Front end splitter two speed gearbox power flow

(Fig. 3.23) Input power to the gearbox is supplied to the first motion shaft. When the splitter synchronizing sliding sleeve is in neutral, both the splitter low and high input gear wheels revolve on their needle bearings independently of their supporting first motion shaft and mainshaft respectively.

3.5.4 Twin counter shaft ten speed constant mesh gearbox with synchromesh two speed rear mounted range change (Fig. 3.24) With the quest for larger torque carrying capacity, closer steps between gear ratio changes, reduced gearbox length and weight, a unique approach to fulfil these requirements has been developed

Fig. 3.23 Sixteen speed synchromesh with range change and integral splitter gears
Fig. 3.24 Twin countershaft ten speed constant mesh gearbox with synchromesh two speed range change

adopting the two countershaft constant mesh gearbox which incorporates a synchromesh two speed rear mounted range change (Fig. 3.24).

The main gearbox is in fact a double stage compound conventional gearbox using two countershafts (layshaft) instead of the normal single arrangement.

Design and construction Referring to Fig. 3.24, there is a countershaft either side of the mainshaft and they are all in the same plane. What cannot be seen is that this single plane is inclined laterally at 19° to the horizontal to reduce the overall height of the gearbox.

The mainshaft is hollow and is allowed to float in the following manner: each end is counterbored, and into each counterbore is pressed a stabilizing rod. The front end of this rod projects into the rear of the input shaft which is also counterbored to house a supporting roller bearing for the stabilizer rod. The rear projecting stabilizer rod has a spherically shaped end which rests in a hole in the centre of a steel disc mounted inside the auxiliary drive gear immediately behind the mainshaft. This gear itself is carried by a ball bearing mounted in the gearbox housing. When torque is transmitted through the gearbox, the centrally waisted 11 mm diameter section of both stabilizers deflects until radial loads applied by the two countershaft gears to the mainshaft gear are equalized. By these means, the input torque is divided equally between the two countershafts and two diametrically opposite teeth on the mainshaft gear at any one time. Therefore, the face width of the gear teeth can be reduced by about 40% compared to gearboxes using single countershafts. Another feature of having a mainshaft which is relatively free to float in all radial directions is that it greatly reduces the dynamic loads on the gear teeth caused by small errors of tooth profile during manufacture. A maximum radial mainshaft float of about 0.75 mm has proved to be sufficient to permit the shaft to centralize and distribute the input torque equally between the two countershafts. To minimize end thrust, all the gears have straight spur teeth which run acceptably quietly due to the balanced loading of the gears.

Each of the five forward speeds and reverse are engaged by dog teeth clutches machined on both ends of the drive hubs. The ends of the external teeth on the drive hubs and the internal teeth in the mainshaft gears are chamfered at about 35° to provide some self-synchronizing action before engagement.

Power flow path Power flows into the main gearbox through the input first motion shaft and gear wheel. Here it is divided between the two first stage countershaft gears and is then conveyed via each countershaft gear wheel to the corresponding second stage mainshaft gears. Each of these rotate at relative speeds about the mainshaft. Torque is only transmitted to the mainshaft when the selected dog clutch drive hub is slid in to mesh with the desired gear dog teeth.

The power flow can then pass directly to the output shaft by engaging the synchromesh high range dog teeth. Conversely, a further gear reduction can be made by engaging the low range synchromesh dog teeth so that the power flow from the mainshaft auxiliary gear is split between the two auxiliary countershafts. The additional speed reduction is then obtained when the split power path comes together through the second stage auxiliary output gear. It should be observed that, unlike the mainshaft, the auxiliary gear reduction output shaft has no provision for radial float.

Reverse gear is obtained by incorporating an idle gear between the second stage countershaft reverse gears and the mainshaft reverse gear so that the mainshaft reverse gear is made to rotate in the opposite direction to all the other forward drive mainshaft gears.

3.6 Transfer box power take-off (PTO) (Fig. 3.25) A power take-off (PTO) provides some shaft drive and coupling to power specialized auxiliary equipment at a specified speed and power output. Power take-offs (PTOs) can be driven directly from the engine's timing gears, but it is more usual and practical to take the drive from some point off the gearbox. Typical power take-off applications are drives for hydraulic pumps, compressors, generators, hoists, derricks, capstain or cable winch platform elevators, extended ladders, hose reels, drain cleaning vehicles, tippers, road sweepers, snow plough blade and throwing operations and any other mechanical mechanism that needs a separate source of power drive output.

The power take-off can be driven either by one of the layshaft cluster gears, so that it is known as a side mounted transfer box, or it may be driven from the back end of the layshaft, in which case it is known as a rear mounted transfer box (Fig. 3.25).

Transfer boxes can either be single or two speed arrangements depending upon the intended application. The gear ratios of the transfer box are so chosen that output rotational speeds may be anything from 50 to 150% of the layshaft input speed.

Fig. 3.25 Six speed constant mesh gearbox illustrating different power take-off point arrangement

3.6.1 Side mounted single speed transfer box

With the single speed side mounted transfer box, the drive is conveyed to the output gear and shaft by means of an intermediate gear mounted on a splined idler shaft which is itself supported by two spaced out ball bearings (Fig. 3.25). Engagement of the transfer output shaft is obtained by sliding the intermediate straight toothed gear into mesh with both layshaft gear and output shaft gear by a selector fork mounted on a gear shift not shown.

3.6.2 Side mounted two speed transfer box

If a more versatile transfer power take-off is required, a two speed transfer box can be incorpor ated. With this gear train layout, the drive is conveyed to the intermediate shaft by a gear wheel which is in constant mesh with both the layshaft gear wheel and the high speed output gear (Fig. 3.25). The output shaft supports the high speed output gear which is free to revolve relative to it when the transfer drive is in neutral or low gear is engaged. Also attached to this shaft on splines is the low speed output gear.

High transfer gear ratio engagement is obtained by sliding the low speed output gear towards the high speed output gear until its internal splines mesh with the dog teeth on the side of the gear. This then transfers the drive from the layshaft to the output shaft and coupling through a simple single stage gear reduction.

Low transfer gear ratio engagement occurs when the low speed output gear is slid into mesh with the smaller intermediate shaft gear. The power flow then takes place through a double stage (compound) gear reduction.

Rear mounted two speed transfer box (Fig. 3.25) In some gearbox designs, or where the auxiliary equipment requires it, a rear mounted transfer box may be more convenient. This transfer drive arrangement uses either an extended monolayshaft or a short extension shaft attached by splines to the layshaft so that it protrudes out from the rear of the gearbox (Fig. 3.25). The extended layshaft supports a pair of high and low speed gears which are in permanent mesh with corresponding gears mounted on the output shaft.

When the transfer box is in neutral, the gears on the extended layshaft are free to revolve independently on this shaft. Engagement of either high or low gear ratios is achieved by sliding the output drive hub sleeve in to mesh with one or other sets of adjacent dog teeth forming part of the transfer box layshaft constant mesh gears. Thus high gear ratio power flow passes from the layshaft to the constant mesh high range gears to the output shaft and coupling. Conversely, low gear ratio power transmission goes from the layshaft through the low range gears to the output drive.

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