By geometry: the cam angle between the two halves is greater than the thread pitch, so backing off tightens the joint instead of slackening it.

The pair is assembled cam face to cam face. Serrations on the outer faces lock each washer to the surface it touches.

If the fastener rotates in the loosening direction, the only place movement can happen is between the two cams.

Climbing a cam steeper than the thread's own helix increases bolt elongation, and therefore tension.

The fastener is effectively fighting itself to come loose, which is why the mechanism does not fade with vibration cycles.

It also means the washer works without relying on friction, spring force or penetrating the joint surface.

Cam faces together, serrated faces outward - and the wrong way round provides no locking whatever.

The cams must be able to slide on each other; that relative movement is the mechanism.

The serrations must grip the fastener face and the joint face so movement is forced to the cam interface.

Reversed, the serrations grind against each other and the pair behaves like two plain washers.

Manufacturers commonly supply the pairs bonded together with a light adhesive so they arrive correctly oriented.

On inspection, the visible outer faces should be the serrated ones on both sides of the stack.

Yes, within the manufacturer's stated number of re-uses, provided the serrations are sharp and the cams undamaged.

Nothing is plastically deformed in normal service, which is the key difference from toothed and split washers.

The serrations do wear and the cam faces can be damaged by grit or by incorrect assembly.

Inspection is straightforward: sharp serrations, clean cam faces, no cracking.

Where a joint is safety-critical, the maintenance regime may still mandate replacement.

Pairs must stay together - mixing halves from different washers or sizes is not acceptable.

Yes - the serrations grip by biting, so they mark both the fastener face and the joint face.

That damage is necessary: without it the washers rotate against the surfaces and the cam mechanism never engages.

On plated or painted surfaces the coating is broken, which is the same corrosion concern as with any locking washer.

Versions with finer serrations or larger bearing areas exist for softer substrates such as aluminium.

Where the surface truly cannot be marked, an adhesive thread-locking compound or a prevailing-torque nut may be more appropriate.

Their finishes are generally chosen with corrosion in mind precisely because the surfaces are broken.

They need to be selected against the fastener's hardness, because the serrations must grip the bearing face.

Against a very hard fastener face the serrations cannot bite, so the washer can turn and the cam interface never becomes the sliding plane.

Manufacturers publish which property classes each washer suits, and harder versions exist for high-strength bolts.

The joint side matters equally: soft substrates need a version with a larger bearing area to avoid sinking.

Head type also matters, with versions made for countersunk and flanged fasteners.

Selecting on thread size alone is the common mistake.

Wherever transverse vibration is real and loosening has consequences - rail, mining, wind, heavy plant, conveyors and engines.

In those environments the alternatives lose effectiveness over time while a cam mechanism does not.

They also suit joints that are difficult or expensive to access for retightening, since the maintenance saved outweighs the component cost.

For static indoor joints with adequate preload, the extra cost buys little.

They are also chosen where a locking method must be demonstrably effective for documentation or approval purposes.

The cheapest effective measure in any joint remains getting the preload right in the first place.

They work immediately, tolerate higher temperatures, and can be dismantled with ordinary tools - but they mark the surfaces.

Thread adhesive needs cure time before the joint is protected, and it is sensitive to clean, dry, oil-free threads.

Its temperature ceiling is set by the chemistry, and high-temperature grades cost more and can make removal difficult.

Wedge washers work the moment the joint is tightened and are unaffected by temperature within the material's range.

Adhesive leaves no marks and suits finished or plated surfaces where serrations are unacceptable.

Some demanding applications use both, which is a decision for the joint's designer rather than a default.