stack shortening
Stack shortening in bolted joints
By ISOKLAMP Engineering, Inc. Editorial Team · Updated
Why micrometres matter
A bolt is a spring. You stretch it during tightening, and the clamp force it applies is the tension in that spring. Shorten the stack it clamps, and the bolt relaxes by the same amount. The clamp force falls by the bolt's stiffness times the shortening, reduced by how much the members share the change.
That relationship is the load factor, Φ:
Φ = k_S / (k_S + k_P)
For the reference joint, k_S = 1,04 × 10⁹ N/m and k_P = 5,71 × 10⁹ N/m, giving Φ = 0,154.
The clamp force lost for a stack shortening of δ is:
ΔF_V = δ · k_S · (1 − Φ) ... in the limit, δ · (k_S · k_P)/(k_S + k_P)
Which for this joint works out at 0,884 kN per micrometre.
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 penalty table
| Stack shortening | Clamp force lost | Residual | % of assembly preload |
|---|---|---|---|
| 5 µm | 3,1 kN | 66,9 kN | 95,5 % |
| 10 µm | 6,3 kN | 63,7 kN | 91,0 % |
| 15 µm | 9,4 kN | 60,6 kN | 86,6 % |
| 20 µm | 12,5 kN | 57,5 kN | 82,1 % |
| 30 µm | 18,8 kN | 51,2 kN | 73,1 % |
| 40 µm | 25,1 kN | 44,9 kN | 64,2 % |
| 50 µm | 31,3 kN | 38,7 kN | 55,2 % |
Read that table next to the DIN 25201-4 Annex B acceptance criterion, which requires 80 percent residual clamp force at 2 000 cycles. Twenty-two micrometres of stack shortening fails that criterion on its own, with the nut never having moved.
Where the micrometres come from
Stack shortening is not one phenomenon. It is four, and they have different budgets and different time constants.
Embedment. Surface asperities on the faying, underhead and nut-bearing faces flatten under contact stress. Fast, mostly complete inside the first 200 load cycles. Budget: 3 to 8 µm per interface for machined steel, 6 to 15 µm for as-rolled or coated surfaces, more for soft members. A typical joint has four to six interfaces.
Coating creep. Zinc flake, PTFE-impregnated and electroless nickel coatings all flow under sustained contact stress. Budget: 5 to 20 µm depending on coating thickness and temperature. Slower than embedment, and it continues.
Gasket and polymer relaxation. PTFE, EPDM, silicone and fibre gaskets creep continuously. Budget: 20 to 60 µm for a 3 mm PTFE gasket over its first months. This is the largest single contributor wherever it applies. See hygienic bolting.
Thermal ratcheting. Where members and fastener have different expansion coefficients, each thermal cycle can leave a small permanent set. Aluminium at 23 × 10⁻⁶ /K against a steel bolt at 12 × 10⁻⁶ /K is the classic case. Budget: accumulates without limit until something arrests it. See busbar joints.
Add them up. A coated steel joint with a PTFE gasket, thermally cycled, comfortably exceeds 40 µm. That is a joint at 64 percent of assembly preload with a perfectly tight nut.
Why the standard fixes do not fix it
Retorquing works exactly once per intervention, and it is only available where the joint is accessible and the downtime is affordable. It also re-embeds fresh asperities each time.
Longer bolts reduce k_S, which reduces the clamp force lost per micrometre. This is real and useful engineering. It does not remove the loss, and there is usually no room.
Belleville stacks add elastic travel, which lowers the effective joint stiffness so the same shortening costs less force. Also real, also useful, and also limited: the force falls along the spring curve as the stack extends, so recovering 0,2 mm costs a large fraction of the force being held. Compare on the Belleville alternative.
Locking devices do nothing at all for stack shortening. A wedge-lock washer holds a nut that was never going to turn. This is the single most common mis-specification in bolted joint design, and the symptom is a joint that got better but not well.
Designing the shortening out
The alternative is to put the length back. A preload-recovering washer carries a reserve of clamp length and deploys it, one way, as the stack shortens.
The ISK-16 carries 0,50 mm of take-up reserve. Against a 40 µm shortening budget that is a reserve factor of 12,5. Against the 60 µm worst case with a thick PTFE gasket it is still better than 8.
| Shortening budget | Reserve consumed | Reserve remaining |
|---|---|---|
| 10 µm | 2,0 % | 490 µm |
| 30 µm | 6,0 % | 470 µm |
| 60 µm | 12,0 % | 440 µm |
| 150 µm | 30,0 % | 350 µm |
The remaining reserve is readable through the indicator window without instruments and without disassembly, which converts an invisible failure mode into an inspection item. See the preload indicator.
Calculating your own budget
Work through it in this order.
- Count the interfaces. Every faying, underhead and nut-bearing face embeds.
- Assign per-interface embedment from surface finish and hardness.
- Add coating creep for every coated surface in the load path.
- Add gasket relaxation from the manufacturer's compression-set data at your service temperature.
- Add thermal ratcheting where expansion coefficients differ across the stack.
- Multiply the total by your joint's clamp force loss per micrometre, from the load factor.
- Compare the result against the residual preload your design actually requires.
If step 7 fails, you have a stack shortening problem, not a loosening problem, and the fix has to supply length. Method comparison on locking methods compared, full derivation in the technical report.
Frequently asked questions
What is stack shortening in a bolted joint?
Stack shortening is any reduction in the clamped length of a bolted joint. It is caused by embedment of surface asperities, creep in coatings, relaxation of gaskets and polymers, and thermal ratcheting between dissimilar materials. Because the bolt relaxes along its own elastic curve as the stack shortens, the clamp force falls even though the nut never rotates.
How much preload does stack shortening cost?
For an M16 × 2,0 class 10.9 joint preloaded to 70 kN with a load factor of 0,154, the loss is 0,884 kN per micrometre. Ten micrometres costs 9,0 percent of clamp force, 30 micrometres costs 26,9 percent, and 50 micrometres costs 44,8 percent. Twenty-two micrometres alone is enough to fail the DIN 25201-4 Annex B criterion of 80 percent residual preload.
What is a typical stack shortening budget?
Embedment runs 3 to 8 micrometres per interface for machined steel and 6 to 15 for as-rolled or coated surfaces, across four to six interfaces in a typical joint. Coating creep adds 5 to 20 micrometres. A 3 mm PTFE gasket adds 20 to 60 micrometres. Thermal ratcheting between aluminium members and a steel bolt accumulates without limit until something arrests it.
Do locking washers prevent stack shortening?
No. Locking washers prevent the nut from rotating, which addresses a completely different loss mechanism. Stack shortening occurs with the nut perfectly tight. Specifying a locking device against a stack shortening problem is the most common mis-specification in bolted joint design, and the characteristic symptom is a joint that improves but does not stop failing.
How do you design stack shortening out of a joint?
Either reduce the penalty or remove the shortening. Longer bolts and Belleville stacks reduce the clamp force lost per micrometre but cannot restore length already lost, and disc spring force falls along the load-deflection curve as it extends. A preload-recovering washer carries a reserve of clamp length, 0,50 mm in the ISK-16, and deploys it one way as the stack shortens, which is a reserve factor of 12,5 against a 40 micrometre budget.
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.