When used in a gearbox the lubricant provides two primary two benefits: to lubricate the teeth and to remove heat generated from the gear operation.   The lubricant is also often used for lubricating the various bearing found in the gearbox.   If the correct lubricant is selected for use in a gear system it will provide slip-free power transmission at high mechanical efficiency, with good reliability, low maintenance, and long life.

Surface fatigue occurs when the lubricant film on the gear wheel teeth is insufficient to protect the surfaces from the stress, resulting in pitting forming in the contact region.    Scuffing can result if the lubricant film is not maintained, with the consequent increase in temperature causing distress and wear of the material surface.

The lowest practical viscosity oil should be selected to minimise friction losses and churning. However for low speed units with high tooth loading higher viscosity oils may be necessary

The oil should be selected is thus a compromise. Gear oil generally includes additives to improve its load bearing properties (Extreme pressure additives), it viscosity (Viscosity Index improvers), corrosion inhibiters etc etc>

Selection of lubrication oils for gearbox application should only be done with guidance from the specialist suppliers.  The following notes are intended as background guidance.

For high speed gears at lower temperatures -10^o to 16^o C is low viscosity oil should be selected e.g ISO VG 46.   
For medium speed at lower centre distances (less than 200mm) a viscosity grade range of ISO VG 68 - ISO VG 100 is reasonable.   
For medium speed at lower centre distances (more than 200mm) a viscosity grade range of ISO VG 68 - ISO VG 220 is reasonable.   
For low speed gears at higher temperatures up to 52^o C ISO VG 150 - ISO VG 320.

For gears subject to high surface pressures and sliding then special grades of oil must be considered including EP grades, synthetics etc.

To meet the lubrication needs of modern enclosed industrial gear drives, a gear lubricant must possess the following key performance properties:

Rust and Oxidation-Inhibited Gear Lubricants
These lubricants are commonly referred to as R & O gear oils.   They are generally petroleum base oils or synthetic blend base oils with additive systems that protect against rust and oxidation.   In addition to rust and oxidation-inhibiting additives, some R & O gear oils contain minute amounts of antiwear additives.   The viscosity grades for R & O corresponds to the ISO viscosity grades 32 to 320. R & O gear oils perform well over a wide range of gear drive sizes and speeds in a temperature range of -15^oC to 121^oC.

Compounded Gear Lubricants
Compounded gear oils are a blend of petroleum base oils with rust and oxidation inhibitors, demulsibility additives and 3 percent to 10 percent fatty or synthetic fatty oils.   These gear oils are frequently used in worm gear drives to provide excellent lubricity and prevent sliding wear.   Compounded gear oils are limited to an upper operating temperature limit of 82^oC.   The oils are supplied in grades corresponding to ISO viscosity grades 460 to 1,000.

Extreme Pressure Gear Lubricants
These lubricants are commonly referred to as EP gear oils.   EP gear oils are petroleum based or synthetic blend based oils that contain special multi-purpose additive systems.   The additive systems contain rust and oxidation inhibitors, EP additives, demulsifiers, antifoam agents, and in some cases solid lubricants that are collodially suspended, such as molybdenum disulfide, borates or graphite.   The EP additive system, which includes sulfur-phosphorous, borates and sulfur-phosphorous-boron chemistries, provides a chemically protective film that protects against welding, scuffing and scoring of the gears during boundary lubrication conditions, which can occur at start-up, stopping and high shock loads.   The EP lubricant grades correspond to ISO viscosity grades 68 to 1,500.   EP gear oils perform well over a wide range of gear drive sizes and speeds in a temperature range of -15^oC to 121^oC .

Synthetic Gear Lubricants
Synthetic gear lubricants differ from petroleum base gear lubricants in that they are formulated using synthetic base fluids.   The most common types of synthetic base fluids used in the formulation of synthetic base gear oils include: polyalphaolefins (PAO), diesters, polyol esters and polyglycols.

Synthetic gear lubricants are used whenever petroleum base gear lubricants have reached their performance limit.  Synthetic lubricants are conventionally reserved for equipment subjected to extreme high or low temperatures, frequent high loading and extended service intervals to avoid both equipment and oil degredation.    Synthetic lubricants have a higher viscosity index (smaller viscosity changes with temperature variations), improved thermal and oxidation resistance and and better lubricity.   Each type of synthetic base fluid has different characteristics and some of them may have disadvantages such as compatibility with elastomers, paints, reactions in the presence of moisture and higher price.

Synthetic gear lubricants can also contain rust and corrosion inhibitors, EP additives, demulsifiers, antifoam agents and in some cases solid lubricants.   They can be supplied in grades which corresponds to ISO viscosity grades 32 to 6,800. synthetic gear lubricants over used at temperature ranges of -46^o up to 125^oC

There are three primary methods of lubricating gears

Grease Lubrication (0 to 6 m/s tangential gear speed )
Grease lubrication is suitable for any gear system that is open or enclosed, so long as it runs at low speed. The grease should have a suitable viscosity with good fluidity especially in a enclosed gear unit.   Grease is not suitable for high loads and continuous operation and there is virtually not cooling effect.  The must be sufficient grease to ensure the gear teeth are lubricated but an excess can result in viscous drag and power losses.

Splash Lubrication(4 to 15 m/s tangential gear speed )
Splash lubrication is the normal method for lubricating spur, helical, bevel and worm gears.  The gears simply dip into a bath of oil as the rotate.   Splash lubrication needs at least 3 m/s tangential speed gear speed to be effective.  It is important that provisions are made to ensure the teeth are not immersed in the bath such that excessive losses result from the oil being churned up.  The oil level should be monitored under static and dynamic conditions to ensure it is correct for the application

Spray Lubrication (above 12 m/s tangential gear speed )
For the higher speed units (10 to 20 m/s peripheral speed) engineered spray lubrication is genally provided using shaped nozzles with oil at a circulated pressure of about 0.7 barg. At higher speeds the system for directing the oil at the teeth needs to be carefully engineering to ensure the oil actually reaches the contacting surfaces as centrifugal forces and escaping air flow will tend to deflect the oil jet.

Worm drives are inefficient because the gears experience sliding rather than rolling contacts, leading to operating temperatures much higher than other gear types.   Spur gears normally operate at 28�C higher than ambient temperatures while worm gear temperatures typically rise 50�C over ambient. This temperature difference is required to aid in dispersing the friction heat generated.

Due to the sideway sliding motion in worm gears, it is difficult to maintain a hydrodynamic oil wedge.   This results in gears operating under boundary lubrication conditions.   Also, high operating temperatures that approach 88�C and higher usually require oils with an ISO VG of 460 and higher.   They also require good thermal and oxidative stability.   The types of oils used to lubricate worm gears are compounded mineral oils, EP mineral gear oils and synthetics.