THRUST
When a worm drives a worm gear, there is a tendency for the worm to “back out” or push forward (depending upon the
direction it is rotating) due to the action of the thread. This is known as “thrust action”. To counteract the friction caused by this thrust action, thrust bearings should be used. Thrust bearings should also be used on the worm gear shaft, although thrust is considerably less on the shaft (due to the slower gear rotation.)
Figure 4.11 shows the direction of thrust when worms and worm gears are rotating in various directions. Thrust bearings are shown in their proper position to absorb the pushing force (thrust).
CENTER DISTANCE
The center distance of a worm and worm gear in mesh is the distance between the center of the two shafts. When mounted on the proper center distance, the worm and worm gear will mesh correctly. (See Figure 4.11A)
Important: For proper operation, the center distance should be equal to one-half the pitch diameter of the worm plus one-half the pitch diameter of the worm gear.
If the exact center distance cannot be maintained, it is better to increase the center distance than it is to decrease it.
BACKLASH
Backlash (See Figure 4.11B) is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles. Backlash may be determined in the tranverse, normal, or axial-planes, and either in the direction of the pitch circles or on the line of action. Such measurements should be corrected to corresponding values on tranverse pitch circles for general comparisons. (See Figures 4.11C)
Important: The operation of Boston gears at proper center distances assures the correct degree of backlash for greatest efficiency and longest life.
