Junker test
The Junker test and DIN 25201-4 Annex B
By ISOKLAMP Engineering, Inc. Editorial Team · Updated
Why transverse, not axial
Gerhard Junker's 1969 contribution was to show that the dangerous input to a bolted joint is transverse, not axial. Axial vibration has to overcome the full preload to do anything. Transverse displacement only has to overcome friction at the thread flanks and the bearing face, and once it does, the thread helix does the rest.
Every serious loosening test since has been a transverse test. That is the single most useful thing to know about the method.
Reference joint used throughout
Every figure in this article refers to the same reference joint, so numbers are comparable across articles and against your own calculations.
| Parameter | Value |
|---|---|
| Bolt | M16 × 2,0, property class 10.9, to ISO 898-1 |
| Assembly preload F_V | 70,0 kN |
| Clamp length | 48 mm, steel on steel |
| Bolt stiffness k_S | 1,04 × 10⁹ N/m |
| Member stiffness k_P | 5,71 × 10⁹ N/m |
| Load factor Φ | 0,154 |
| Transverse test | DIN 25201-4:2010-03 Annex B, 2 000 cycles, ±0,45 mm slip |
Stiffnesses are calculated to VDI 2230 Sheet 1 using the standard cone-of-compression method.
The apparatus
| Element | Function |
|---|---|
| Fixed base plate | Reacts the transverse load |
| Sliding plate | Driven transversely by an eccentric or hydraulic actuator |
| Test bolt | Passes through both plates, clamps the joint |
| Load cell | Measures clamp force continuously, in line with the bolt |
| Displacement transducer | Measures relative plate slip |
| Drive | Sets amplitude and frequency |
The bolt is preloaded to a specified value. The sliding plate is then driven back and forth at a fixed displacement amplitude, and clamp force is recorded against cycle count.
The parameters that decide the result
Junker results are not comparable between laboratories unless the parameters match. These are the ones that matter.
| Parameter | Typical DIN 25201-4 Annex B setting |
|---|---|
| Bolt size | M8 to M20, most commonly M10 or M12 |
| Property class | 8.8, 10.9 or 12.9 |
| Assembly preload | 60 to 75 percent of proof load |
| Transverse amplitude | ±0,3 to ±0,6 mm, specified per test |
| Frequency | 10 to 15 Hz |
| Cycle count | 2 000 |
| Acceptance | Residual clamp force ≥ 80 percent |
| Clamp length | Specified, affects k_S and therefore sensitivity |
Amplitude is the parameter that decides everything. Below the critical slip distance for the joint, nothing happens at all and every product passes. Above it, results separate sharply. A test report that does not state amplitude is not a test report.
The reference joint used throughout this site is tested at ±0,45 mm, which is comfortably above critical slip for an M16 joint and therefore discriminating.
Reading a Junker curve
Plot residual clamp force as a percentage of assembly preload against cycle count on a linear x-axis. Four shapes appear.
Cliff. Rapid loss in the first few hundred cycles, then a low plateau. Plain washers, split washers. The joint slipped, the nut rotated, and it kept rotating until friction re-established at low preload.
Slow decay. Steady, roughly linear decline that does not flatten. Prevailing-torque nuts. The nylon insert resists rotation and is progressively worn or extruded.
Early loss then flat. A drop of 5 to 12 percent in the first 200 cycles, then a horizontal line. Wedge-lock washers and ribbed flange nuts. Rotation is arrested immediately; the early drop is embedment, which the device cannot recover.
Dip and recovery. A drop to a minimum around cycle 100 to 150, then a rise, then flat at a value above the minimum. This shape requires a source of new clamp length and does not occur with any purely retentive device.
| Cycles | Plain washer | Nyloc | Wedge-lock pair | ISOKLAMP CFR |
|---|---|---|---|---|
| 0 | 100,0 % | 100,0 % | 100,0 % | 100,0 % |
| 50 | 88,1 % | 91,4 % | 96,4 % | 97,8 % |
| 120 | 71,3 % | 84,7 % | 94,8 % | 96,9 % |
| 300 | 48,6 % | 76,2 % | 94,0 % | 98,1 % |
| 800 | 26,4 % | 66,8 % | 93,5 % | 99,1 % |
| 2 000 | 12,8 % | 57,2 % | 93,1 % | 99,4 % |
ISOKLAMP figures are predicted by finite-element analysis of the ISK-16 geometry coupled to VDI 2230 Sheet 1 mechanics. Baselines are calibrated against published Junker data with per-row sources on Full residual clamp force dataset.
The 80 percent criterion
DIN 25201-4 Annex B sets 80 percent residual at 2 000 cycles as the pass threshold. Two things are worth understanding about that number.
It is a securing-function criterion, not a design allowance. Passing means the device prevented rotational self-loosening to an acceptable degree under the specified transverse input. It does not mean your joint will be at 80 percent in service, because service adds gasket creep, coating creep and thermal cycling that the test does not apply.
It is also reachable by embedment alone. Twenty-two micrometres of stack shortening takes the reference joint below 80 percent with no rotation whatsoever. A joint can therefore fail in service having passed the test, if its dominant loss mechanism is one the test does not exercise.
What the test does not measure
This is stated clearly in the standards themselves and is worth repeating because it is widely misread.
ISO 16130:2015 explicitly scopes itself to rotational self-loosening. NASA STD-5020 treats preload loss from relaxation as a design allowance separate from self-loosening and requires it to be budgeted independently.
A Junker rig applies transverse displacement at 10 to 15 Hz for 2 000 cycles, which is roughly three minutes. In three minutes a PTFE gasket does not creep, a zinc flake coating does not flow, and no thermal cycle occurs. Those mechanisms are real, they are large, and the test is silent on all of them. See Non-rotational preload loss.
Running or commissioning a Junker test
- Specify amplitude explicitly and justify it against the critical slip distance for your joint geometry.
- Specify clamp length, because k_S sets how much clamp force each micrometre costs.
- Specify assembly preload as a percentage of proof load, and state the tightening method.
- Require continuous clamp force logging, not endpoint measurement. The curve shape carries most of the information.
- Require competitor articles to be purchased on the open market rather than supplied by the manufacturer, and named in the report.
- Require the raw data, not just the plot.
- Use an ISO/IEC 17025 accredited laboratory and have the test witnessed.
Points 5 through 7 are what separates a test report from marketing. Our own verification programme is specified on exactly those terms, and is described in The ISOKLAMP technical report.
Method comparison on Locking methods compared.
Frequently asked questions
What does the Junker test measure?
The Junker test applies controlled transverse displacement to a preloaded bolted joint while continuously measuring clamp force. It measures resistance to rotational self-loosening, the mechanism Gerhard Junker identified in 1969 in which transverse micro-slip momentarily breaks thread friction and lets the thread helix drive the nut backwards.
What is the DIN 25201-4 Annex B acceptance criterion?
At least 80 percent residual clamp force after 2 000 transverse cycles. It is a securing-function criterion rather than a service design allowance: passing means the device adequately prevented rotational self-loosening under the specified input, not that a joint will sit at 80 percent in service.
Why is transverse amplitude the most important test parameter?
Below the joint's critical slip distance nothing happens and every product passes. Above it, results separate sharply. Amplitude therefore decides the outcome more than any other setting, which is why Junker results are not comparable between laboratories unless amplitude, clamp length, preload and tightening method are all stated.
How do you read a Junker curve?
Four shapes appear. A cliff followed by a low plateau means the nut rotated freely, typical of plain and split washers. A steady decline that never flattens indicates a prevailing-torque nut being progressively worn. An early drop of 5 to 12 percent then a flat line means rotation was arrested immediately and the drop was embedment, typical of wedge-lock washers. A dip to a minimum followed by a rise requires a source of new clamp length.
What does the Junker test not capture?
Non-rotational preload loss. A Junker run is 2 000 cycles at 10 to 15 Hz, roughly three minutes. In that time a PTFE gasket does not creep, a zinc flake coating does not flow, and no thermal cycle occurs. ISO 16130:2015 explicitly scopes itself to rotational self-loosening, and NASA STD-5020 requires relaxation to be budgeted as a separate design allowance.
Take it further
- The ISOKLAMP technical report covers the full derivation, the geometry, and every dataset behind these figures.
- Full residual clamp force dataset gives the residual clamp force numbers for ten securing methods, with sources.
- Design partner programme is open. Eight slots, two per sector. Bring us a joint that keeps failing and we will run a VDI 2230 Sheet 1 analysis on it.
Engineering questions go to engineering@isoklamp.com. An engineer answers, not a form.