Load-testing machine features can affect the accuracy, precision, repeatability, and reproducibility of test results.

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.

Figure 1: Springs Require Unique Testing considerations because they generate non-axial forces under load.

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.

Figure 2: Instron® 2 kN (450-lb.) capacity Test Frame outfitted with a 20 N load cell for testing small springs.

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.