Gear Oil Basics
by Kevin Dinwiddie
AMSOIL Drivetrain Specialist Kevin Dinwiddie is a veteran of 28 years in
the oil industry and a Certified Lubrication Specialist (CLS) by the
Society of Tribologists and Lubrication Engineers (STLE).
This article appeared in ASO Magazine, March 2007
High quality gear oils must lubricate, cool and protect geared systems.
They must also carry damaging wear debris away from contact zones and
muffle the sound of gear operation. Commonly used in differential gears
and standard transmission applications in commercial and passenger
vehicles, as well as a variety of industrial machinery, gear oils must
offer extreme temperature and pressure protection in order to prevent
wear, pitting, spalling, scoring, scuffing and other types of damage
that result in equipment failure and downtime. Protection against
oxidation, thermal degradation, rust, copper corrosion and foaming is
Gear Oil and Motor Oil Are Not the Same
Gear oil differs from motor oil. Most people assume that SAE 90 gear oil
is much thicker than SAE 40 or 50 grade motor oil. However, they are the
same viscosity. The difference is they are calculated by different
classifications, SAE gear lube and SAE engine oil. Another main
difference is the additives used to produce them.
Motor oil has to combat byproduct chemicals from gasoline or diesel
ignition and should contain additives such as detergents and
dispersants. Since an internal combustion engine has an oil pump and
lubricates the bearings with a hydrodynamic film, the need for extreme
pressure additives such as those used in gear oils does not exist in
Engine oils and gear oils both have anti-wear additives, and they both
must lubricate, cool and protect components, but gear oils are placed
under extreme amounts of pressure, creating a propensity for boundary
lubrication, a condition in which a full fluid lubricating film is not
present between two rubbing surfaces. For example, differentials in cars
and trucks have a ring and pinion hypoid gear set. A hypoid gear set can
experience boundary lubrication, pressures and sliding action that can
wipe most of the lubricant off the gears. To combat this extreme
environment, extreme pressure additives are incorporated into the oil.
Companies like AMSOIL use an extra treatment of extreme pressure
additives in its gear oils in order to reduce wear and extend the gear
and bearing life.
Because many of the components found in the drivetrain consist of
ferrous material, the lubricant is required to prevent rust and possible
corrosion to other materials. Rust and corrosion problems are not nearly
as prevalent in engines.
The many small and intricate components that make up gear sets found in
the drivetrain can be quite noisy and may be subjected to shock loading.
The viscosity and extreme pressure formulation of gear oil quiets gears
and dissipates shock loading.
The rotating motion of the gear sets also tends to churn the lubricant,
resulting in foaming. If a gear lube foams, the load carrying capacity
is significantly reduced because the air suspended within the oil is
compressible. For example, when the gear teeth come into contact with
each other any trapped air bubbles will compress, therefore reducing the
thickness of the separating oil film. In turn, this reduction could lead
to direct metal-to-metal contact between gear teeth and result in
accelerated wear. The gear oil must have the ability to dissipate this
entrapped air, insuring a sufficient lubricating film exists to protect
the gears from contact wear.
Typical Drivetrain Fluid Additives
Much like engine oil, the chemical compounds, or additives, added to
drivetrain base stocks either enhance existing properties or impart new
ones. Some of the additives that may be found in a drivetrain fluid
include the following:
• Extreme pressure and/or antiwear agents - These additives are used to
minimize component wear in boundary lubrication situations.
• Pour point depressants - This type of additive is used to improve low
• Rust and corrosion inhibitors - These are used to protect internal
• Oxidation inhibitors - These additives are used to reduce the
deteriorating effects of heat on the lubricant, increasing the
lubricant’s service life.
• Viscosity index improvers - These allow a lubricant to operate over a
broader temperature range.
• Anti-foam agents - These are used to suppress the foaming tendency and
dissipate entrapped air.
• Friction modifiers - The required degree of friction reduction can
vary significantly between differing pieces of equipment in drivetrain
applications. In some cases, friction modifiers may be required to
obtain the desired results.
Gear Design Dictates Lube Design
Gear designs vary depending on the requirements for rotation speed,
degree of gear reduction and torque loading. Manual transmissions
commonly use helical gears for forward speeds and spur gears for
reverse, differentials utilize hypoid ring and pinion gears gear designs
and side and spider bevel gears.
Helical gears differ from spur gears in that their teeth are not
parallel to the shaft axis; they are cut in a helix or angle around the
gear axis. During rotation, parts of several teeth may be in mesh at the
same time, which reduces some of the loading characteristics of the
standard spur gear and provides quiet gear operation.
Spur (straight cut) gears are widely used in parallel shaft
applications, due to their low cost and high efficiency. The design
allows the entire gear tooth to make contact with the tooth face at the
same instant. As a result, this type of gearing tends to be subjected to
high shock loading and uneven motion. Design limitations include
excessive noise and a significant amount of backlash during high-speed
operation. Because of these limitations modern transmissions normally
only use Spur gears for reverse.
Hypoid gear sets are a form of bevel gear, but offer improved efficiency
and higher ratios over traditional straight bevel gears. Commonly found
in axle differentials and used as the ring and pinion gears, hypoid
gears are used to transmit power from the driveline to the axle shafts.
Because of the spiral design and sliding action of Hypoid gears extreme
pressure additives are required.
Bevel gears (straight and spiral cut) transmit motion between shafts
that are at an angle to each other. Primarily found in various types of
industrial equipment, as well as some automotive applications such as
side and spider gears in differentials. Side and spider gears provide
differential speeds between the wheels and allow for smooth turning,
such as when turning a corner when the outside wheel turns farther than
the inside wheel.
The differences in gear design create the need for significantly
different lubrication designs, which is why manual transmissions
sometimes use much different lubrication than differentials. For
instance, hypoid gears normally seen in automotive differentials require
API GL-5 concentration and performance of extreme pressure additives
because of their spiral sliding action. For everyday driving API GL-5
performance and SAE 75W-90 viscosity is recommended. Heavy towing or
hauling may require the use of API GL-5, 75W-140 viscosity since
pressure between the ring and pinion gears are elevated.
As for manual transmission gearing, how they are set up and the service
factor dictates the use of many different oils. OEMs sometimes
recommend automatic transmission fluid such as MERCON or ATF+4,
specialty lubes such as synchromesh fluids and API GL-4, 75W-90
viscosity gear lube. The difference in GL-4 and GL-5 is that GL-4 gear
lubes have half the extreme pressure additives of GL-5. Because the
gear types in manual transmissions do not necessitate the use of GL-5
gear lube, GL-4 is the correct recommendation called for by most OEM’s
when gear lube is required.
In all cases synthetic oils and gear lubes provide better fluidity at
cold temperatures and higher oxidation resistance at elevated
temperatures. Synthetics also provide longer service intervals than
petroleum lubes. By recommending the correct synthetic lube for each
application, customers will see the difference and feel comfortable
about leaving their vehicle with you for future service.