Virtually all spring manufacturers must test their products with some kind of load testing machine. Testing ensures that the product is produced and shipped within the customer's specifications. Or does it? Disputes over test results still occur that can cause a multitude of costly problems: returned product, delayed payment, product liability, or customer dissatisfaction. The quality of a spring testing program can largely affect these problems and their associated cost.
Most spring tests call for the measurement of a load at a specific height. This requires the testing machine to measure load and height accurately and simultaneously in order to get meaningful results. Consequently, the accuracy and repeatability of both measurements is equally important to the capability of the tester.
Spring manufacturers need to be aware of the unique loading conditions caused by springs. Due to a spring's coil design, it generates side loads and moments in addition to axial loads. The presence of these extraneous (non-axial) loads can cause significant errors in load readings. All load measurement systems are not created equal in this respect. Using a load cell system that minimizes these errors is essential. "Pancake" style strain-gauged load cells are typically more resistant to non-axial forces.
Spring testing professionals also need to ensure that the load cell being used is properly sized for the spring being tested. Modern load cell systems can normally be used from 1% to 100% of capacity with sufficient resolution and accuracy to generate repeatable results.
Probably the most important part of maintaining an accurate load tester is the calibration and verification of these cells. Verification involves comparing the force indication of the testing machine to that of a standard. A "standard" can include weights or load cells traceable to a national standard. If the force indicator does not match the standard, adjustments are made to bring the indicator within the allowable error tolerance. This adjustment is the calibration.
ASTM E4, Standard Practices for Force Verification of Testing Machines, is the governing standard for load calibration and verification in North America. Some general requirements of the standard include:
- Minimum load accuracy requirement of ±1% of reading;
- Verification of load is recommended at least annually.
The most common method to measure the length of a spring is to use the position of the frame crosshead. As the crosshead moves, its travel is tracked by either a linear or rotary encoder. Unfortunately, this method does not take into account the deflection of the load cell and the frame under testing conditions.