SELECTION OF THE OIL

ENCLOSED SPUR, HELICAL AND BEVEL GEARS

In these gears the choice of oil mainly depends on the tooth loading and the pitch line speed. In general it can be said that higher the tensile strength of the gear material, higher will be tooth loading and greater the required oil viscosity. It is also generally true that higher the pitch line speed, lower will be the tooth loading and less is the required oil viscosity. It is therefore practicable to use oils of low viscosity for high speed gears, which is desirable from the point of view of cooling and frictional losses also. Fluid friction and the heat produced by it, increase as the gear speed and oil viscosity increase. Lower viscosity oils are also preferable because they give better separation of water and other insoluble contaminants and have less tendency to foam.

Straight mineral oils are suitable for most gears of this type. In the case of turbine gears and similar high speed units employing a pressure circulating system, the use of a turbine oil with high resistance to oxidation is generally advisable. In certain medium and low-speed applications, e.g. steel mill gears, where tooth loading may be abnormally high or where shock loads occur, straight mineral oils may allow a higher rate of wear than usual and fail to prevent some deterioration in the condition of the contact surfaces of the teeth. In such circumstances oils containing additives which confer additional load-carrying and anti wear properties, without having true E.P. properties, may be used with advantage. As a E.P. lubricants are not required for gears of this type, though they may be recommended for some designs, e.g. spiral bevel units, where conditions of tooth engagement are liable to be severe.

HYPOID GEARS

These are designed to transmit high power in proportion to their size. High degree of sliding, introduced due to offset of axles, along the line of contact between meshing teeth, coupled with the heavy loading generally associated with this type of gear makes lubrication conditions particularly severe and places an extreme requirement on the gear oil.

Straight mineral oils or oils containing relatively inactive additives of the anti wear type are inadequate and will generally allow severe scuffing to take place as these gears operate under boundary or mixed film conditions essentially all the time.

Hence lubricants containing active extreme pressure additives are invariably recommended for these gears.