Industvrial gears may be either of the enclosed or of the open type. The enclosed type 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 re-circulated either directly from the bottom of the gearbox or by the way of oil tank.
Splash lubrication is suitable where pitch line speeds are low, upto 5 m/sec for spur, helical and bevel gears and upto 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 splash lubrication to minimise 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 is generally applied to teeth by hand (brush). Alternatively dip shallow pool, automatic drip or spray methods (continuous or intermittent) can also be used.
The main functions of a gear lubricant are to reduce the friction and 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 hypoid 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 such 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 and forming deposits in consequence of conditions existing in the gearbox. In addition, fluid friction tends to be higher with 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 some 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 temperatures are high.
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Selection of the Oil
Enclosed Spur, Helical And
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 the 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 oil containing additives which confer additional load-carrying and anti wear properties, without having true extreme pressure properties, maybe used with advantage. As a rule extreme pressure 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.
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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.
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These are usually splash lubricated as these do not usually exceed a pitch line
velocity of 4 m/sec, but if they do so, spray lubrication is essential. Owing to almost pure sliding between the teeth, worm gears generally run at a higher temperature than other types.
As worm gear efficiency depends upon the operating temperature, increase in temperature is a limiting factor for the power transmitting capacity of the worm gear unit. It is necessary to reduce tooth friction as much as possible by the use of comparatively heavy oils of carefully selected type. However, in order to limit the temperature rise due to churning, the viscosity selected should be lowered with increasing speed of gears. Normally straight mineral oils are preferred but, under high loading conditions, compounded oils may be advantageous notwithstanding their inferior chemical stability.
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These are generally spur or bevel type and the lubricant is applied manually to the
A special type of lubricant with good adhesive properties is required to prevent its being flung off the teeth or being squeezed out.
These requirements are met by heavy, adhesive type, residual oils and greases. Heavy straight mineral residual oils need to be heated or thinned with solvent before they can be applied to the gears. The solvent evaporates after application, leaving the teeth coated. The viscosity of the oil alone gives good protection to the gears. For improved load carrying capacity, oils containing extreme pressure additives are also recommended. Sometimes straight mineral oils are also compounded with fatty material to give improved field strength and to provide adequate lubrication where water is present.
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Two sources of troubles directly related to lubrication in gear systems are high temperature and wear.
Excessive wear of teeth can result from misalignment or incorrect machining of the teeth; it may also result from the use of an oil of too low viscosity, which would not provide an adequate film of lubricant between the two contacting surfaces. In addition, oil starvation, or the pressure of abrasive impurities in the oil can cause high gear wear.
High temperatures may be due to any of the following causes :-
Cleanliness is an all important factor if efficient and trouble free lubrication of gears is to be achieved. Special attention should be given to the thorough cleaning of any gear system before initial operation. Therefore, the oil should be purified regularly and renewed when necessary.
- Use of an oil of too high viscosity.
- Excessive churning of the oil because of the level being too high (this applies to splash lubricated types).
- Insufficient supply of oil to the teeth (where a force feed circulating system is employed), usually caused by the choking of oil pipes or oil strainer, causing high metallic frictional heat.
- Inefficiency of the cooling system, if one is provided.
The range of lubricants for gear application marketed by Bharat Petroleum Corporation Limited suiting varied application is given below. For more information on the product click on the product name