Bell Crank

What it is

A bell crank is a lever that turns on a fixed pivot and redirects a push-pull movement through an angle, often 90 degrees, so that a rod moving in one direction drives another rod in a different direction. In a steering linkage it is used where the steering gear cannot be lined up with the wheels: the steering gear moves one arm of the bell crank, usually through a drag link, and the other arm drives the tie rods. Bell cranks appear in less common layouts such as centre-point steering; most passenger cars with rack-and-pinion steering do not have one.

How it works

Turning motion through an angle

Two arms sit at an angle around one pivot. When a rod pushes one arm, the lever rotates and the other arm pushes its rod in a new direction. The ratio of the two arm lengths sets the ratio of the movements: equal arms keep the travel, unequal arms increase or reduce it.

A bell crank in the steering linkage

The steering gear drives one arm of the bell crank, usually through a drag link. The other arm, or a plate on the same pivot, carries the tie rods to each knuckle. Because both tie rods attach close to one central pivot, this layout can keep the left and right wheels symmetrical (centre-point steering).

Construction

A steering bell crank is a forged or fabricated steel lever, often L-shaped or triangular, turning on a pivot shaft held in a bracket on the frame or crossmember. The pivot runs in bushings or bearings, often with a grease fitting, and the rods connect to the lever through ball joints with tapered studs.

Common failure symptoms

Play at the pivot

Worn bushings or bearings let the lever rock on its shaft, which is felt as loose steering.

Knocking over bumps

A loose pivot or worn ball joint knocks as the load on the linkage reverses.

Stiff steering or poor self-centring

A dry or seized pivot adds friction to the whole linkage.

Uneven tyre wear

Play at the bell crank lets the toe change while driving.

Frequently asked