It takes the rotation instead of the component, so the surface is neither marked nor scuffed.

A plain or serrated nut drags its bearing face across the component as it tightens.

A washer that turns independently absorbs that motion between nut and washer rather than between washer and component.

That protects finished, plated and painted surfaces from the circular scuff marks.

It also means the locking mechanism does not need to bite the joint face at all.

The washer still spreads load in the usual way while doing it.

A serrated flange locks by marking the joint face; this locks on the bolt and protects the face.

Both integrate a bearing surface into the nut, so both remove a separate loose washer.

The flange nut's teeth are its locking mechanism, so the marking is not optional.

In a lock nut with washer, the locking element is on the thread and the washer simply bears.

That makes it the correct choice on exactly the surfaces where a flange nut should not be used.

The flange nut is usually cheaper and faster to run down, which is why it dominates steel-on-steel assembly.

No - it is captive, retained by a rolled lip, and the assembly is a single part.

That is what stops it being lost, which is the other reason to choose one.

Where a larger bearing area is needed, a separate oversized washer must be added underneath.

Stacking an additional washer works but adds another interface for the joint to settle into.

The captive washer's material and finish are fixed by the part, so mixed-metal considerations follow the whole assembly.

If the washer is damaged, the nut is replaced.

Any of them - nylon insert, distorted thread or flexible top - and the properties follow the mechanism.

A nylon-insert version carries the same temperature ceiling and reuse limit as a plain nylon insert nut.

An all-metal version has no polymer ceiling and reuses accordingly.

So the selection question is really two questions: which locking mechanism, and whether the captive washer is wanted.

Prevailing torque and assembly torque figures come from the underlying mechanism.

Manufacturers state both the mechanism and the washer specification.

Better than most locking nuts, because the washer spreads load and nothing bites the surface.

Aluminium, plastics and thin sheet are all vulnerable to serrated flange nuts embedding and losing preload.

A free-spinning washer bears flat and distributes the load over its full area.

It also covers oversized and slotted holes, which small nuts alone do not.

For very soft material a larger separate washer may still be needed under the captive one.

The clamping load itself must still suit the material, since no washer prevents a soft substrate from creeping.

Extra height, a fixed washer diameter, and a higher price than a nut and washer bought separately.

The assembly is taller than a plain nut, which matters in congested positions.

The washer's outside diameter cannot be changed, so unusual bearing requirements are not covered.

Cost per piece is higher, though assembly time and lost-washer costs often offset it.

Availability is narrower than for plain locking nuts, particularly in less common sizes and materials.

Where the surface does not need protecting, a plain locking nut is simpler.

On the head side, yes if that surface needs protecting - the captive washer only serves the nut end.

A joint has two bearing faces, and this nut addresses one of them.

Where the bolt head turns against a finished or soft surface, it needs its own washer.

Where the head bears on a hard machined face and does not turn, nothing may be required.

Flanged bolts solve the head side the same way this nut solves the nut side.

Adding washers increases grip length, so bolt length should account for them.