preload-recovering washer
The preload-recovering washer
By ISOKLAMP Engineering, Inc. Editorial Team · Updated
A category, not a product feature
Fastener securing devices divide into three families. The first two are well populated. The third had one member until recently.
Locking devices stop the nut from rotating. Wedge-lock washers, prevailing-torque nuts, thread lockers, ribbed flange nuts, safety wire. Every one of them addresses rotational self-loosening.
Compliant devices add elastic travel so a given amount of stack shortening costs proportionally less clamp force. Belleville stacks, conical spring washers, wave springs, long-grip bolts. They soften the penalty. They do not remove it.
Recovering devices put clamp force back after the joint has lost it. They need a source of new clamp length and a mechanism for deploying it without operator intervention. This is where the preload-recovering washer sits.
The distinction matters because the three families fail differently, and buying from the wrong family is the most common expensive mistake in joint design.
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 problem a recovering washer solves
Preload is lost two ways.
Mode A, rotational self-loosening. The nut turns. This is what Junker demonstrated in 1969 and what the entire locking-device industry was built to prevent. It is well understood and well served.
Mode B, non-rotational preload loss. The nut does not turn at all. The clamped stack gets shorter, through embedment of surface asperities, creep in coatings, relaxation of gaskets, or differential thermal contraction. The bolt follows the shortening stack down its own elastic curve and the clamp force falls.
Mode B is not a small residual effect. For the reference joint, VDI 2230 Sheet 1 gives the clamp force penalty per micrometre of stack shortening directly from the load factor:
| Stack shortening | Preload lost | Residual clamp force |
|---|---|---|
| 5 µm | 4,5 % | 66,9 kN |
| 10 µm | 9,0 % | 63,7 kN |
| 20 µm | 17,9 % | 57,5 kN |
| 30 µm | 26,9 % | 51,2 kN |
| 50 µm | 44,8 % | 38,7 kN |
Thirty micrometres. That is less than the thickness of a sheet of paper, and it takes better than a quarter of the clamp force out of an M16 class 10.9 joint. No locking washer on the market addresses it, because the nut never moved.
Full treatment on preload loss.
What makes a washer "recovering"
Three requirements, all necessary.
A reserve of length. Something has to supply the missing 30 µm. In the ISK-16 this is 0,50 mm of helical take-up travel, a reserve twelve to thirty times the expected embedment budget of a well-made steel joint.
A one-way deployment mechanism. Clamp length that can be given back is not recovery, it is compliance. The ISK-16 ramp satisfies tan α_c < μ_r with a 1,8:1 margin, so advance is irreversible.
An energy source that does not need an operator. Recovery on a maintenance schedule is just retorquing with extra steps. Vibration-Actuated Take-up draws on the transverse micro-slip already present in the application.
Recovering versus compliant, in numbers
The honest comparison is against a Belleville stack, because that is what a competent engineer reaches for when a joint keeps relaxing.
| Belleville stack | Preload-recovering washer | |
|---|---|---|
| Mechanism | Elastic compliance | One-way helical take-up |
| Travel | 0,15 to 0,40 mm typical | 0,50 mm |
| Force across travel | Falls along spring curve | Flat, quasi-zero stiffness |
| Recovers clamp force | No, reduces the penalty | Yes, restores the length |
| Arrests rotation | No | Yes, α_c > β |
| Reserve is readable | No | Yes, indicator window |
| Stack height added | 2 to 6 mm for a stack | 3,40 mm base |
Disc spring figures are reconstructed from published manufacturer load-deflection curves, sourced on the test data page.
The conceptual difference is simple. A Belleville stack lowers the effective joint stiffness so 30 µm of shortening costs less. A recovering washer removes the 30 µm.
Performance
Under DIN 25201-4:2010-03 Annex B transverse loading on the reference joint:
| Method | Residual at 2 000 cycles | Handles Mode A | Handles 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 | Partially |
| Nyloc | 57,2 % | Yes | No |
| Conical spring washer | 44,6 % | No | Partially |
| Plain washer | 12,8 % | No | No |
ISOKLAMP figures are predicted by finite-element analysis of the ISK-16 geometry. Baselines are calibrated against published data with per-row sources on the test data page.
When to specify one
Reach for a preload-recovering washer when the joint has a length problem:
- Gaskets, seals or polymer spacers in the stack. See hygienic bolting.
- Coated or plated faying surfaces that creep under contact stress.
- Aluminium or copper members against steel bolts, thermally cycled. See busbar joints.
- Joints you cannot retorque without significant cost or downtime.
- Joints where a previous wedge-lock or nyloc solution reduced but did not eliminate the problem. That outcome is the classic signature of a Mode B joint treated as Mode A.
For a dry, stiff, steel-on-steel joint at constant temperature, a wedge-lock washer remains correct and cheaper. Compare the families on locking methods compared.
Frequently asked questions
What is a preload-recovering washer?
A preload-recovering washer restores clamp force that a bolted joint has already lost, rather than only preventing further loss. It contains a self-locking helical take-up biased by a constant-torque spring. When the clamped stack shortens through embedment, coating creep or gasket relaxation, the take-up advances and refills the lost clamp length.
How is a preload-recovering washer different from a locking washer?
A locking washer stops the nut from rotating, which addresses rotational self-loosening. It has no mechanism for restoring clamp force lost when the nut does not turn. A preload-recovering washer does both: it arrests rotation using a wedge geometry and adds a helical take-up that recovers lost clamp length.
Is a Belleville washer a preload-recovering washer?
No. A Belleville washer is a compliant device. It adds elastic travel so that a given amount of stack shortening costs proportionally less clamp force, but it cannot restore length the joint has already lost, and the force it applies falls along its load-deflection curve as it extends. A recovering washer removes the lost length rather than softening the penalty.
How much preload does a joint actually lose without the nut turning?
For an M16 × 2,0 class 10.9 joint preloaded to 70 kN with a load factor of 0,154, 30 micrometres of stack shortening costs 26,9 percent of clamp force, taking the joint to 51,2 kN. Ten micrometres costs 9,0 percent. Typical joints lose 10 to 40 percent of preload in the first 200 load cycles this way.
When should I specify a preload-recovering washer?
When the joint has gaskets, seals or polymer spacers in the stack, when faying surfaces are coated or plated and creep under contact stress, when dissimilar metals are thermally cycled, when retorquing is expensive or impossible, or when a wedge-lock or prevailing-torque solution reduced the problem without eliminating it. That last case is the classic signature of a non-rotational loss mechanism being treated as a rotational one.
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.