Industrial gears may be either of the enclosed or of the open types. The enclosed type may be lubricated by splash, in which case the oil level in the gear box is maintained so that the teeth of the bottom wheel just dips into the oil Alternatively a pressure circulating system may be used in which oil is sprayed on the teeth close to the point of engagement and is recirculated either directly from the bottom of the gearbox or by the way of separate oil tank.
Splash lubrication is suitable where pitch line speeds are low, up to 5 m /sec. for spur, helical & bevel gears and up to 4 m /sec. for worm gears. With splash lubricated gears it is most important that the oil level should not be too high otherwise excessive churning of oil will occur with consequent rise in oil temperature and power loss. The depth to which the bottom wheel should dip into the oil, when stationary, is generally between 2 cms. to 4 cms. depending upon the size of the gear. Usually twice the tooth depth is sufficient for the splash lubrication to minimize excessive churning. Where high powered gear sets running at high speeds are used, pressure circulating systems with oil coolers are preferred to reduce churning.
In the case of open gears the lubricant are to reduce the friction & wear by providing a lubricant film between working surfaces of meshing teeth and, in case of enclosed gears, to carry away the heat developed during tooth contact or meshing of gears.
To perform these functions properly, in cases where the conditions of tooth engagement are not abnormally severe and where straight mineral oils are suitable, the lubricant must be viscous enough to maintain the film and yet sufficiently free flowing to give adequate heat dissipation. These conflicting requirements are best met by oil as thin as is consistent with proper lubrication of teeth. It must be borne in mind that in most cases one oil is used not only for the gear teeth but also for the gear wheel and pinion bearings and that the bearings will tend to overheat with too thick an oil.
Where conditions of tooth engagement are particularly severe, as for example, in hybrid gears, the lubricant is unable by virtue of its viscosity alone to provide a film that can entirely prevent metal to metal contact between the teeth. In cha cases it is necessary to employ extreme pressure lubricants. These lubricants contain chemical substances which at relatively higher surface temperatures developed at the points of metallic contact react with the metal of the gear teeth at these points to form solid or semi-solid films possessing anti-welding properties.
The use of greases in enclosed gears should be avoided in all but exceptional cases, because of their tendency to 'channel' and leave the teeth dry, also because of the possibility of their separating out the forming deposits in consequence of conditions existing in the gearbox. In addition, fluid friction tend to be higher with a grease than with oil of suitable viscosity and heat dissipation with the former is very low because of its sluggishness. However, it may be necessary to use a grease for enclosed medium and low speed gears where the box is not oil tight. For enclosed gears the lubricant is required to perform satisfactorily over long periods and high quality oils of good stability towards oxidation are therefore required. This is particularly so where conditions are such that aeration of the oil is pronounced for example, in high speed gears or where operating temperature are high.
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.
