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.
WORM GEARS
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.
OPEN GEARS
These are generally spur or bevel type and the lubricant is applied manually to the gear teeth.
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 EP 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.
OPERATING TROUBLES
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:
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.)
inefficiency of the cooking system, if one is provided.
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
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.
