Don't Forget the Gear Oil!
Maintaining a vehicle requires the use of many lubricants, each specifically designed to perform a certain task or set of
tasks. The most common lubricant requiring routine attention from motorists is engine oil. Gear oil, on the other hand, is
often-times overlooked when it comes to scheduled maintenance.
Gear Oil Basics
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 also important.
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 motor oil.
However, they are the same viscosity. According to AMSOIL Technical Drivetrain Products Manager Kevin Dinwiddie, the
difference is in the additives.
"Motor oil has to combat byproduct chemicals from gasoline or diesel ignition and should contain additives such as
detergents and dispersants," said Dinwiddie. "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. AMSOIL uses 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
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 temperature
- 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
- Anti-foam agents - These are used to suppress the foaming tendency and dissipate
- 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.
|Worm Gear Set
Gear Reduction Boxes
Gear Design Dictates Lube Design
Gear designs vary depending on the requirements for rotation speed, degree of gear reduction and torque loading.
Transmissions commonly use spur gears, while hypoid gear designs are usually employed as the main gearing in differentials.
Common gear types include the following:
Spur (straight cut) gears are widely used in parallel shaft applications, such as transmissions, 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.
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 (differentials), they offer efficient
operation and are easy to manufacture. As with spur gears, they are limited due to their noisy operation at high speeds and
are not the top choice where load carrying capacity is a requirement.
Worm gear sets employ a specially-machined "worm" that conforms to the arc of the driven gear. This type of design increases
torque throughput, improves accuracy and extends operating life. Primarily used to transmit power through non-intersecting
shafts, this style of gear is frequently found in gear reduction boxes as it offers quiet operation and high ratios (as high
as 100:1). Downfalls with this type of gear set are its efficiency, high price per HP and low ratios (5:1 minimum).
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, hypoid gears are used to transmit power from the driveline to the axle
Planetary gear sets, such as those found in automatic transmissions, provide the different gear ratios needed to propel a
vehicle in the desired direction at the correct speed. Gear teeth remain in constant mesh, which allows gear changes to be
made without engaging or disengaging the gears, as is required in a manual transmission. Instead, clutches and bands are
used to either hold or release different members of the gear set to get the proper direction of rotation and/or gear
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. However, this style of gearing can produce thrust forces parallel to
the axis of the gear shaft. To minimize the effects, two helical gears with teeth opposite each other are utilized, which
helps to cancel the thrust out during operation.
Herringbone gears are an improvement over the double helical gear design. Both right and left hand cuts are used on the same
gear blank, which cancels out any thrust forces. Herringbone gears are capable of transmitting large amounts of horsepower
and are frequently used in power transmission systems.
The differences in gear design create the need for significantly different lubrication designs. For instance, hypoid
gears normally seen in automotive differentials require GL-5 concentration and performance of extreme pressure
"This is because of the spiral sliding action that hypoid gears have," said Dinwiddie.
In differential applications that utilize hypoid gears, AMSOIL typically recommends one of the following: SEVERE GEAR Synthetic Extreme Pressure 75W-90 (SVG),
SEVERE GEAR Synthetic Extreme Pressure75W-140 (SVO),
SAE 80W-90 Synthetic Gear Lube (AGL), Long Life Synthetic gear Lube SAE 75W-90 (FGR), or Long Life Synthetic Gear Lube SAE 80W-140 (FGO).
Most manual transmissions have helical gears and do not require GL-5 performance.
"The helical gear is almost a straight cut gear, but on an angle," said Dinwiddie. "There is spiral action and very
little sliding action, hence there is less need for extreme pressure additives."
GL-4 gear lubes have half the extreme pressure additives of GL-5 lubes.
In manual transmissions utilizing helical gears, AMSOIL typically recommends one of the following: Synthetic Manual Synchromesh Transmission Fluid (MTF)
or Synthetic Manual Transmission and Transaxle Gear Lube
AMSOIL Provides Gear Oil Options
AMSOIL carries drivetrain lubricants to meet nearly every application. The synthetic base stocks and top-quality additive
packages found in AMSOIL gear lubes and transmission fluids provide the ultimate in wear protection for cars, trucks
outboards, heavy-duty and racing applications.
AMSOIL Drivetrain and Transmission Fluid Index
For more information also see: