T-223
Mounting distance er-
ror will cause a change of
backlash; positive error will
increase backlash; and
negative, decrease. Since
the mounting distance error
of the pinion affects the
surface contact greatly, it is
customary to adjust the
gear rather than the pinion
in its axial direction.
19.3 Surface Contact Of
Worm And Worm Gear
There is no specific
Japanese standard con-
cerning worm gearing, ex-
cept for some specifications
regarding surface contact in
JIS B 1741.
Therefore, it is the general practice to
test the tooth contact and backlash with a
tester. Figure 19-5 shows the ideal contact
for a worm gear mesh.
From Figure 19-5, we realize that the
ideal portion of contact inclines to the
receding side. The approaching side has a
smaller contact trace than the receding side.
Because the clearance in the approaching
side is larger than in the receding side, the
oil film is established much easier in the
approaching side. However, an excellent
worm gear in conjunction with a defective gear
box will decrease the level of tooth contact
and the performance.
There are three major factors, besides the gear itself, which may influence the surface
contact:
1. Shaft Angle Error.
2. Center Distance Error.
3. Mounting Distance Error of Worm Gear.
Errors number 1 and number 2 can only be corrected by remaking the housing. Error
number 3 may be decreased by adjusting the worm gear along the axial direction. These
three errors introduce varying degrees of backlash.
19.3.1. Shaft Angle Error
If the gear box has a shaft angle error, then it will produce crossed contact as shown
in Figure 19-6.
A helix angle error will also produce a similar crossed contact.
Fig. 19-4 Poor Contact Due to Error in
Mounting Distance
(+) Error
(–) Error
Pinion
Gear
Pinion
Gear
Fig. 19-5 Ideal Surface Contact of
Worm Gear
Rotating Direction
Recess Side
Approach Side
