GB2176754A

GB2176754A - Two/four wheel vehicle transmission

Info

Publication number: GB2176754A
Application number: GB08615008A
Authority: GB
Inventors: Brian Tolan
Original assignee: Leyland Vehicles Ltd
Current assignee: Leyland Vehicles Ltd
Priority date: 1985-06-24
Filing date: 1986-06-19
Publication date: 1987-01-07
Also published as: GB8515880D0; GB8615008D0

Abstract

A vehicle transmission offering automatic selection of four wheel drive from two wheel drive comprises a differential of the type which only drives both output shafts when the ratio of their rotational speeds falls within a predetermined range around the value 1:1. The input of the differential is connected to the prime mover of the vehicle and the output shafts are connected to respective sets of wheels, at different drive reduction ratios such that in normal use with all wheels rotating at the same speed the output shafts do not rotate at speeds producing a ratio within the said range. In normal use, therefor, only one shaft will be driven. The wheels driven with two wheel drive are connected to the output shaft which normally rotates at the lesser speed, and thus if wheelslip occurs, that shaft will speed up until the speeds of the two shafts fall within the said ratio and both are driven, providing four wheel drive. <IMAGE>

Description

SPECIFICATION Vehicle transmission The present invention relates to a vehicle transmission. More particularly it relates to a vehicle transmission which changes automatically from two wheel to four wheel drive in dependence upon driving conditions.

There are known a number of systems providing manually selectable two or four wheel drive in vehicles. These however have the disadvantage that a drive inappropriate for given conditions may be engaged. For example four wheel drive may be engaged during normal road use, causing undue strain on the driveline components, or two wheel drive may be engaged in conditions where slipping will occur.

There have also recently been developed sophisticated systems which select between two and four wheel drive by using a computer to detect when wheel slip occurs and to engage four wheel drive in that situation. These systems, however, are costly and complex.

According to the present invention there is provided a vehicle transmission for a vehicle having first and second sets of wheels, one set being mounted to the front and the other set being mounted to the rear of the vehicle, the transmission comprising a first differential gear of the type which drives both output shafts only when the ratio of their rates of rotation falls within a predetermined range, the input shaft being connected to a prime mover of the vehicle and the respective output shafts being connected to drive the respective sets of wheels.

The normally driven set of wheels may be either the front set or the rear set.

In an embodiment of the invention the differential is of the type that drives both output shafts when their rotational speeds are approximately in the ratio 1:1, but otherwise drives only the slower rotating shaft. In this case the set of wheels which are normally not driven are connected to the differential by a gear train having a drive ratio between the wheels and the differential output shaft less than that of the corresponding gear train for the normally driven wheels. Normally, therefor, only one set of wheels will be driven, as one shaft will be rotating faster than the other. If the driven set of wheels start to slip, however, the speed of the shaft connected to the slipping wheels will increase until it matches that of the undriven wheels. At this point both shafts will start to be driven, automatically engaging four-wheel drive.Once wheel slip stops, the speed differential between the shafts will be re-established, and four wheel drive will again be disengaged.

Reference will now be made, by way of example, to the accompanying drawing which shows a schematic view of a transmission embodying the present invention.

The drawing shows a first driveshaft 1 which transmits power from an engine (not shown) to a transfer gearbox 2. The gearbox 2 drives rear wheels 3 through a rear drive shaft 4 and rear ring gear set 5. It is also connected to the front wheels 9 by a front drive shaft 6 and front ring gear set 7, but does not normally drive the front wheels. Incorporated into the gearbox 2 is a differential gear 8 of the type which drives only the slower rotating output shaft unless the rotation speeds of both shafts are nearly equal, when it drives both shafts. The reduction ratio of the front ring gear set 7 is higher than that of the rear ring gear set 5.

When travelling on good road sufarces the front wheels 9 and the rear wheels 3 will be rotating at substantially the same speed. The front drive shaft 6 will therefor be rotating faster than the rear drive shaft 4 because of the different ring gear ratios, and only the rear shaft will be driven. When travelling on poor surfaces, where the friction forces between the rear wheels 3 and the ground cannot sustain the traction forces which drive the vehicle, wheelslip will occur. This wheelslip will cause the rear drive shaft 4 to rotate faster; when it reaches synchronous speed with the front drive shaft 6 the differential gear 8 will start to drive both output shafts, effectively engaging four-wheel drive. At such time as traction is regained, the speed differential between the front and rear shafts will be re-established, and the vehicle will revert to two wheel drive.

When the vehicle is stationmary rear drive shaft 4 and front drive 6 are not rotating; this is also a synchronous condition, so differential gear 8 will potentially drive both output shafts together. This means that on initial move off from rest, the vehicle will always be in allwheel drive mode, until sufficient speed has built up to disconnect the drive to the front wheels 9.

Of course, if the reduction ratio of the rear ring gear set 5 is set higher than that of the front eing gear set 7 the vehicle will be predominantly front wheel drive with automatic reversiom to all-wheel drive when required.

1. A vehicle transmission for a vehicle having first and second sets of wheels, one set being mounted to the front and the other set being mounted to the rear of the vehicle, the transmission comprising a first differential gear of the type which drives both output shafts only when the ratio of their rates of rotation falls within a predetermined range, the input shaft being connected to a prime mover of the vehicle and the respective output shafts being connected to drive the respective sets of wheels.

2. A transmission as claimed in claim 1 wherein the said ratio is substantially 1:1 and

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Vehicle transmission The present invention relates to a vehicle transmission. More particularly it relates to a vehicle transmission which changes automatically from two wheel to four wheel drive in dependence upon driving conditions. There are known a number of systems providing manually selectable two or four wheel drive in vehicles. These however have the disadvantage that a drive inappropriate for given conditions may be engaged. For example four wheel drive may be engaged during normal road use, causing undue strain on the driveline components, or two wheel drive may be engaged in conditions where slipping will occur. There have also recently been developed sophisticated systems which select between two and four wheel drive by using a computer to detect when wheel slip occurs and to engage four wheel drive in that situation. These systems, however, are costly and complex. According to the present invention there is provided a vehicle transmission for a vehicle having first and second sets of wheels, one set being mounted to the front and the other set being mounted to the rear of the vehicle, the transmission comprising a first differential gear of the type which drives both output shafts only when the ratio of their rates of rotation falls within a predetermined range, the input shaft being connected to a prime mover of the vehicle and the respective output shafts being connected to drive the respective sets of wheels. The normally driven set of wheels may be either the front set or the rear set. In an embodiment of the invention the differential is of the type that drives both output shafts when their rotational speeds are approximately in the ratio 1:1, but otherwise drives only the slower rotating shaft. In this case the set of wheels which are normally not driven are connected to the differential by a gear train having a drive ratio between the wheels and the differential output shaft less than that of the corresponding gear train for the normally driven wheels. Normally, therefor, only one set of wheels will be driven, as one shaft will be rotating faster than the other. If the driven set of wheels start to slip, however, the speed of the shaft connected to the slipping wheels will increase until it matches that of the undriven wheels. At this point both shafts will start to be driven, automatically engaging four-wheel drive.Once wheel slip stops, the speed differential between the shafts will be re-established, and four wheel drive will again be disengaged. Reference will now be made, by way of example, to the accompanying drawing which shows a schematic view of a transmission embodying the present invention. The drawing shows a first driveshaft 1 which transmits power from an engine (not shown) to a transfer gearbox 2. The gearbox 2 drives rear wheels 3 through a rear drive shaft 4 and rear ring gear set 5. It is also connected to the front wheels 9 by a front drive shaft 6 and front ring gear set 7, but does not normally drive the front wheels. Incorporated into the gearbox 2 is a differential gear 8 of the type which drives only the slower rotating output shaft unless the rotation speeds of both shafts are nearly equal, when it drives both shafts. The reduction ratio of the front ring gear set 7 is higher than that of the rear ring gear set 5. When travelling on good road sufarces the front wheels 9 and the rear wheels 3 will be rotating at substantially the same speed. The front drive shaft 6 will therefor be rotating faster than the rear drive shaft 4 because of the different ring gear ratios, and only the rear shaft will be driven. When travelling on poor surfaces, where the friction forces between the rear wheels 3 and the ground cannot sustain the traction forces which drive the vehicle, wheelslip will occur. This wheelslip will cause the rear drive shaft 4 to rotate faster; when it reaches synchronous speed with the front drive shaft 6 the differential gear 8 will start to drive both output shafts, effectively engaging four-wheel drive. At such time as traction is regained, the speed differential between the front and rear shafts will be re-established, and the vehicle will revert to two wheel drive. When the vehicle is stationmary rear drive shaft 4 and front drive 6 are not rotating; this is also a synchronous condition, so differential gear 8 will potentially drive both output shafts together. This means that on initial move off from rest, the vehicle will always be in allwheel drive mode, until sufficient speed has built up to disconnect the drive to the front wheels 9. Of course, if the reduction ratio of the rear ring gear set 5 is set higher than that of the front eing gear set 7 the vehicle will be predominantly front wheel drive with automatic reversiom to all-wheel drive when required. CLAIMS

2. A transmission as claimed in claim 1 wherein the said ratio is substantially 1:1 and wherein the said sets of wheels are connected to their respective drive shafts at different drive reduction ratios.

3. A transmission as claimed in claim 2, wherein the drive reduction ratio of the front set of wheels is higher than that of the rear set of wheels.