How Do You Choose Between an Extension and a Compression Spring?
Your design needs a spring, but which one? Choosing incorrectly leads to bulky designs, unexpected failures, and a product that just doesn't feel right, costing you time and money.
A compression spring is designed to be pushed, storing energy when compressed and resisting a compressive force. An extension spring is designed to be pulled, storing energy when stretched and providing a return force to bring components back together. They are mechanical opposites.
In my 14 years of manufacturing custom springs, the most common source of early-stage design failure[^1] is a misunderstanding of this fundamental choice. I once visited a small company that had designed a new type of exercise machine. They used two large compression springs to provide resistance. The problem was, the mechanism had to pull on these springs using a complex and bulky system of levers and cables. The machine was heavy, expensive, and felt awkward to use. We redesigned it using extension springs, which simplified the entire mechanism[^2], cut the weight in half, and made the motion feel smooth and natural. They were trying to make a pulling mechanism[^2] work with a pushing spring, and it was a perfect lesson in why choosing the right type from the start is so critical.
When Should You Use a Pushing Force Instead of a Pulling Force?
You need to create resistance in your device, but the mechanism[^2] is becoming overly complex. This adds unnecessary parts, increases the chance of failure, and drives up your manufacturing costs.
Use a compression spring for pushing force[^3] when you need to provide support, absorb shock, or separate two components. Use an extension spring for pulling force when you need to return a mechanism[^2] to its original position or hold two components together.
The choice between pushing and pulling defines your entire mechanical system. A compression spring's job is to resist being squeezed. Think of the suspension in a car. The springs are compressed by the weight of the car and absorb shock by pushing back. An extension spring[^4]’s job is to resist being stretched. Think of a classic screen door closer. The spring is stretched when you open the door, and its pulling force is what closes it behind you. Compression springs excel in load-bearing and shock-absorbing roles. Extension springs are the default choice for return mechanism[^2]s. Trying to use one for the other's job, like in that exercise machine, almost always results in a more complicated and less efficient design. The most elegant mechanical solutions are often the ones that use the most direct type of force.
The Function Defines the Form
The right choice simplifies your design and improves its performance.
- Compression for Support and Shock: These springs are designed to sit under a load. Their coiled structure is inherently stable when being pushed from either end.
- Extension for Return and Tension: These springs are designed to pull from their ends. Their hooks are critical components that transmit the pulling force[^5].
| Function | Best Choice | Common Examples | Why It Works |
|---|---|---|---|
| Absorb Shock | Compression | Vehicle suspension, pogo stick | The spring can take a direct impact and push back, dampening the force. |
| Provide Support | Compression | Mattress coils, battery contacts | The spring holds up a constant load and maintains outward pressure. |
| Return to Center | Extension | Trampoline mat, screen door | The spring is stretched from its resting state and pulls the mechanism[^2] back. |
| Hold Together | Extension | Garage door balance, carburetor linkage | The spring's pulling force[^5] keeps tension on the system to hold it in place. |
Which Spring Type is More Prone to Failure?
Your spring-loaded product works perfectly, but then it fails unexpectedly. This sudden failure can damage your product, create a safety risk, and ruin your brand's reputation for reliability.
Extension springs are generally more prone to catastrophic failure than compression spring[^6]s. The hooks on an extension spring[^4] are areas of high stress concentration. If a hook fails, the spring completely detaches, releasing all its stored energy at once.
The weak point of an extension spring[^4] is almost always the hook. The bend where the hook transitions into the spring body is a natural point of stress concentration. Over many cycles, this is where microscopic cracks can form and eventually lead to a complete fracture. When an extension spring[^4] breaks, it's a sudden, total failure. The spring can fly off, and the mechanism[^2] it was holding will snap back. A compression spring, on the other hand, tends to fail more gracefully. If a compression spring is overloaded or fatigues, it will usually just sag or take a permanent "set." It stops providing the correct force, but it rarely breaks into pieces. It remains captured in the assembly, and the failure is less dramatic. This is why for safety-critical applications, I always advise engineers to design their system around a compression spring[^6] if possible.
Designing for Durability
Understanding how each spring fails is key to building a safe and reliable product.
- The Risk of Hooks: An extension spring[^4] is only as strong as its hooks. We can use different hook designs (like crossover hooks or extended hooks) and processing methods (like shot peening) to improve fatigue life, but the risk remains.
- The Stability of Compression: A compression spring is supported by its own structure. As long as it is properly guided to prevent buckling, it is a very stable and predictable component.
| Spring Type | Common Failure Mode | Consequence of Failure | Design Consideration |
|---|---|---|---|
| Earrach síneadh | Hook fracture due to fatigue. | Sudden, complete release of force. The spring can become a projectile. | The hook design and material must be carefully selected for the required cycle life. |
| Earrach comhbhrú | Fatigue cracking, sagging, or "taking a set." | Gradual loss of force. The spring typically remains in place. | Ensure the spring is not compressed beyond its solid height and is guided to prevent buckling. |
Conclúid
Choose compression for support and shock absorption and extension for return force, always considering the different ways each spring type can fail to ensure a safe and reliable design.
[^1]: Understanding design failures can help prevent costly mistakes in product development.
[^2]: Explore the principles of mechanical mechanisms to enhance your design skills.
[^3]: Learn about the importance of pushing forces in simplifying designs and improving performance.
[^4]: Explore the role of extension springs in mechanisms that require pulling forces and return functions.
[^5]: Discover how pulling forces can enhance the functionality of various mechanical applications.
[^6]: Understanding compression springs is crucial for applications requiring support and shock absorption.