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 is very different 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 engines.”

Engine oils and gear oils both have anti-wear additives, they both must lubricate, cool and protect components, but gear oils are placed under extreme amounts of pressure, creating a propensity for boundary lubrication. 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 treat of extreme pressure additives in its gear oils in order to reduce wear and extend the gear and bearing life.

Additional Differences
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 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 stocks either enhance existing properties or impart new ones. Some of the additives that may be found in a drivetrain fluid include:

• 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 performance.

• Rust and corrosion inhibitors - These are used to protect internal components.

• Oxidation inhibitors - These additives are used to reduce the deteriorating effects of heat on the lubricant which will increase the lubricant’s service life.

• Viscosity index improvers - These allow a lubricant to operate over a broader temperature range.

• Anti-foam agents - The churning effects of gears running in oil causes a foaming effect in the oil. To reduce the harmful effects of air trapped in a lubricant, anti-foam additives speed the separation process.

• 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. Transmissions commonly use spur gears, while hypoid gear designs are usually employed as the main gearing in differentials. Common gear types include:

Spur
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 for 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
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
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 they offer 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
Hypoid gear sets are a form of bevel gears, 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 shafts.

Planetary
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 for 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 ratio.

Helical
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
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, gears normally seen in automotive differentials are hypoid gears and require GL-5 concentration and performance of extreme pressure additives.

“This is because of the spiral sliding action that hypoid gears have,” said Dinwiddie.

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 of the extreme pressure additives of GL-5 lubes.

AMSOIL Provides Gear Oil Options
AMSOIL carries drivetrain lubricants to meet nearly any 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, differentials, heavy-duty and racing applications.