Gear oil is heavy oil, rated at 80 to 100 weight viscosity that is used in enclosed units containing gears. Industrial gear oils are multigrade lubricants containing sophisticated extreme pressure additives. The viscosity of the oil should be high enough to muffle gear teeth and protect against failure. The viscosity is selected based on the system's requirements. Viscosity is the most important property of gear oil. The rate of change of viscosity with temperature is called the viscosity index. The ability of oil to maintain a small viscosity differential over the operating range of gearbox provides a more consistent lubricating films to the gears and more predictable wear performance.
Gear oil performs a multitude of functions. They are designed to reduce friction and wear, act as heat transfer agents, and protect against corrosion and rust. They also contain additives to minimize oil oxidation, inhibit foaming and separate water readily.
There are several specifications used to define the physical characteristics and performance attributes of many gear oil lubricants. Few among these specifications are as follows:
SAE J306 : gear lubricants are defined in SAE J306. This standard was updated in July 1998.
API Category MT-1 : was issued in 1995, and describes the performance requirements of lubricants in manual transmission and lightly loaded final axle drives.
MIL-PRF-2105E : was issued in 1995, maintains all existing physical and chemical requirements, stationary axle test requirements and field test requirements.
There is now a worldwide acceptance of the International Organization for Standardization's (ISO) to establish viscosity measurements in centistokes (Cst) at 40° C and 100° C temperatures. AGMA have set up a numbering system to define gear oil viscosity. The optimum viscosity is the ideal viscosity at the operating temperature.
| Centistokes | - |
| 25 | Hydraulic systems |
| 30 | Plain bearings |
| 40 | Spur and helical gears (e.g. ISO- VG 150 @ 60° C) |
| 75 | Worm gears (e.g. 460 @ 75° C) |
Industrial gear oils are formulated to meet the operating requirements of the gear set. In enclosed gear set, high-speed gear oils are required with inhibited oils that contain rust and oxidation inhibitors and anti-wear agents. Worm gears require a blend of base oil with synthetic fatty oils to provide lubricity for sliding motion under heavy pressure. Open gear sets require high viscosity lubricant with extreme pressure and anti-wear additives. Most commonly used gear oils are 75W90, 80W90, and 85W140. Penzoil, evinrude, sierra, volva and duraplus are few of the gear oils used.
Synthetics and mineral-based oils are used in several industries. Synthetic gear oil is superior to mineral-based oil in many ways. Synthetic oils have high resistance to heat, higher film strength, and added lubricity. Synthetics prevent start-up engine wear and are pumped to critical surfaces and bearings much faster than mineral-based oils. On the other hand, with mineral-based oils small amount of wear occurs every time the car is started. These small amounts of wear then adds up to larger amounts, resulting in increase in clearance, reduced oil pressure, increased oil consumption and reduced life of engine.
The oil inside the differential runs relatively cool and doesn't get dirty, so it is capable of lasting the life of the differential. An inspection plug is usually located on the side or rear of the differential for checking the oil level inside. The level should be at or near the plug opening. If the oil level is low, the specific gear oil should be added until the level is reached.
Gears oils should be handled carefully and stored to guard against contamination, which can drastically reduce the performance and life of a lubricant. It should be stored in a place with moderate temperature away from industrial contamination. Labels, stencils and dispensing equipment should be kept legible to avoid incorrect application. Turbine oils, hydraulic oils and extreme pressure oils should use separate dispensing equipment. Care should be taken to avoid skin contact and inhalation of oil mists during use.