A “worm” gear has teeth cut in the shape of a helix.
The steering box is a gearbox. It converts the rotary motion of the steering wheel to the linear motion needed to control the wheels. Its gear ratio increases output torque, and reduces the effort the driver has to apply.
The input, attached to the steering column, is called a worm shaft. It is meshed with a sector, or portion, of a gear, mounted on its own shaft, at right angles to the worm. The outer end of the sector shaft has a tapered spline which mates with an internal spline on the Pitman or drop arm. As the steering wheel rotates, the worm-shaft causes the sector to move through an arc, and transfer the motion, through the Pitman arm, to the steering linkage.
Variations to this principle include the worm-and-roller, and the worm-and-nut.
The recirculating ball steering box is a popular development of the worm-and-nut, and worm-and-sector principle.
Both ends of the worm shaft are supported in the housing by angular bearings, which are pre-loaded to reduce end-float, and side-thrust movements of the worm, when it is under load.
A ball nut rides on the worm, supported on the spiral grooves of the worm, and the inside of the nut, by many balls. The balls form a low-friction internal thread, which causes the nut to move up or down on the worm as it rotates.
With rotation, the balls are rolled along the grooves, partly in the worm and partly in the nut, and they circulate by passing through ball-return-guides at each end of the nut.
External teeth on one side of the nut mesh with the teeth of the sector gear formed on the sector shaft, or Pitman shaft, and this transfers the motion through the Pitman arm to the steering linkage.
The sector gear and nut teeth are designed so that when the teeth are in the straight-ahead position, they have minimum clearance. This reduces free play in that position.
The Pitman shaft is supported by 2 caged, needle roller-bearings, in the steering box housing.
The sector teeth are angled, and an adjustment screw on the steering housing cover provides proper engagement of the sector gear and nut teeth.
The worm shaft in a worm-and-roller steering box is supported in a similar manner to the recirculating-ball type, but the worm has an hour-glass shape, and it meshes with a double track roller, mounted on bearings, on a pin attached to the Pitman shaft.
As the worm rotates, the roller moves in an arc, following the hour-glass shape, and transferring the motion to the Pitman shaft. The hour-glass shape changes the steering ratio slightly as the steering wheel is turned from the central position towards each lock.
