25.12.
Gearbox Lubrication
The moving parts in the gearbox are lubricated by partially filling the box with the correct grade oil through a level plug hole located on the side of the casing, until the oil starts to drain back out of the hole. The plug is then screwed on to prevent spilling out of oil during operation of the gearbox. The oil level submerges the layshaft or secondary-shaft cluster gears, so that the oil is dragged around with the gearwheel teeth when the gears revolve. This helps the oil to spread and flow between the individual gearwheels, output main shaft and primary shaft, dog-clutch assemblies, and support bearings. The selector mechanisms are lubricated by oil splashing up from the gar teeth (Fig. 25.28). A drain-hole and a screw plug are usually provided at the lowest point in the oil-bath casing, to drain used oil. Overfilling the gearbox creates a pumping action, which builds-up pressure within the box and eventually forces oil past the input shaft and output shaft oil seals. For smooth flow of oil between the shafts and the gears revolving relative to them, two or three holes are drilled radially in each gearwheel.
Heavy-duty commercial vehicles in special cases uses a forced-feed lubrication system in which a gear pump pressurises oil along an axial hole in both the primary and main shafts (Fig. 25.29).
Radial holes intersect this central hole and feed oil outwards between the shaft and the gears, both when engaged and when disengaged. It is not necessary to pressurize the lay-shaft and gear cluster.
In front-wheel-drive cars, a single oil supply is generally provided for both the gearbox and the final drive. The lubricant used must be the one recommended, and its required viscosity depends on whether it is to be used as a common oil for both the gearbox and the final drive crown wheel and pinion or just for the straight or helical gear teeth. In the former case higher-viscosity oil may be selected. On the other hand, in synchromesh gearboxes, thinner oil is generally preferred to provide sound and quiet gear changes.
