Gear Lubricants

It is a big challenge to select a proper industrial gear lubricant. This is important in the long-term, smooth and an optimal gear performance. In their lubrication requirements, gears can vary greatly because of the design considerations. Gear lubricants ensures among others the following. Reduction of wear and friction, prevention of corrosion, reduction in the operating noise, improving the heat transfer etc.

Factors in Selecting the Right Gear Lubricants
There are innuberable factors that goes in the selection of an appropriate industrial gear lubricants for any application. These factors can be summarized in a table form.

Factors	Requirements
Gear Types	Spur and Bevel Helical and Spiral bevel Hypoid Worm Low slide and low speed Moderate slide, with moderate-high loading High slide and loading excessive sliding, with moderate-high loading
Loading	Highly loaded industrial gear drives needs extreme pressure gear lubricants
Surface finish	High viscosity oils-rougher surfaces Low viscosity oils-smoother surfaces
Power transmission	With increase in load viscosity must also be increased
Gear Speed	With high speed of the gear drive, lighter viscosity needs
Temperature	The lubricants selected should be based on the lowest/highest/ambient operating temperature

Viscosity of Gear Lubricants
Viscosity is the most critical feature of a lubricating oil. Viscosity is able to give the requisite thickness of the oil film. It is able to keep apart the mating surfaces of the gears and bearings on the hydrodynamic lubrication conditions. It also permits a proper flow of the lubricant for taking away the frictional heat from the stress points. It also takes along all the wear debris or contaminants. Further, the viscosity of the gear lubricant selected is vital to its overall load-carrying ability. It is not difficult to comprehend that higher the viscosity, higher would be the load-carrying contribution of the industrial gear lubricants. That is the reason why attention must be given in selecting a proper viscosity for the gear application. A too much heavy viscosity results in excessive heat generation, power losses, lower gearbox efficiency and an improper oil flow.

Gear oils ideally must have a minimum pour point of 5° C (9° F) which is less than the lowest expected temperature. A typical pour point of a mineral gear oil is -7° C (20° F). In cases of lower pour points synthetic gear oils that has pour points of -40° C (-40° F) can be the answer.

The following equation provides a technique to verify the necessary viscosity for a particular gear that is based on operating velocity:

V40 = 70/v^0.5
V40 = lubricant kinematic viscosity at 40° C (105° F) (Cst)
V = pitch line velocity (ft/min) given by:
V = 0.262nd
n = pinion speed in rev/min
d = pitch diameter (inches)
Cst = Centistoke

Lubricants for Open Gearing
Open gearing applications, in particular those that are associated with applications like ball mill or finishing mill, are tough applications that a lubricant has to challenge. There are numerous industry-wide standards and specifications for open gear lubricants. These has been developed by the American Gear Manufacturers Association (AGMA) and other different OEMs (original equipment manufacturers). However, The most popularly used is the one that has the class of AGMA 251.02 (This has now been incorporated into the AGMA 9005-D94 and AGMA 9005-E02) also popular is the U.S. Steel 226 and 236.

An open gear lubricant should fulfill the following properties:

Tackiness (cohesive/adhesive/properties) – A good adhesion towards gears.
Resistant to washout by water and spray-off.
Load-carrying capability- For protection against friction or wear.
Cushioning ability (vibration reduction).
Resistance towards fling-off.
Roots of the gear teeth to be free from any build up.

Types of Gear Lubricants
There can be the following types of Gear lubricants:

Asphaltic type
Semi fluid greases
Semi fluid grease cutbacks
Gel/polymer-thickened ones
High-viscosity synthetics
Rust and oxidation (R&O) inhibited oils
Extreme pressure (EP) gear oils
Compounded gear oils
Synthetic gear oils