GEAR (GEARBOX) is a mechanical assembly transmitting energy of rotational motion and changing its parameters (torque, speed, direction) by means of toothed wheels and cylinders.
GEAR OIL is a fluid lubricant used in gears (gearboxes) for reduction of friction and wear of the gear tooth surfaces, removal of the heat generated by the operating gear and corrosion protection of the gear parts.
The following properties of gear oils are important for their operation:
- Proper viscosity;
- Ability to withstand extreme pressures (EP);
- Thermal and oxidation stability;
- Corrosion and rust protection;
- Compatibility with seal materials.
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, spilling, 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 oils should provide reliable, efficient (low friction), low maintenance operation of gears at different speeds, temperatures, oil contaminations.
SAE DESIGNATION OF GEAR OILS BY VISCOSITY
The Society of Automotive Engineers (SAE) established a viscosity grading system for gear and Engine oils.
According to the SAE viscosity grading system, all oils are divided into two classes:
Mono-grade Gear oils
Mono-grade gear oils are designated by one number (70, 90, 140, 250, etc.). The number indicates a level of the oil viscosity at a particular temperature. The higher the grade number, the higher the oil viscosity.
Viscosity of gear oils designated with a number only without the letter “W” (SAE 80, SAE 90, SAE 140 etc.) was specified at the temperature 212°F (100°C). These gear oils are suitable for use at high ambient temperatures
Viscosity of gear oils designated with a number followed by the letter “W” (SAE 70W, SAE 75W, SAE 80W etc.) was specified at the temperature 0°F (-18°C). The letter “W” means winter. These grades are used at low ambient temperatures.
Multi-grade Gear oils
Viscosity of gear oils may be stabilized by polymeric additives (viscosity index improvers). Viscosity of such gear oils is specified at both high and low temperature. These oils are called multi-grades and they are designated by two numbers and the letter “W” (SAE 75W-90, SAE 80W-90, SAE 85W-140 etc.).
The first number of the designation specifies the oil viscosity at cold temperature and the second number specifies the oil viscosity at high temperature. For example: SAE 85W-140 oil has a low temperature viscosity similar to that of SAE 85W, but it has a high temperature viscosity similar to that of SAE 140.
Multi-grade gear oils are used in a wide temperature range.
DESIGNATION OF GEAR OILS BY PERFORMANCE
American Petroleum Institute (API) established a performance grading system for gear oils (mostly automotive gear oils). According to the system, gear oils are designated by the letters GL (Gear Lubricant) followed by a number 1,2,3,4 & 5:
GL-1 gear oils have rust and oxidation protection effect but do not contain extra pressure (EP) additives. These oils are used in low load applications only.
GL-2 gear oils contain more additives than GL-1, but without EP effect. These are used in medium loaded worm gears.
GL-3 gear oils possess light EP effect & are used in non-hypoid gears.
GL-4 gear oils possess moderate EP effect & are widely used.
GL-5 gear oils possess high EP effect. These are used in hypoid and other highly loaded gears.