wedge locking washers
Wedge-locking washers, honestly assessed
By ISOKLAMP Engineering, Inc. Editorial Team · Updated
Give the incumbent its due
Wedge-locking washers work. Any honest assessment starts there.
Against a plain washer under DIN 25201-4 Annex B transverse loading, a wedge-lock pair takes residual clamp force from 12,8 percent to 93,1 percent on the reference joint. That is not a marginal improvement, it is the difference between a joint that fails and one that does not. The principle is sound, the products are well made, and for a large class of joints they are the right answer and the cheapest one.
This article is about the boundary of that class.
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.
How the wedge principle works
A wedge-lock washer is used in pairs, cam face to cam face.
- The outer faces carry radial teeth or a high-friction texture that grips the bolt head and the member.
- The inner faces carry cam ramps at an angle α_c.
The design condition is:
α_c > β
where β is the thread helix angle, 1,47° for M16 × 2,0. Commercial wedge washers use cam angles in the region of 5 to 7°, giving a comfortable margin.
If the nut attempts to rotate loose, the cam pair must climb its own ramps. Because the ramp is steeper than the helix, climbing the cam demands more axial extension than backing off the thread provides. The nut therefore cannot rotate without increasing bolt tension, which it has no means of doing. Rotation is arrested.
It is an elegant piece of mechanism design and it does exactly what it claims.
Where wedge-locking washers are the correct choice
Specify them without hesitation when all of the following hold:
- The failure mode is confirmed rotational. Witness mark broken. See Non-rotational preload loss.
- The stack is metal on metal, no gaskets, no polymers, no composites.
- Faying and bearing surfaces are uncoated, or the coating is thin and hard.
- Service temperature is roughly constant, or all members share an expansion coefficient.
- Bearing surfaces may be marked. Painted structural steel, machined mild steel.
- Cost per joint matters and volumes are high.
That is a very large fraction of general engineering joints, and in every one of those cases a wedge-lock pair is the correct engineering answer.
The boundary
The wedge principle addresses rotation. It has no mechanism that produces clamp length. So wherever the loss mechanism is stack shortening rather than rotation, a wedge-lock washer arrests something that was not the problem.
| Loss mechanism | Wedge-lock washer response |
|---|---|
| Transverse micro-slip, nut rotating | Arrested. This is what it is for |
| Embedment of asperities | No effect. Typically 5 to 12 % lost |
| Coating creep | No effect |
| Gasket and polymer relaxation | No effect |
| Thermal ratcheting | No effect, accumulation continues |
| Composite matrix creep | No effect |
The characteristic Junker curve tells the story. A wedge-lock pair drops 5 to 12 percent in the first 200 cycles and then runs perfectly flat for the remaining 1 800. The flat section is the wedge principle working. The initial drop is embedment, and the washer has no way to get it back.
| Cycles | Wedge-lock pair | ISOKLAMP CFR |
|---|---|---|
| 0 | 100,0 % | 100,0 % |
| 50 | 96,4 % | 97,8 % |
| 120 | 94,8 % | 96,9 % |
| 300 | 94,0 % | 98,1 % |
| 800 | 93,5 % | 99,1 % |
| 2 000 | 93,1 % | 99,4 % |
ISOKLAMP figures are predicted by finite-element analysis of the ISK-16 geometry coupled to VDI 2230 Sheet 1 mechanics. Wedge-lock baselines are calibrated against published Junker data, sourced on Full residual clamp force dataset.
Under pure transverse loading over three minutes the gap is 6,3 percentage points, which is not a compelling argument on its own. The gap opens where the test does not go: 60 CIP and SIP cycles, 250 aluminium thermal cycles, six months of PTFE creep. Those are the conditions in which the two curves separate by 20 to 40 points.
The second boundary: surface marking
Wedge-lock washers grip through radial teeth that bite into the bearing surface. That is integral to the design, not a defect, and on painted structural steel it is irrelevant.
It disqualifies them on:
- Passivated stainless. Breaking the passive layer initiates pitting. See Stainless galling.
- Plated busbars. Exposed aluminium oxidises and contact resistance rises sharply. See Busbar and battery joints.
- Anodised aluminium. The anodic layer is the corrosion protection.
- Composite laminates. Severed surface fibres initiate delamination.
- Hygienic equipment. Tooth roots are unreachable undercuts under 3-A and EHEDG guidance. See Hygienic equipment bolting.
Detail on No surface damage.
Choosing between them
| Your joint | Specify |
|---|---|
| Steel on steel, dry, constant temperature, marking acceptable | Wedge-lock pair |
| As above but with a coated interface in the load path | Wedge-lock plus uncoated hardened washers |
| Gasket, polymer or composite in the stack | Take-up mechanism |
| Dissimilar metals, thermal cycling | Take-up mechanism |
| Plated, passivated or anodised bearing surfaces | Non-marking take-up mechanism |
| No service access over design life | Take-up with readable reserve |
| Wedge-lock already fitted and joint still degrading | Take-up. The mechanism was never rotational |
The last row is the one we see most often, and it is a diagnosis rather than a sales argument. A wedge-lock washer that half-solved a problem has told you the problem was not rotation.
Full side-by-side on Locking methods compared. Diagnostic procedure on Non-rotational preload loss.
Frequently asked questions
How does a wedge-locking washer work?
It is used in pairs, cam face to cam face. The outer faces grip the bolt head and member through radial teeth, and the inner faces carry cam ramps steeper than the thread helix angle. If the nut tries to rotate loose it must climb its own cam ramps, which demands more axial extension than backing off the thread provides, so rotation is arrested.
When is a wedge-locking washer the right specification?
When the failure mode is confirmed rotational, the stack is metal on metal with no gaskets or polymers, surfaces are uncoated or thinly and hardly coated, temperature is roughly constant, and the bearing surfaces may be marked. That covers a large fraction of general engineering joints, and in those cases a wedge-lock pair is both correct and the cheapest option.
What do wedge-locking washers not fix?
Anything that shortens the clamped stack. Embedment, coating creep, gasket and polymer relaxation, thermal ratcheting and composite matrix creep all reduce clamp force with the nut perfectly still. The characteristic Junker curve shows this directly: a 5 to 12 percent drop in the first 200 cycles from embedment, then a flat line, because the wedge principle arrests rotation but produces no clamp length.
Why can't wedge-locking washers be used on stainless or plated surfaces?
They grip by biting radial teeth into the bearing surface. On passivated stainless that breaks the passive layer and initiates pitting. On plated busbars it exposes aluminium that oxidises and raises contact resistance. On anodised aluminium it removes the corrosion protection, and on composites it severs surface fibres and initiates delamination.
My wedge-locking washers helped but the joint still degrades. What now?
That outcome is diagnostic. A wedge-lock washer that partially solved a problem has demonstrated that rotation was not the dominant mechanism, because rotation is the one thing it arrests completely. The remaining loss is stack shortening, and the fix has to supply clamp length rather than restrain rotation.
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.