MACHINERY LUBRICATION- INDIA SEPTEMBER-OCTOBER 2019 | Page 28

LESSONS IN LUBRICATION
Non-throated (cylindrical) worm
Three starts (threads)
Side view
Input
Throated (enveloped)
worm wheel
Worm wheel
Figure 4. Single-throated (single-enveloping) worm drive
As the contact surface area increases,
the torque capacity, load-holding ability
(shock load resistance) and durability
are improved. Enveloping gear designs
also have a lower anticipated wear rate
as a result of the load distribution.
Worm drive manufacturers attempt to
optimize this contact relationship
between the two gear elements for
improved reliability.
Other notable advantages of worm
drives over potential gear system
alternatives include:
• • A worm drive can be designed with a
gear ratio of more than 200-to-1, in
comparison to that of a helical gear,
which may be limited to 10-to-1 on a
single reduction. The gear ratio for
worm drives is the number of teeth
on the worm wheel to the number of
threads (or starts) on the worm.
• • The high gear ratio and configuration
of the two gear elements allow for a
compact design, making the worm
drive a great option for space-
26 | September - October 2019 |
limited areas. In addition, the
number of moving parts is reduced
along with the opportunities for
failure. However, this may be
partially offset by a loss in efficiency
from large increases in torque.
• • Due to the extreme helical angle,
switching the direction of power is
nearly impossible. The worm wheel
cannot
easily
be
rotated
independently to force movement
on the worm. This self-locking ability
eliminates the need for a backstop,
which may be required in alternative
gear systems.
• • With the precise movement of worm
drives, particularly in double-
enveloping designs, backlash (play
between gear teeth) can be greatly
minimized. This is crucial in certain
applications such as robotics.
160
150
contact area (in the shape of a radial
band) with three or more teeth in
contact.
140
130
polyglycol
120
110
100
90
mineral oil
80
70
300
500
1,000
5,000 10,000
Oil Change Interval (hours)
30,000
Figure 5. A comparison of lubricant life and oil change intervals for mineral oil,
PAOs and PAGs over a range of oil sump temperatures
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