The joint face, not the bolt - which is why the nut runs down the thread freely.

Radial teeth on the underside of the flange bite into the component as the nut is tightened.

Their asymmetric form resists rotation in the loosening direction more than in the tightening direction.

Because nothing grips the thread, there is no prevailing torque and assembly is fast.

That is a real production advantage over nylon insert and distorted thread nuts.

It also means the lock depends entirely on the joint face being suitable.

Yes - and for many assemblies it is the deciding argument.

The conical flange spreads clamping load over a wider area, which is what a flat washer does.

One part instead of two removes a component from the bill of materials and an operation from assembly.

It also removes a washer that can be dropped or omitted.

Where the surface must not be marked, however, a washer plus a different locking method is the correct arrangement.

Flange nuts also need clearance around the fastener, so they do not suit counterbores or tight recesses.

Plated, painted and finished surfaces, and soft materials such as aluminium and plastics.

The teeth break protective coatings exactly where water sits, which starts corrosion at the fixing.

On a visible surface the ring of marks is permanent and obvious.

In soft materials the teeth embed too far, the material flows, and preload is lost over time.

Steel and other medium-hardness materials are where they work as intended.

Against a hardened surface the teeth cannot bite and the nut becomes a plain flange nut.

Easier than a prevailing-torque nut, because the locking is one-directional and at the face.

The serrations resist loosening but are designed to release when torque is applied deliberately.

There is no prevailing torque along the thread, so once the teeth release the nut spins off.

That is convenient in service and means less resistance to a joint being deliberately disturbed.

Where a fastener must resist deliberate removal, a different approach is needed.

It also means the nut gives no warning through feel that its lock has been compromised.

The teeth may still be sharp, but the joint face is already marked - and that is what fails first.

Refitting into an existing serrated impression gives very little additional lock.

Rotating the nut slightly so the teeth meet fresh material helps but is not controllable.

Teeth flatten over successive installations, particularly against harder surfaces.

They are inexpensive, so replacement at disassembly is normal.

In documented maintenance the nut is usually treated as single-use.

Less than prevailing-torque nuts, but the flange's larger bearing radius increases friction under the nut.

There is no prevailing torque to add, which simplifies the calculation.

However, the flange bears further out from the axis, so a bigger share of the effort goes into friction at the face.

A torque figure developed for a plain nut and washer therefore over-estimates the preload achieved.

Manufacturers publish figures for flange nuts specifically.

The teeth also bed in during tightening, so a short pause and re-check is worthwhile on critical joints.

Wedge washers resist transverse vibration far better; serrated flange nuts are faster and cheaper.

A wedge pair works by cam geometry, which does not fade with vibration cycles.

Serrations rely on maintaining bite in the joint face, which depends on preload being retained.

Both mark the surfaces, so neither solves the finished-surface problem.

The flange nut is one part with an integral washer, which the wedge pair is not.

For ordinary machinery assembly the flange nut is usually sufficient; for severe vibration it is not.