What is Transfer Gearbox?
Transfer Gearbox is used for choosing between rear-wheel-drive and four-wheel-drive, as well as you can chose between a street and a off road transmission. Made with helical gears, an aluminium die cast gearbox and with a thoughtful shift linkage.
Construction of Transfer Gearbox
On vehicles where four wheel drive is employed; i.e. where the driving power can be applied to the front axle as well as the rear, it is necessary to transfer the drive from the main gear box to another gearbox known as the transfer gearbox. Three propeller shafts are fitted, one from the main to the transfer gearbox, one from the transfer gearbox to the rear axle, and the other from the transfer gearbox to the front axle.
(a) The transfer gearbox has four shafts. The main shaft driven by the propeller shaft from the main gearbox is splined and carries a toothed gear wheel which has dogs machined on its rear face. The gear-wheel can be moved along the splined shaft by selector mechanism to engage similar dogs machined on the front face of another gear wheel also mounted on the main shaft but not keyed to it. The sliding gear wheel can also be moved in the opposite direction, so that the teeth engage those of a large gear wheel fixed on the lay shaft (Idler shaft).
(b) The lay shaft carries two fixed gear wheels. A large one, which when engaged by the sliding gear wheel of the main shaft transmits the drive and gives a gear reduction. A smaller gear wheel at the rear end of the lay shaft is in constant mesh with a wheel on the main shaft and with one on the output shaft connected to the rear axle propeller shaft. Wheel on the main shaft is freely mounted, but the wheel on the output shaft is fixed on that shaft.
(c) There are two output shafts, the front one being connected to the front axle propeller shaft and the rear one to the rear axle propeller shaft. The front output shaft is splined and carries a sliding dog which can be moved along the splines by selector mechanism to engage dogs cut on the front face of the gear wheel fixed to the rear axle output. The rear end of the front output shaft is supported in spigot formed in the rear output shaft.
Operation of Transfer Gearbox
When the high gear and rear axle drive only is selected, the sliding gear wheel on the main shaft is moved along the splines and its dog engage those of the constant mesh gear that is freely mounted on the main shaft. The drive is then transmitted through the main shaft and constant mesh gears to the rear output shaft and there is no gear reduction. When the low gear and four wheel drive is selected the positions of the gear after as shown in image above. The sliding gear on the main shaft moves along the splines in a forward direction and its teeth engage those of the large gear wheel fixed to the lay shaft. At the same time the sliding dog on the output shaft connected to the front axle propeller shaft, is moved rear-wards to engage the dogs cut on the front face of the constant mesh wheel fixed to the output shaft which is in turn connected to the rear axle propeller shaft. Therefore, the driveline is from the main shaft to the lay shaft (with gear reduction) the constant mesh gears to both output shafts, so that the drive is conveyed to the front and rear axles.
Note: Transfer gearboxes are some time known as Auxiliary gearboxes. But although a transfer gear box is necessarily a transfer box. Gear changes in transfer gearboxes are affected by one, two, gear levers, and normal selector mechanism. Change of gear should be carried out when the vehicle is stationary.
General Defects and Their Causes
|Noisy operation in Neutral||Wear on constant mesh gear.|
Wear on primary shaft and lay shaft bearing.
|Noisy operation in one gear only||Worn gear in the ratio concerned.|
Loose fit gears or dogs
|Noisy operation in all gears||Worn teeth or bearings.|
Loose fit gears or dogs.
Lack of oil.
Wrong grade oil.
|Gears difficult to engage||Worn selector or fork.|
Selector shaft ball lock wrongly adjusted.
Poor bedding of synchro-meshing gear.
Synchro meshing pressure balls wrongly adjusted.
|Gear jump out of engagement||Weak selector shaft ball lock springs.|
Gear teeth or dogs worn.
Loose fit of gears on splines.