Less reliably than their popularity suggests, and against transverse vibration the evidence is poor.

Once tightened, the coil flattens and the washer behaves much like a solid washer, so the spring force it is credited with has largely been used up.

Independent vibration testing repeatedly finds that joints with split washers loosen at broadly the same rate as those without.

Several engineering authorities now recommend prevailing-torque nuts or wedge washers in their place for vibration-critical joints.

Where they retain some value is in joints that are not fully preloaded, in softer materials, and against slow thermal cycling rather than rapid transverse movement.

The most effective protection against loosening remains adequate preload in a correctly designed joint.

No - it removes the mechanism, because the cut ends have nothing soft to bite into.

The washer works, to the extent it works at all, by its sharp ends gouging the surfaces either side.

A hardened flat washer underneath gives the cut end a smooth hard face to skate on instead.

This is a very common assembly habit, usually adopted to protect the joint face - which is a reasonable aim achieved by defeating the part fitted to prevent loosening.

If the surface must be protected, the honest answer is a different locking method rather than a stacked washer.

The split washer goes against the part that turns, directly on the joint face.

Only if they still spring open when removed - and replacing them is cheap enough that it is rarely worth the check.

A washer that stays flat after removal has taken a permanent set and has no spring left.

The cut ends also blunt after biting once, which reduces whatever gripping effect they had.

Corrosion between the coil faces is common in used washers and can crack them on re-tightening.

Where a joint is being dismantled anyway, the washer is the least expensive part of the assembly to renew.

In safety-related or documented maintenance, single-use is often mandated regardless of condition.

Finished, plated, painted or soft surfaces - the damage is the mechanism, so it cannot be avoided.

The cut ends gouge both the fastener face and the joint face, leaving marks that stay for the life of the component.

On plated or painted surfaces that damage breaks the protective coating and starts corrosion at the fixing.

In soft materials such as aluminium or plastics the ends can embed so deeply that the washer sinks and preload is lost.

On visible fixings the marks are simply unacceptable in most finished work.

Where the surface matters, a locking nut, thread adhesive or a wedge washer set is the appropriate substitute.

Either way round mechanically, but always directly against the surfaces it is meant to bite, and under the part that turns.

The coil is symmetrical, so orientation of the split itself is not critical.

What matters is that both cut ends can reach a surface: one bearing on the nut or head, one on the joint face.

Placing it under the stationary part means the biting happens where no relative movement occurs, which reduces its effect.

Stacking two split washers does not double the effect and creates an unstable joint.

The washer should be flattened by correct tightening; a joint tightened only until the washer closes is not necessarily correctly preloaded.

Stainless versions exist and behave the same mechanically, with the usual stainless caveats.

Stainless split washers are softer than hardened carbon steel ones, so the cut ends blunt more readily.

Galling between a stainless washer and a stainless nut is a real risk, and anti-seize compound is the usual mitigation.

Outdoors, the gouged surfaces are a corrosion starting point in any material, which matters more than the washer's own finish.

Plated carbon steel washers used outdoors corrode quickly at the cut ends where the plating is thinnest.

For exposed joints where loosening genuinely matters, a wedge washer set or a prevailing-torque nut in a matched material is the sounder choice.

Cost, habit, and the fact that most joints they are fitted to are not actually subject to serious vibration.

They are among the cheapest components in any assembly and have been standard for generations.

A great many joints never see the transverse vibration that defeats them, so the washer appears to work.

Drawings and specifications carry them forward from earlier designs without the question being revisited.

Where they genuinely add nothing, they still add cost and assembly time, which is the practical argument for reviewing them.

Where vibration is real, the substitution to a wedge washer or prevailing-torque nut is straightforward and well proven.