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Tuesday, September 15, 2009

Lube Notes: Gear Lubes and Synthetic Gear Oil

In this issue of Lube Notes, I will respond to the numerous questions I receive from month to month about gear lubes. From the questions I receive, I realize that folks are not really sure what gear lubes are or exactly why they are different from motor oils. I want to briefly introduce gear lubes and discuss their classification system. Then we will look at proper applications for gear lubes.

First, a little knowledge of gears is important in order to understand the function of gear lubes (please refer to Figure One on the next page for this discussion). Gears transmit motion and power from one rotating shaft to another rotating shaft providing multiple applications of power transmission. There are several types and various geometric shapes for gears but I will only address automotive applications. In Figure One, spur gears, bevel gears and hypoid gears are displayed; sun and planetary gears will be discussed with automatic transmission systems. Spur gears are simple gears with easily meshing gear teeth that transfer power between parallel shafts. Bevel gears allow intersecting shafts to transmit power. Hypoid gears facilitate the transfer of power between non-intersecting shafts at right angles. The important concept to grasp in these gear sets is the action of contact and sliding motion. The spur and bevel gears are engaging and rolling in motion whereas the pinion and ring in the hypoid gears are contacting and sliding. This sliding action allows the Hypoid gears to transmit greater power (the force is distributed over the sliding area), providing for smaller differentials in auto and truck applications.
Gear Types (graphic courtesy AMSOIL)

Design and Function of Petroleum/Synthetic Gear Oil

With this summary introduction to gears, we can now go forward with the design and function of gear lubes. Gear lubes must achieve the following:
  • Provide for proper shifting in manual transmissions at all temperatures
  • Maintain fluid separation of moving metal surfaces
  • Reduce friction and wear
  • Lubricate associated bearings
  • Prevent scoring of highly stressed gears
  • Provide fluid flow in cold temperatures
  • Remove heat during operation to maintain safe temperatures
  • Demulsify (separate from water)
  • Prevent Rust and Corrosion
  • Resist foaming and dissipate entrained air bubbles
  • Be compatible with all seals
Gear lube is a complex product accomplishing a multitude of functions and if any of these functions are ignored it can result in damage to the components.

Classification of Petroleum and Sythetic Gear Oil

The American Petroleum Institute (API) establishes the service classifications for transportation gear lubes as follows:
  • GL-1 through GL-3 designates gear lubes for light loads on spiral and bevel gears. This classification is usually satisfied by motor oil
  • GL-4 designates the type of service characteristics of gears, particularly hypoid, in passenger cars and other automotive equipment operated under high speed/ low torque and low speed/high torque conditions.
  • GL-5 designates the type of service characteristics of gears, particularly hypoid, in passenger cars and other automotive equipment operated under high speed/shock load, high speed/low torque, and low speed/high torque conditions.
The Society of Automotive Engineers (SAE) establishes the system to classify gear lubes by viscosity grades. The exact values for the viscosity grades are beyond the scope of this article, however, understanding the designation is important. Gear Lubes use a designation such as 75W-90 to indicate the viscosity grade. The 75W is the winter rating and it establishes the cold weather performance of the lubricant. The 90 is the operational viscosity (measured at 210ºF). The lower the W number, the better the lube oil functions in cold weather. The higher the second number, the more viscous (thicker) the oil.
Manufacturers stipulate the required GL and SAE classifications for a particular gear set. The recommendation is based on the geometry (spur, bevel, hypoid etc) as well as on the load and environmental conditions.
Automotive applications usually come down to transmissions and differentials. Materials used in the construction of the components like transmission synchronizers can require different types of additives in the gear lubes.
Gear Lubes are formulated by selecting base oils from petroleum or, in the case of sythetic gear oil lubes, synthetic oils and then adding in specific chemicals to achieve the rated classification. The additives consist of anti-wear, anti-foaming, anti-oxidation, demulsifiers, corrosion inhibitors, friction modifiers, viscosity improvers and extreme pressure additives and specialty additives where required. The specific formulations are determined by the manufacturer and all gear lubes are not created equal.
OEMs (Original Equipment Manufacturers) determine the required gear lubes for the gear sets in their manufactured products. You can find the specified gear lube listed in the owner’s manual and, in general, this is the gear lube you should use. Look for a viscosity rating, like 75W-90, and a classification, such as GL-4 or GL-5.
A classic error made by consumers is to buy thicker gear lube in order to get “better” protection. While there are times when using a 75W-140 or 75W-110 instead of the specified 75W-90 is appropriate, such a change should only be made on the advice of a lubrication professional. The failure rate for gears is very low but the failure rate for bearings is not so low. Bearings are better lubricated by thinner oils and so the change to a thicker lube in the gears increases the likelihood of bearing failures. Towing is a heavy duty use of the vehicle and, depending on the combined gross vehicle weight (including trailer), it may be prudent to use a heavier weight gear lube. The action of shifting transmission gears can hammer the gears in the differential when pulling heavy loads. Thicker gear lubes provide better shock absorption at the gear face and prevent spooning or cupping gear teeth. Some gear lube manufacturers have started to offer 75W-110 in order to provide better protection when pulling heavy loads without sacrificing as much fuel efficiency as 75W-140. If you plan to use your vehicle for heavy towing, my advice is don’t guess. Contact a lubrication specialist and get the recommendation that is right for your vehicle and your driving conditions.
The transmission is nothing like the differential and rarely will GL-5 gear lube be required. When there is a gear lube specification, it will routinely be for GL-4. GL-4 gear lubes contain less extreme pressure (EP) additives and are suitable for brass alloys. Some transmissions use brass synchronizers and the sulfur-based EP additives can react with brass and destroy the synchronizers. A great many manual transmissions are now using automatic transmission fluid or motor oil. Others even use certain synchromesh fluids developed by the manufacturers. The best policy is to stick with the recommendations of the transmission manufacturer.

Petroleum or Synthetic Gear Oil?

When selecting gear lubes for your vehicle you will have to choose between petroleum and sythetic gear oil. There is no argument in the lubrication profession over this question; synthetic gear oil is overwhelmingly superior in every category measured. Large trucking companies, like UPS for instance, use nothing but synthetic gear oil for better fuel economy and to extend equipment life.
In a future article I plan to provide an in-depth comparison between petroleum and synthetic gear oil.

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