Key takeaways
- Free length is measured unloaded, standing on a flat surface, with the axis vertical.
- A fixture that holds the spring square and applies a known light preload removes most operator variation.
- Put a tolerance on free length only where the assembly needs it, and pair it with a load check at the installed height.
- Run a gauge R&R on the actual fixture before judging a supplier, because a large share of variation is often in the measurement.
What free length means
Free length is the overall length of a compression spring with no load applied, measured parallel to its axis. It is a simple definition and a surprisingly difficult measurement. The spring is an elastic body, the ends may not be square, and the wire round means the spring contacts a flat surface on a small area rather than a full face.
The measurement also depends on the condition of the part. A spring measured straight from the coiler has not taken its final set. A plated spring is slightly longer and thicker. The drawing should state the condition, and the inspection should match it.
Free length matters because it sets the installed height and the preload. Two springs with the same rate but different free lengths will sit at different heights in the same cavity, and one may not reach its working load.
The fixture decides the reading
The basic setup is a flat base plate, a square vertical stop and a height gauge or a linear transducer with a light contact. The spring stands on the base, the gauge touches the top, and the reading is taken without side load. A granite plate and a dial height gauge are enough for most workshop checks.
The spring must stand square. A spring with unground ends tends to lean, and a leaning spring reads longer than a square one. A small V block or a recessed pocket can hold the end square, but the pocket must not constrain the coils or add friction. The best fixtures hold the spring on its end faces, not on its outside diameter.
A light preload is often used to settle the spring and close the gauge contact. If a preload is used, it must be stated and consistent, and the reading must be corrected for the compression it causes. A 1 N preload on a 2 N/mm spring compresses it by 0.5 mm, which is larger than many free length tolerances. That correction cannot be left to judgement.
Setting a free length tolerance
Free length tolerance should come from the assembly, not from a general table. The question is how much variation the cavity can absorb before the preload moves outside its limit. If the spring is preloaded to a stop, free length may be almost irrelevant and the load at height is the real call.
A common approach is to tolerance free length generously and put the tight call on the load at the installed height. The spring can then be made within normal coiling capability, and the function is still controlled. This also avoids rejecting parts that work, which is the usual result of a tight dimensional call with no functional reason.
Where free length is critical, state it with the measurement method, the preload and the condition of the part. A tolerance without a method is an invitation to argue. Two inspectors can be 0.5 mm apart on the same spring if one measures it lying down and the other stands it up.
Running a gauge R&R for free length
A gauge R&R study separates the variation from the gauge and the operators from the variation between parts. For free length, select ten springs that span the production range, mark them so they can be identified, and have three operators measure each one three times in a random order. Record the results without letting the operators see the previous readings.
The study gives a repeatability figure, which is the same operator measuring the same part, and a reproducibility figure, which is the difference between operators. Combined, they show how much of the observed variation is measurement rather than part variation. The usual guide is under 10% of the tolerance to be acceptable, 10 to 30% to be conditional, and over 30% to be unacceptable for the decision being made.
As an illustrative example, a free length call of 38.0 mm plus or minus 0.5 mm has a tolerance of 1.0 mm. A gauge R&R of 0.18 mm is 18% of that tolerance, which is conditional. Before tightening the supplier's tolerance, check whether the fixture, the preload or the operator technique can be improved. Those figures are illustrative only.
What moves a free length reading
The table lists the influences that most often explain a disagreement. Most of them are controlled by the method rather than by the spring.
| Influence | Effect on the reading | Control |
|---|---|---|
| Spring lying on its side | Reads long because the coils sag | Stand the spring with the axis vertical |
| Unground or leaning ends | Reads long and varies by position | Use a square stop or a recessed pocket |
| Gauge contact force | Compresses the spring before reading | State the preload and correct for it |
| Part temperature | Changes length through expansion | Measure at a stated temperature |
| Plating thickness | Adds length and wire diameter | Measure after plating |
| Not fully set | Reads long until the spring settles | Measure after setting and stress relief |
| Operator technique | Different contact and alignment | Fixture the part and train the operators |
The method belongs on the inspection sheet. Without it, the number has no meaning.
The errors that cause most disputes
The first is measuring a spring on a bench with a rule. The coils sag, the end is not square and the reading depends on where the rule touches. The second is using a caliper across the coil stack of a spring lying on its side, which is not the free length.
The third is measuring before setting. A spring that has not been preset can read long and then settle in service, which changes the installed height. The fourth is mixing plated and unplated parts. Plating adds to the wire and the overall length, so the measurement condition must match the drawing.
The fifth is a fixture that grips the outside diameter. That can straighten a bent spring or add friction, and it hides the variation the measurement is meant to find. Hold the spring on its ends.
Recording the result and writing the call
The inspection record should show the free length, the gauge or fixture used, the preload, the temperature if it matters, and the condition of the part. If the spring is measured as part of a first article, add the drawing revision and the sample size. That makes the result repeatable by someone else.
On the drawing, state free length with a tolerance and a note on the method: unloaded, standing on a flat surface, after setting. Add the load at the installed height as the functional call. If the part is plated, say whether the dimension applies before or after plating.
- Free length with tolerance, after setting and stress relief
- Measurement method and preload, if any
- Load at installed height and at working height
- Condition: plated or unplated, as measured
- Sample size and inspection level for the first article
Frequently asked questions
How should free length be measured on a compression spring?
Stand the spring on a flat surface with its axis vertical, apply no load or a stated light preload, and measure with a height gauge or a linear transducer. Correct for the preload and record the method.
What is an acceptable gauge R&R for free length?
Under 10% of the tolerance is clearly acceptable, 10 to 30% is conditional, and over 30% is usually unacceptable for a pass or fail decision. The right threshold depends on how the measurement is used.
Why does my free length reading change when I measure again?
The spring may be settling, the fixture may not hold it square, or the gauge contact may be compressing it. Check the preload and the alignment first, then run a repeatability test on the same part.
Should free length be checked before or after plating?
After plating, if the drawing dimension applies to the finished part. Plating adds to the wire and the overall length, so measuring before plating can pass a part that is outside the finished tolerance.
Sources and further reading
- ISO 14253-1:2017, Geometrical product specifications, Inspection by measurement of workpieces and measuring equipment
- AIAG Measurement Systems Analysis reference manual, 4th edition
- Qumek spring inspection and gauge R&R notes
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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.
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