MACHINERY LUBRICATION- INDIA SEPTEMBER-OCTOBER 2019 | Page 30

LESSONS IN LUBRICATION
oils generally results in improved gear
efficiency
and
lower
operating
temperatures. Figure 5 illustrates
lubricant life and oil change interval
expectations
for
polyalphaolefins
(PAOs), polyalkylene glycols (PAGs) and
mineral oils over a range of oil sump
temperatures. This is supported by the
Arrhenius Rate Rule, which states that
for every increase of 10 degrees C in the
average oil temperature, the chemical
reactions double.
The energy transmission efficiency of
the gear system’s input and output can
be significantly influenced by the
lubricant selected. Figure 6 specifies the
improved efficiency when choosing a
synthetic over a mineral oil, particularly
PAGs, which have an inherently low
coefficient of friction. PAGs are also
known
to
reduce
operating
temperatures
and
total
losses.
Additional
comparisons
between
mineral and PAG base oils are seen in
Figure 7.
PAGs do have some drawbacks, most
notably their higher costs. They also are
not compatible with some seal
materials, plastics and paint coatings,
so always confirm compatibility when
switching to PAGs.
The Right Additives
One of the most important jobs of a
gear oil additive is to form a protective
or sacrificial barrier between contacting
surfaces when the conditions exceed
that of the bulk oil’s film strength. An
additive package for a lubricant in a
worm drive must be selected with care,
since the yellow metals often contained
within worm wheels can be adversely
affected by corrosion from the activated
sulfur within the extreme-pressure (EP)
additive, particularly in the presence of
heat. Nevertheless, advancements in
additive formation with deactivated
sulfur have helped to reduce or eliminate
these corrosive attacks.
28 | September - October 2019 |
Worm drives can present a unique
boundary lubrication challenge, with
the focus more on friction reduction
than on the effects of wear. In these
applications, a specific type of mineral-
based
lubricant
known
as
a
compounded oil can be used. This
lubricant is formulated with up to 10
percent fatty acid (natural oil) or
acidless tallow as the compounding
agent along with rust and oxidation
inhibitors and other additives. This
results in improved lubricity, reduced
friction and decreased sliding wear.
EP oils are still commonly used in worm
drive applications where they are
formulated
with
yellow
metal
ISO VISCOSITY GRADES
PITCHLINE VELOCITY OF FINAL
REDUCTION STAGE
A Ambient temperature (°C)
-40 to -10 -10 to 10
10 to 55
Less than 2.25 m/s 220 460 680
More than 2.25 m/s 220 460 460
NOTES: Worm gear applications involving temperatures outside the limits shown above or
speeds exceeding 2,400 rpm or 10 m/s sliding velocity should be addressed by the manufac-
turer. In general, for higher speeds, a pressurized lubrication system is required along with
adjustments in the recommended viscosity grade.
This table applies to lubricants with a viscosity index of 100 or less. For lubricants with a
viscosity index greater than 100, wider temperature ranges may apply. Consult the lubricant
supplier.
Figure 8. ISO viscosity grade guidelines for enclosed cylindrical
worm gear drives
CENTER DISTANCE
OF FINAL
REDUCTION
STAGE
Up to 305 mm
More than
305 mm to
610 mm
More than
610 mm
ISO VISCOSITY GRADES
WORM SPEED
OF FINAL
REDUCTION
STAGE (RPM) -40 to -10 -10 to 10 10 to 35 35 to 55
<300 460 680 1000 1500
300-700 320 460 680 1000
>700 220 320 460 680
<300 460 680 1000 1500
300-500 320 460 680 1000
>500 220 320 460 680
<300 460 680 1000 1500
300-600 320 460 680 1000
>600 220 320 460 680
A Ambient temperature (°C)
NOTES: Worm gear applications involving temperatures outside the limits shown above or
speeds exceeding 2,400 rpm or 10 m/s sliding velocity should be addressed by the manufac-
turer. In general, for higher speeds, a pressurized lubrication system is required along with
adjustments in the recommended viscosity grade.
This table applies to lubricants with a viscosity index of 100 or less. For lubricants with a
viscosity index greater than 100, wider temperature ranges may apply. Consult the lubricant
supplier.
Figure 9. ISO viscosity grade guidelines for enclosed
globoidal worm gear drives
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