GB2313815A - Front-wheel speed-change apparatus in a four-wheel-drive vehicle - Google Patents

Abstract

Apparatus for automatically increasing the speed of the front wheels of a four-wheel-drive vehicle on turning a corner comprises means for sensing steering angle connected to an electronic control unit which controls a multi-speed gearbox via hydraulics. The gearbox (6) is driven by the front wheel drive shaft (5) and has an output shaft (25) which drives the front differential. The gearbox has two ratios, direct drive and a higher ratio which is selected automatically when a given steering angle, eg 45{, is reached. The gearing is planetary, comprising input (14) and output (16) suns and compound planets (17), and is shifted by means of a brake (13) and a clutch (23). The clutch is engaged by a spring (27) for direct drive and is disengaged, with engagement of brake (13), by a hydraulicallyactuated piston (29) for selection of the higher ratio.

Description

Front Wheel Speed Change Apparatus For Four Wheel Drive Vehicle The present invention relates to the adjustment of the relative speed of the front and rear wheels of a front wheel steer, four wheel drive vehicle when turning a corner. The invention has particular application for four wheel drive agricultural tractors.

It is well known that, when turning a comer, particularly a sharp corner, a vehicle's front wheels need on average to accelerate relative to the rear wheels. This is governed simply by the geometry of the paths which each of the wheels follow when a vehicle turns a comer. For eg a 40 to 500 steering angle, the front wheel speed needs to increase by around 30 to 50%.

In a four wheel drive vehicle, therefore, there will be considerable wheel slip at the front wheels when turning a tight comer, unless the four wheel drive is disengaged. In a four wheel drive agricultural tractor, it is standard practice to disengage four wheel drive when turning a sharp comer, eg when changing direction on field headlands.

This is inconvenient because there are always several other operations which need to be performed simultaneously, eg steering the tractor into a tight turn, lifting and lowering the tractor three-point linkage, changing gear, etc.

A well know solution to this problem is to provide change speed gearing for the front axle drive whereby the speed of the front axle is increased relative to the rear axle when a comer is turned. A linkage to the steering mechanism controls the ratio of this gearing automatically, so that the front axle speed is increased when the steering angle passes a certain point.

The gearing can be arranged to increase front wheel speed to approximately compensate for the effect of the turn. Alternatively, the front wheel speed can be increased beyond that necessary to compensate for the geometry of the turn. This has the effect of decreasing the turning circle by pulling the front of the tractor into the turn, although the resulting wheel spin can have a damaging effect on the soil. An example of such a system is described in GB-A-2, 195,590. Such systems are popularly known as "bi-speed" systems.

As is apparent from a cursory examination of GB-A-2,195,590 (see eg Figure 2), the bi-speed gearing in this design is incorporated integrally into the front axle housing.

The demand for tractors having bi-speed systems is at present relatively small, at least in Europe, and it represents an uneconomical solution to have two separate designs of front axle housing for "bi-speed" and "non bi-speed" tractors.

Other designs of bi-speed system exist having the bi-speed gearing located differently, eg as part of the front wheel drive shaft assembly (see eg JP-B-7-35129). However, this normally has the effect of increasing the overall length of the drive line with respect to existing non bi-speed designs, which necessitates re-design of a substantial number of components.

Many current designs of bi-speed gearing, including that described in IP-B-7-35 129, are also very difficult to alter to provide alternative ratios. As explained above, bi-speed gearing may simply provide the front wheel speed increase necessary to avoid wheel slip, or may provide a greater speed increase to facilitate tight turning. In the latter case, different ratios may be preferred for different applications particularly for different ground conditions and different desired degrees of turning assistance to be provided by the bi-speed.

There thus exists a need for a bi-speed system which can be relatively easily added to a current tractor design without significantly adding to the drive line length (and hence tractor wheel base), and also being compact enough not to interfere with the tractor's function and able to be located so as not to interfere with the tractor's operation.

Particularly, it is desirable for such a system not to detract from the front axle ground clearance. There also exists a need for a system which is readily changeable to provide different bi-speed ratios.

According to the present invention, front wheel speed change apparatus for a four wheel drive vehicle is provided according to Claim 1. The apparatus according to Claim 1 is particularly compact and therefore readily able to be incorporated into a current, conventional design of tractor drive line without unduly affecting ground clearance and substantially without affecting the wheel base, with the resulting advantage that most of the tractor componentry does not have to be re-designed to accommodate the bi-speed facility.

According to another aspect of the present invention front wheel speed change apparatus for a four wheel drive vehicle is provided according to Claim 4. The use of a relatively simple planetary gear set of the type claimed results in a system which is exceptionally easy to modify to produce alternative versions of the same system having different bi-speed ratios.

Preferable features are set out in the dependent claims. The advantages of these and other features will be apparent from the following description of one exemplary specific embodiment which is now given with reference to the accompanying drawings in which: Figure 1 is a schematic plan view of apparatus according to the invention, and Figure 2 is a side sectional view showing planetary gearing, clutches, input and output shafts, all contained within a housing.

Referring firstly to Figure 1, a bi-speed system according to the present invention is shown schematically with respect to a tractor front axle. Front wheels I are mounted on a front axle casing 2. A steering linkage 3 connects the wheels 1 to a hydraulic steering cylinder 4.

The front wheel drive shaft 5 from the tractor main transmission enters a casing 6 housing the bi-speed gearing/clutches via which it is connected to the front axle differential (not shown).

Electronic angular sensors 7 (eg rotary potentiometers), are mounted on the king pins (not shown) of the wheel mounting assembly on each side of the axle casing 2. The sensors 7 are connected via signal lines 8 to an electronic control unit 9 whose output is connected to a solenoid actuated hydraulic valve 10 of a conventional type. The control unit comprises simple logic circuitry or possibly a microprocessor and means for providing a signal of sufficient magnitude to energise the solenoid valve 10 The valve 10 controls the supply of hydraulic fluid from a supply 11 to clutches in the bi-speed casing 6.

Referring now to Figure 2, the bi-speed casing is provided with a flange 3 1 for bolting onto a conventional front axle casing 2. The casing 6 houses bearings supporting the drive shaft 5 and an output shaft 25 concentric with the drive shaft. These two shafts are coupled by planetary gearing and clutches also housed in the casing 6. The casing 6 is shaped so that the steering cylinder 32 fits snugly into an exterior recess 33 immediately behind the flange 3 1.

The front wheel drive shaft 5, inside the bi-speed casing 6, is splined to a first set of clutch plates 12 of a first wet clutch assembly 13. Also splined to the drive shaft 5 is an input gear 14 comprising the first sun gear of a compound planetary gear set, generally shown at 15.

The planetary gear set 15 comprises the input sun 14, an output sun 16 and three (or more) compound planet gears 17, each having a first portion 18 in mesh with the input sun and a second portion 19 in mesh with the output sun 16. The planet gears are journalled in a planet carrier 20.

The output shaft 25, coaxial with the front wheel drive shaft 5, is in mesh with the front differential (not shown) via crown wheel 26. The output sun 16 of the planetary 15 is splined to the output shaft 25.

The planet carrier 20 of the planetary 15 is connected to the second set 21 of clutch plates making up the first wet clutch assembly 13. A first set 24 of plates of a second clutch 23 are fixedly mounted on the interior of the casing 6. The carrier 20 is also connected to a second set 22 of plates making up the second clutch 23. One of these plates is common between the first and second clutches and will be referred to as an "interplate", designated 28 in Figure 2. This construction, with both sets of clutch plates on the same side of the common interplate, is particularly compact.

The first clutch 13 is biased into engagement by Belleville spring 27 which acts against the clutch plates 12, 21 via the intermediate plate or interplate 28. The interplate 28 biased by the Belleville 27, normally keeps the plates of the second clutch 23 apart.

A hydraulic piston 29 is actuated via hydraulic fluid inlet 30 from the valve 10. The piston 29, when actuated, bears against the plates of the second clutch 23 thereby connecting the planet carrier 20 to "ground", ie to the casing 6. At the same time, the interplate 28 is moved against the action of the Belleville to disengage the first clutch 13. The piston 29 is stationary with respect to the housing 6, removing the need for rotating seals which inevitably leak. This is particularly desirable in this application where any leakage would result in the front axle casing 2 filling with hydraulic oil.

The bi-speed gearing is thus shiftable between a first state in which the drive from the drive shaft 5 is transmitted directly to the output shaft, and a second state in which the drive is transmitted via a ratio greater than 1. The first state is entered when the first clutch 13 is engaged, thus rotationally fixing the planet carrier 20 to the drive shaft 5 and hence the input sun 14. In this state, the planetary gearing rotates as one body, and drive is transmitted directly to the output sun 16. The second state is entered when the first clutch 13 is disengaged and the second clutch 23 is engaged. In this state, the planet carrier 20 is braked to ground and the ratio provided by the gearing is determined by the number of teeth on the input and output suns 14, 16 and the first and second portions 18, 19 of the planet gears 17.

It will be appreciated that the use of parallel axis spur tooth gears means that altering the bi-speed ratio involves only a very simple re-design of the gearing 14, 16, 18, 19 without necessarily even changing the position of the axial centres of the planet gears 17. The dimensions of the casing 6 need not be altered. Accordingly, a number of alternative bi-speed systems may be produced at relatively low cost which may be readily substituted for each other. This is in sharp contrast to the bevel gear system of JP-B-7-35129, for example, which would allow a 1:1 or 1:2 ratio only and would have to be completely re-designed to allow a different ratio. This gearing may also easily be changed, if desired, to provide a gear change between 1:1 and an overdrive ratio. In this case, overdrive would be used normally, and 1:1 engaged on turning the tractor.

It will also be appreciated from what has been described above that the inclusion of a bi-speed feature according to the invention into a conventional tractor need not involve any change to the front axle casing and associated componentry. All that is necessary is to re-design the front wheel drive shaft and to substitute the bi-speed casing 6 and componentry therein for the corresponding casing on a conventional tractor, which would house only the junction between the drive shaft and front differential and a bearing to support the drive shaft. The compactness of the planetary gearing and the clutch arrangement with both clutches on the same side of the interplate 28 allows the bi-speed casing 6 to be substituted for the equivalent casing on a conventional tractor without significantly effecting wheel base or ground clearance. It is, of course, also necessary to provide the sensors 7 on the king pins and the control electronics and hydraulic lines and a valve(s) to supply the piston 29.

In an alternative embodiment a mechanical linkage could be provided between the steering cylinder 32 and a mechanically operated valve controlling the hydraulic supply to the piston 29. The mechanical linkage to the steering cylinder would in effect "detect" the steering angle and control the actuation of the piston 29 directly rather than via an electronic system. This would provide an even simpler modification to a conventional tractor to provide bi-speed since everything necessary for the bi-speed feature would be associated with the casing 6, including the linkage which could be mounted on the outside of the casing.

In another alternative, the linkage could be connected directly to the first and second clutches, eliminating the need for hydraulic operation of the system.

In operation, the bi-speed system does not, as a practical matter, need to engage until the steering angle becomes very large, since the purpose of the system is essentially to facilitate tight turning and to avoid soil damage on tight turning. For most of the time, the angular sensors 7 will indicate to the control unit a zero or relatively small steering angle, and the control unit will control the valve 10 such that the piston 29 is not actuated. In the alternative systems described above, the linkage will be designed so that it only actuates the clutches in the bi-speed system (whether directly or via hydraulics) when the steering cylinder has reached a certain point corresponding to a large wheel angle.

The normal maximum interior steering angle for a tractor (that is the angle of the wheel on the inner side of the turning circle) is around 55" When an interior angle of between 40 and 50 , say for example 45" is reached, the control unit will energise the valve 10 so as to actuate the piston 29 and engage a higher than normal gear ratio in the drive to the front axle.

In this example, the control unit averages the magnitude of the reading from the left and right sensors 7. Since the interior and exterior steering angle will not be the same, averaging has the advantage that the control unit does not need to "know" what direction the tractor is being steered in. However, it would equally be possible to sense the steering angle of only one wheel provided the control unit is capable of distinguishing between signals from the sensor indicating steering in one or the other direction. It would equally be possible to sense steering angle in many other ways, eg from the movement of the hydraulic steering cylinder 4 as described above, or from movement of the steering linkage 3.

In this embodiment the control unit 9 is designed to output a simple on/off signal to the valve 10. This is found to work satisfactorily. However, in an alternative embodiment, the transition could be made to occur gradually, ensuring a smooth transfer between normal and "bi-speed" operation. This would, of course, involve more sophisticated control, with related cost implications.

When bi-speed mode is entered, ideally no wheel slip should occur since this tends to damage soil, eg on a field headland where most sharp turns in a tractor are performed.

To minimise wheel slip, a bi-speed ratio of the order of 1 1.2 to 1 1.5 is employed eg approximately 1 1.3. This would be the desired type of ratio for a normal application in Europe and will approximately compensate for the front/rear wheel speed differences dictated by the geometry of the turn. In certain applications, however, notably for paddy field work in Japan, a higher bi-speed ratio is desirable, eg in the range 1 1.6 to 1:2.0, more preferably 1 1.7 to 1 1.9 eg around 1 1.8. This inevitably results in a certain degree of wheel slip since the front wheels will be accelerated more than is required by the geometry of the turn. This magnitude of ratio also tends to produce a jerk when the bi-speed is engaged. However, both of these disadvantages are outweighed by the decreased turning circle made possible by the higher ratio, and soil damage is not such a problem in a paddy field environment.

Claims (11)

1. Claims

   1Front wheel speed change apparatus for a four wheel drive vehicle, the apparatus comprising steering means for steering the vehicle's wheels, a front wheel drive shaft and a front axle casing housing a front differential, the apparatus further comprising a bi-speed gear system which comprises: (a) a planetary multi-speed gear set concentric with the front wheel drive shaft and drivingly connected with it; (b) an output shaft drivingly connected between the gearset and the front differential, and (c) means for automatically changing the ratio of the gearset dependent on the steering angle of the vehicle's front wheels characterised in that: the said ratio changing means comprise first and second clutches, each located on the same side of a common interplate via which each clutch is actuatable.
2. 2 Apparatus as claimed in Claim 1, wherein the said interplate is spring biased in one direction and movable in the opposite direction by means of a hydraulic or mechanical actuator.
3. 3 Apparatus as claimed in Claim 2, wherein a Belleville spring is located on one side of the interplate to provide the said bias.
4. 4 Front wheel speed change apparatus for a four wheel drive vehicle, the apparatus comprising steering means for steering the vehicle's wheels, a front wheel drive shaft and a front axle casing housing a front differential, the apparatus further comprising a bi-speed gear system which comprises: (a) a planetary multi-speed gear set concentric with the front wheel drive shaft and drivingly connected with it, (b) an output shaft drivingly connected between the gearset and the front differential, and (c) means for automatically changing the ratio of the gearset dependent on the steering angle of the vehicle's front wheels characterised in that: the planetary gearset comprises a first sun gear connected to the front wheel drive shaft, a second sun gear connected to the output shaft and dual diameter planet gears in mesh with the first and second sun gears.
5. 5 Apparatus as claimed in any preceding claim including a housing mounted on the front axle casing, into which the front wheel drive shaft extends, and inside which is located the planetary gearset and ratio changing means.
6. 6 Apparatus as claimed in any preceding claim comprising means for generating an electronic signal representative of steering angle, and means for actuating the ratio changing means in dependence on the value of this signal.
7. 7 Apparatus as claimed in any of Claims 1 to 5 comprising a mechanical linkage connected to the said vehicle steering means and to either a hydraulic valve controlling flow to a hydraulic actuator for changing the ratio of the gearset or directly to a clutch for changing the ratio of the gearset.
8. 8 Apparatus as claimed in any preceding Claim including a hydraulically actuated piston for effecting movement of a clutch or clutches for changing the ratio of the gearset, the said piston being rotationally fixed with respect to the front axle casing.
9. 9 Apparatus substantially as herein specifically described with reference to the accompanying drawings.
10. 10 A four wheel drive vehicle including apparatus as claimed in any preceding claim.
11. 11 An agricultural tractor including apparatus as claimed in any of Claims I to 9.