T-130
fills the entire tooth space in all mesh positions.
2. Backlash Removal by Forced Center Distance Closing
This is derived from static Case III. A spring force is applied to close the center
distance; in one case as a linear force along the line-of-centers, and in the other case as
a torque applied to the swing arm.
In all of these dynamic methods, the applied external force should be known and
properly specified. The theoretical relationship of the forces involved is as follows:
F > F1 + F2
(14-24)
where:
F1 = Transmission Load on Tooth Surface
F2 = Friction Force on Tooth Surface
If F < F1 + F2, then it would be impossible to remove backlash. But if F is excessively
greater than a proper level, the tooth surfaces would be needlessly loaded and could lead
to premature wear and shortened life. Thus, in designing such gears, consideration must
be given to not only the needed transmission load, but also the forces acting upon the
tooth surfaces caused by the spring load. It is important to appreciate that the spring
loading must be set to accommodate the largest expected transmission force, F1, and this
maximum spring force is applied to the tooth surfaces continually and irrespective of the
load being driven.
3. Duplex Lead Worm
A duplex lead worm mesh is a special design in which backlash can be adjusted by
shifting the worm axially. It is useful for worm drives in high precision turntables and
hobbing machines. Figure 14-12 presents the basic concept of a duplex lead worm.
The lead or pitch, pL and pR , on the two sides of the worm thread are not identical.
The example in Figure 14-12 shows the case when pR > pL. To produce such a worm
requires a special dual lead hob.
The intent of Figure 14-12 is to indicate that the worm tooth thickness is progressively
bigger towards the right end. Thus, it is convenient to adjust backlash by simply moving
the duplex worm in the axial direction.
pR
pR
pR
pR
pL
pL
pL
pL
Fig. 14-12 Basic Concepts of Duplex Lead Worm
