MACHINERY LUBRICATION- INDIA SEPTEMBER-OCTOBER 2019 | Page 31

MLI
Photos courtesy Agnee Transmissions
POSITION Worm-under
(worm on the bottom) Worm-over
(worm on the top) Vertical
(worm on the side)
OIL LEVEL Wheel immersed at approximately
one-third of its diameter Wheel immersed to approximately the
center of the meshing zone Half the wheel immersed to at least
worm height
Figure 10. The three most common worm drive positions (Ref: The Lubrication Engineers Manual)
compatibility.
However,
both
compounded oils and EP gear oils have
a working temperature limitation of
approximately 80 degrees C before
oxidation rates rapidly increase,
resulting in acidic products that can
attack cupric worm wheel materials.
The Right Viscosity
Aside from the ambient and operating
temperature, the correct viscosity will
depend on several variables of the final
worm wheel, including the pitchline
velocity, center distance and revolutions
per minute. Figures 8 and 9 provide
recommendations for the ISO viscosity
grade selection on cylindrical and
double-enveloped
worm
drives
according to the American Gear
Manufacturers Association (AGMA)
9005-E02 standard.
As these recommendations and the oil
change interval chart show, temperature
has a significant impact on effective
lubrication. Not only are the lubricant
and machine longevity negatively
affected by higher temperatures, but
worm drives in particular have trouble
with temperature spikes. As a result, if
higher temperatures are expected, more
effective alternatives for base oils and
additives should be selected. Synthetic
oils such as PAOs and PAGs perform
better than mineral oils due to their
naturally higher resistance to thermal
degradation. Nevertheless, an increase
of 32 degrees C above the ambient
temperature in single-throated worm
drives (37 degrees C for double-throated
worm drives) is not considered excessive
for the operating conditions.
When the pitchline velocity of the worm
elements exceeds 10 meters per second,
particularly with double-enveloping
worm drives, a force-feed lubrication
system is recommended to spray the
entire face of the worm.
The Right Visual Inspections
Besides monitoring the oil level, a sight
glass should be regarded as a window
into the oil’s condition. This may include
visual checks for unusual oil darkening
(a sign of oxidation), visible sludge, solid
particles
and
moisture.
These
inspections can be performed more
efficiently when the sight glass is
extended out from the gear housing so
light can be passed through it, as in the
sight glass shown on the left.
The Right Oil Level
As with most splash-lubricated gear
systems, the oil level in a worm drive is
essential to maintain accuracy.
Depending on the position of the worm
relative to the worm wheel, a small drop
in oil level could be the difference
between ideal lubrication and no
lubrication. When monitoring the oil
level in the three most common worm
drive positions (Figure 10), adhere to
the manufacturer’s recommendations,
which will often be in line with the
standards for depth of oil immersion.
If possible, a bottom sediment and
water bowl should also be used. This
will help capture any solid particles or
liquids that are heavier than the oil and
provide a daily visual inspection point.
The Right Choice
The goal of any chosen lubricant should
be to protect the worm drive from
undesirable levels of friction, the
dangerous effects of corrosion and
inefficient operation. Assessing and
achieving the optimum reference state
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