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 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 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 engines.

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.

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 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 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, 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

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

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

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

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. 

Conclusion

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.