Important Considerations When Testing Springs with Force Systems By James M. Clinton
S
pring testing is one of the most difficult testing processes. A common misconception is that because springs are viewed as relatively simplistic components in terms of design and function, they must be just as easy to test and verify. There are also many deleterious factors and improper techniques that affect the accuracy and methodology used when testing springs. This article covers critical considerations and basic force testing precautions when testing extension and compression springs.
Testing System Load Capacity Using a load sensor that is suitable for the spring being tested is a major consideration in accurately testing springs for spring rate, spring constant, initial tension, free length, etc. Do not attempt to test a spring with a load rating that is less than 20 percent of the load system’s load cell sensor. For example, if the force system has a 500N (110lbf, 50kgf) load cell, that sensor is acceptable for testing between 100N (22lbf, 10kgf) and 500N.
Load Cell Deflection A load cell sensor must deflect in either compressive or tensile directions in order for the sensor to provide a measured output. Additionally, the entire load string used in the test will have some deflection ‒ for example, the testing fixture, the test frame, the crosshead, etc. Deflection may need to be compensated for when testing springs. Some force testers have a deflection compensation feature within the corrections setting, which is extremely helpful.
Grip String Alignment The grip string is the combined load cell sensor and testing fixture (top and bottom) that is used to test the spring. Hooks are often used for testing extension springs. Platens and specialized testing fixtures that ensure spring containment during compression are used for compression springs. It is absolutely critical that the spring being tested is perfectly aligned to the grip string. The spring should be in the center of the platens to ensure even loading.
Correct Spring Preparation Accurate measurement of spring rate, spring constant, free length and initial tension require that the spring is designed and manufactured correctly. Compression springs should have a perfectly flat surface at both ends (top and bottom). If not, length measurements are compromised, which can lead to inaccurate and inconsistent spring rates. Compression springs with ground ends can often be tested with simple platens. Springs without ground ends can have trouble standing unsupported, and typically require a guide rod that runs through the center of the spring to keep it in place during testing.
SPRINGS / Fall 2020 / 33
