Steering worm is a thread screw-like cylindrical device with fully rotational capability. As the steering wheel rotates, the steering column moves the steering worm up and down and along with it, it also moves the steering parts attached to it. Steering system transmits the left right rotation of the steering wheel by the truck driver to the steering worm, which in turn moves the part attached to the steering worm bolt.
The steering wheel connects to a shaft, which rotates the worm inside of the steering worm block. Instead of twisting further into the block, the worm gear is fixed so that when it rotates, it moves the block, which transmits the motion through the gear to the Pitman arm, causing the roadwheels to turn.
Some of the video links Im sharing below to see how the steering worm works: