Automotive Dictionary: Gearbox Oil

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Automotive Dictionary: Gearbox Oil

Throughout this site we use many technical terms, and given the breadth of readership our site enjoys, sometimes we are remiss and incorrectly assume everyone knows what we are referring to. For those that do not, here are some explanations of the technical terms use.
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Gearbox Oil

A four-speed all-synchromesh gearbox consists of more than 60 moving parts. All these components, from the mainshaft down to the small needle rollers, need lubrication. Gearbox lubrication is invariably a combination of oil bath and splash lubrication; the gearbox internals are partly immersed in oil and the moving parts are used to carry the oil to the exposed components. For many parts of the gearbox the lubrication process is quite straightforward. For instance the gearbox bearings carry a thin film of oil just as the engine bearings do. It is the gears themselves that present the most difficult lubrication problem. Lubrication processes can be divided into three groups - hydrodynamic, boundary, and extreme pressure. Depending on the type of gear and the geometry of the teeth, gears may induce any of these different states of lubrication. The selection of a particular gearbox oil is governed by the type of material used in the gears and the surface properties of the gear teeth themselves.

Above all the gearbox oil must prevent metal-to-metal contact between the gear teeth and the most satisfactory method of doing this is hydrodynamic lubrication. Helical gears, for instance, usually induce a state of hydrodynamic lubrication and in this case straight mineral oil or ordinary engine oil is perfectly adequate. Other types of gears, worm gears for example, place far more stress on the lubricant. Worm gears subject the lubricant to a sliding or wiping action. In this case hydrodynamic lubrication breaks down and boundary lubrication is relied on to maintain separation and slip. Certain additives - the oiliness agents - adhere to the surfaces of the gear teeth and form into chains, thus preventing metal-to-metal contact.

In many gearboxes the contact area between the gears is so small and so highly loaded that the oil temperature rises to a point where the oiliness agents melt. This would lead to metal-to-metal contact between the gear teeth and this is avoided by the use of extreme pressure lubricants. Extreme pressure (EP) oils contain compounds of sulphur, chlorine and phosphorus in solution. These compounds are in fact activated by high temperatures rather than the direct effect of pressure. When the temperature of the gear teeth surfaces rises, the EP additives form solid coatings of metallic chlorides, sulphides or phosphides on the gear teeth. The coatings have a good 'shear' characteristic and maintain clearance and slip. Extreme pressure lubricants usually have a high viscosity rating (normally 80 or 90) and they can be readily distinguished by their thick consistency and distinctive rich smell.

Some manufacturers specify ordinary high viscosity oil, not EP. The Fiat 128 for example uses SAE 90 oil. The use of high viscosity oils, however, can lead to problems with the lubrication of other parts of the gearbox such as the bearings. Many cars use ordinary 20W50 engine oil in the gearbox. The correct type of gearbox oil to use in a particular car can only be found by reference to the manufacturer's servicing instructions. In Leyland front-wheel drive vehicles such as the Mini and 1100, the gearbox and final drive gears share the same lubricating oil as the engine. Because of this helical spur gears are used to convey the power of the third-motion shaft of the gearbox to the differential and drive shafts. If these cars incorporated hypoid bevel gears, which would run much quieter, EP additives would be needed and this would be difficult to reconcile with the demands of engine lubrication. In other more modern front-wheel drive vehicles such as the Ford Fiesta, the gearbox is a separate component and there is less compromise in both lubrication and gearbox design.

Gearbox oil does not become contaminated in the way that engine oil does and therefore the change intervals may be longer. However the process of wear within the gearbox means that small metal particles accumulate within the oil, and it should be changed at recommended intervals and filled to the manufacturer's specified level.
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