Why Do Compression Springs Take a Permanent Set? 6 Common Causes and How to Prevent It

A set of deformed and damaged compression springs

Why does a compression spring fail to return to its original free length? Learn how excessive stress, compression stroke, material selection, spring geometry, heat treatment, and operating conditions can cause permanent set.

Why Doesn't a Compression Spring Return to Its Original Length?

A properly designed compression spring should return close to its original free length after the applied load is removed.

In real applications, however, engineers may encounter problems such as:

• reduced free length after operation;

• loss of spring force after fatigue testing;

• incomplete recovery after compression;

• gradual spring height reduction;

• visible sagging after durability testing.

  
  

This condition is commonly referred to as permanent set.

A spring does not need to fracture before it has failed functionally. Permanent set can already change preload, working force, travel, positioning accuracy, and overall system performance.

More importantly, permanent deformation is rarely caused by one factor alone.

Spring geometry, operating stress, compression stroke, material properties, heat treatment, and actual operating conditions must usually be considered together.

What Is Permanent Set in a Compression Spring?

Compression springs operate primarily through elastic deformation.

When a load is applied, the spring wire experiences torsional stress and stores mechanical energy. Within its elastic working range, the spring should recover when that load is removed.

If the stress becomes high enough to produce irreversible deformation in the material, however, the spring may no longer return completely to its original geometry.

For example, if a compression spring originally has a free length of 65 mm but measures only 60 mm after a durability test — and remains at 60 mm after unloading — the 5 mm reduction is a clear permanent set.

This can affect:

• assembly preload;

• operating force;

• available travel;

• mechanism positioning;

• load balance;

• product service life.

For suspension, return mechanisms, automation equipment, and other cyclic applications, controlling permanent set is therefore a critical design requirement.

1. Excessive Operating Stress

Excessive stress is one of the first factors engineers should investigate when a compression spring takes a permanent set.

When a compression spring is loaded, the wire is subjected primarily to torsional stress.

If the working load or deflection is too high, the material can operate close to or beyond a stable elastic range. The spring may not fail immediately, but repeated loading can gradually produce permanent deformation.

One important point is often overlooked:

A spring does not have to reach solid height before its stress becomes excessive.

A spring may still have visible space between its coils while operating at an unfavorable stress level because of the combination of:

• wire diameter;

• mean coil diameter;

• active coil count;

• applied load;

• working deflection.

  
  

For high-load applications, the important question is therefore not simply:

Can the spring be compressed to this height?

The more useful engineering question is:

Can the spring repeatedly operate at this height for the required service life?

Those are very different requirements.

2. Excessive Deflection or Operation Too Close to Solid Height

Every compression spring has a solid height, where the coils are essentially stacked together.

Operating too close to this condition reduces the available design margin and can significantly increase stress.

This becomes particularly important in dynamic applications such as:

• e-bike suspension;

• mobility systems;

• industrial buffers;

• reciprocating machinery;

• high-frequency return mechanisms.

  
  

Actual service loads are not always equal to the nominal static load used during initial calculations.

Road impact, sudden mechanical contact, vibration, or rapid equipment movement may compress a spring beyond its expected normal position.

A design that appears acceptable under a slow bench test may therefore behave very differently under dynamic loading.

For this reason, the spring should normally have an appropriate margin between its regular operating height and its mechanical compression limit.

3. Incorrect Material Selection

Spring materials differ in:

• tensile strength;

• elastic limit;

• fatigue performance;

• temperature resistance;

• corrosion resistance.

  
  

A general-purpose spring steel may perform adequately in moderate applications but may not be the best choice for high-stress, high-cycle, or long-duration loading.

Applications involving:

• repeated high loads;

• suspension systems;

• high cycle counts;

• demanding fatigue life;

• continuous compression

  
  

may require careful comparison of spring materials such as 72A, 82B, music wire, stainless steel, or other grades depending on the environment and mechanical requirements.

However, a common mistake is to assume:

“If the spring deforms, simply use a stronger material.”

That is not always enough.

Material strength is only one variable.

If the spring has:

• an unfavorable diameter ratio;

• unsuitable active coil count;

• excessive working travel;

• high calculated stress;

  
  

changing material alone may improve performance without solving the fundamental problem.

Material selection and spring geometry should therefore be evaluated together.

4. Spring Geometry Is Not Suitable for the Required Load

Compression spring performance depends strongly on several geometric parameters:

• wire diameter;

• mean coil diameter;

• active coil count;

• free length;

• pitch;

• working height.

  
  

These parameters interact with one another.

In general, increasing wire diameter can significantly increase spring stiffness, while increasing the number of active coils reduces spring rate.

This becomes important when a spring is required to generate a high load inside a restricted installation envelope.

For example, suppose the design specifies:

• a fixed outer diameter;

• limited free length;

• a very low working height;

• limited wire diameter;

• high required force.

  
  

The available design space may become very narrow.

To meet the load target, the spring may then be forced to operate at a relatively high stress.

When permanent deformation occurs, engineers should therefore review the complete spring geometry rather than focusing on a single dimension or material grade.

5. Inadequate Heat Treatment or Stress Relieving

Spring wire experiences substantial forming deformation during coiling.

This process introduces residual stresses into the material.

For many spring designs, appropriate heat treatment or stress relieving is therefore an important part of the manufacturing process.

Its purpose is to help:

• reduce residual stress;

• stabilize dimensions;

• improve elastic recovery;

• improve performance consistency.

  
  

If the heat-treatment process is not appropriate for the selected material or spring geometry, possible consequences can include:

• dimensional instability;

• free-length variation;

• inconsistent force;

• greater performance loss after cycling;

• increased permanent-set risk.

  
  

For production orders, consistency is equally important.

A prototype may perform correctly, but variation in heat-treatment conditions during volume manufacturing can create differences between batches.

This is why stable spring production requires process control, not only final dimensional inspection.

6. Real Dynamic Loads Were Underestimated

A spring that passes a static compression test does not automatically have sufficient durability in the final product.

Real operating environments may also introduce:

• impact loads;

• vibration;

• off-axis loading;

• spring buckling;

• temperature changes;

• corrosion;

• assembly misalignment.

• 
  

A suspension spring, for example, does not experience one constant vertical load during vehicle operation. It experiences continuously changing dynamic loads, including sudden impact events.

If a spring is not properly guided or is subjected to eccentric compression, local stress may also become significantly higher than expected from a simplified theoretical calculation.

When a spring performs well in a laboratory test but develops permanent deformation during field testing, the investigation should therefore include the entire mechanical system rather than the spring alone.

A Practical Example: Suspension Spring Deformation After Durability Testing

One compression spring project for a light-mobility suspension application involved the following specifications:

• Wire Diameter: 6.0 mm

• Outer Diameter: 41 mm

• Pitch: 19.7 mm

• Active Coils: 3

• Total Coils: 4.5

• Free Length: 65 mm

• Material: 65Mn

• Ends: Closed and Ground

• 
  

After approximately 200 km of riding durability testing, significant permanent deformation was observed.

Without complete load and testing data, it would be incorrect to attribute this result to one confirmed cause.

A proper investigation should consider:

• maximum operating load;

• maximum deflection;

• minimum working height;

• calculated operating stress;

• material fatigue properties;

• heat-treatment condition;

• spring guidance;

• potential impact loading.

The key engineering lesson is straightforward:

Permanent set after durability testing is rarely explained by “poor material” alone.

Spring design, material, manufacturing process, and real operating conditions must be evaluated as a system.

How Can Permanent Set Be Reduced?

Several steps can reduce the risk during spring development.

1. Define the Real Operating Conditions

A spring drawing is important, but dimensions alone may not provide enough information to evaluate durability.

For demanding applications, a spring manufacturer should ideally understand:

• minimum load;

• maximum load;

• working travel;

• maximum compression;

• required cycle life;

• operating temperature;

• impact conditions;

• whether the component is safety-critical.

If only geometric dimensions are provided, a supplier can manufacture the spring according to the drawing but may have limited ability to evaluate whether the design is appropriate for the actual application.

2. Maintain Sufficient Margin From the Compression Limit

The normal working height should not be unnecessarily close to solid height.

This is especially important for dynamic and high-cycle applications where unexpected impact loads may create additional deflection.

A reasonable design margin helps reduce the risk of excessive operating stress.

3. Match the Material to the Stress and Service Life

There is no single “best” spring material.

Material selection should consider:

• load;

• operating stress;

• cycle requirements;

• temperature;

• corrosion;

• manufacturing requirements;

• cost.

Using a stronger material may help in some applications, but it should not replace proper spring design.

4. Test Function, Not Just Dimensions

A compression spring can meet every dimensional tolerance and still fail functionally.

Depending on the application, prototype validation may include:

• force at specified heights;

• spring rate;

• maximum compression;

• free-length recovery;

• load retention after fatigue testing.

For critical applications, testing should reproduce real operating conditions as closely as practical before volume production.

5. Control Mass-Production Consistency

A good prototype does not automatically guarantee stable production performance.

Volume manufacturing requires control over:

• raw material;

• machine setup;

• coil diameter;

• free length;

• active coil count;

• heat treatment;

• force testing;

• final inspection.

For B2B buyers, this distinction is important: the objective is not only to produce one acceptable spring, but to maintain consistent performance across production batches.

What Information Should You Send Your Spring Supplier?

When reporting a permanent-set issue, saying only:

“The spring became soft.”

does not provide enough information for effective analysis.

More useful data includes:

• original free length;

• free length after testing;

• maximum compression height;

• working load;

• number of test cycles;

• point at which deformation appeared;

• actual operating environment;

• force measurements before and after testing;

• photos of the deformed spring.

Providing this information can significantly shorten the investigation process.

Conclusion

Permanent deformation in a compression spring generally indicates that the spring has not remained within a sufficiently stable elastic operating range under actual service conditions.

Common contributing factors include:

1. excessive operating stress;

2. excessive deflection;

3. unsuitable material selection;

4. unfavorable spring geometry;

5. inadequate heat treatment or stress relieving;

6. underestimated dynamic operating loads.

In practice, several of these factors may interact.

For that reason, permanent-set problems should not be investigated by looking only at material grade or spring dimensions. The more reliable approach is to evaluate material, geometry, operating load, manufacturing process, and actual application conditions together.

J&L Spring has manufactured custom springs and metal components in Dongguan, China since 1999. Our spring manufacturing capabilities cover wire diameters from 0.2 to 7.0 mm, including custom compression springs for industrial and OEM applications.

For new compression spring projects, we recommend verifying not only dimensions but also required load, working height, compression travel, and actual operating conditions during the prototype stage.

Developing a custom compression spring?

Send us your drawing, wire diameter, outer diameter, free length, working height, target load, material requirements, and application conditions. These details allow the project to be reviewed more effectively before prototype or volume production.