A gearbox, a bar running the sash length, cams along it and keeps on the frame.

The gearbox converts handle rotation into linear movement of the bar.

Cams or mushrooms on the bar engage the keeps.

That happens at several points simultaneously.

It is a system rather than a component.

All the parts are designed to work together at matching spacings.

Because a single fastening leaves the corners free to be levered.

Engagement along the edge resists that far better.

It also pulls the sash evenly onto its seals.

Security and weather performance improve together.

Draughts and rain ingress both reduce.

That combination is why the arrangement became standard.

Generally not - cam spacings differ between systems.

A different bar produces cams that do not align with existing keeps.

Measuring the length alone is not enough.

Identifying the existing system matters more than the dimensions.

Markings on the gearbox or bar usually identify it.

Where the system is discontinued, replacing gear and keeps together may be necessary.

They set how firmly the sash is drawn onto its seals.

Too little compression and the window draughts despite locking.

Too much and the handle becomes stiff and the gear wears.

Most cams offer adjustment for exactly this reason.

Adjusting seasonally is normal on timber windows.

Small changes make a noticeable difference to how the window feels.

They resist levering considerably better than plain cams.

The mushroom head engages a matching keep profile.

Pulling the sash outward pulls the head against the keep.

Plain cams can ride out of their keeps under leverage.

Enhanced security standards commonly call for mushrooms.

Both the cam and the keep have to be the matching type for the benefit to apply.

Because the bar is working against resistance somewhere.

Debris and corrosion in the gear are common causes.

A dropped sash makes every cam work against its keep.

A distorted frame does the same thing.

Forcing a stiff handle breaks the gearbox.

That is a much larger repair than adjusting whatever caused the stiffness.

Cams, keeps and handles are frequently replaceable individually.

A failed gearbox usually means replacing the gear.

Bars can sometimes be obtained separately.

Availability depends heavily on the manufacturer.

Older systems are frequently discontinued.

Where nothing matches, a complete replacement gear of a current system is the usual route.