There is no specific Japanese standard concerning worm gearing, except 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.
19.3.2 Center Distance Error
Even when exaggerated center distance errors exist, as
shown in Figure 19-7, the results are crossed end contacts. Such
errors not only cause bad contact but also greatly influence backlash.
A positive center distance error causes increased backlash. A
negative error will decrease backlash and may result in a tight mesh, or
even make it impossible to assemble.
19.3.3 Mounting
Distance Error
Figure 19-8 shows the resulting poor contact from
mounting distance error of the worm gear. From the figure, we can see the
contact shifts toward the worm gear tooth's edge. The direction of shift
in the contact area matches the direction of worm gear mounting error.
This error affects backlash, which tends to decrease as the error
increases. The error can be diminished by micro-adjustment of the worm
gear in the axial direction.
SECTION 20 LUBRICATION OF GEARS
The purpose of
lubricating gears is as follows:
1. Promote sliding between teeth to reduce the
coefficient of friction (m).
2. Limit the temperature rise caused by rolling
and sliding friction.
To avoid difficulties such as tooth wear and premature failure, the
correct lubricant must be chosen.
20.1 Methods Of Lubrication
There are
three gear lubrication methods in general use:
1. Grease lubrication.
2. Splash lubrication (oil bath method).
3. Forced oil circulation lubrication.
There is no single best lubricant and method. Choice depends
upon tangential speed (m/s) and rotating speed (rpm). At low speed, grease
lubrication is a good choice. For medium and high
