non-rotational preload loss
Non-rotational preload loss in bolted joints
By ISOKLAMP Engineering, Inc. Editorial Team · Updated
Two failure modes, one industry
Gerhard Junker published his transverse vibration work in 1969 and changed fastener engineering permanently. He showed that transverse micro-slip, not axial vibration, is what backs a nut off, and he gave the industry a test for it. Seventy years of product development followed, and it has been overwhelmingly successful. Modern wedge-lock washers hold better than 90 percent residual clamp force through 2 000 transverse cycles.
That work solved one failure mode. There are two.
Mode A, rotational self-loosening. The nut rotates relative to the bolt. Diagnosed by a witness mark that has moved. Solved by wedge-lock washers, prevailing-torque nuts, thread lockers and ribbed flange nuts.
Mode B, non-rotational preload loss. The nut does not rotate at all. The witness mark is exactly where you left it, and the joint is loose anyway. Solved by nothing currently on the shelf, because everything on the shelf is a Mode A device.
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 mechanism
Preload is elastic energy stored in the bolt. Clamp length and clamp force are directly coupled through the joint's stiffnesses.
Φ = k_S / (k_S + k_P) = 0,154
Shorten the stack by δ and the clamp force falls by 0,884 kN per micrometre for this joint. The nut has nothing to do with it.
| Stack shortening | Residual clamp force | Passes DIN 25201-4 Annex B? |
|---|---|---|
| 10 µm | 91,0 % | Yes |
| 20 µm | 82,1 % | Yes, marginally |
| 22 µm | 80,1 % | At the limit |
| 30 µm | 73,1 % | No |
| 50 µm | 55,2 % | No |
Twenty-two micrometres is the entire margin. That is roughly the embedment budget of three as-rolled steel interfaces, before you have added a single coating or gasket.
How to tell which mode you have
This is the diagnostic that changes the answer, and it takes ten minutes.
Mark the joint. Paint or lacquer a witness line across the nut, the washer and the member.
Run it. Whatever your normal duty cycle is.
Read the mark.
| Observation | Diagnosis | What will fix it |
|---|---|---|
| Mark broken, nut has rotated, joint loose | Mode A | Wedge-lock washer, prevailing torque, thread locker |
| Mark intact, nut has not moved, joint loose | Mode B | Length recovery |
| Mark broken and the joint was already loose | Mode A following Mode B | Recovery, then rotation control |
| Mark intact, joint still tight | No problem | Nothing |
The third row is the common one and the reason so many joints get mis-specified. A joint first loses clamp force to stack shortening. Below a certain clamp force the friction that resists Junker slip is no longer sufficient, and the joint starts to slip and then rotate. The engineer sees a rotated nut, diagnoses Mode A, fits a wedge-lock washer, and the joint improves substantially without ever becoming reliable, because the underlying length loss was never addressed.
If a locking device helped but did not solve it, you have a Mode B joint. That single sentence resolves most of the difficult bolted joint problems we see.
Where Mode B dominates
| Application | Dominant mechanism | Typical budget |
|---|---|---|
| Hygienic and food process equipment | PTFE and EPDM gasket creep, CIP/SIP cycling | 30 to 70 µm |
| Busbar and battery pack joints | Aluminium to steel thermal ratcheting | Accumulating |
| Coated structural steel | Zinc flake and PTFE coating creep | 10 to 25 µm |
| Composite and CFRP joints | Matrix creep, through-thickness relaxation | 25 to 80 µm |
| Cast iron and soft members | High embedment at faying faces | 15 to 35 µm |
| Machined steel, dry, isothermal | Embedment only | 8 to 20 µm |
Only the last row is comfortably served by a conventional locking device.
What the literature says
Mode B is not a novel observation. It is well described and poorly served.
ISO 16130:2015 explicitly scopes itself to rotational self-loosening, which is a precise and honest statement of what the aerospace dynamic test measures and what it does not. NASA STD-5020 treats preload loss from relaxation as a separate design allowance from self-loosening and requires it to be budgeted independently. Jiang and co-workers characterised the two-stage nature of preload loss, with a rapid early relaxation phase preceding any rotational phase. Eraliev and colleagues surveyed loosening mechanisms and separated non-rotational relaxation as a distinct category.
The literature agrees the mechanism exists, matters, and is separate. What has been missing is a product.
What a Mode B fix requires
Not compliance. Recovery. Three requirements:
- A reserve of clamp length, sized against the shortening budget with margin. The ISK-16 carries 0,50 mm, a factor of 12,5 against a 40 µm budget.
- One-way deployment. A mechanism that can give length back is a spring, not a recovery device. Self-locking requires tan α_c < μ_r, satisfied at 1,8:1 margin.
- Autonomous actuation. Recovery that needs a technician is retorquing. Vibration-Actuated Take-up uses the transverse slip already present in the duty cycle.
Add Mode A control on the same part, since a joint that has recovered its clamp force is still exposed to Junker slip, and you have a joint that holds on both mechanisms.
The comparison
Reference joint, DIN 25201-4:2010-03 Annex B, 2 000 cycles:
| Method | Residual | Mode A | Mode B |
|---|---|---|---|
| ISOKLAMP CFR | 99,4 % | Yes | Yes |
| Wedge-lock pair | 93,1 % | Yes | No |
| Ribbed flange nut | 88,4 % | Yes | No |
| Belleville stack | 71,4 % | No | Partial |
| Nyloc | 57,2 % | Yes | No |
| Conical spring washer | 44,6 % | No | Partial |
| Plain washer | 12,8 % | No | No |
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 test data with per-row sources on the test data page.
Full treatment on preload loss. Mechanism on how it works.
Frequently asked questions
What is non-rotational preload loss?
Non-rotational preload loss is clamp force lost while the nut remains perfectly tight. The clamped stack gets shorter through embedment, coating creep, gasket relaxation or thermal ratcheting, and the bolt relaxes along its elastic curve. It accounts for 10 to 40 percent of preload in the first 200 load cycles of a typical joint.
How do I tell non-rotational preload loss from self-loosening?
Paint a witness line across the nut, washer and member, run your normal duty cycle, then read the mark. If the mark is broken and the nut has rotated, the mechanism is rotational self-loosening. If the mark is intact and the joint is loose anyway, the mechanism is non-rotational. If a locking device previously helped but did not solve the problem, the joint is almost certainly losing preload non-rotationally.
Why do wedge-lock washers not stop non-rotational preload loss?
A wedge-lock washer works by making it impossible for the nut to rotate without lifting the entire clamp load. That is an effective answer to rotational self-loosening. Non-rotational loss occurs with the nut perfectly tight, so there is no rotation for the device to arrest and no mechanism by which it can restore the lost clamp length.
Which applications are dominated by non-rotational preload loss?
Hygienic and food process equipment with PTFE or EPDM gaskets under CIP and SIP cycling, busbar and battery pack joints with aluminium against steel, coated structural steel with zinc flake or PTFE finishes, composite and CFRP joints with matrix creep, and any joint with soft or cast members. Dry machined steel at constant temperature is the main case where embedment alone stays within a conventional device's capability.
What does the standards literature say about non-rotational loss?
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. Published work by Jiang and by Eraliev characterises the two-stage nature of preload loss and separates non-rotational relaxation as a distinct category.
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.
- Contact sales puts you in touch with the engineering team. 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.