32 Five speed and reverse synchromesh gearboxes

With even wider engine speed ranges (1000 to 6000 rev/min) higher car speeds (160km/h and more) and high speed motorways, it has become desirable, and in some cases essential, to increase the number of traditional four speed ratios to five, where the fifth gear, and sometimes also the fourth gear, is an overdrive ratio. The advantages of increasing the number of ratio steps are several; not only does the extra gear provide better acceleration response, but it enables the maximum engine rotational speed to be reduced whilst in top gear cruising, fuel

Table 3.1 Typical four and five speed gearbox gear ratios

Five speed box

Four speed box

Gear

Ratio

Gear

Ratio

top

0.8

top

1.0

4

1.0

3

1.3

3

1.4

2

2.1

2

2.0

1

3.4

1

3.5

R

3.5

R

3.5

consumption is improved and engine noise and wear are reduced. Typical gear ratios for both four and five speed gearboxes are as shown in Table 3.1.

The construction and operation of four speed gearboxes was dealt with in Vehicle and Engine Technology. The next section deals with five speed synchromesh gearboxes utilized for longitudinal and transverse mounted engines.

3.2.1 Five speed and reverse double stage synchromesh gearbox (Fig. 3.3) With this arrangement of a five speed double stage gearbox, the power input to the first motion shaft passes to the layshaft and gear cluster via the first stage pair of meshing gears. Rotary motion is therefore conveyed to all the second stage layshaft and mainshaft gears (Fig. 3.3). Because each pair of second stage gears has a different size combination, a whole range of gear ratios are provided. Each mainshaft gear (whilst in neutral) revolves on the mainshaft but at some relative speed to it. Therefore, to obtain output powerflow, the selected mainshaft gear has to be locked to the mainshaft. This then completes the flow path from the first motion shaft, first stage gears, second stage gears and finally to the mainshaft.

In this example the fifth gear is an overdrive gear so that to speed up the mainshaft output relative to the input to the first motion shaft, a large layshaft fifth gear wheel is chosen to mesh with a much smaller mainshaft gear.

For heavy duty operations, a forced feed lubrication system is provided by an internal gear crescent type oil pump driven from the rear end of the layshaft (Fig. 3.3). This pump draws oil from the base of the gearbox casing, pressurizes it and then forces it through a passage to the mainshaft. The oil is then transferred to the axial hole along the centre of the mainshaft by way of an annular passage formed between two nylon oil seals.

Lubrication to the mainshaft gears is obtained by radial branch holes which feed the rubbing surfaces of both mainshaft and gears.

3.2.2 Five speed and reverse single stage synchromesh gearbox (Fig. 3.4) This two shaft gearbox has only one gear reduction stage formed between pairs of different sized constant mesh gear wheels to provide a range of gear ratios. Since only one pair of gears mesh, compared to the two pairs necessary for the double stage gearbox, frictional losses are halved.

Power delivered to the input primary shaft can follow five different flow paths to the secondary shaft via first, second, third, fourth and fifth gear wheel pairs, but only one pair is permitted to transfer the drive from one shaft to another at any one time (Fig. 3.4).

The conventional double stage gearbox is designed with an input and output drive at either end of the box but a more convenient and compact arrangement with transaxle units where the final drive is integral to the gearbox is to have the input and output power flow provided at one end only of the gearbox.

In the neutral position, first and second output gear wheels will be driven by the corresponding gear wheels attached to the input primary shaft, but they will only be able to revolve about their own axis relative to the output secondary shaft. Third, fourth and fifth gear wheel pairs are driven by the output second shaft and are free to revolve only relative to the input primary shaft because they are not attached to this shaft but use it only as a supporting axis.

When selecting individual gear ratios, the appropriate synchronizing sliding sleeve is pushed towards and over the dog teeth forming part of the particular gear wheel required. Thus with first and second gear ratios, the power flow passes from the input primary shaft and constant mesh pairs of gears to the output secondary shaft via the first and second drive hub attached to this shaft. Gear engagement is completed by the synchronizing sleeve locking the selected output gear wheel to the output secondary shaft. Third, fourth and fifth gear ratios are selected when the third and fourth or fifth gear drive hub, fixed to the input primary shaft, is locked to the respective gear wheel dog clutch by sliding the synchronizing sleeve in to mesh with it. The power flow path is now transferred from the input primary shaft drive hub and selected pair of constant mesh gears to the output secondary shaft.

Power f low patti

Fig. 3.3 Five speed and reverse double stage synchromesh gearbox

Power f low patti

Fig. 3.3 Five speed and reverse double stage synchromesh gearbox

Transference of power from the gearbox output secondary shaft to the differential left and right hand drive shafts is achieved via the final drive pinion and gear wheel which also provide a permanent gear reduction (Fig. 3.4). Power then flows from the differential cage which supports the final drive gear wheel to the cross-pin and planet gears where it then divides between the two side sun gears and accordingly power passes to both stub drive shafts.

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