GB2151565A

GB2151565A - Vehicle transmission system

Info

Publication number: GB2151565A
Application number: GB08430134A
Authority: GB
Inventors: Robert Anthony Bryson
Original assignee: Vickers PLC
Current assignee: Vinters Ltd
Priority date: 1983-12-14
Filing date: 1984-11-29
Publication date: 1985-07-24
Also published as: GB8333278D0; GB2151565B; AU3655684A; AU560954B2; GB8430134D0

Abstract

A transmission system for vehicles having two or more axes, and particularly such vehicles to be used both on and off the road, includes mechanical drive means (12, 14) between the vehicle engine (10) and a first axle (4), said drive means (12, 14) being driven by the engine (10) but being disengageable therefrom. The system further includes a first hydrostatic drive means (16, 20) acting in parallel with the mechanical drive means (12, 14) between the engine (10) and the first axle (4), and further hydrostatic drive means (24, 26, 28, 30) between the engine (10) and the further axes. The system has two modes of operating providing: a) mechanical drive coupled with variable hydrostatic drive to the first axle (4) with the further hydrostatic drive means (24, 26, 28, 30) inoperative; b) hydrostatic drive from the engine (10) to all axes with the mechanical drive (12, 14) to the first axle (4) inoperative. <IMAGE>

Description

SPECIFICATION Improved vehicle transmission system This invention relates to transmission systems for vehicles and has particular though not exclusive application to such systems for incorporation on vehicles to be used both on and off the road.

Vehicles to be used to traverse rough terrain in addition to their normal use on the highway are often provided, in the case of fourwheeled vehicles, with drive to both axles and, in the case of six- or more- wheeled vehicles, with drive to all axles.

An all-mechanical drive to the various axles offers efficient performance but provides many disadvantages. Such an arrangement is mechanically complex requiring mechanical drive connections to all wheels, the provision for relative slip between the various axles and, consequently, complex differential arrangements.

The provision of hydrostatic drive transmissions in four- or more- wheel drive vehicles can overcome a number of the disadvantages inherent in all-mechanical system, particularly in that the hydraulic system obviates the necessity for complex mechanical couplings between the engine and the wheels by having hydraulically-operated motors close to the wheels, power being distributed from the engine to the wheels by hydraulic pipework.

Further advantages of such hydrostatic systems, making them particularly suited to offroad vehicles, are that the transmission ratio between the engine and the wheels is infinitely variable enabling maximum torque to be developed at minimum engine revolutions, the control of a hydraulic drive is easy with response being immediate and positive in both directions and high starting torque can be maintained over a wide engine speed range.

However hydrostatic drive systems have a number of disadvantages compared with conventional gearbox transmissions in that the efficiency of the transmission is generally lower particularly at high speeds, while the top speed of most hydrostatically driven vehicles is limited.

It would be desirable to be able to provide a vehicle transmission system which coupled the inherent advantages of a hydrostatic drive with the improved efficiency and higher speeds available from a mechanical drive.

According to the present invention there is provided a transmission system for a vehicle having at least two axles, the system comprising a disengageable mechanical drive means between the vehicle engine and a first axle, a first hydrostatic drive means between the vehicle engine and said first axle acting in parallel with said mechanical drive means, and further hydrostatic drive means between the vehicle engine and the or each further axle, said transmission system having a first mode of operation providing mechanical drive coupled with variable hydrostatic drive to the first axle, the hydrostic drive means to the or each other axle being inoperative, and a second mode of operation providing hydrostatic drive from the engine to all axles.

Preferably the first hydrostic drive means comprises a first hydrostatic pump driven by the vehicle engine, said pump driving a hydrostatic motor coupled to the first axle.

Conveniently the first hydrostatic pump is of the variable displacement type while the hydrostatic motor coupled to the first axle is of the fixed displacement type.

In one embodiment of the invention, the mechanical drive means and the hydrostatic motor drive the first axle by way of a differential arrangement. Alternatively, said differential arrangement could be replaced by an epicyclic gear train.

The mechanical drive means to the first axle is conveniently disengageable by the provision of a clutch between the engine and the first axle.

The further hydrostatic drive means may comprise, for the or each further axle, a pair of hydrostatic motors one closely adjacent to each wheel on the axle, and at least one further engine-driven hydrostatic pump coupled to, to drive said motors.

By way of example only, an embodiment of the invention will now be described in greater detail with reference to the accompanying drawing which shows, schematically, a transmission system according to the invention.

Transmission systems according to the invention have application to any wheeled vehicle to be used both on and off the road whether they be four- or more-wheel drive vehicles. The illustrated system is shown applied to a six-wheel drive vehicle having a pair of front wheels 2 either end of a front axle 4, a pair of intermediate wheels 6 and a pair of rear wheels 8. The vehicle engine is indicated generally at 10 and can provide a primary drive to the front axle 4 through a disengageable clutch/brake arrangement 1 2 and differential 14.

The system includes a hydrostatic drive arrangement to the front axle 4 which can be used in conjunction with said mechanical drive to said axle. The hydrostatic drive arrangement comprises a variable displacement pump 1 6 driven by the engine and coupled by hydraulic pipework 1 8 to a fixed displacement motor 20 which can provide drive to the axle 4 through the differential 14 by way of an input 22 to said differential 1 4.

The engine 10 also drives further variable displacement pumps 24,26 associated respectively with the intermediate wheels 6 and rear wheels 8. More particularly, the pump 24 is coupled by hydraulic pipework to a pair of wheel motors 28 which drive the wheels 6, while the pump 26 is similarly coupled to a pair of wheel motors 30 which drive the wheels 8.

The described transmission system has two primary modes of operation, one for on-theroad situations and the other for off-the-road performance. In the first mode of operation, for on-the-road performance where all-wheeldrive is not required, the clutch of the arrangement 1 2 is engaged and the engine drives the axle 4 mechanically through the differential 14. The hydrostatic drive to the axle by way of the motor 20 is used to supplement or reduce the mechanical drive as desired, the speed of the vehicle being determined by the sum of the mechanical and hydrostatic inputs to the differential 14-i.e.

a hydrostatic shunt to the mechanical drive is provided.

In the second mode of operation, which provides six-wheel drive for off-the-road situations, the engine 10 is declutched from the axle 4 with the mechanical input to said axle locked, while the pumps 16,24 and 26 are all powered by the engine to drive the associated motors 20,28 and 30 and therefore all six wheels 2,6,8. By suitable control of the displacement and direction of flow of the pumps 16.24 and 26, the vehicle can be driven hydrostatically in a forward or reverse direction at a desired speed.

Thus there is provided a transmission system which combines mechanical and hydrostatic drives in such a manner as to utilise the advantages of both these types of drive whilst at the same time eliminating their disadvantages. The mechanical drive is applicable only to a single axle thus eliminating the mechanical couplings to all but the wheels 2. The hydraulic motors 28,30 can be built into the wheels 6,8 thus further simplifying the design of the transmission and suspension, the only connections to the wheels being the hydraulic pipework.

The illustrated hydraulic circuitry is shown in its basic form. In order to prevent slip of the wheels 6,8, a flow divider (not shown) will be provided between each pair of wheel motors 28,30.

The provision of hydrostatic drive to all wheels when the vehicle is to be used crosscountry ensures adequate torque to all wheels even at low engine speed. Speed in this mode would typically be limited to about one-third of maximum on-road speed.

On roads and tracks, the vehicle would normally be powered by the mechanical shunt hydrostatic drive to the front axle 4 only. This enables some power to be transmitted via the mechanical drive line thus increasing the overall speed and efficiency of the hydrostatics, and, conversely, by varying the speed and direction of the hydrostatic drive, the overall ratio of the mechanical drive can be varied thus eliminating the need for a stepped mechanical drive. In this mode the motors fitted to the wheels 6,8 are free-wheeling.

At cruising speeds on the highway, the vehicle is propelled mainly by the engine driving the axle 4 with little transmission power loss, maximum speed being obtained with the aid of the hydrostatic drive.

If preferred, low on-the-highway speeds may be achieved using hydrostatic drive only to the axle 4.

Claims

1. A transmission system for a vehicle having at least two axles, the system comprising a disengageable mechanical drive means between the vehicle engine and a first axle, a first hydrostatic drive means between the vehicle engine and said first axle acting in parallel with said mechanical drive means, and further hydrostatic drive means between the vehicle engine and the or each further axle, said transmission system having a first mode of operation providing mechanical drive coupled with variable hydrostatic drive to the first axle, the hydrostatic drive means to the or each other axle being inoperative, and a second mode of operation providing hydrostatic drive from the engine to all axles.

2. A tranmission system as claimed in claim in which the first hydrostatic drive means comprises a first hydrostatic pump driven by the vehicle engine, said pump driving a hydrostatic motor coupled to the first axle.

3. A transmission system as claimed in claim 2 in which the first hydrostatic pump is of the variable displacement type while the hydrostatic motor coupled to the first axle is of the fixed displacement type.

4. A transmission system as claimed in any one of claims 1 to 3 in which the mechanical drive means and the hydrostatic motor drive the first axle by way of a differential arrangement.

5. A transmission system as claimed in any one of claims 1 to 3 in which the mechanical drive means and the hydrostatic motor drive the first axle by way of an epicyclic gear train.

6. A transmission system as claimed in any one of claims 1 to 5 in which the mechanical drive means to the first axle is disengageable by means of a clutch provided between the engine and the first axle.

7. A transmission system as claimed in any one of claims 1 to 6 in which the further hydrostatic drive means comprise, for the or each further axle, a pair of hydrostatic motors one closely adjacent to each wheel on the axle, and at least one further engine-driven hydrostatic pump coupled to, to drive said motors.

8. A transmission system for a vehicle having at least two axles, the system being substantially as described with reference to and as illustrated by the accompanying drawing.