Lesson 1/Learning Event 1
In a steering gear, the part that is like the bolt is called the worm. The
worm is secured to the lower end of a shaft with the steering wheel on the
opposite end so that the worm and steering wheel turn together. The
steering gear part that is like the section of a nut is called the sector,
and its shaft is called the pitman arm shaft. The pitman arm is splined to
the pitman arm shaft.
The steering gear worm (bolt) and the sector (nut section) are machined so
that there is very little lash or clearance between their threads in the
midposition. However, as the worm is turned to steer the vehicle either to
the right or the left, the amount of lash increases. This makes up for the
unequal wear that occurs in normal use. Vehicles are operated in the
straightahead position most of the time, so most of the wear is in the
center of the steering gear worm.
It requires 2 1/2 to 3 1/2 turns of the steering wheel and worm to move the
pitman arm shaft through its entire allowable movement, an arc of about 70.
That pivots the front wheels from a hard turn in one direction to a hard
turn in the opposite direction. The steering wheel has to be turned farther
because of the mechanical advantage gained by the worm and sector. Most
steering gears are designed so that they provide more mechanical advantage
in the midposition than when turned to the extreme right or left, so they
are said to have a "variable" ratio.
Many different kinds of steering gears are used, but they all work in about
the same manner.