Inspecting Coil Springs – What to Know

Custom Spring Manufacturing ProcessLearning how to test coil springs is an important quality control measure manufacturers use during production. Testing and inspecting coil springs is far more than eliminating any guesswork in spring design. Every spring has individual design specifications related to application requirements, the materials used, the manufacturing methods, and any particular type of finish. The importance of testing and inspecting is to identify any defects or irregularities that could affect a spring’s functionality and safety and verify compliance with industry-standard specifications.

Types of Coil Springs

Coil springs are those familiar open-wound or closed-wound helical-shaped mechanical devices used in numerous commercial, industrial, and consumer applications. Made of elastic materials ranging from plastics to a wide variety of wound metals, coil springs are engineered to store potential mechanical energy that is released when a force, be it tension, torsional, or compressive, is applied. Though coil springs come in many designs, the three most common types are the extension spring, the torsion spring, and the compression spring.

How to Test and Inspect Coil Springs 

Testing and inspecting coil springs is a multi-step process used to ensure a spring’s design meets its performance requirements. Specific testing and inspection methods may vary on the application or industry standard specifications. Also, each manufacturer may have their quality control procedures in place to ensure the springs meet the required specifications and standards. Nonetheless, as testing and inspecting are multi-step processes, here are eight methods commonly employed to make certain coil springs function as designed:

1. Visual Inspection

The first step would typically begin with a visual inspection of the spring. Inspectors check for any visible defects such as surface imperfections, cracks, or material and processing defects like scratches or folding that could affect the spring’s performance, functionality or safety.

2. Dimensional Measurements

Dimensional accuracy ensures a coil spring meets its required design specifications for an application. Typical measurements include key parameters such as inner and outer wire diameter using a micrometer, the coil diameter using a vernier caliper, its free length, solid height, and the number of active coils that are required for the device.

3. Load Testing

Testing a coil spring’s load-carrying capacity helps verify that the spring can withstand the expected loads it is designed for without excessive deformation or failure. Load testing is typically performed using a testing machine that measures and records the force applied and the corresponding spring deflection. For example, the load-carrying capacity of a compression spring—the force required to compress the spring—must be proportional to its spring rate, i.e., its stiffness. Therefore, compression springs designed to carry a heavy load will have a high spring rate that will require more force to compress it.

4. Spring Rate Testing

Rate testing is designed to evaluate a spring’s force-deflection characteristics necessary to determine a spring’s load. As mentioned above, spring rate, a measure of a spring’s stiffness, is essential to ensure the spring’s performance within a specified range and to provide the desired force for a given deflection.

5. Free Length Testing

The free length testing of a coil spring is useful to analyze and improve a spring design. It refers to the overall length without any loads or forces applied to it. Thus, the free length of an extension spring for example, includes the coiled body plus the hooked ends measuring from one end to another.

6. Solid Height Testing for Compression Springs

Solid height refers to the length of a fully compressed spring with all coils in contact. The solid height test makes sure that the spring does not experience any permanent deformation during compression and that it returns to its original shape once the load is removed. Also, with regard to compression springs, the difference between the free length and solid height is the spring’s maximum compression distance.

7. Fatigue Testing 

Fatigue testing is used for critical applications or high-performance springs. The test involves subjecting the spring—compressed, extended or rotational—to repetitive loading cycles to determine its fatigue life and to measure its performance over an extended period of time.

8. Material Analysis Testing

As the strength and elasticity of materials can vary widely, material analysis may be performed to verify and ensure the composition and properties of the material meet the application requirements. Coil springs are commonly made from various materials, such as steel, stainless steel, phosphor bronze, or other alloys. The choice of material depends on factors like the application’s requirements, load capacity, environmental conditions, and desired spring life.

James Spring & Wire Company 

Coil springs are mechanical devices that play an important role in countless applications large and small. Engineers and designers at James Spring & Wire Company carefully test coil springs to ensure they meet specific requirements for a given application, to identify any defects that could affect a spring’s performance, and to verify compliance with industry-standard specifications. , and to achieve the desired performance and safety standards for our customers. For more information on coil springs and coil spring design, contact us today.