GB2142593A - Transmission for a four wheel drive vehicle - Google Patents

Abstract

The transmission includes a conventional gearbox (5) which drives the cage (36) of a differential gear (22) by way of a synchromesh-controlled simple epicyclic gear (32) to provide drive to the front drive shaft (40) and/or to the rear drive shaft (50). A three-position selector (70) is controlled to connect the front drive shaft (40) to the differential cage (36) (for two wheel drive to the front axle) or to the front quill shaft (52) of the differential (allowing normal central differential action for four wheel drive to both axles) or to both the differential gear cage (36) and the front quill shaft (52) (whereby the differential is locked and both front and rear axles are driven). The differential gear (22) may be planetary and the transmission could include a viscous coupling unit between the drive shafts. The gearbox (5) could be manual or automatic or a continuously-variable-ratio unit. The transmission is more compact and lighter than any other four wheel drive transmission and, when used in a front wheel drive vehicle, can be designed not to intrude into the passenger compartment. <IMAGE>

Description

SPECIFICATION Transmission for a four wheel drive vehicle The present invention relates to a transmission for use in a vehicle capable of being driven either by two wheels or by four wheels.

Previous transmissions which provided the vehicle with selectable two or four wheel drive, with the further option of locking a central differential gear, have been complex and bulky. it is an object of the invention to provide a transmission having all these options but which is capable of greater compactness and is simpler, cheaper and lighter to produce.

A transmission in accordance with the invention giving selective two- or four-wheel drive with lockable centre differential for use in a vehicle having an engine driving a main gearbox, and first and second axles, comprises: a central differential gear whose cage is adapted to be driven by the main gearbox; first and second quill shafts for conveying drive from the differential gear to the first and second axles respectively; and a three-position selector arranged selectively to couple a final drive shaft for the first axle either: (1) to the differential gear cage, for two-wheel drive to the first axle, or (2) to the first quill shaft, for four-wheel drive with central differential action, or (3) to both the differential gear cage and the first quill shaft, thereby locking the differential, for four-wheel drive without central differential action.The transmission preferably also includes means for automatically disconnecting the second axle when the vehicle is driven only by the first axle (two-wheel drive (1)).

In the preferred embodiment of the invention, the main gearbox of the vehicle has an output which is connected to drive the differential gear cage by way of a simple epicyclic gear and a synchromesh unit for controlling the selection of low and high gear ratio ranges. In the preferred form, for low ratios the drive from the gearbox passes to the sun of the simple epicyclic gear, while the annulus is held stationary, resulting in the drive being conveyed from the planet carrier to the differential cage at a predetermined ratio. For the high range of gears, the synch romesh unit instead couples the annulus to the planet carrier, locking the epicyclic and resulting in drive being conveyed from the sun to the differential cage at a ratio of one to one and causing the whole unit to rotate as one mass.

The transmission is preferably contained within one gear case with the main gearbox, for the greatest compactness. The transmission is especially useful for two-wheel drive to the front axle, in which case the first and second axles are respectively at the front and rear of the vehicle.

In order that the invention may be better understood a preferred embodiment will now be described with reference to the accompanying drawings, wherein: Figure 1 is a cross-section of lie transmission system of a vehicle, including a conventional two shaft gearbox (the lower shaft only being shown) and a transmission including a differential gear, a 3-position selector and a synchromesh unit in accordance with the preferred embodiment of the invention; and Figures 2, 3, and 4 are each schematic diagrams of the selector and differential gear of Figure 1, Figure 2 showing the selector in a first position for two-wheel drive, Figure 3 showing the selector in a second position for normal four-wheel drive, and Figure 4 showing the selector in a third position for fourwheel drive with locked central differential.

Referring now to Figure 1, a conventional twoshaft gearbox 5, of which the lower shaft only is shown, receives drive from an engine crank shaft (not shown). Drive from the first shaft of the gearbox, which is coupled directly to the crankshaft, is transmitted to the second shaft 20 and leaves the gearbox 5 by way of final drive gears 21. The transmission arrangement according to the preferred embodiment rotates around a third shaft 30 parallel to the gearbox shafts. The transmission essentially consists of a differential gear 22, a drive control selector 70, a simple epicyclic gear 32, and a gear ratio control synchromesh unit 60. The main gearbox 5 and transmission components are contained in the same gear case 10.

Drive from the gearbox 5 is transmitted by the final drive gears 21 to a tubular casing 31 permanently connected to a sun gear of the simple epicyclic gear 32. The simple epicyclic gear 32 includes at least two planet gears 33, a planet carrier 35 and an annulus 34. The gear ratio control synchromesh unit 60 includes an axially slidable annulus 61 and splined gears 62, 63. The selection of a low range of gear ratios moves the annulus 61 to the left in Figure 1.

This has the effect of locking the annulus 34 to the stationary gear 63. As a result, drive at a predetermined ratio, typically 2.5 to 1, is conveyed in the epicyclic unit 32 from the sun through the planets 33 to the planet carrier 35. The planet carrier 35 is permanently attached to rotate the cage 36 of the differential unit 22.

When the high range of gear ratios is selected, the annulus 61 of the synchromesh unit 60 locks the annulus 34 of the epicyclic gear 32 to the gear 62 which is connected by way of splines to the planetary carrier 35. This effectively locks the simple epicyclic gear 32 and results in drive at the ratio of 1 to 1 being conveyed from the tubular casing 31 to the planet carrier 35 which is connected to the cage 36 of the differential unit 22.

Thus drive at either the low or the high ratio range is conveyed from the gearbox 5 to the extended cage 36 of the differential gear 22.

It will be appreciated that in Figure 1 only the top half of the cross-section of the differential gear 22, simple epicyclic gear 32, selector 70 and synchromesh unit 60 is shown. Each unit is symmetrical about the axis 30.

Drive from the extended cage 36 of the differential gear 22 is transmitted selectively to a first drive shaft 40 and to a second drive shaft 50. In this example, the first drive shaft 40 drives the front axle through a further differential unit (not shown), and the second drive shaft 50 is connected to a rear axle differential unit (not shown). The complete transmission, including the gearbox 5, is so compact that when mounted at the front of a front wheel drive vehicle only the rear prop shaft (joined to driveshaft 50 via a universal joint) need lie beneath the passenger compartment of the vehicle. The vehicle may be designed so that none of the actual transmission extends into the passenger compartment.

A first quill shaft 52 of the differential gear 22 drives the first drive-shaft 40 through the drive control selector 70. A second quill shaft 51 of the differential gear 22 drives the second drive-shaft 50 through splines and a coupling flange. The differential gear 22 includes at least two bridging bevels 37 on one or more cross pins 38 mounted in the cage 36. The cage 36 extends as a tubular casing at one end into the drive control selector 70 and at the other end to be attached to the planetary carrier 35.

The selection of two or four-wheel drive or four-wheel drive with locked central differential is accomplished by the drive control selector 70, which will now be described in greater detail with reference to Figures 2,3 and 4. The three-position selector 70 includes an annulus71 slideable axially between first quill shaft 52 and the first drive shaft 40. In each of Figures 2,3 and 4 drive from the differential cage is indicated by an arrowed line D. Splines which are under load are indicated by heavy shading.

Figure 2 shows the selector 70 in position for two-wheel drive. Drive D is transmitted from the differential cage 36 through the annulus 71 to the drive shaft 40. The differential bevels, and both quill shafts 51,52, are free to rotate independently. In order that no power should be lost during two-wheel drive, in this case to the front axle via the first drive shaft 40, the rear axle is preferably automatically disconnected. This disconnection may be achieved by any known means, for example by an overrunning clutch which acts as a free wheel device only when the selector is in two-wheel drive mode.

Normal four-wheel drive is achieved with the drive control selector 70 in the position shown in Figure 3.

Normal differential action conveys drive to the second quill shaft 51 to drive the rear axle, and to the first quill shaft 52 to drive the front axle by way of the annulus 71 and first drive shaft 40.

Figure 4 illustrates a third position of the drive control selector 70 whereby the annulus 71 locks the differential cage 36 to the first quill shaft 52, effectively locking the differential. Thus drive is conveyed to both axles, but differential action between the axles is prevented by the lock.

Drive from the planet carrier of the simple epicyclic gear 32 to either axle is achieved in this referred example by means of a normal differential unit 22 with bevelled gears. The differential unit 22 could however be relaced by a planetary differential unit, using epicyclic gearing instead of bevelled gearing thereby enabling torque biasing between the axles to be incorporated. Moreover, the transmission has been described as taking drive from a two shaft gearbox 5, chosen for its compactness. The transmission of the invention could however be used with any gearbox, manual or automatic, or with a continuously-variable-ratio transmission unit, for example.

The transmission of this embodiment could op tionaliy include a viscous coupling unit between the two drive shafts. The advantages of such a unit in four-wheel drive vehicles are weli known; essentially, the viscous coupling unit resists sudden changes in the relative shaft speeds associated with the front and rear axles.

It is envisaged that the drive control synchromesh 70 and the gear ratio range control synchromesh 60 could be controlied separately either through manually controlled levers or by automatic means. Both units can be linked mechanically.

A transmission in accordance with the invention can be made more compact and iighter than any other four-wheel drive transmission. Moreover it can be made more efficient and quiet since there is no relative movement between transfer gears and low range gears when the normal high range mode is engaged. Moreover the driveline is low and lies on the centre line of the vehicle for both front and rear drives, thus allowing the rear prop shaft angles to be small and avoiding intrusion of the transmission into the vehicle compartment as described above. Due to its compactness, the arrangement does not create power unit bending problems often associated with front/rear wheel drive and high/low range configurations.

Claims (8)

1. A transmission giving selective two- or four- wheel drive with lockable centre differential for use in a vehicle having an engine driving a main gearbox, and first and second axles, comprising: a central differential gear whose cage is adapted to be driven by the main gearbox; first and second quill shafts for conveying drive from the differential gear to the first and second axles respectively; and a three-position selector arranged selectively to couple a final drive shaft for the first axle either: (1) to the differential gear cage, for two-wheel drive to the first axle, or (2) to the first quill shaft, for four-wheel drive with central differential action, or (3) to both the differential gear cage and the first quill shaft, thereby locking the differential, for four-wheel drive without central differential action.

2. A transmission in accordance with claim 1, further including means automatically disconnecting the second axle for two-wheel drive to the first axle.

3. A transmission in accordance with claim 1 or claim 2, further including a simple epicyclic gear adapted to be driven by the said main gearbox and conveying drive to the differential cage.

4. Atransmission in accordance with claim 3, further including a gear ratio range control synchromesh unit co-operating with the simple epicyclic gear to provide either a low range or a high range of gear ratios to the differential cage.

5. A transmission in accordance with any preceding claim, wherein the three-position selector comprises a geared annul us co-operating selectively with splines on the said final drive shaft for the first axle, on the first quill shaft, and on an extension of the differential gear cage.

6. A transmission in accordance with claim 5, wherein four-wheel drive without central differential action for the said annulus of the three-position selector locks the differential cage extension to the first quill shaft, both of which drive the final drive shaft through the said annulus.

7. A four-wheel drive vehicle comprising an engine, a gearbox transmission, front and rear axles and a transmission in accordance with any preceding claim connected to receive drive from the gearbox transmission.

8. A transmission substantially as described herein with reference to the accompanying drawings.