|Energy is transmitted from a power source to
a terminal point, through gears that change speeds,
directions, and torque. Gear lubricants are formulated and
applied to prevent premature component failure, assure
reliable operation, reduce operating cost, and increase
service life. The important objectives accomplished by these
lubricants include: reduction of friction and wear,
corrosion prevention, reduction of operating noise,
improvement in heat transfer, and removal of foreign or wear
particles from the critical contact areas of the gear tooth
Gears vary greatly in their design and in
their lubrication requirements. Proper lubrication is
important to prevent premature wear of gear tooth surfaces.
When selecting a lubricant for any gear application the
following issues must be considered: type and materials of
gear, operating conditions, including rolling or sliding
speed, type of steady load and temperature, method of
lubricant application, environment and type of service.
Enclosed gears -- those encased in an oil-tight housing --
usually require an oil with various additives, depending on
the operating conditions. Rust, oxidation, and foam
inhibitors are common. Extreme pressure (EP) additives are
also used when loads are severe.
Worm gears are special because the action
between the worm and the mating bull gear is sliding rather
than the rolling action common in most gears. The sliding
action allows fluid film lubrication to take place. Another
significant difference is that worm gears are usually made
of dissimilar materials, which reduces the chance of galling
and reduces friction. EP additives usually are not required
for worm gears and may actually be detrimental to a bronze
worm gear. Lubrication can be improved by oiliness
In open gear applications, the lubricant
must resist being thrown off by centrifugal force or being
scraped off by the action of the gear teeth. A highly
adhesive lubricant is required for most open gear
applications. Most open gear lubricants are heavy oils,
asphalt-based compounds, or soft greases. Depending on the
service conditions, oxidation inhibitors or EP additives may
be added. Caution must be exercised when using adhesive
lubricants because they may attract and retain dust and
dirt, which can act as abrasives. To minimize damage, gears
should be periodically cleaned.
Failure and Lubrication
The most critical function provided by
lubricants is to minimize friction and wear to extend
equipment service life. Gear failures can be traced to
mechanical problems or lubricant failure. Lubricant-related
failures are usually traced to contamination, oil film
collapse, additive depletion and use of improper lubricant
for the application. The most common failures are due to
particle contamination of the lubricant. Dust particles are
highly abrasive and can penetrate through the oil film,
causing “plowing” wear or ridging on metal surfaces.
Water contamination can cause rust on working surfaces of
the gears and eventually destroy metal integrity. To prevent
premature failure, gear selection requires careful
consideration of the following: gear tooth geometry, tooth
action, tooth pressures, construction materials and surface
characteristics, lubricant characteristics and operating
|Gear Pitting and Lubrication
Pitting occurs when fatigue cracks are
initiated on the tooth surface or just below the surface.
Usually pits are the result of surface cracks caused by
metal-to-metal contact of asperities or defects due to low
lubricant film thickness. High-speed gears with smooth
surfaces and good film thickness may experience pitting due
to subsurface cracks. These cracks may start at inclusions
in the gear materials, which act as stress concentrators,
and propagate below and parallel to the tooth surface. Pits
are formed when these cracks break through the tooth surface
and cause material separation. When several pits join, a
larger pit (or spall) is formed. Another suspected cause of
pitting is hydrogen embrittlement of metal due to water
contamination of the lubricant. Pitting can also be caused
by foreign particle contamination of lubricant. These
particles create surface stress concentration points that
reduce lubricant film thickness and promote pitting. The
following guidelines should be observed to minimize the
onset of pitting in gear units:
Reduce contact stresses through load
reduction or by optimizing gear geometry.
Steel should be properly heat-treated to
high hardness. Carburizing is preferable.
Gear teeth should have smooth surfaces
produced by grinding or honing.
Use proper quantities of cool, clean and dry lubricant with the required viscosity.
Gear Lubricant Characteristics and Viscosity
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 provides a method
for verifying the required viscosity for a specific gear
based on the operating velocity:
V40 = lubricant kinematic viscosity at 40° C (105° F) (cSt)
V = pitch line velocity (ft/min) given by :
V = 0.262nd
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
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
Temperature: Ambient and operating
temperatures also determine the selection of gear
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
Open gears: In addition to the general
requirements, lubrication for open gears must meet the following
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
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.