A proper amount of gear lubrication is necessary for the satisfactory performance of gears and durability of gears. Inadequate lubrication becomes responsible for scoring failure of gears.
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Purpose of Gear Lubrication
Lubricants used in Gears
The open type of gears which are operating at very low pitch line velocities are lubricated by grease.
The transmission gears which are used in multi-speed gearboxes are lubricated by motor oils. In fact, we can say that they are lubricated with SAE motor oils. The motor oils contain strong additives.
The grades of different motor oils used are SAE 5W, SAE 10W, SAE 20, SAE 30, SAE 40 and SAE 50.
Straight Mineral Oils
Normally, straight mineral oils are used for medium-duty gears lubrication purposes. No additives are included in it.
Heavy duty gears are lubricated with the help of gear oils. These oils have stronger additives of the Extreme Pressure type. The SAE grades of different gear oils used are SAE 75W, SAE 80W, SAE 90, SAE 140, and SAE 250.
Modes of Gear Lubrication
As discussed above, grease is used as lubricant for very low pitch line velocities. Hence this type of lubrication is used when hydrodynamic action is not possible.
For low pitch line velocities, drip-feed lubrication under gravity can be used.
This type of lubrication is used for medium pitch line velocities. In this type of lubrication, a portion of gear (running gear) dips into oil sump and oil is splashed. This oil splash lubricates the gear pair.
Note that for spur and helical gears, the lower gear should be dipped by four to six times the module of that gear in the oil bath.
Spray or Jet Lubrication
For high pitch line velocities, pressurized spray lubrication is used.