GB2208217A

GB2208217A - An automatic system for selecting 4-wheel drive when braking a farm tractor

Info

Publication number: GB2208217A
Application number: GB8810421A
Authority: GB
Inventors: Michael James Drew; Leslie Colin Phillips
Original assignee: Vapormatic Co Ltd
Current assignee: Vapormatic Co Ltd
Priority date: 1987-07-03
Filing date: 1988-05-03
Publication date: 1989-03-15
Also published as: GB8715759D0; GB8810421D0

Abstract

A farm tractor which has the facility for selection of 2-wheel or 4-wheel drive has a manually operable supplementary switch (40) which, in conjunction with the conventional 2-wheel/4-wheel drive selector switch (14) and the back or "brakes off" contact (30) on the brake lights switch (22), ensures that when the tractor is being driven in 2-wheel drive mode, the solenoid valve (10) in the oil pressure circuit of the transmission clutch actuating motor is de-energised as soon as the brake pedal is depressed, so that the braking effort applied to the rear wheels is also transferred through the transmission clutch to the front wheels. A back-up circuit includes the handbrake lever switch for ensuring that 4-wheel braking is maintained when the driver takes his foot off the brake pedal. For tractors with dual brake pedals for selective braking of either rear wheel, a pedal interlock sensor (88) is wired into the 4-wheel braking circuit to give warning of failure to interlock the pedals when running in 2-wheel drive mode with the supplementary switch (40) set to the "road" position (42-46). <IMAGE>

Description

AN AUTOMATIC SYSTEM FOR SELECTING 4-WHEEL DRIVE WHEN BRAKING A FARM TRACTOR This invention relates to four-wheel drive farm tractors or like vehicles (hereinafter referred to for convenience as "tractors") having provision for selecting 2-wheel or 4-wheel drive at will, such tractors being dual-purpose vehicles which can be driven on roads or across country. Tractors tend usually to have smaller front than back wheels, and while the transfer "box' is normally geared to the disparity in rolling diameters of the front and back wheels, precise accuracy is rarely achieved, and among other things, is subject to variations in rolling diameter due to tyre wear and changes in tyre pressures.

In use of such tractors, by reason of the almost inevitable mis-match of gear ratios and tyre condition, it is conventional to switch to 2-wheel drive for road work because of the inevitable scuffing" of front tyres in 4-wheel drive mode, due to incorrect speeds of rotation for the road speed of the tractor. In the majority of designs of farm tractor, the only braking provision is the inboard brake fitted on each output shaft of the rear wheel differential, although a few models are available with additional disc brakes on the front wheels. The present invention is aimed at the former design so as to give the same effect as the latter when braking while travelling in 2-wheel drive mode - especially when towing a loaded trailer on public roads.

Accordingly, the present invention provides a system for addition to, or incorporation as initial equipment in, the transmission control of a tractor in which the drive to the front wheels is engaged or disengaged by an oil-actuated clutch, the oil supply to which is under the control of a solenoid valve which, in turn, is under the control of the driver. The conventional arrangement of such controls is that the 4-wheel drive clutch is positively held in the disengaged position by oil pressure admitted to the clutch ctuating motor when the solenoid valve is energised.

Thus, the transmission is kept in the2wheel drive mode by positive action of the solenoid valve and of the clutch actuating motor, so that if there is a failure in the electrical circuit of the solenoid, or the oil circuit of the clutch motor-, the clutch is released to couple the front wheels to the rear wheels and leave the tractor in the 4-wheel drive mode until the fault is found and rectified.

The system according to the present invention makes use of two conventional designs of switch forming parts of the standard equipment of at least most 4-wheel drive tractors. These switches are (i) the pedalactuated switch in the brake lights circuit and (il) the manual selector switch for selecting 2-wheel or 4-wheel drive. It is common practice for these switches to have normally unused terminals on "switch open" or "back" contacts, corresponding respectively with "brakes off" and "4-wheel drive engaged" positions.

The invention relies on at least the brake lights switch having -such an accessible (and normally opencircuited) "brakes off" or back contact terminal, and if this is not available on the original equipment switch on a given tractor it will be necessary to substitute a switch having such a terminal if the tractor is to be fitted with the present invention for better braking on the road, because an electrical signal representing "brakes off" is required.

It has already been proposed to override the normal action of a 2-wheel/4-wheel drive selector switch, 90 far as concerns providing 4-wheel braking automatically while running in the 2-wheel drive mode, by sensing the road speed of the tractor and using the signals from the sensor to change from 2-wheel drive to 4-wheel drive when travelling at above a given speed, A speed-responsive braking system, however, is not relevant to the present invention, which is designed to give automatic 4-wheel braking when running in the 2-wheel drive mode at any speed.

The invention consists essentially in the provision of a change-over switch connected in circuit between the live pole of the tractor battery and the conventional 4-wheel drive selector switch, one fixed contact of the change-over switch being directly connected to the battery and the other fixed contact being connected via the normally open-circuit "brakes off" fixed contact of the brake lights switch. The moving contact of the change-over switch is connected to the moving contact of the 2-wheel/4-wheel drive selector switch.

Preferably, the change-over switch is a doublepole, double-throw switch having two pairs of fixed contacts and two moving contacts ganged together, the first moving contact and one pair of fixed contacts constituting the change-over switch of the preceding paragraph and the second moving contact being connected in circuit with a tell-tale indicator for showing that the system is set for automatic 4-wheel braking on the road. The battery is also connected to the fixed contact engageable by the second moving contact when the first moving contact is engaged with the fixed contact connected to the brakes off" contact of the brake lights switch, the other fixed contact engageable by the second moving contact being on open circuit.

From another aspect the invention consists in a tractor having dual brake pedals each coupled to the brake on a respective rear wheel and fitted with means for mechanically interlocking the pedals at will, a sensor for detecting when the interlock means are inoperative and an indicator responsive to the sensor for indicating one of the inoperative or operative conditions of the interlock means.

A practical embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows the existing circuit of a 2-wheel/ 4-wheel drive selector switch as currently found on a commercial make of tractor; Figure 2 is a circuit diagram of a system according to the present invention incorporating the components in Figure 1; Figure 3 shows a modified circuit diagram incorporating a handbrake-operated switch, and Figures 4 and 5 are scrap perspective views of a pair of independent foot brake pedals in the unlocked and locked condition, respectively, and fitted with a sensor connected in the circuit of Figure 3.

Referring first to Figure 1, a solenoid valve 10 controls the flow of oil under pressure to the actuating motor (not shown) of a multi-plate clutch (also not shown) arranged to couple or uncouple the drive to or from the front wheels. The solenoid 10 is energised from the live pole 12 of the tractor battery through a selector switch 14 having a moving contact 16 engageable with either of two fixed contacts 18, 20. In the position shown in Figure 1, the contact 18 is energised by the moving contact 16 so that the solenoid 10 is energised and the solenoid valve is open to feed oil to the clutch actuating motor. This in turn pulls the multi-plate clutch out so that the drive to the front wheels is broken, and the tractor is set in the 2-wheel drive mode which is customarily used for road travel.

This is the conventional fail-safe arrangement whereby any failure of the electrical or hydraulic equipment allows the multi-plate clutch to engage and leaves the tractor in 4-wheel drive mode. 4-wheel drive can be deliberately selected by moving the selector switch contact 16 into engagement with the contact 20 which is open-circuit.

she normal footbrake switch 22 has its moving contact 24 directly connected to the live battery terminal 12.

The brake lights circuit is made when depression of the brake pedal (not shown) moves the moving contact 24 into engagement with the contact 26 to which the brake lights 28 are connected. When the pedal is released, the moving contact 24 engages a "back" or "off" contact 30 which is on open-circuit, so that the switch 22 acts as a simple on/off switch.

Referring now to Figure 2, the solenoid 10 is controlled, as in Figure 1, by the manual selector switch 14 so that the driver retains the same degree of control of the selection of 2-wheel or 4-wheel drive, except that a further or supplementary switch 40 is interposed between the selector switch 14 and the live terminal 12 of the battery. Also, a parallel path from the battery terminal 12 is now available through the "back" or 'loft contact 30 on the brake lights switch 22 and contacts on the switch 40.

The switch 40 is shown as a double-pole double-throw switch having two moving contacts 42, 44 which are ganged to move in unison. Each moving contact 42, 44 engages either of a respective pair of fixed contacts 46, 48 and 50, 52. The fixed contact 46 is directly connected to the "back" or "off" contact of the brake lights switch 22, which is otherwise connected as in Figure 1. The other fixed contact 48 of the pair engageable by the moving contact 42 is connected directly to the live battery terminal 12. Thus, so far as concerns the half 42, 46, 48 of the supplementary switch 40, the live terminal 12 of the battery will be connected to the selector switch 14 whichever position the switch 40 is in provided that the brakes are not on.This half of the supplementary switch 40 thus acts purely as a changeover switch, one position (42-46) of which puts the solenoid 10 under the control of the brake pedal if the selector switch is in the 2-wheel drive setting (16-18) as would be normal when travelling on the road. Thus, under these circumstances, 4-wheel drive is automatically engaged when the brakes are applied, and the extra braking power of the front wheels is added to that of the rear wheels via the transmission.

The other half 44, 50, 52 of the switch 40 is, from a functional point of view, optional. However, it is used in Figure 2 to control a warning or tell-tale indicator - such as a light 54 - to tell the driver when he has set the system to the "Road" condition. In this condition, 4-wheel braking is automatic when the selector switch 14 is in-the 2-wheel drive-position.

The circuit of Figure 2 is effective to give 4-wheel braking when the tractor is running in 2-wheel drive so long as the foot brake is actuated. There are, however, two quite separate situations in which safeguards may be required: (a) the need to hold the tractor fully braked - for example, on a steep hill when towing a loaded trailer, and (b) the failure of the foot brake mechanism - for example by rupture of the piston seal in the master cylinder of a conventional hydraulic master and slave circuit for actuating the brake cylinders on the output shafts of the main differential.

Referring first to (a), this situation may be paralleled on sloping fields where the surface is muddy, and the rear wheel brakes may not be sufficient by themselves to prevent the tractor from sliding. Whether on the road or the field, it becomes vitally necessary to hold 4-wheel brakes. This can only be done when the transmission is in 4wheel drive, and one way in which the driver can secure this is by changing the 2-wheel/4-wheel drive selector switch to 4-wheel drive when he stops.

This then ensures that the clutch which couples the rear wheels to the front wheels via the transfer box is engaged, so that the effect of the hand brake is then felt at all 4 wheels.

Referring to situation (b), it has to be remembered that there is only one source of braking effort on most tractors, viz: the brake disc on each output shaft of the main differential driving the back wheels. It is the braking effort generated at these'discs which, in 4-wheel drive, also acts on the front wheels. If, through hydraulic pressure failure, the foot brake fails to apply the brake pads to the brake discs, braking effort will be applied to any wheels, whatever the state of drive of the transmission.Every tractor, however, is equipped with a handbrake which acts independently of the footbrake on the brake discs and, as in the case of most road vehicles, is less powerful for slowing the vehicle than the foot brake but, thanks to static friction being higher than dynamic friction at each brake disc, is well able to hold the tractor stationary once it has been stopped.

On most tractors, there is a handbrake tell-tale which visually warns the driver that the hand brake is on. This is energised by the (electrical) counterpart on the handbrake lever of the brake lights switch 22 at least the 24-26 half thereof. This handbrake switch can be utilised in a modification of the present invention, to duplicate the action of the electrical circuit through the footbrake switch 22 as described above. This modification is illustrated in Figure 3 of the drawings.

Ah illustrated in Figures 4 and 5 is a conventional design of locking latch for (optionally) mechanically coupling both pedals of a 2-pedal braking unit whereby the driver can, at will, brake either rear wheel independently so as to decrease the tractor turning circle when the inside rear wheel is braked. When travelling on a public road in 2-wheel drive mode, it is vitally important that the independent brake pedals 80, 82 are locked mechanically to ensure simultaneous opera tin,~ If this is not done, and only one pedal is depressed, the tractor will tend to veer right or left according to which pedal is inadvertently operated alone.

It is therefore an additional feature of the present invention to provide a tall-tale or other warning to the driver if the pedals 80, 82 are not mechanically coupled when the tractor is being driven in 2-wheel drive with the supplementary "field/road" switch 40 in the "road" position (contacts 42-46 made).

Referring now to Figure 3, the handbrake switch 60 is fed from the live pole 12 of the battery and has its "brake-on" contact 62, which is normally connected to the brake warning light 64, connected also to the coil 66 of a-relay in parallel with the light 64.This relay controls a pair of normally closed contacts 68, 70 in series with the "back" or "brakes off" contact 30 of the brake lights switch 22 and the "road" contact 46 of the supplementary "field/road" switch 40. The contacts 68, 70 are closed when the relay 66 is deenergised - i.e. when the handbrake is off and the switch 60 is open. The circuit of Figure 3 is thus the same as that of Figure 2 with the interposition between the contacts 30 and 46 of the normally closed contacts 68, 70.As soon, however, as the handbrake is applied, the relay 66 is energised and the power supply to the solenoid 10 via the "brakes off" contact 30; the "road" contacts 46, 42 of the supplementary switch 40, and the 2-wheel setting 16, 18 of the 2-wheel/4-wheel selector switch 14 is broken. This causes the de-energisation of the transmissiqn clutch motor which thus allows the clutch to couple the drives to the front and rear wheels. Thus, when the driver dismounts, the handbrake switch 60, 62 keeps the circuit to the solenoid 10 broken while the closed switch 60 energises the handbrake warning light 64 to give the usual visual warning that the handbrake is on.

Figures 4 and 5 show schematically a conventional "dual pedal arrangement of an independent rear wheel braking system. Each pedal 80, 82 is grooved at 86 along its upper edge to allow a rigid latch bar 84 pivoted on one pedal 80 to be swung over to nest snugly in the groove 86 on the other pedal 82. When thus nested, pressure on either pedal 80, 82 alone is transmitted to the other and both pedals will always be depressed when either is actuated to produce equal 2wheel braking. A sensor 88 is secured to the pedal 82 on which the latch bar 84 is not pivoted to detect the presence of the rigid latch bar 84 in its operative or pedal-locking position (Figure 4).A lead 90 strapped to the stem 92 of the pedal 82 carries the output sig nals of the sensor 88 to the supplementary switch 40 (Figure 3) where a third moving contact 45 engages or disengages a single fixed contact 47. The arrangement is such that the sensor 88 will energise a warning device 94 when the pedals 80, 82 are not linked and the supplementary switch 40 is in the "Road" position. The driver's attention is thus drawn to a potentially hazardous situation when driving in 2-wheel drive on a road. It will be understood that this sensing and indicating arrangement is also generally applicable to any tractor having "split" brakes. The indicator may be on whenever the brakes are uncoupled, or it may be inhibited in certain conditions, for example t as des- cribed above or when the tractor travels below a certain speed.

A tractor fitted with equipment in accordance with the present invention as described above,with reference to Figures 2 - 5 of the drawings can thus be driven in 2-wheel drive on a public road with the reassurance of having 4-wheel braking available as soon as the brakes are applied by locked pedals 80, 82, and that the tractor can be parked with all four wheels braked, although no brake drums or discs are provided on the front wheels.

It will be understood that the circuit diagrams of Figures 2 and 3 serve to illustrate how the system works electrically rather than the physical construction of the components 14, 22, 40 and 60 themselves. A competent electronics engineer will have no difficulty in substituting optical, proximity or other electronic switching device for a mechanical switch if the circum stances of any particular case indicate the desirability of such a substitution.

The rear wheel brakes of a tractor are usually directly mechanically or hydraulically coupled to the brake pedal so that there is no delay in function of the brakes when the brake pedal is depressed. With the present invention in use on a tractor, however, there will be a slight delay in the achievement of full power by the transmission clutch, so that front wheel braking force will build up progressively compared with that at the rear wheels. This is particularly advantageous in slippery road conditions, since the absence of shock braking force on the front wheels will tend to obviate front wheel slides.

It will be understood that the selection system described above is equally applicable whether, on a particular tractor, the four wheel drive is engaged by energising or de-energising the solenoid. Further, it will be seen that the interlock system is particularly important on tractors in which operation of either one of the brakes causes the brake light to illuminate.

Claims

( I' A I M S

1. A system for automatically applying, through the transmission, braking effort to the front wheels of a tractor having a facility for electrtoally selecting at will either 2-wheel or 4-wheel drive comprising connecting a supplementary change-over switch in the electrical circuit between the live terminal of the battery and the manual 2-wheel/4-wheel drive selector switch, one contact of a pair of fixed contacts of the supplementary change-over switch being connected directly to the live terminal of the tractor battery while the other fixed contact of the pair is connected to the normally open-circuit back or "brakes off" contact of the brake lights switch while the moving contact of the supplementary change-over switch is connected to the moving contact of the selector switch.

2. A system according to claim 1 wherein the supplementary change-over switch has a second set of fixed and moving contacts, the second moving contact being ganged to the moving contact of the first set of contacts and connected to a tell-tale indicator to show automatic 4-wheel braking while the tractor is in 2-wheel drive awhile the fixed contact with which it is engaged in the position of the switch for automatically applying 4wheel braking when the tractor is in 2-wheel drive is connected to the live terminal of the battery.

3. A system according to claim 1 or 2 wherein a switch operated by the handbrake is effective to energise, when the handbrake is pulled on, a relay having normally closed contacts connected in circuit between the "brakes off" contact of the brake lights switch and the moving contact of the 2-wheel/4-wheel drive selector switch.

4. A system according to claim 1, 2 or 3 for use in a tractor having dual brake pedals each coupled to the brake on a respective rear wheel and fitted with means four mechanically interlocking the pedals at will, wherein a sensor is adapted to detect when the interlock means is inoperative and is connected to a third set of on/off contacts on the supplementary "field/ road" change-over switch so that a "pedals unlocked" warning device is energised when the supplementary switch is set to the "road" condition and the interlock means is in its inoperative position.

5. A system for automatically applying, through the transmission, braking effort to the front wheels of a tractor having a facility for electrically selecting at will either 2-wheel or 4-wheel drive substantially as hereinbefore described with reference to Figure 2 or 3 of the accompanying drawings.

6. A kit of parts for converting a tractor having a manual selector switch for selecting 2-wheel or 4-wheel drive at will comprising a brake lights switch for operation by the brake pedal and having a terminal on the "back1, or "switch open" contact; a change-over switch having a moving contact and two fixed contacts alternatively engageable therewith; means for connecting one fixed contact of the change-over switch to the "back" or "switch open" contact of the brake lights switch, and means for connecting the other fixed contact of the change-over switch to the live terminal of the battery.

7. A system for automatically applying through the transmission, braking effort to the front wheels of a tractor having a facility for electrically selecting at will either 2-wheel or 4-wheel drive, substantially as hereinbefore described, with reference to the accompanying drawings.

8. A tractor having a facility for the selection of either 2-wheel or 4-wheel drive and provided with a system according to any preceding claim for automatically selecting 4-wheel drive when braking while being driven in the 2-wheel drive mode.

9, A tractor having dual brake pedals each coupled to the brake on a respective rear wheel and fitted with means for mechanically interlocking the pedals at will, a sensor for detecting when the interlock means are inoperative and an indicator responsive to the sensor for indicating one of the inoperative or operative conditions of the interlock means.

10. A tractor as claimed in Claim 9, including inhibit means for the indicator such that it will only indicate the condition when the tractor is in a "Road" condition.

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11. A tractor as claimed in Claim 9, including inhibit means for the indicator such that it will only indicate the condition when the tractor is in 2-wheel drive.

12. A tractor as claimed in Claim 9, including inhibit means for the indicator such that it will only indicate the condition when the tractor exceeds a predetermined speed.

13. A tractor substantially as hereinbefore described, with reference to the accompanying drawings.