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Fig. 7.23 Outboard epicyclic bevel gear two speed double reduction axle

Fig. 7.24 Two speed double reduction helical gear axle

drive. For example, with a single reduction final drive the gear reduction can be so chosen as to provide a high cruising speed on good roads with a five speed gearbox. Conversely, if the truck is to be used on hilly country or for off-road use then a double reduction axle may provide the necessary gear reduction.

Therefore, to enable the vehicle to operate effectively under both motorway cruising and town stopping and accelerating conditions without overloading or overspeeding and without having to have an eight, ten or twelve speed gearbox, a dual purpose two speed gear reduction may be built into the final drive axle.

Combining a high and low ratio in the same axle doubles the number of gears available from the standard gearbox. The low range of gears will then provide the maximum pulling power for heavy duty operations on rough roads, whereas the high range of gears allows maximum speed when conditions are favourable. From the wide range of gear ratios the driver can choose the exact combination to suit any conditions of load and road so that the engine will always operate at peak efficiency and near to its maximum torque speed band.

7.5.1 Two speed double reduction helical gear axle (Rockwell-Standard) (Fig. 7.24)

This two speed double reduction helical gear axle has a conventional crownwheel and bevel pinion single speed first reduction with a second stage speed reduction consisting of two pairs of adjacent pinion and wheel helical cut gears. These pinions mounted on the crownwheel support shaft act as intermediate gears linking the crownwheel to the differential cage final reduction wheel gears (Fig. 7.24).

Low ratio (Fig. 7.24) Low ratio is engaged when the central sliding dog clutch splined to the crownwheel shaft slides over the selected (left hand) low speed smaller pinion dog teeth. Power from the propellor shaft now flows to the bevel pinion where it is redirected at right angles to the crownwheel and shaft. From here it passes from the locked pinion gear and crownwheel to the final reduction wheel gear bolted to the differential cage. The drive is then divided via the differential cross-pin and planet pinions between both sun gears where it is transmitted finally to the half shafts and road wheels.

High ratio (Fig. 7.24) High ratio is engaged in a similar way as for low ratio but the central sliding dog clutch slides in the opposite direction (right hand) over the larger pinion dog teeth. The slightly larger pinion meshing with a correspondently smaller differential wheel gear produces a more direct second stage reduction and hence a higher overall final drive axle gear ratio.

7.5.2 Two speed epicyclic gear train axle (Eaton) (Fig. 7.25)

With this arrangement an epicyclic gear train is incorporated between the crownwheel and differential cage (Fig. 7.25).

High ratio (Fig. 7.25) When a high ratio is required, the engagement sleeve is moved outwards from the differential cage so that the dog teeth of both the sleeve and the fixed ring teeth disengage. At the same time the sun gear partially slides out of mesh with the planet pinions and into engagement with the outside pinion carrier internal dog teeth. Subsequently, the sun gear is free to rotate. In addition the planet pinions and carrier are locked to the sun gear, so that there can be no further relative motion within the epicyclic gear train (i.e. annulus, planet pinions, carrier, differential cage and sun gear). In other words, the crownwheel and differential cage are compelled to revolve as one so that the final drive second stage gear reduction is removed.

Low ratio (Fig. 7.25) When the engagement sleeve is moved inwards its dog clutch teeth engage with the stationary ring teeth and the sun gear is pushed fully in to mesh with the planet pinion low ratio that has been selected. Under these conditions, the input drive from the propellor shaft to the bevel pinion still rotates the crownwheel but now the sun gear is prevented from turning. Therefore the rotating crownwheel with its internal annulus ring gear revolving about the fixed sun gear makes the planet pinions rotate on their own axes (pins) and roll around the outside of the held sun gear. As a result of the planet pinions meshing with both the annulus and sun gear, and the crownwheel and annulus rotating while the sun gear is held stationary, the planetary pinions are forced to revolve on their pins which are mounted on one side of the differential cage. Thus the cage acts as a planet pinion carrier and in so doing is compelled to rotate at a slower rate relative to the annulus gear speed. Subsequently, the slower rotation of the differential cage relative to that of the crownwheel produces the second stage gear reduction of the final drive.

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