensures accurate readings during the test. To load the spring, a calibrated tire weight should be placed on the cradle, and the corresponding deflection from the dial gauge should be noted.
After this initial measurement, more tire weights can be added one by one, while noting the dial gauge reading each time a weight is placed. This process should continue until the spring has been fully loaded.
To complete the test, the spring should be unloaded, and the dial gauge readings should be observed during this process as well. By carefully following these steps, it is possible to gather accurate data about the behavior of the close coiled spring under different loads.
Compression Test (Test on open coiled spring)
To fix the spring on the bearing collars between the middle and lower crossheads, the clutch mechanisms can be used. Once the spring is in place, the clutch can be released, allowing the spring to rest freely between the collars. To measure the spring’s deflection, a dial gauge can be adjusted to read zero on the dial before the spring is loaded. The loading process can be done similarly to how a tension spring is loaded. After loading the spring, its deflection can be noted.
Observation and Calculation
The task requires plotting the load-deflection graph for both close coiled and open coiled springs. The load-deflection graph is a common way of representing the relationship between the load applied to a spring and the resulting deflection or deformation of the spring.
A close coiled spring refers to a type of spring where the coils are tightly wound together with minimal space between them. On the other hand, an open coiled spring has larger spaces between its coils, giving it a more open appearance.
To plot the load-deflection graph for both types of springs, it is necessary to apply varying loads to the springs and measure the resulting deflection. The data obtained can then be plotted on a graph with the load on the x-axis and the deflection on the y-axis.
By plotting the data for both close coiled and open coiled springs on the same graph, it is possible to compare and contrast their behavior under different loads. This can help to identify the strengths and weaknesses of each type of spring and determine which one is better suited for a particular application.
Overall, plotting the load-deflection graph for close coiled and open coiled springs is an important step in understanding their behavior and performance characteristics.
1. For Closed Coil Spring
