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Gear
lubricant must possess the following characteristics:
Good viscosity is essential to
ensure cushioning and quiet operation. An oil viscosity that is too high will
result in excess friction and degradation of oil properties
associated with high oil operating
temperature. In cold climates gear lubricants should flow
easily at low temperature. Gear oils should
have a minimum pour point of 5° C (9° F) lower than the
lowest expected temperature. The pour point
for mineral gear oil is typically -7° C (20° F). When
lower pour points are required, synthetic gear oils with
pour points of -40° C (-40° F) may be necessary. The
following equation from the ASM Handbook provides a method
for verifying the required viscosity for a specific gear
based on the operating velocity:
Where:
where
n is the pinion speed in rev/min and d is the pitch diameter
(inches).
Film
strength: Good film strength helps prevent metal contact and
scoring between the gear teeth.
Lubricity
(oiliness): Lubricity is necessary to reduce friction.
Adhesion. Helps prevent loss of lubrication
due to throw-off associated with gravity or centrifugal force especially at
high speeds.
Gear speed. The now superseded Industrial
Gear Lubrication Standards, AGMA 250.04, used center
distance as the primary criterion for gear lubricant
selection. The new version of this standard, designated
AGMA 9005-D94 Industrial Gear Lubrication, has adopted pitch
line velocity as the primary selection
criterion. As noted above, gear speed is a factor in the
selection of proper oil viscosity. The pitch line
velocity determines the contact time between gear teeth.
High velocities are generally associated with light
loads and very short contact times. For these applications,
low-viscosity oils are usually adequate. In contrast,
low speeds are associated with high loads and long contact
times. These conditions require higher-viscosity
oils. EP additives may be required if the loads are very
high.
Temperature. Ambient and operating
temperatures also determine the selection of gear
lubricants. Normal
gear oil operating temperature ranges from 50 to 55° C (90
to 100 °F) above ambient. Oils operating at high
temperature require good viscosity and high resistance to
oxidation and foaming. Caution should
be exercised whenever abnormally high temperatures are
experienced. High operating temperatures are
indicative of oils that are too viscous for the application,
excess oil in the housing, or an overloaded condition.
All of these conditions should be investigated to determine
the cause and correct the condition. Oil
for gears operating at low ambient temperatures must be able
to flow easily and provide adequate viscosity. Therefore
these gear oils must possess high viscosity indices and low
pour points.
Open gears. In addition to the general
requirements, lubrication for open gears must meet the following
requirements:
Drip
resistance. Prevents loss of lubricant, especially at high
temperatures which reduce viscosity.
Brittle resistance. Lubricant must be
capable of resisting embrittlement, especially at very low temperatures.
Enclosed gears. In addition to the general
requirements, lubrication for enclosed gears must meet the following
requirements:
Chemical stability and oxidation resistance.
Prevents thickening and formation of varnish or sludge.
This requirement is especially significant in high-speed
gears because the oil is subjected to high operating
oil and air temperatures.
Extreme pressure protection. Provides
additional galling and welding protection for heavily loaded gears when the
lubricant film thickness fails. Extreme pressure lubricants
are available for mild and severe
(hypoid) lubricant applications.
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