In a gearboxes operating at a speed above 15 metres per second or in the case of use of plain bearings, forced lubrication system (FLS) is necessary. Oil is directed onto teeth and bearing by jets or sprays or through oil pockets in the saddle for plain bearing. Forced lubrication is employed to enhance the thermal rating of the gear unit or to get hydrodynamic effect for plain bearings. Quantity of oil for lubrication is determined on the basis of frictional losses of bearings and toothmesh.

The FLS consists of the following sub-components:

1. Suction strainer : this is used to trap bigger (250 micron) foreign particles.
2. Gate valve : this is required to increase/decrease the flow of oil.
3. Pump motor set : this is of positive displacement type, mainly used to pressurize lubricating oil inside the gearbox.
4. Non-return valve : this prevents the flow of oil in the reverse direction when pump trips or when the oil pressure is very low.
5. Oil cooler : this is water-cooled shell - and tube-type heat exchanger
6. Filters : this is either cartridge-type or self-cleaning filter (150 to 250 micron)
7. Thermometers : dial-type for measurement of temperature of oil before its entry into and after exit from oil cooler.
8. Pressure gauge : this is bourdon tube-type diaphragm operated gauge with pressure range 0-7 kg/cm²
9. Pressure switch : This is mainly used for monitoring the pressure level in the delivery line of the gearbox by the way of releasing alarm in case of pressure drop.

The operation of two pumps is interlocked with each other as well as with the main motor to ensure trip-off in the event of low pressure. On request additional features like flow switch and RTDS for monitoring the bearing temperature can be supplied.