Spring Testing: Load, Rate and Fatigue Checks Before You Approve
Before you approve a spring for production, four tests matter: load at working height (the functional spec, measured on a load tester), the force-deflection curve for rate, dimensional checks on OD and free length, and a fatigue sample run if the spring cycles. A batch inspection report with measured load values is the deliverable that separates a real spring factory from a reseller — demand it, read it, and keep it.
Springs are among the most tested small components in manufacturing, yet also among the most loosely verified. The reason is that the functional output — force — cannot be read with a caliper; it needs a load tester and a defined test procedure. Two springs with identical free length can differ in load by 20% if the wire diameter drifted. Testing is not a quality add-on; it is the only way the spring's function is confirmed at all.
The Core Test: Load at Working Height
The single most important spring test is force at a specified working height (compression springs) or at a specified extension (extension springs). The spring is compressed or pulled to the drawing's working dimension and the force is read on a load tester. This is the number the mechanism feels, so it is the number to control and the number to put on the drawing. Test procedure matters: the deflection rate must be slow and consistent, the part must settle before reading, and the height must be set from a real reference surface, not eyeballed.
| Test | What it measures | Typical acceptance |
|---|---|---|
| Load at working height | Force at the functional dimension | Class 2: ±7–10% of spec |
| Load at second height | Force curve shape | Optional — for progressive parts |
| Free length | Unloaded length | Per drawing tolerance |
| Outside/inside diameter | Envelope fit | Per drawing tolerance |
| Solid height | Fully compressed length | Maximum, for bind check |
| Squareness / parallelism | End quality | Per standard, class-dependent |
Takeaway: when a drawing shows both a free length and a load tolerance, test the load — the assembly feels force, not length. A factory's load tester is its most important spring instrument, and the tester's own calibration (typically ±1% of reading or better) sets the floor for every load claim. Ask what tester is used and when it was last calibrated; the answer is a fast quality signal.
Rate and the Force-Deflection Curve
Spring rate — force per unit deflection — is measured by taking load readings at several deflections and fitting the line. On a linear spring, two well-separated points define it; on conical, variable-pitch or other progressive springs, the full curve must be recorded because the rate changes along the travel. Rate is far more sensitive to wire diameter than to anything else — the rate scales with wire diameter to the fourth power — which is why a tiny wire variation shows up as a rate shift large enough to reject a batch.
| Check | Method | What it catches |
|---|---|---|
| Rate on linear spring | Load at 2–3 deflections | Wire diameter drift, wrong coil count |
| Full force curve | Load at 5+ points | Progressive-form shape errors |
| Hysteresis | Load and unload curve | Friction, improper set |
| Rate at temperature | Test in heated fixture | Material modulus issues, hot service |
Takeaway: ask for the rate or the curve rather than only the single-point load when the spring travels through a range — a spring can pass at one height and miss badly at another if the rate drifted. For critical mechanisms, specify a rate window plus the working-height load, and let the factory's curve data prove both. Hysteresis between loading and unloading is normal in small amounts; large hysteresis signals coil friction or internal damage.
Dimensional Checks and End Quality
Force data sits on top of dimensional basics. Outside diameter controls the fit in a bore, free length controls preload in fixed-cavity assemblies, and end quality — squareness, parallelism, closed and ground ends — controls how the load is introduced. A spring whose end is not square loads eccentrically, which changes the effective force and invites buckling. These checks are quick with a caliper, a surface plate and a square, but only if someone actually performs them and records the results.
| Dimension | Instrument | Why it matters |
|---|---|---|
| OD / ID | Caliper or optical | Bore fit, rod clearance |
| Free length | Height gauge | Preload in cavity |
| Squareness of ends | Surface plate + square | Even load introduction |
| Parallelism of ground ends | Surface plate | Stability standing |
| Coil count | Visual / optical | Rate and solid height |
| Wire surface | Microscope on samples | Scratches → fatigue risk |
Takeaway: dimensional scatter and force scatter are correlated through the wire and the coiling process, so a supplier that records both gives you the full picture. A good report lists measured OD, free length and load per batch with the specification and the pass/fail against it — not a single line saying "OK". The tolerance class on the drawing sets the limits these measurements are judged against.
Fatigue Testing: Sample-Based, Defined by End Point
Fatigue cannot be tested on every part — it is a sample-based test that runs representative springs through cycles to a defined end point: a target cycle count without failure, or cycles to a specified load loss or crack. Define the test conditions on the drawing: deflection range (minimum to maximum working position), cycle frequency, and the failure criterion. A spring that passes 100,000 cycles at 2 mm travel may fail at 4 mm, so the tested range must match the real one. Shot peening is the production step that most improves fatigue results, and the fatigue test is what proves whether the peening and the design together hit the life target.
| Fatigue test parameter | Typical value | Note |
|---|---|---|
| Sample size per lot | 3–5 pcs | Representative, not statistical |
| End point | Cycle target or load-loss % | Define before testing |
| Frequency | 5–20 Hz typical | Avoid self-heating on small parts |
| Acceptance | No failure to target cycles | Cracks or break = fail |
| Report | Cycles completed + condition | Keep with batch record |
Takeaway: a fatigue claim without the test conditions is marketing. Get the deflection range, the cycle target and the failure criterion in writing, and keep the sample test report with the batch records — it is your evidence if a field failure is ever disputed.
BQUQ tests springs the way they are used: load at working height on calibrated testers, rate curves for every geometry, dimensional and end checks per batch, and sample fatigue runs to your cycle target — all recorded in a batch inspection report that ships with the goods. Compression, extension and torsion springs all pass the same gate under ISO9001 in Dongguan. Send the drawing with test requirements to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours, and ask to see a sample batch report before you commit.
Email sc@bquq.com or WhatsApp +86 137 1315 7787 with your PDF/DXF/STEP file. An engineer reviews it and replies with price, lead time and DFM notes on working days.
Q: What is the most important spring test before approval?
Load at working height — it verifies the spring's actual function in your mechanism. Everything else (length, diameter, ends) supports that force. Test it with a calibrated load tester and require the measured values in the batch report.
Q: How many springs should be fatigue tested?
Typically 3–5 samples per lot, cycled to the defined end point. Fatigue testing is sample-based because it destroys the parts; the sample report plus consistent production control (peening, material, wire) is what protects the whole batch.
Q: What is a reasonable load tolerance to specify?
Class 2 production holds roughly ±7–10% of the specified load at working height; class 1 reaches about ±5% with individual measurement. Specifying below ±5% means selecting springs into bins — plan the cost and logistics of that if your mechanism needs it.
Q: Can I test springs myself instead of trusting the factory report?
Yes — a load tester and basic gauges let you verify incoming batches in minutes, and spot-checking against the factory report is good practice. The report gives you the values to compare; your own tester confirms the supplier's data and calibration.
Q: What does a proper batch inspection report contain?
Measured values, not just "OK": OD, free length and load at working height per sample or per batch, compared with the drawing limits, plus the test date and instrument. Ask for it with every shipment and file it with the batch records.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
