Gear Oils - Fork & Bearing Oils

Gear oil is an automotive motor oil made specifically for use in the transmission systems including differentials in motor vehicles of all types, including cars, vans and trucks.

Gear olis are also used in other machinery where there is a need for a higher viscosity fluid to better protect the gears in the equipment, the stresses involved being very high. The high viscosity in the gear oil ensures transfer of lubricant throughout the whole gear train. This is necessary since the devices involved do not have pumps for transferring the oil upwards and along the gear train. With only a small portion of the lowermost gears bathed in the oil sump, if the oil was too thick it would not transfer and damage would occur.

However, this heavy oil can create viscous drag on the components, leading to inefficiencies and thus higher fuel consumption (in motor vehicles). Therefore modern automatic trans-axles (integrated transmission and differential) do not use a heavy gear oil at all, instead lubricating with low viscosity hydraulic fluid. Most lubricants for manual gearboxes and differentials are hypoid gear oils. These contain extreme pressure (EP) additives and antiwear additives to cope with the sliding action of hypoid bevel gears.

Gear Oil API Ratings

Gear oils produced today are classified by the American Petroleum Institute (API) using 'GL ratings'.

The gearboxes in most modern cars require a GL-4 oil, any separate differential requiring a heavier GL-5 oil. To make it slightly more difficult, purchasers are recommended to check any oil against the vehicle manufacturer's specifications, as it is possible that it could contain aggressive chemicals which could attack the metal gear components.

It should also be born in mind that API viscosity ratings for gear oils are not directly comparable with those for motor oils, plus they are thinner than the figures suggest.

This confusion is easy to see when you consider that some modern gearboxes use a 75W90 gear oil, which has the same viscosity as a 10W40 motor oil, but is different in other important ways.

Multigrade gear oils

Today Multigrade gear oils are becoming increasingly available as while gear oil does not reach the temperatures of motor engine oil, it does warm up as the car is driven, this mostly due to the shear friction involved.

Fully synthetic gear oils

Fully synthetic gear oils can also used in many vehicles, these having a greater resistance to the breakdown caused by the shearing forces endured in operations than mineral oils.

API Gear Oil Grades

API classification subdivides all transmission oils into 6 grades or classes:-

• API GL-1. These oils are suitable for light use only. They consist of base oils (i.e. no additives) only, although in some cases they can contain small amounts of some anti oxidizing additives, corrosion inhibitors, depressants and antifoam additives. Designed for spiral-bevel, worm gears and manual transmissions without synchronizers, these oils are used in trucks and farming machinery.
  
  
• API GL-2. An oil for moderate use conditions. These contain anti wear additives and are designed for use with worm gears. Many manufactures Recommend these (over API GL-1) to ensure proper lubrication of tractors and farm machinery transmissions.
  
  
• API GL-3. Another oil for moderate conditions, this variation contains up to 2.7% antiwear additives and are designed for lubricating bevel and other gears of truck transmissions, but are not recommended for hypoid gears.
  
  
• API GL-4. An oil suitable for a variety of conditions ranging from light to heavy. They contain up to 4.0% effective anti-scuffing additives. Designed for some bevel and hypoid gears, normally found in the gearboxes and axles of trucks.
  
  
• API GL-5. These oils are for severe use conditions. They contain up to 6.5% of effective anti scuffing additives. They are universal oils to many mechanical transmissions (but not gearboxes). Oils in this class, which have the special approval of vehicle manufacturers, are only used in synchronized manual gearboxes only.
  
  
• API GL-6. GL-6 oils for very heavy conditions only (significant shock loading). Containing a full 10% of high performance antiscuffing additives, they are designed for hypoid gear.
  
  However, Class API GL-6 is not used these days as the class of API GL-5 oils meets the most severest of requirements.

As you can see there is a lot to the selection of the right gear oils and we would advise that if you are unsure in this matter that you contact us so that our experts can ensure that you make the right choice of oil.

Base Oils

The building blocks of all lube (lubrication) oils are known as base oils. These base oils are normally a mixture of the various fractions from the crude oil refining process.

Additives are then added to these base oils so that all the required properties are present (i.e. it can do the job required).

Base oils are refined by solvent extraction (this normally being achieved using propane at a pressure high enough to keep the oil in liquid form), hydro-treatment (the reaction with hydrogen) also being involved.

These processes remove the unwanted heavy hydrocarbons and aromatics (benzene based chemicals) from the oil, making them suitable for use as a base oil.