Torque-Clamp-Force Test: How to Perform and Interpret GB/T 16823.3
Executive Summary
The torque-clamp-force test is the only direct verification of fastener friction performance: it measures K, thread friction coefficient and bearing-surface friction coefficient at the same time. Following GB/T 16823.3-2010, this article explains specimen preparation, test steps, data analysis methods and acceptance criteria, with practical experience in reading curves and spotting anomalies.
Table of Contents
A Batch of "Qualified" Bolts Snapped on the Line
A project assembled M14 grade 10.9 bolts at the drawing torque of 120 N·m; the first sample batch all passed. During mass production, the second batch, a few bolts broke during tightening, with obvious plastic shear necking at the fracture. Investigation showed that the second-batch supplier had raised the coating film thickness from 10 μm to 18 μm to improve corrosion protection, dropping K from 0.15 to 0.11. At the same torque, preload rose by about 36%, driving the bolts into the plastic zone.
The problem: these bolts passed all mechanical, dimensional and appearance checks, yet their friction characteristics had changed, and no routine inspection item could detect it. Only one test can find it—the torque-clamp-force test, performed per GB/T 16823.3-2010.
What the Test Actually Measures
The test uses a tightening machine that simultaneously acquires torque and clamp force, screwing the bolt into a simulated clamped part fitted with a force sensor, and continuously recording the whole curve. Four key results can be extracted from the curve:
| Output Parameter | Meaning | Engineering Use |
|---|---|---|
| Total friction coefficient μtot (the friction part of K) | Combined thread-pair and bearing-surface friction | Convert to tightening torque; judge coating batch consistency |
| Thread-pair friction coefficient μth | Contribution from thread flank friction only | Judge seizure risk and stripping tendency |
| Bearing-surface friction coefficient μb | Contribution from bearing-surface friction only | Judge washer and bearing-surface condition |
| Torque coefficient K | K = T / (F × d) | Directly used in tightening-torque calculation |
Another layer of value is that the test gives the complete curve shape of the tightening process, including the seating point, the elastic-stage slope, and the yield onset. This information cannot be obtained from single-point torque data, yet it is the direct basis for judging whether a bolt will be over-tightened.
Specimen and Test-Condition Preparation
| Item | Requirement | Note |
|---|---|---|
| Specimen condition | All surface treatment completed, same as mass-production state | Semi-finished test specimens have no judgment authority |
| Specimen quantity | ≥ 5 pieces per batch, 10 recommended | Too few pieces cannot evaluate scatter |
| Simulated clamped part | Material, hardness and bearing-surface condition matching the real part | Bearing-surface hardness directly affects μb |
| Washers | Same as actual assembly (use if present, omit if not) | Omitting a washer causes K deviation of 15%+ |
| Threaded hole | Tolerance class matching the real part, usually 6H | Nuts should be replaced each time or use dedicated test nuts |
| Tightening speed | Same as production line, usually 20~60 r/min | Speed affects the shear state of the lubricating film |
| Pretreatment | No cleaning or extra lubrication before tightening | Extra treatment changes the friction state |
Test Steps and Data Acquisition
- Mounting and preloading: mount the specimen on the tester, first apply about 5% of target torque to eliminate clearance, and check coaxiality and bearing-surface seating.
- Continuous loading: tighten continuously at the specified speed until past the target torque, acquiring torque and clamp-force signals throughout; recommended sampling rate ≥ 100 Hz.
- Record to failure or set point: record at least to the target preload; if the purpose is to evaluate ultimate capability, continue tightening to yield or fracture to obtain the ultimate torque.
- Data processing: take the linear segment near the target preload to calculate K. The common two-point method or least-squares fit is used; the choice of fit interval affects the result and must be kept consistent.
- Separate friction components: by varying the clamp length or using a dedicated force-measuring clamped part, obtain the contributions of thread friction and bearing-surface friction separately.
Data conventions must be unified. The same bolt batch, fitted over two different intervals, can yield K values differing by more than 5%. The test report should clearly state: sampling rate, speed, fit interval, and calculation software version.
How to Read the Curve: Four Typical Shapes
| Curve Shape | Possible Cause | Follow-up Action |
|---|---|---|
| Excessively long seating segment, slow slope rise | Large clearance, burrs, compressed coating, clamped part not seated | Check mating-surface cleanliness and flatness |
| Step or kink in the slope midway | Coating fracture, local bearing-surface indentation, thread flank slip | Check bearing-surface hardness and coating adhesion |
| Slope gradually falling, no clear yield point | Soft clamped part crushing or low-strength bolt plasticity | Calculate bearing-surface pressure; assess size match |
| Wide scatter among curves in the same batch, large K range | Uneven lubricant application, film-thickness variation, poor thread accuracy | Return to supplier to investigate coating process |
Acceptance Criteria and Batch Management
A single data point proves nothing; what it proves is batch stability. It is recommended to establish judgment rules along the following lines:
| Criterion | Recommended Limit | Action When Exceeded |
|---|---|---|
| Deviation of mean K from design assumption | ≤ 10% | Correct tightening torque or adjust coating |
| Within-batch K range | ≤ 30% of mean | Investigate coating consistency; downgrade the batch |
| Ratio of μth to μb | Keep within 0.6~1.5 | Imbalanced ratio indicates abnormal friction somewhere; check threads or bearing surface |
| Batch-to-batch K drift | ≤ 15% across 3 consecutive batches | Supplier process-capability issue; on-site audit required |
| Ultimate torque to fracture | ≥ 1.3× target torque | Insufficient safety margin; adjust torque or size |
Conclusion
The torque-clamp-force test is one of the few fastener tests that "shows both cause and effect": the effect is the clamp force on the clamped part, the cause is the friction distribution between threads and bearing surface. It provides K input for design, friction-state control basis for process, and a comparison baseline for failure analysis.
What must be stressed is its positioning—this is not a sampling re-inspection item, but an engineering capability verification item. Running the test at the sample stage is verification; running it on every batch is control; running it after a problem is remediation. The value of the three decreases in that order, while the cost runs exactly the opposite.
