Key takeaways
- Specify the load at a stated length, not a load at an unspecified compression.
- The fixture, the platen and the test speed all affect the reading, so they belong in the test method.
- Test after the final process, including set, stress relief and plating.
- A load test at two lengths separates a rate error from a free length error.
What load at length means
A load at length call says that when the spring is compressed to a stated length, it must produce a stated force within a tolerance. It is a functional call. It captures the wire diameter, the coil geometry, the material and the free length in one measurement, which is why it is more useful than a stack of linear dimensions.
The length must be defined. Is it the installed length, the working length, or a length at a stated deflection from free? A drawing that says load 20 N without a length leaves the supplier to assume a compression, and two suppliers can then meet the note with springs that behave differently.
The test fixture
A load test needs a rigid base, a platen that stays parallel, and a load cell that is calibrated for the range. The spring has to sit square on the platen, because a tilted spring bends and reads a lower load. A shallow recess or a guide pin can hold the spring in place without restricting its diameter.
The platen size matters for a spring with a large mean diameter. A small platen can let the end coil overhang and deflect differently. A platen that is too large can trap the end coil and add friction. The fixture should support the end coil across its full face, or as close to it as the design allows.
For a spring that is guided in service, the test fixture should reproduce the guide. A spring tested without its guide may buckle and read low. If the service condition includes a rod or a bore, test with a representative guide.
Specifying the load point
Give the load at the installed length and at the working length. The first tells you whether the spring will seat correctly. The second tells you whether it will do its job at the end of the stroke. If the assembly has a preload, the installed length is the preload length.
State the tolerance on the load and the tolerance on the length. A load tolerance without a length tolerance is incomplete, because the length measurement contributes to the load reading. A typical call might be a load band at a length with a tolerance, but the exact values depend on the function and the process capability.
If the spring is used in a pair or a set, say so. A pair that is matched at the installed length may need a selecting step, and the drawing should allow it. A set of springs that are individually in band can still be mismatched if the band is wide relative to the assembly.
Measurement and tolerance
Measure the length at the point where the load is read, not before or after. The load cell and the length sensor should be read together, or the platen should be stopped at the target length and the load recorded after it settles. A continuous reading during compression includes friction and inertia.
The test speed should be slow enough that the spring settles, and the same speed should be used for every sample. A fast compression can read high because of damping and friction. A slow one gives the spring time to seat. State the speed in the test method so the result is repeatable.
Temperature matters for a precision call. A spring tested on a cold bench and installed in a warm enclosure can move. For a tight load band, record the test temperature and the service temperature.
Sample size and frequency
The sample size should match the risk. A prototype needs enough parts to prove the shape and the load. A first article needs a full report on at least one part, plus a small sample to show the spread. A production lot needs a sampling plan that reflects the process capability and the consequence of a failure.
The table below is an illustrative starting point. It is not a standard, and it is not a substitute for a control plan agreed with the supplier.
| Stage | Illustrative sample | What to check | Record |
|---|---|---|---|
| Prototype | 3 to 5 parts | Load at installed and working length | Length, load and wire size |
| First article | 1 part, full report | All drawing calls and the functional load | Full dimensional and load report |
| Pilot batch | 5 to 10 parts | Spread across the batch | Mean and range, with the wire lot |
| Production lot | Per the control plan | Load at the critical length | Sample size, result and disposition |
| Periodic audit | Per the control plan | Load and free length drift | Trend chart and action limit |
Illustrative only. Set the sample size from the process capability and the cost of a failure.
Common errors
The most common error is testing before the final process. A spring that is tested before set, stress relief or plating can read differently from the delivered part. The test should follow every process that changes the load or the free length.
The second error is a fixture that does not hold the spring square. A tilted spring reads low, and the error is easy to miss because the part still looks correct. A flat platen and a shallow guide solve most of it.
The third error is a load cell that is too large for the range. A cell at 10 per cent of its capacity can carry a significant error. Use a cell that reads in the middle of its range, and calibrate it on the quality system schedule.
- Testing before set, stress relief or plating
- A platen that is not parallel or too small for the coil
- A load cell that is oversized for the load
- Reading the load before the spring settles
- Using a different speed for different samples
- Recording the load without the length or the wire size
When the load test disagrees with the calculation
If the measured load is outside the calculated band, check the simple causes first. Measure the wire diameter on the sample. Count the active coils. Measure the mean diameter. Confirm that the test length is the length the calculation used. A small wire error or an extra dead coil can explain a result that looks like a formula failure.
If the geometry matches, check the material. A different modulus or a different temper changes the rate. A stainless substitution for music wire is a common cause of a low load. A wire lot at the top of its tensile range can also move the result.
Finally, check the process. A set operation, a stress relief or a plating bake can change the free length and the load. If the test was taken before the process, repeat it after. The calculation is a design tool. The test is the evidence.
Frequently asked questions
At what length should I specify spring load?
Specify the load at the installed length and at the working length. The installed length checks the preload and the seating. The working length checks the function at the end of the stroke. Give a tolerance on both the load and the length.
How many springs should be load tested?
It depends on the stage and the risk. A prototype may need 3 to 5 parts, a first article needs a full report, and a production lot needs a sampling plan based on the process capability. Agree the plan with the supplier rather than using a fixed number.
Why does the load test read lower than the calculation?
Check the wire diameter, the active coil count and the mean diameter first. Then check the material modulus and the test length. A wire at the low end of its tolerance or an extra dead coil is a common cause. A test taken before set or stress relief can also read low.
Should the spring be settled before testing?
If the assembly preloads the spring or the part has a set operation, yes. Test in the condition the customer will see. State in the test method whether the spring is compressed once before the reading, and use the same method for every sample.
Sources and further reading
- EN 13906-1, Cylindrical helical springs made from round wire and bar, compression springs
- ISO 7500-1, Metallic materials, calibration and verification of uniaxial testing machines
- Qumek spring load test records and first article templates
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Need a load test method for a spring drawing?
Send the drawing, the installed length and the working load. The engineering desk will set out the fixture, the sample and the record to ask for.
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