Traditional round wire coil springs are often defined by a tolerance on the free height (or uncompressed height of the spring) and a tolerance on the spring rate (or how much load is gained per unit of travel as the spring is compressed). Once compressed in your assembly, however, the spring will operate at one or more compressed heights. We call these the working heights of the spring. The load at these working heights can be inconsistent, as they depend completely on the free height tolerance and the spring rate tolerance.
At Smalley, we control our wave springs with a tolerance on that load at the compressed height. This provides precise control of how the spring will function in your assembly. During manufacturing, we will compress the spring to the specified working height to verify that the load meets the tolerance.
The load tester consists of two parallel plates attached to a load cell. These readouts show the distance between the plates, which would be the height of the spring, and the load measured by the load cell.
In this example test, we are using a standard CS Series Crest-to-Crest wave spring. It is rated to provide 20 lb at 0.866-inch work height. With a +/-10% tolerance, we are aiming for an 18 to 22-lb load. To inspect the spring, place it between the parallel plates and lower the upper plate using this wheel. As the upper plate contacts the spring, you can see the load readout increasing. At a height of 0.866 inches, the load came in at 19.9 lb, which is in tolerance.
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