GB2230573A

GB2230573A - A four-wheel drive motor vehicle having bridged freewheel coupled to rear axle

Info

Publication number: GB2230573A
Application number: GB9005952A
Authority: GB
Inventors: Gerhard Bock
Original assignee: Volkswagen AG
Current assignee: Volkswagen AG
Priority date: 1989-03-20
Filing date: 1990-03-16
Publication date: 1990-10-24
Also published as: GB9005952D0; JPH02121336U

Abstract

A four-wheel-drive motor vehicle of the type having a front engine, in which the wheels of the front axle (1), Fig 1, are permanently driven, while the drive for the rear axle (3) wheels is either engageable or is establishable permanently by way of a lockable centre differential or automatically by means of a viscous clutch (5) disposed between the front and rear axles in the drive train (4), in such a way that the front (1) and rear (3) axles are coupled to one another substantially rigidly with respect to driving torque, comprises at least one freewheel device (6) having a freewheel lock in the torque-transmitting drive train (4) between the front and rear wheels, which device is such that, in the unlocked condition, the part (61) of the freewheel device coupled to the rear axle (3) drive can overtake the freewheel part (62) coupled to the front axle (1) drive when driving forwards. The freewheel device (6) is bridged by a spring-loaded friction device (6) such as a multi plate clutch, one end of which acts on the part (61) and the other end acts on the part (62). In Fig 2, the devices (6) and (8) are mounted in the rear axle and the clutch (5) is disposed at the input side of said axle torque introduced by shaft (10) is conveyed by bevel gears or angular transmission (11) and hollow shaft (14) to the housing of differential (12), the services (6) and (8) being connected between the shaft (14) and the housing of said differential. <IMAGE>

Description

DESCRIPTION A MOTOR VEHICLE HAVING ALL-çEEEL DRIVE The invention relates to a motor vehicle of the type having an all-wheel drive and a front-mounted engine. More specifically it concerns such a vehicle of the type in which the wheels of the front axle are permanently driven, whilst the drive for the wheels of the rear axle is engageable or is establishable permanently by way of a lockable centre differential or automatically establishable by means of a viscous clutch disposed between the front and rear axles in the drive train, in such a way that the front and rear axles are coupled to one another substantially rigidly with respect to driving torque, wherein at least one freewheel device having a freewheel lock, is provided in the torque-transmitting drive train between the front and rear wheels, which freewheel device is disposed and designed such that, in the unlocked condition, the part of the freewheel device coupled to the rear axle drive can overtake the freewheel part coupled to the front axle drive when driving forward.

A motor vehicle of this type is known, for example, from GB-A- 2,189,753. In this known motor vehicle, for example, at least one freewheel deivce, for example a clamping roller freewheel device, in the torque-transmitting drive train between the front and rear wheels, is disposed and designed in such a way that a reduction in rotational speed of the front wheels, for example by actuating the service brake, cannot be transmitted by way of the drive train to the rear wheels. As a result, it is reliably ensured that the travelling stability of the motor vehicle is not impaired, even in the event of overbraking of the front wheels, because any locking of the front wheels cannot, as a result of the freewheel device, act on the rear wheels.

In order to provide all-wheel drive even during reverse travel, the freewheel device is provided with a freewheel lock which can be locked either manually or automatically, for example on engaging the reverse gear.

The invention seeks further to improve the travelling behaviour of a motor vehicle of the type mentioned.

According to the present invention, the freewheel device is bridged by a spring-loaded friction device, one end of which acts on the freewheel part which is coupled to the rear axle drive, and the other end of which acts on the freewheel part which is coupled to the front axle drive.

The travelling behaviour of a motor vehicle of the type described, which is already very good, is improved still further when taking bends by the proposed friction device, which co-operates with the freewheel device. If deceleration takes place, for whatever reason, when taking a bend, that is if overrun operation occurs, the rear wheels are not, as previously, immediately completely decoupled by the freewheel device, but rather do they continue to participate in the overrun operation of the friction device, which is subjected to spring force, that is the thrust forces are transmitted not only exclusively by way of the front wheels, as in the known drive, but also to a certain extent by way of the rear wheels, with the result that the vehicle's stability on taking the bend is substantially maintained.

No deliberate switching or control operations or the like have to be carried out, since the friction deivce, which is subjected to spring force and which acts as it were as a "freewheel bridging brake", operates as a latent passive component and, given corresponding operating conditions, that is in the event, for example, of overrun operation occurring on deceleration, is directly effective.

In another known all-wheel-drive motor vehicle of the type described, however, in order to improve the travelling behaviour of the all-wheel-drive motor vehicle, the freewheel device is mechanically locked during driving, both in traction and overrun operation, and is not released until the service brake is actuated (e.g. actuation of the brake pedal) (DE-PS 37 08 193).

In this known all-wheel-drive motor vehicle, there is thus either total rigid bridging of the freewheel device (normal vehicle operation) or total floating or overrun of the freewheel device (actuation of the service brake), which is ideal for the desired improvement in travelling behaviour. It is, however, a disadvantage that additional expenditure on control is necessary to carry out this locking and unlocking of the freewheel device, and that the freewheel lock is comparatively heavily stressed because of comparatively frequent actuation, which can result in corresponding additional wear or necessitate larger dimensioning. Moreover, in conjunction with the frequent switching on and off of the freewheel lock there may, of course, also be certain noticeable intrusions to comfort, for example, in the form of switching or engaging/disengaging noises or slight jolts.It must also be ensured, through corresponding structural expenditure, that the freewheel lock of the freewheel device really does open quickly enough when the service brake is actuated in order to fulfil the actual task of the freewheel device, which is namely to ensure that, for example, if the front wheels become locked as a result of heavy emergency braking, this locking cannot be transmitted by way of the drive train to the rear wheels.

The invention will now be described by way of examples with reference to the accompanying drawings which show two embodiments of the invention partly schematically.

Fig.l shows the drive arrangement of an all-wheeldrive motor vehicle having a front engine 2, in which the wheels of the front axle 1 are permanently driven, whereas the drive for the wheels of the rear axle 3 is automatically established by means of a viscous clutch 5 in the drive train 4 between the front axle and the rear axle, in such a way that the front and rear axles are normally permanently coupled to one another substantially rigidly with respect to driving torque.

Such a viscous clutch is known to be able to transmit only small torques if only slight differences in rotational speed occur between its input and output shafts. It is, however, able to transmit high torques as soon as somewhat greater differences in rotational speed begin to occur between the input and output shafts. It is only when the permanently directly driven wheels of the front axle 1 rotate with a very small amount of slip as a result of particularly good road conditions, that is to say, the rotational speeds of the front and rear wheels are substantially equal, that the viscous clutch will transmit virtually no torque and the wheels of the rear axle 3 will consequently just freewheel. In all other road conditions, torque is transmitted by way of the viscous clutch 5 to the wheels of the rear axle 3.

The four wheels of the motor vehicle are hence generally permanently driven, the ratio of the force transmission to the front and rear axles being automatically adapted to prevailing road conditions.

Given such operating conditions, there is hence a substantially rigid coupling between the front and rear wheels, such that, for example, locking of the front wheels as a result of over-braking would also cause coresponding locking of the wheels of the rear axle 3 by way of the drive train 4, unless particular measures are taken to counteract this.

According to GB-A- 2,189,753, therefore, a device which can be coupled and decoupled in terms of torque, namely a freewheel device 6 having a freewheel lock, is provided in the torque-transmitting drive train 4 between the front axle 1 and the rear axle 3, which device may be in the form, for example, of a clamping roller freewheel device. In the embodiment of Fig.l, the freewheel function and the freewheel lock function of this known freewheel device are shown only schematically and are labelled 7 and 9 respectively.

It will be appreciated that, in contrast to the embodiment illustrated, the freewheel device 6 may be disposed at any other suitable point of the drive train 4 leading to the wheels of the rear axle 3, and this also applies to the viscous clutch 5.

According to the invention, the freewheel device 6 is bridged by a friction deivce, which is subjected to spring force and which is designated 8, one end of which acts on the freewheel part 61 which is coupled to the rear axle drive, and the other end acts on the freewheel part 62 which is coupled to the front axle drive. Preferably, a known friction coupling, which is subjected to spring force, of the multi-plate clutch type is used.

The friction torque of this friction device 8, which is subjected to spring force, is designed such that, on the one hand, there is negligible impairment of the freewheel function which is required on actuation of the service brake, but that, on the other hand, there is such a noticeable coupling torque that the front axle and rear axle are prevented from being completely and suddenly separated from one another on deceleration.

By using the freewheel device 6 with a parallel connected friction deivce 8, which is subjected to spring force and which acts so to speak as a bridging brake, it is ensured in a quite simple structural manner, without any switching measures or manual operatives, that on the one hand any locking of the front wheels on actuation of the service brake which may be due, for example, to panic (heavy emergency) braking, can never be transmitted - by way of the drive train 4 - to the rear wheels and that, on the other hand, in the event of deceleration there is always a certain coupling between the front and rear axles.

In this simple way it is thus ensured that the directional stability of the motor vehicle on braking (actuation of the service brake) is preserved, and that the travelling behaviour when taking a bend is, in comparison to vehicles without a friction device, substantially improved, in that the thrust forces occurring on deceleration are transmitted not only by way of the front wheels, but, within the scope of the adjusted friction torque of the friction device 8, also by way of the rear wheels.

Preferably, the friction torque of the friction device 8 is dimensioned such that it is around 10 to 20% of the maximum torque to be transmitted by the freewheel device during normal vehicle operation (tractive operation).

In Fig.l, the drive half-shafts leading to the wheels of the rear axle 3 are designated 13, the rear axle differential is designated 12 and the shaft part leading from the freewheel device 6 to the rear axle differential 12 is designated 10.

In the embodiment of the invention shown in Fig.2, the freewheel device 6 is mounted with the spring loaded friction device 8 according to the invention, in the rear axle transmission of the motor vehicle, and the viscous clutch 5 is disposed at the input side of the rear axle tranmission.

The torque introduced by way of the shaft part 10 is conveyed by way of bevel gears or an angular transmission 11 and a hollow shaft 14 to the housing of the rear axle differential 12, wherein the freewheel device 6, with the parallel-connected spring-loaded friction device 8 is connected between the hollow shaft 14 and the housing of the rear axle differential 12. In this embodiment, a clamping roller freewheel 7 is provided, whose freewheel inner ring, which is coupled to the front axle drive, that is the hollow shaft 14, is designated 62 and its freewheel outer ring, which is coupled to the rear axle drive, that is the housing of the rear axle differential 12, is designated 61. The friction device 8 is a multi-plate clutch which is spring loaded, one end of which acts on the freewheel outer ring 61 and the other on the freewheel inner ring 62.

Claims

1. A motor vehicle having a front engine and all-wheel drive, in which the wheels of the front axle are permanently driven, whilst the drive for the wheels of the rear axle is engageable or is establishable permanently by way of a lockable centre differential or automatically establishable by means of a viscous clutch disposed between the front and rear axles in the drive train, in such a way that the front and rear axles are coupled to one another substantially rigidly with respect to driving torque, wherein at least one freewheel device having a freewheel lock, is provided in the torque-transmitting drive train between the front and rear wheels, which freewheel device is disposed and designed such that, in the unlocked condition, the part of the freewheel device coupled to the rear axle drive can overtake the freewheel part coupled to the front axle drive when driving forward, characterised in that the freewheel device is bridged by a spring-loaded friction device, one end of which acts on the freewheel part which is coupled to the rear axle drive, and the other end of which acts on the freewheel part which is coupled to the front axle drive.

2. A motor vehicle as claimed in claim 1, wherein the spring-loaded friction device is a spring-loaded friction clutch.

3. A motor vehicle as claimed in claim 2, wherein the spring-loaded friction device is a spring-loaded multi-plate clutch.

4. A motor vehicle as claimed in claim 1, 2 or 3, wherein the friction torque of the friction device is around 10 to 20% of the maximum torque to be transmitted during normal vehicle operation by the freewheel device.

5 A motor vehicle as claimed in any one of claims 1 to 4, wherein the freewheel device is of the clamping roller type.

6. A motor vehicle substantially as herein described with reference to, and as illustrated in, Fig.l or Fig.2 of the accompanying drawings.