Why Compression Springs Lose Force After Fatigue Testing

Compression springs are often required to maintain stable force after repeated compression. In many mechanical products, a spring may pass the initial load test, but after fatigue testing, the load becomes lower, the free length becomes shorter, or the spring no longer returns to its original height.

This is a common issue in custom spring pro

jects, especially when the spring is used in shock absorption systems, locking mechanisms, return structures, electric mobility equipment, automotive components, tools, or other moving assemblies.

When a compression spring loses force after fatigue testing, the problem is usually not caused by one single factor. It may be related to spring design, material selection, working stroke, heat treatment, surface condition, or the way the spring is installed.

This article explains the main reasons why compression springs lose force after fatigue testing and what engineers and buyers should check before mass production.

1. What Does “Losing Force” Mean?

When a compression spring loses force, it usually means that the spring provides less load than required at a specific working height.

For example, a spring may be required to provide a certain load when compressed to a fixed height. Before fatigue testing, the load may be acceptable. But after thousands or tens of thousands of cycles, the measured load becomes lower.

This problem may appear together with other signs:

• The free length becomes shorter

• The spring does not fully return after compression

• The load at working height decreases

• The spring tilts or compresses unevenly

• The coil gap changes

• The spring becomes permanently deformed

• In severe cases, cracks or breakage appear

For custom compression springs, the most important point is this:

A spring should not only meet the initial load requirement. It should also maintain acceptable load after repeated working cycles.

2. Over-Compression Is One of the Most Common Causes

One of the most common reasons for load loss is over-compression.

If a compression spring is repeatedly compressed too close to its solid height, the internal stress becomes too high. After many cycles, the spring may take a permanent set. This means the spring becomes shorter and can no longer return to its original free length.

This directly reduces the load at the required working height.

For example, if the spring’s maximum working height is very close to its solid height, there is not enough safety margin. Even if the spring works during initial testing, fatigue performance may be poor.

A better design should check:

• Free length

• Working height

• Maximum compressed height

• Solid height

• Working stroke

• Stress level

• Required load after fatigue testing

The spring should have enough compression margin during actual use. If the working stroke is too aggressive, simply increasing wire diameter may not fully solve the problem. The entire spring structure may need to be reviewed.

3. Material Selection Affects Fatigue Performance

Material selection has a direct effect on spring performance.

Different materials have different tensile strength, elasticity, fatigue resistance, corrosion resistance, and cost. If the selected material is not suitable for the working condition, the spring may lose force after fatigue testing.

Common spring materials include:

• High carbon spring steel

• Music wire

• Stainless steel

• Alloy spring steel

For general mechanical applications, high carbon spring steel may be cost-effective. For higher fatigue performance, music wire or suitable alloy spring steel may be considered. For humid or outdoor environments, stainless steel or proper surface treatment may be required.

However, material selection should not be based only on price. If the spring needs to support high load, repeated compression, or long-term movement, the material must match the stress level and service environment.

A low-cost material may pass dimensional inspection, but still fail during fatigue testing.

4. Spring Design May Create Excessive Stress

Even with the correct material, poor spring design can still cause force loss.

Important design factors include:

• Wire diameter

• Outer diameter

• Mean coil diameter

• Number of active coils

• Free length

• Spring index

• Solid height

• End type

• Load requirement

• Working stroke

A small change in active coils or wire diameter can significantly affect the spring rate and stress level.

If the spring rate is too high, the spring may feel strong at first, but the internal stress may also be high. If the working stroke is large, the spring may lose force more easily after repeated compression.

If the spring has too few active coils, stress may concentrate during compression. If the spring is too slender, it may buckle or compress unevenly.

Therefore, spring design should not only focus on “achieving the required load.” It should also consider whether the spring can maintain that load after repeated use.

5. Heat Treatment and Stress Relief Are Critical

After a compression spring is coiled, internal stress remains inside the material. Proper heat treatment or stress relief helps stabilize the spring and improve performance.

If heat treatment is insufficient, the spring may show unstable load, poor rebound, or larger force loss after fatigue testing.

Common problems related to poor heat treatment include:

• Load decreases after cycling

• Free length changes after compression

• Spring force is unstable between batches

• Fatigue life is shorter than expected

• Spring takes permanent set more easily

For high-load or fatigue-sensitive springs, heat treatment should be controlled carefully. In some cases, pre-setting or load stabilization before final inspection may also be necessary.

6. Surface Damage and Corrosion Can Reduce Fatigue Life

Surface condition is another important factor.

Compression springs often fail from the surface because cracks usually start from small defects. Scratches, tool marks, rust, corrosion pits, or plating defects can all reduce fatigue performance.

For springs used outdoors, in humid environments, or near corrosive substances, surface protection is especially important.

Common surface treatment options include:

• Zinc plating

• Nickel plating

• Black oxide

• Painting

• Powder coating

• Passivation for stainless steel

However, surface treatment should be selected carefully. For some high-strength spring materials, electroplating processes must be controlled properly to reduce the risk of hydrogen embrittlement. If the spring is used in a fatigue-sensitive application, the manufacturer should consider both corrosion resistance and mechanical performance.

A spring with a bright surface is not always a spring with better fatigue life. The surface finish must match the actual working condition.

7. Poor End Grinding or Uneven Loading Can Cause Deformation

For compression springs with closed and ground ends, end quality affects how the spring sits during compression.

If the ends are not flat enough, or if the spring is not properly guided in the assembly, the spring may tilt under load. Uneven compression can create side force and local stress concentration.

This may lead to:

• Spring bending

• Uneven coil contact

• Faster load loss

• Shorter fatigue life

• Assembly noise

• Abnormal wear

In many applications, the spring does not work alone. It works together with a shaft, housing, seat, guide rod, or mechanical structure. If the installation space is not stable, even a correctly manufactured spring may perform poorly.

Therefore, spring fatigue problems should be analyzed together with the assembly condition.

8. How to Diagnose Force Loss After Fatigue Testing

When a compression spring loses force after fatigue testing, the first step is to compare data before and after testing.

Useful inspection items include:

• Free length before and after testing

• Load at the same working height

• Outer diameter before and after testing

• Solid height

• Coil gap

• Surface condition

• Whether the spring tilts during compression

• Whether the spring has cracks or corrosion

• Whether the test stroke is the same as the real working condition

It is also important to confirm the fatigue test conditions:

• Compression height

• Number of cycles

• Test speed

• Load range

• Test temperature

• Whether the spring is guided

• Whether the spring is tested vertically

• Whether the test height is too close to solid height

If the fatigue test condition is much more severe than the actual application, the spring may fail the test even though it can work in normal use. On the other hand, if the real application is harsher than the test condition, the spring may fail after assembly.

The test condition should match the real working condition as closely as possible.

9. How to Improve Compression Spring Fatigue Performance

To reduce force loss after fatigue testing, the spring design and manufacturing process may need to be optimized.

Common improvement methods include:

Reduce Working Stress

The spring should not be repeatedly compressed too close to solid height. Increasing the safety margin can help improve fatigue performance.

Adjust Spring Structure

Changing wire diameter, outer diameter, active coils, free length, or working height can help reduce stress and improve stability.

Select a Better Material

For high-load or high-cycle applications, stronger or more fatigue-resistant spring materials may be required.

Improve Heat Treatment

Proper stress relief and heat treatment can improve spring stability and reduce permanent deformation.

Use Pre-Setting When Necessary

For some compression springs, pre-setting can help stabilize the spring before final inspection and reduce force loss during later use.

Improve Surface Protection

If the spring works in a humid or outdoor environment, suitable surface treatment can help reduce corrosion and surface damage.

Check Assembly Design

Guiding structure, spring seat flatness, installation space, and compression direction should be checked to avoid uneven loading.

10. What Information Should Buyers Provide?

To avoid force loss problems, buyers should provide more than basic dimensions.

Important information includes:

• Drawing or sample

• Wire diameter

• Outer diameter or inner diameter

• Free length

• Total coils or active coils

• Material requirement

• Surface treatment requirement

• Working height

• Required load at working height

• Maximum compressed height

• Solid height limitation

• Fatigue cycle requirement

• Working environment

• Installation method

• Whether the spring is guided during compression

If the spring must maintain a certain load after fatigue testing, this requirement should be clearly stated before sampling.

For example, instead of only saying “the spring needs 50 kgf,” it is better to specify:

• 50 kgf at a certain compressed height

• Before or after fatigue testing

• Required number of cycles

• Acceptable force tolerance

• Test method and test condition

Clear requirements help the spring manufacturer design a more suitable product and reduce the risk of repeated sampling.

Conclusion

Compression springs may lose force after fatigue testing because of over-compression, high stress, unsuitable material, poor heat treatment, surface damage, corrosion, uneven loading, or incorrect assembly conditions.

For custom compression springs, initial load is only one part of the requirement. Long-term stability, fatigue life, working stroke, and installation condition are also important.

A reliable compression spring should be designed according to the real application, not only according to basic dimensions.

If you are developing a compression spring that needs stable load after fatigue testing, send us your drawing, sample, working height, load requirement, and fatigue cycle requirement. Our engineering and production team can help review the design before sampling and mass production.