WO2003046409A1

WO2003046409A1 - Automotive transmissions

Info

Publication number: WO2003046409A1
Application number: PCT/GB2002/005292
Authority: WO
Inventors: Eric Alexander Pengilly
Original assignee: Worrall, Trevor, James
Priority date: 2001-11-26
Filing date: 2002-11-25
Publication date: 2003-06-05
Also published as: GB2384540B; AU2002343078A1; GB0128298D0; CN1615410A; GB2384540A

Abstract

An output section (10) of an automotive gearbox comprises an output gear (22) forming part of a final drive differential (23), and an input shaft (12) in driving engagement with an intermediate shaft (18) via a pair of bevel gears (14, 16). A torque sharing gear (20) is connected to the intermediate shaft (18). A pair of intermediate gears (28, 30) simultaneously transmit torque between the torque sharing gear (20) and the output gear, to provide parallel power transmission paths. The torque sharing gear (20) is movable to share torque between those paths. In preferred embodiments, the output section also provides selective motion reversing means.

Description

AUTOMOTIVE TRANSMISSIONS

This invention concerns automotive transmissions, especially those used in auto sport and other high performance applications, operating at high engine RPM. In certain preferred forms, the invention is also applicable to reversible drives.

A known gearbox for high performance autosport applications has close ratios with servo- operated gear shifts achieved without the use of a friction clutch or synchronisers. The transmission includes a friction clutch but this is used only when the vehicle is stationary. The known gearbox configuration gives a need for a high reduction ratio in the output portion, typically of about 6.25:1. This is achieved in two stages: firstly by a bevel reduction gear set driving an intermediate or transverse shaft, and finally by a spur reduction gear set, the output gear of which forms part of a differential drive to the rear wheels of the vehicle. As speed reduction does not take place solely at the final gear set, the intermediate shaft, associated gears, and their bearings are subjected to relatively high torque loads and must therefore be made large and heavy. The two stage reduction gearing also occupies a significant portion of the overall length of the gearbox.

The present invention seeks to improve upon the above gearbox configuration by the use of split torque gearing, and accordingly provides an output section of an automotive gearbox comprising an output gear forming part of a final drive differential, and an input shaft in driving engagement with an intermediate shaft via a pair of bevel gears, wherein a torque sharing gear is drivingly connected to the intermediate shaft, and a pair of intermediate gears simultaneously transmit torque between the torque sharing gear and the output gear to provide parallel power transmission paths. Preferably the torque sharing gear is movable to share torque between those paths.

The size of the output gear will be dictated by the axle torque and the space needed to accommodate the differential gear elements. By providing parallel torque transmission paths, tooth loads at the output gear and at the torque sharing gear are substantially halved. The torque sharing gear and output gear may therefore be made smaller, leading to a reduction in the gearbox size and weight. The ratio between the output gear and the torque sharing gear may also be made larger than that between the output gear and the co- operating spur gear of the prior art, so that substantially all of the speed reduction in the gearbox output section is achieved at the torque sharing/intermediate/output gear set. The intermediate shaft is therefore subjected to lower torque loads and can be smaller and lighter. The bevel gears may be simple mitre gears of the same size and module, simplifying manufacture. No heavy bearings are needed for these gears.

In preferred embodiments, the torque sharing gear is a spur gear which undergoes torque compensating movements in a radial direction, so as to be self-centring between the intermediate gears. There is no need to provide the intermediate shaft with bearings at its end in driving engagement with the torque sharing gear. Other known torque compensating movements are possible as appropriate, e.g. where the torque sharing gear has helical or double helical ("herringbone") teeth. Alternatively, torque may be shared sufficiently evenly between the two intermediate gears with fixed shafts by using very accurate manufacture and assembly.

Conventionally, automotive gearboxes are provided with a reverse mechanism comprising an idler gear with its own shaft and bearings, to transmit reverse motion between the input and output shafts in the ratio selection portion of the gearbox.

In a further preferred form of the invention, the bevel gear on the output section input shaft meshes simultaneously with a pair of bevel gears, these being selectively connectable in driving engagement with the intermediate shaft, e.g. by dog clutches, to provide reversible motion. The conventional reversing mechanism with its idler shaft and bearings is thereby eliminated. The two selectively engageable bevel gears (and advantageously also the co-operating gear on the input shaft) are of the same size and module, again simplifying manufacture. All three of these gears are preferably mitre gears. The intermediate shaft may be axially movable to effect the selective driving engagement with the bevel gears. Such movement may be servo, e.g. pneumatically, actuated, and linked to the same control system as used to select the gear ratios.

Variable ratio embodiments of the invention incorporating the reverse action have multiple forward and multiple reverse ratios and are capable of handling the same output torques and speeds in either output direction of motion. Such a gearbox configuration is suitable for twin cab rail vehicles, for example. In vehicles with a single driver's position an appropriate safety interlock may be provided so that reverse motion can only be selected in first gear (or perhaps the lowest few gears).

In yet other embodiments, the input bevel gear of the gearbox output section co-operates with a bevel gear connected to drive the sun gear of an epicyclic gear set. The sun gear or connected bevel gear is selectively engageable to drive the intermediate shaft in one direction, and a ring gear of the epicyclic set is selectively engageable to drive the intermediate shaft in the other direction at a lower ratio. Planet gears of the epicyclic set are fixed, e.g. journalled on the gearbox casing. Such a mechanism can of course supply very low ratios.

The invention and its further preferred features are described below with reference to illustrative embodiments shown in the drawings, in which:

Fig. 1 is a diagram of the output section of a prior art high performance gearbox such as is used in autosport;

Fig. 2 is a corresponding diagram of a first embodiment of the invention;

Fig. 3 is a view on arrow A in Fig. 2; Fig. 4 is a scrap section showing mounting of the intermediate shaft for torque sharing movement;

Fig. 5 is a scrap section showing an alternative torque sharing arrangement for use with helical gears;

Fig. 6 is a diagram corresponding to Fig. 2 but showing a second embodiment incorporating a reversing mechanism;

Fig. 7 is a scrap section showing details of the mechanism of Fig. 6;

Fig. 8 is a scrap section showing an alternative reversing mechanism for use with the invention;

Fig. 9 is a section of line IX-IX in Fig. 8, and Fig. 10 is a cross-sectional view of a further autosport gearbox, with certain parts rotated out of position for clarity. In various of these drawings, for simplicity the gear teeth have been omitted, with the gears concerned illustrated using their pitch circles.

Fig. 1 shows the general layout of the output section 10' of a known high performance gearbox as used for example in Formula 1 and Le Mans racing cars. An input shaft 12' carries a bevel gear 14', meshing with a somewhat larger diameter bevel gear 16' on an intermediate shaft 18', to provide a first speed reduction stage. The intermediate shaft 18' also carries a pinion spur gear 20', meshing with an output gear 22', transmitting power to half shafts 24', 26' via suitable differential and traction control means (not shown).

In the embodiment of the invention shown in Fig. 2, the layout of the gearbox output section 10 is somewhat similar, having an input shaft 12, intermediate shaft 18 and half shafts 24, 26. The following modifications are made with respect to Fig. 1. Firstly, the bevel gears 14, 16 are similarly sized. For example, they may be identical mitre gears, for simplicity of manufacture. All of the speed reduction is therefore provided between the spur gear 20 on the intermediate shaft 18 and the output gear 22. The shaft 18. is therefore subjected to lower torque loads and may be smaller and lighter than in the Fig. 1 arrangement. The bearings for gear 16 (34, Fig. 4) may also be smaller and lighter. The gear 20 needs no bearings, as further explained below.

Secondly, torque is transmitted from gear 20 to output gear 22 through two parallel paths, formed by further gears 28 and 30, Fig. 3, each meshing simultaneously with the gears 20 and 22. The relative sizes of the gears 20 and 22 are chosen to provide the required speed reduction ratio of the gearbox output section 10. The relative sizes and numbers of teeth for all four gears 20, 22, 28 and 30 are calculated (e.g. by numerical means) to permit assembly in simultaneous mesh. To permit even torque sharing through the mesh zones with each of gears 28 and 30, the axis of gear 20 is substantially co-planar with the axes of gears 28 and 30, but is permitted to move slightly in a direction normal to that plane, as indicated by the double headed arrow 32 in Figs. 2 and 3. Such movement compensates in known manner for manufacturing and assembly errors, which would otherwise give rise to uneven sharing of torque between the two available paths. The differential gearing is indicated schematically in Fig. 3 by dotted lines 23. Fig. 4 shows in a little more detail one possible mounting arrangement which permits such torque sharing movement of the gear 20. Gear 16 is journalled in bearings 34. Shaft 18 is held in driving engagement with gear 16 by crowned splines 36, permitting the shaft 18 to pivot slightly. The gear 20 is held centred between the gears 28 and 30 5 without the use of a bearing; the gears 28 and 30 being maintained in fixed phase by the gear 22. The shaft 18 is restrained against axial movement by suitable means (not shown).

Fig. 5 shows an alternative torque sharing arrangement between the gears 20 and 28 (the

10 gear 30 being omitted for simplicity, but still having its axis substantially co-planar with the axes of gears 20 and 28). This arrangement is for use with single helical gears. The compensating movement is provided by mounting gear 20 to shaft 18 via a torque transmitting sleeve 38. Gear 20 is formed as a ring gear having internal helical splines engaging complementary splines 40 on the sleeve 38. Internal helical splines on the

15 sleeve 38 in turn engage complementary splines 42 on the shaft 18. The splines 40, 42 are short and slightly crowned, to allow pivoting of the axes of shaft 18, sleeve 38 and gear 20 relative to each other. The lead and hand of the splines 40, 42 are equal to the lead and hand of the helical gear teeth 44. This ensures that the axial forces on the sleeve 38 and gear 20, arising from the transmitted torque, balance out. With this arrangement,

20 the gear 20 is not only free to pivot slightly out of the plane normal to the shaft 18, but can also translate slightly, both axially and normal to the plane defined by the axes of the gears 28, 30. Other known torque sharing arrangements are of course possible, such as disclosed in GB1434928, or as in EP0244263, for double helical or "herringbone" gears, or simply by accurate manufacture and assembly.

25

Fig. 6 shows a modification of the Fig. 2 embodiment, permitting reversible rotation (represented by double' headed arrow 46) of the half shafts 24, 26 for a given direction of motion (represented by arrow 48) of the input shaft 12. For this purpose, the input shaft gear 14 meshes simultaneously with a pair of identical bevel gears 16a, 16b. The shaft

30 18 is selectively clutchable to either gear 16a or 16b by a dog clutch or a similar selectable drive connection 50. The selective engagement of the dog clutch 50 may be effected by axial movement of the shaft 18, as indicated by double headed arrow 52. For example, the shaft 18 may be moved axially by a double acting pneumatic actuator, connected to it via suitable thrust bearings. For racing and sports car applications, the actuator control can be via a paddle on the steering column, also used for selection of forward gear ratios. A suitable interlock can be provided, to ensure that reverse gear can only be selected from neutral, with the variable ratio portion of the gearbox in first gear. 5 Fig. 7 is a half-sectional view with further details of one possible arrangement of the gears 16a, 16b, journalled in the gearbox casing 54. The dog clutch comprises involute stub teeth 50c on the shaft 18. The teeth 50c have conventional back taper, to ensure that under load they are held in engagement with co-operating stub teeth 50a, 50b on the gears

10 16a or 16b. As shown, the gear 20 is sufficiently broad to remain in mesh with the full width of gear 28, as the shaft 18 moves axially. Alternatively, gear 20 may be held axially fixed, with a splined connection or the like permitting relative sliding movement between the shaft 18 and gear 20 during operation of the dog clutch. The gears 20, 28 may then be of the same width.

15

Figs. 8 and 9 show an epicyclic reversing arrangement for use with the present invention, giving "forward" and "reverse" ratios which are different. Input shaft gear 14 (not shown) meshes with a single bevel gear 16c, carrying an integrally formed sun gear 56 of the epicyclic arrangement. Planet gears 58 are journalled on the gearbox casing 54, and

20 mesh with the sun gear 56 and with a rotating ring gear 60. The dog clutch teeth 50c on shaft 18 co-operate selectively with teeth 50e on the combination bevel/sun gear 16c/56 for "forward" motion, and with corresponding teeth 50d on the ring gear 60 for "reverse".

Fig. 10 shows a gearbox output section for a Formula 1 racing car. Like the Fig. 2 25 arrangement, this does not include a reversing mechanism, and has an input shaft 12 driving mitre gears 14 and 16. Gear 16 is carried on a hollow stub shaft 62 mounted in bearings 34a, 34b. Intermediate shaft 18 is held in driving engagement with gear 16 by crowned splines 36, retained by a washer 62 and circlip 64, permitting slight lateral movement of torque sharing gear 20. Gear 20 meshes simultaneously with a pair of 0 further gears, only one of which, 28a, is shown in Fig. 10. For clarity, gear 28a is shown rotated 90 degrees about shaft 18; in reality it and the other further gear will lie on opposite sides of gear 20, above and below the plane of the page as shown. The further gear 28a is rigidly fixed (e.g. welded) to a still further gear 28b, both journalled on a stub shaft 66 fixed in the gearbox casing 54. The other further gear (not shown) is similarly fixed to a still further gear. Both still further gears (28b and the one not shown) mesh simultaneously with an output gear (also not shown) similar to gear 22 in Figs. 2 and 3, to provide parallel power transmission paths. The relative tooth positions on gears 28a and 28b (or on the equivalent further and still further gears not shown) can be adjusted prior to fixing them together, to permit proper assembly of the further and still further gears in simultaneous mesh with the torque sharing gear 20 and the output gear respectively. Alternatively, the further and still further gears can be manufactured from a single, unitary blank, with the required angular offset between the sets of teeth for each gear. In Figures 2 and 3, the numbers of teeth on gears 20, 28, 22, 30 must be calculated to ensure proper meshing and assembly.

Claims

1. An output section of an automotive gearbox, comprising an output gear forming part of a final drive differential, and an input shaft in driving engagement with an mteπnediate shaft via a pair of bevel gears, wherein a torque sharing gear is drivingly connected to the intermediate shaft, and a pair of intermediate gears simultaneously transmit torque between the torque sharing gear and the output gear to provide parallel power transmission paths.

2. A gearbox output section as defined in claim 1 wherein the torque sharing gear is movable to share torque between those paths.

3. A gearbox output section as defined in claim 1 or 2 wherein the bevel gears are substantially the same size as each other.

4. A gearbox output section as defined in claim 3 wherein the bevel gears are mitre gears.

5. A gearbox output section as defined in any preceding claim, wherein the input shaft bevel gear meshes simultaneously with a pair of bevel gears which are selectively connectable in driving engagement with the intermediate shaft.

6. A gearbox output section as defined in claim 5, wherein the selectively connectable pair of bevel gears, and preferably the input shaft bevel gear are of the same size and module.

7. A gearbox output section as defined in claim 5 or 6, wherein the intermediate shaft is axially movable to effect the selective driving engagement with the bevel gears.

8. A gearbox output section as defined in any of claims 1-4, wherein the input shaft bevel gear co-operates with a bevel gear connected to drive the sun gear of an epicyclic gear set.

9. A gearbox output section as defined in claim 8, wherein the sun gear or connected bevel gear is selectively engageable to drive the inteπnediate shaft in one direction and a ring gear of the epicyclic set is selectively engageable to drive the intermediate shaft in the other direction.