Accurate Spring Testing

Instron Corporation, Canton, Massachusetts

Saturday, March 01 2003 In most cases, when using equipment designed for spring testing, these deflections are small in relationship to the spring's deflection and behave in a linear fashion. Thus, the load-dependent deflection can be determined and the height corrected using adequate spring testing software and electronics.

Small displacement springs such as disc or wave springs may require more accurate height measurement. In such cases, a stiff frame and electronic compensation for deflection may not be enough. These applications typically require direct measurement of the distance between the compression plates using an auxiliary position transducer placed in the test space. This eliminates the need for height correction. With these devices, accuracies of ±1 μm can easily be achieved.

Spring testing fixtures normally include compression plates, tension devices, or special tooling designed for a specific type of spring. These fixtures can have a significant effect on the quality of the results. The fixtures may be a significant contributor to frame deflection. This is especially true for extension springs. Ensuring that test fixtures are installed when determining the frame deflection rate can reduce errors.

The compression plates on a spring tester also need to be flat and parallel. This prevents deviation in height if the spring is not placed in the same position on the plate for each test, and also eliminates errors that may be caused by uneven loading.

Spring testing is not always as easy as it sounds. Several key factors are critical to measuring load and height accurately and getting meaningful test results. The items discussed here cover just a few of the important features of an accurate spring testing machine. There are many other features that can affect accuracy, convenience, and efficiency. A good testing program can ensure that one is shipping quality product with respect to spring specifications.

This work was done by Chris Wilson, Product Manager, and Patty Hartzell, Assistant Product Manager, at Instron Corporation. To access the complete paper on accurate spring testing, visit: www.instron.com.