Springs are small parts with big consequences. A valve spring, a clutch spring or a spring in a switchgear mechanism that is slightly too weak or too stiff can change how a whole assembly behaves. That is why spring manufacturers and the companies that buy springs check them on a spring testing machine. This article explains what a spring test measures, how to run one, and what to look for in the results.
What a spring test measures
For a helical spring, the key relationship is between the force on the spring and how far it compresses or extends. Within its working range, an ideal helical spring follows Hooke's law: force is proportional to deflection. The constant of proportionality is the spring rate (also called stiffness or spring constant), usually expressed in N/mm.
- Load at a given length: the force when the spring is compressed or extended to a specified working height. This is the most common acceptance check on drawings.
- Length at a given load: the reverse check, used when the function depends on position under a known force.
- Spring rate: the change in force divided by the change in length between two test points.
- Free length and solid length: measured to confirm the geometry of the spring.
- Initial tension: for extension springs wound with coils pressed together, the force needed before the coils begin to separate.
Calculating spring rate
Spring rate is calculated from two points on the load-deflection line rather than from zero, because the first part of the curve is affected by seating of the end coils:
k = (F2 β F1) / (L1 β L2)
where F1 and F2 are the loads measured at lengths L1 and L2. A common practice is to take the two points between roughly 30% and 70% of the available deflection, where the spring behaves most linearly. Your drawing or the relevant spring standard (for example IS 7906 for helical compression springs) may specify the test points and tolerances to use.
Running a spring test
- Inspect and measure: check free length, wire diameter and outer diameter, and look for visible defects.
- Pre-set (if specified): many specifications ask for the spring to be compressed to solid length, or to a set height, once or several times before testing, so that any initial settling does not distort the result.
- Seat the spring: place it centrally on the platen, with ground ends square to the plate. For extension springs, use suitable hooks or fixtures so the load acts along the spring axis.
- Zero the readings: establish the reference length, usually at a small contact load.
- Load slowly: compress or extend to the test lengths, record loads and calculate the rate.
- Compare with tolerance: report each result against its specified value and tolerance band.
Common sources of error
| Error | Effect | Remedy |
|---|---|---|
| Spring off-centre or buckling | Low or erratic load readings | Use a centring guide or arbor, especially for long slender springs |
| Non-parallel platens | Uneven coil contact, scatter | Check platen parallelism during maintenance |
| Measuring rate from zero | Rate too low due to end-coil seating | Use two points within the linear range |
| Skipping pre-set | Results drift on repeat tests | Follow the specified pre-set procedure |
| Uncalibrated load or length system | Systematic error | Calibrate load and displacement measurement regularly |
Manual, motorised and computerized spring testers
Spring testers range from simple manual units, where the operator applies the deflection and reads a gauge or digital display, to motorised and computerized machines that run a programmed sequence and print or store a load-deflection graph. The choice depends on your volume and on whether customers ask for recorded curves.
For small springs in high volume, a compact bench tester with fast cycle time matters most. For larger springs, such as those used in heavy vehicles, rail and industrial equipment, the capacity of the load cell and the daylight (space between platens) become the deciding factors. If you need to test both, a universal testing machine with compression plates can handle larger springs, while a dedicated spring tester handles routine checks.
Akuracy offers spring testers and spring testing machines; you can see the range in the spring testing machine collection.
Spring fatigue and relaxation
A static rate test confirms that a spring is made correctly. It does not tell you how the spring will behave after millions of cycles or after long periods under load at temperature. Fatigue tests cycle the spring between two heights and count cycles to failure or measure load loss; relaxation tests hold a spring at a fixed length and measure how load drops over time. These tests are usually part of product development or qualification rather than routine QC.
Record keeping
A good spring test record lists the spring part number and batch, the free length, any pre-set applied, the test lengths and measured loads, the calculated rate, the tolerances and a pass or fail result. Keeping the load-deflection data, not just the pass or fail result, makes it much easier to investigate drift in a supplier's process later.
Frequently asked questions
Why is spring rate not calculated from zero deflection?
At the start of compression the end coils are still seating, so the curve is not linear. Using two points within the working range gives a rate that reflects how the spring behaves in use.
Can a UTM test springs?
Yes. With compression plates or suitable fixtures, a universal testing machine can measure spring load and deflection, which is useful for large springs. A dedicated spring tester is usually faster for routine checks.
How often should a spring tester be calibrated?
Follow your quality system and customer requirements; many labs calibrate load and displacement measurement at least once a year and after any repair or relocation.
Request a quote
Akuracy (Ratnakar Enterprises) can advise on a configuration for your materials, standards and test volume. Share your specimen details and the standard you test to, and our team will reply with a suitable option. Request a quote online or message us on WhatsApp at +91 94038 91579.