Worm and Worm Gear Mechanisms - Continued
This is something that can be felt when the worm knob is held between the thumb and forefinger and
turned back and forth a few mils. Every time the rotation of the knob is reversed, the movement is loose
for a short distance (1 to 3 mils) until a click or bump is felt in the knob when the lost motion has
reached its limit. The reason for this is that the dog-point screw fits loosely in the threads of the
housing, or in the groove of the socket, or a combination of both (fig. 3-23). Every time the motion of the
knob is reversed the socket will move in the housing until the dog-point screw has cocked over to the
other side. Once it is started, this clicking will grow progressively worse. This condition cannot be
permitted because the loose spot is always at the setting point. Turn the knob in the opposite direction
from which it was brought to the setting point, or fire the gun, and the instrument will be off the target.
This loose spot in the movement is not lost motion which affects the accuracy of the instrument, but it
makes the instrument unreliable because it will not hold a set reading. If this condition is found in an
elevation or cross-leveling movement, watch the level vial bubble move off center when the knob is
moved between the limits of the loose portion.
Figure 3-23. Dog-Point Screw Loose In Housing