GB2031531A

GB2031531A - A gearbox

Info

Publication number: GB2031531A
Application number: GB7903490A
Authority: GB
Original assignee: Rolls Royce Motors Ltd
Current assignee: Bentley Motors Ltd
Priority date: 1978-07-28
Filing date: 1979-02-01
Publication date: 1980-04-23
Also published as: GB2031531B

Abstract

A gearbox providing five forward and one reverse ratio is intended to be of reduced length and comprises input shaft 1, output shaft 2 and layshaft 3. The output shaft 2 carries five gears 15 to 19 one (19) of which is fixed to the output shaft and in constant mesh with one (21) of two gears 20,21 fixed to rotate with the layshaft 3 and the other four of which are arranged in two pairs 15,16 and 17,18 with two dog-toothed couplings 28,22 associated with respective pairs and each slidable into a position in which it locks one of the gears of the associated pair to the output shaft. The input shaft 1 has four gears 11 to 14 fixed to it meshing with the four gears on the output shaft. The other gear 20 on the layshaft is axially slidable and adopts one of three positions, a first position (as shown) in which it meshes with one (17) of the four gears on the output shaft to provide a forward ratio, a second position in which it meshes with a gear 14 on the input shaft to provide the reverse ratio and a third neutral position in all the other forward ratios of the box. <IMAGE>

Description

SPECIFICATION A gearbox The present invention relates to a gearbox.

The usual design of a four-speed gearbox employs four gears on each of the principal shafts, two slidable dog-toothed couplings to engage the four forward gears and a slidable reverse layshaftto provide reverse gear. This design is sound and attractive; though to provide the first ratio in a single reduction stage required high stresses and sparse contact conditions for the first ratio pair.

Afive-speed gearbox of similar design would employ five gears on each of the principal shafts and three slidable dog-toothed couplings; which would result in long principal shafts deflecting greatly when under load and a gearbox length which might be difficult to install.

According to the present invention, there is provided a gearbox providing a plurality of forward drive ratios comprising an input shaft, an output shaft and a layshaft, a plurality of gears on the input shaft, a plurality of corresponding gears on the output shaft; and two gears on the layshaft one of which is slidable between a first position in which it meshes with one of the gears fixed to the input shaft so as to provide a reverse drive ratio, a second position in which it meshes with one of the gears on the output shaft to provide an additional forward drive ratio the said one gear being freely rotatable with respect to the output shaft, and a third neutral position which it occupies when any other drive ratio is engaged.

A preferred embodiment of the invention may comprise any one or more of the following preferred features: (a) The other gear on the layshaft is in constant mesh with a gear on the output shaft.

(b) There are four gears on the output shaft in addition to the gear referred to in (a).

(c) There are four gears on the input shaft corresponding to the four gears on the output shaft mentioned in (b).

(d) The four gears on the output shaft of (b) are arranged in two pairs and two dog-tooth couplings are arranged between the gears of respective pairs and are slidable into position in which one or other of the gears of the pair can be fixed to rotate with the output shaft.

(e) The four gears on the input shaft are fixed to rotate with the input shafts.

(f) Six gears may be arranged in three pairs with three dog-toothed couplings.

In order that the invention may be more clearly understood, one embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 diagrammatically shows a gearbox with the various gearbox parts arranged for the second gear ratio, Figure 2 diagrammatically shows the gearbox of Figure 1 with the various gearbox parts arranged for the first gear ratio, Figure 3 diagrammaticaliy shows the gearbox of Figure 1 with the various gearbox parts arranged for the reverse gear ratio, Figure 4 is a diagrammatic representation of the relative positions of the main shafts of the gearbox of Figures 1 to 3, and Figure 5 diagrammatically shows another form of gearbox to that shown in Figures 1 to 4.

Referring to Figures 1 to 4, the gearbox, which is indirect, comprises an input shaft 1, output shaft 2 and intermediate layshaft 3. The input shaft 1 is supported in bearings 4 and 5, the output shaft is supported in bearings 6,7 and 8 and the intermediate shaft is supported in bearings 9 and 10. The input shaft 1 supports four gears 11,12,13 and 14 of differing diameters, the output shaft 2 supports five gears 15 to 19 of differing diameter and the layshaft 3 supports two gears 20 and 21 of differing diameters. Gear 20 is axially slidable along the layshaft 3. In the position shown in Figure 1 which is the second ratio position, gear 20 rotates with the layshaft 3 but does not mesh with any other gear.

Gears 19 and 21 on respective shafts 2 and 3 are in constant mesh and gears 13 and 17 on respective shafts 1 and 2 mesh. Gear 17 is coupled to the shaft 2 by means of a dog-tooth coupling 22 and rotates with it. Gear 13 is permanently coupled to the shaft 1 so as to rotate with it, as are gears 11, 12 and 14. The powerflowpath is thus as shown by the heavy black line 24 in Figure 1 of the drawings.

In the position shown in Figure 2 (first gear ratio selected), gear 20 is slid axially along the layshaft 3 into a position in which it meshes with gear 17. Gear 17 also meshes with gear 13 on the input shaft 1 but is freely rotatable with respect to shaft 2 and thus serves only to transmit drive to the layshaft 3. The power flowpath is thus as shown by the heavy black line 25 in Figure 2, originating from what is normally the second ratio gearwheel 13 on the input shaft 1, passing to the second ratio gearwheel 17 on the output shaft 2 (rotating freely on that shaft) and then to the gearbox output via the layshaft 3 and gears 21 and 19.

Figure 3 shows the gearbox of Figures 1 and 2 with the gears arranged for the reverse ratio. Gear 14 on input shaft 1 meshes with gear 20 on the layshaft 3 to drive this shaft and, through it and the constant meshing of gears 19 and 21, output shaft 2. Again the power flow path is denoted by a heavy black line 26 in Figure 3. The remaining gear ratios, are selected in normal manner by appropriate selection of the gears and operation of the dog-tooth coupling 22 already mentioned, and a second dog-tooth coupling 28 which can rotatably lock either the gear 15 or the gear 16 with respect to the output shaft 2.

Thus, with the above arrangement, an additional forward drive ratio is achieved turning a conventional four-speed gearbox into a five-speed gearbox without any of the attendant disadvantages as regards dimensions or complexity which would normally be associated with such a development.

Referring to Figure 4, which illustrates very diagrammatically in end elevational view, the relative disposition of the input, output and layshafts 1,2 and 3. The forces exerted on the second ratio gearwheel 17, when it is rotating freely on its shaft 2, by the gear wheels 13 and 20 with first ratio selected can be made to be substantially equal and opposite by suitable choice of gear wheel sizes. This almost eliminates the forces on the bearing of the wheel 17 which would otherwise be present, correspondingly reducing bearing wear. In Figure 4 the points of application of the forces applied by the gearwheels 13 and 20 on the gearwheel 17 are referenced P1 and P2 and the forces themselves, indicated by arrows, F1 and F2.

The actual spatial arrangements of the input, output and layshafts is shown in Figure 4 but these shafts are shown "laid out" with respect to one another in the other figures in order to simplify the description.

In a tracked vehicle the gearbox is required to transmit power for both propulsive effort and for steering effort. Furthermore the gearbox output shaft lies transversely across the vehicle and is thus restricted in length - so restricted that it would be difficult to produce afive-speed gearbox of normal design with adequate capacity for the power current- ly required. Shorter shafts, as can be achieved by the above described arrangement are thus crucial. The first ratio train and reverse train described earlier can be applied to a transversely-positioned gearbox incoporating input bevels for the drive from the engine together with a tracked-vehicle steering mechanism.It can be shown that, though the novel features of the invention are obtained only be observing suitable relationships between the axes of the three shafts and between certain gear radii, acceptable values for forward and reverse gear ratios can be achieved. The transmission shown in Figure 5 is specifically adapted for use with a tracked vehicle where steering of the vehicle is achieved by a differential drive to the vehicle tracks. In providing the various gear speed ratios therefor the arrangement is similar to the embodiment of Figure 1 but it differs from this arrangement in the provisions of those parts necessary to obtain the above mentioned differential drive. Thus, as with the embodiment of Figure 1, the arrangement comprises an input shaft 51, output shaft 53 and layshaft 53.The layshaft 53 supports two gears 520 and 521 of differing diameter one gear 520 of which is axially slidable along the layshaft 53 between first, second and neutral positions as already described. Output shaft 52 supports five gears 515 to 519. Input shaft 51 comprises two axially aligned component shafts 51A and 51 B coaxially surrounded by a sleeve 51C on which four gears 511,512,513 and 514 are fixedly mounted. The input to this sleeve 51C is through a shaft 550 driven by the engine through a clutch, not shown, and bevel gearing 551 and the output from the output shaft 52 is via annuli 571 and 572 to planet pinion carriers 573 and 574 of respective planetary gear trains 552 and 553 and shafts 567 and 568 to respective vehicle tracks (not shown).

Inside the sleeve 51 C is a planetary gear train indicated generally by the reference numeral 560.

This comprises two sun wheels 561 and 562 fixed to rotate with respective component shafts 51A and 51 B. These sun wheels 561 and 562 mesh with planet pinion 563 and 564 which also mesh with each other and are mounted on spindles 565 and 566 carried by a planet pinion carrier forming part of the sleeve 51C and referenced 51 CC.

In operation the planetary gear train 560 acts as a differential the sun wheels 561 and 562 being connected to the input by pinions 563 and 564 and planet pinion carrier 51CC and also through respective component shafts 51A and 51 B and respective spur gears 55, 56 to the sun wheels 569 and 570 of output planetary gear trains 552 and 553, which also connect to brake discs 575 and 576. In order to make shaft 567 rotate faster than 568 to steer the vehicle (the torques on the shafts are mechanically balanced through 560) the friction brake 576, and thereby the sun wheel 570, is slowed or stopped by application of the brake, the differential 560 causing an equal increase in the speed of the sun wheel 569, the speed of the annuli 571 and 572 remaining unchanged.

Similarly shaft 568 may be made to rotate faster than shaft 567 by braking the disc 575 and sunwheel 569 of gear train 552. This differential thus enables the vehicle to be steered through the tracks.

It is possible by providing an additional sliding dog clutch in the gearbox to selectively hold shaft 52 and annuli 571 and 572 stationary, thereby providing an additional low ratio in which the shafts 567 and 568 are driven entirely through the differential gearing 560. This may be a reverse or forward ratio according to whether gear trains 55 and 56 do or do not include an idler gear. If idler gears are included to produce an additional reverse ratio the effect of applying the brakes is reversed, application of brake 575 increasing instead of decreasing the speed of shaft 567.

Claims

1. A gearbox providing a plurality of forward drive ratios comprising an input shaft, an output shaft and a layshaft, a plurality of gears on the input shaft, a plurality of corresponding gears on the output shaft; and two gears on the layshaft one of which is slidable between a first position in which it meshes with one of the gears fixed to the input shaft so as to provide a reverse drive ratio, a second position in which it meshes with one of the gears on the output shaft to provide an additional forward drive ratio the said one gear being freely rotatable with respect to the output shaft, and a third neutral position which it occupies when any other drive ratio is engaged.

2. A gearbox as claimed in Claim 1, in which the other of the said two gears on the layshaft is in constant mesh with a gear fixed to the output shaft.

3. A gearbox as claimed in Claim 2, in which there are four gears on the output shaft in addition to the gear fixed to the output shaft which is in constant mesh with the other of the said two gears on the layshaft.

4. A gearbox as claimed in Claim 3, in which there are four gears on the input shaft corresponding to and in constant mesh with the four gears on the output shaft.

5. A gearbox as claimed in Claim 3 or 4, in which the four gears on the output shaft are arranged in two pairs and two dog-tooth couplings are arranged between the gears of respective pairs and are slidable into a position in which one or other of the gears of the pair can be fixed to rotate with the output shaft.

6. A gearbox as claimed in Claim 4, in which the four gears on the input shaft are fixed to rotate with the input shaft.

7. A gearbox as claimed in Claim 2, in which six gears additional to the gear on the output shaft which is in constant mesh with the other of the said two gears on the layshaft are arranged on the output shaft in three pairs with three dog-toothed couplings disposed between the gears of respective pairs and slidable into a position in which one or other ofthe gears of the pair can be fixed to rotate with the output shaft.

8. A gearbox as claimed in Claim 7, in which there are six gears on the input shaft corresponding to the six gears on the output shaft.

9. A gearbox as claimed in any preceding claim, in which the main gearbox axis is adapted to lie transversely to the front-rear axis of the vehicle in which the gear is to be fitted and the drive input to the input shaft is via bevel gearing.

10. A gearbox as claimed in Claim 9, in which the input shaft comrises two component shafts drivably connected together and to an input shaft sleeve via planetary gearing comprising two sun wheels fixedly connected to respective component shafts, two planet pinions meshing with each other and with respective sun wheels and mounted on a common spindle, the planet pinion carrier being formed by a part of the input shaft sleeve whereby to provide for differential speeds in the component shafts.

11. A gearbox substantially as hereinbefore de scribedwith reference to Figures 1 to 4 or to Figure 5 of the accompanying drawings.

12. Atracked vehicle comprising a gearbox as claimed in Claim 9 or 10, in which the output shaft is connected to respective tracks on opposite vehicle sides through respective planetary gear trains.