MACHINERY LUBRICATION- INDIA SEPTEMBER-OCTOBER 2019 | Page 27

Bennett Fitch | Noria Corporation
LESSONS IN LUBRICATION
Gear Lubrication
THE RIGHT WAY to
Lubricate WORM GEARS
O
f all the different types of
gear
configurations,
worm gear systems are
considered some of the
most
problematic
because they present unique lubrication
challenges due to their distinct design.
To overcome these challenges, you must
understand not only the complexities of
worm gears but also which qualities to
take into account when choosing a
worm gear lubricant.
Worm Gear Designs
A worm gear is a non-parallel,
non-intersecting axis design consisting
primarily of two gear elements: the
worm, which is the driving gear in the
shape of a spiral or screw, and the worm
gear or worm wheel, which is the driven
gear in the shape of a common spur
gear.
Technically, the entire worm gear system
should be called a worm drive or worm
gearset to avoid confusion. The worm
always drives the worm wheel. This
design characteristic is due to the
extreme helical angle, which is nearly 90
degrees. The worm drive resembles the
design of the crossed helical gear
configuration, except the gear teeth on
the worm of a worm drive will circle
around the circumference of the worm
at least once. Since the worm may have
as little as one tooth that spirals radially
around the helix, the number of teeth
on the worm is more appropriately
identified by the number of starts
or threads.
There are three categories of worm
drive designs that describe the degree
to which the gears mesh together:
non-throated (non-enveloping), single-
throated
(single-enveloping)
and
double-throated
(double-enveloping
or globoidal).
Figure 1. Non-throated
(non-enveloping)
Non-throated or non-enveloping is the
most basic design in which the worm
and worm wheel are both cylindrical in
shape. This allows for simplistic
manufacturing, but the limited contact
zone of a single point on one or two
gear teeth can become problematic.
In single-throated or single-enveloping
designs, one of the gear elements (most
commonly the worm wheel) has
concave helical teeth for contour or
envelopment of the gear teeth onto the
worm. This enables the contacting zone
to increase to a line.
Double-throated (double-enveloping)
or globoidal designs not only have
concave helical teeth on the worm
wheel, but the worm is also shaped like
an hourglass so the two gear elements
wrap around each other during motion.
This results in nearly eight times more
Figure 2. Single-throated
(single-enveloping)
Figure 3. Double-throated
(double-enveloping)
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