STEERING CONTROL SYSTEM FOR A TRACKED VEHICLE

Abstract: A tracked vehicle having a three differential transmission system is steered by selectively applying brakes (38) to brake discs (1,13) of transmission outputs on opposite sides of the vehicle. A turn is controlled by the driver by steering means comprising a handwheel (20) having a shaft (21) bearing cam plates (22, 24). A first cam plate (22) controls pressure modulating valves (23) to pass modulated hydraulic line pressure to the brake (38) according to the turn signal from the steering means. Upon selection of low range gearing in a change speed gearbox of the transmission, a by-pass activating switch (37) activates an electric circuit for control of solenoid-operated selection slide valves (26). With the operating switch (37) closed, operation of the steering means causes the second cam plate (24) to close a by-pass operating switch (25) which results in the selection valve (26) being adjusted to pass unmodulated line pressure directly to the brake (38) for application of full braking pressure.

Description: This invention relates to a steering control system for a tracked vehicle in which a brake can be applied hydraulically at one or the other of two transmission outputs on opposite sides of the vehicle to steer the vehicle, the control system comprising steering means whereby a driver can initiate application of the brakes, and can signal the tightness of a turn required, and pressure modulating means which is responsive to operation of the steering means to control the level of hydraulic pressure applied according to the tightness of the turn called for. The invention may in particular be applied to the steer control of a tracked vehicle having a change speed gearbox incorporating a three differential steer mechanism such as the Merritt-Wilson transmission, widely used on military tracked vehicles. Steering of the vehicle is achieved by establishing a track speed difference, commonly through the selected application of caliper brakes to discs attached to outputs of the steer differential at opposite sides of the gearbox. The caliper brakes are operated by hydraulic pressure, line pressure being modulated by the amount of steer demanded by the driver. This modulation of pressure is critical to the effective operation of the brakes. It is found that in some installations of the three differential steer system there is an accelerated rate of wear in the brake friction pads when steering with lower speed ratios of the gearbox operating. It is the purpose of this invention to provide an improved control system that may be utilised to reduce the problem of brake pad wear. The invention is characterised in that the system comprises by-pass means which enables the pressure modulating means to be by-passed when required for unmodulated line pressure to be applied directly upon operation of the steering means. In a preferred arrangement the by-pass means becomes activated to respond to operation of the steering means, by-passing the pressure modulating means, automatically upon. selection of low range gearing of a change speed gearbox of the vehicle drive line. In a convenient arrangement, selection of the low range gearing causes switching in an electrical circuit which renders the by-pass means operational in response to the steering. means, the by-pass means becoming disabled again upon deselection of the low range gearing. The by-pass means can comprise a selection valve which is adjustable between a first condition in which modulated line pressure is applied in response to operation of the steering means and a second condition in which unmodulated line pressure is applied. The valve may be a solenoid-operated slide valve comprising two inlet ports, for the modulated and unmodulated hydraulic supplies respectively, and a single outlet port leading to the brake. For operation of the valve the by-pass means can comprise an electrical circuit comprising a first (activating) switch and a second (operating) switch which becomes closed upon operation of the steering means, closure of the operating switch causing adjustment of the valve from its first condition to its second condition only when the activating switch is already closed. The steering means would conveniently comprise a steering handwheel, with a steering shaft extending from the wheel. The shaft can bear a first cam arrangement for operation of the pressure modulating means and a second cam arrangement for operation of the operating switch of the by-pass means. There now follows a detailed description, to be read with reference to the accompanying drawings, of a steering control system which illustrates the invention by way of example. In the accompanying drawings: Figure 1 illustrates a three differential steer system for a tracked vehicle; Figure 2 is a diagrammatic illustration of the arrangement of a known steering control system for such a vehicle; Figure 3 is an illustration similar to Figure 2 but of a control system in accordance with the present invention; Figure 4 is an axial section through a selection valve of by-pass means of the system shown in Figure 3; and Figure 5 is a graph illustrating performance of the system of Figure 3. Referring to Figure 1, in a typical Merritt-Wilson transmission power from the vehicle power unit is transmitted via an input shaft 6 and through spur and bevel gearing to a steer differential gear 7. At this point power is divided in three directions, through steer shafts 5 and 9 and through gear 8. Gear 8 drives gear 16, this latter gear being on an input shaft of change speed gearing 17. There are two outputs from the change speed gearing, at 15 and 18, rotating at a common speed to drive annuli 4 and 10 of summing epicyclic gears 3 and 11. Sun wheels 2 and 12 of these epicyclic units are driven from the steer shafts 5 and 9 by spur gear reduction trains. Brake discs 1 and 13 are attached to the same shafts as the sun wheels. Output shafts 14 and 19 are attached to the planet carriers of these epicylic gears and the rotational speeds of the shafts is a summation of the sun and annulus speeds. Application of a braking force by brake caliper 38 to either of the discs 1 or 13 will cause the speed of the output shafts 14 and 19 to change. Because of the differential gear 7, as one disc and its associated sun wheel slows down the other disc and its sun wheel increases speed, so creating a speed differential between the outputs 14 and 19 on opposite sides of the vehicle and resulting in a steer turn of the vehicle. Figure 2 shows a previously known steer control system. Steering means comprises a driver-operated steering handwheel 20 mounted upon a shaft 21, the other end of the shaft having attached to it a cam plate 22. This part of the shaft is mounted within a controller unit 36 and the cam plate 22 contacts operating spindles of two pressure modulating valves 23L and 23R. There is one valve for each brake caliper; for a right turn the cam plate 22 will operate on the spindle of valve 23R in response to a right hand rotation of the handwheel 20, and similarly for a left hand turn the valve 23L is operated. Thus the line pressure from the vehicle hydraulic system entering the controller unit 36 at A is modulated according to the degree of turn applied to the handwheel 20 and typical examples of pressure/control movement are shown by figure 5 at B and C. Whilst the characteristics of these curves (B and C) have proved to be adequate in many applications, it has been shown in some cases that accelerated rate of wear occurs in the brake friction pads. Figure 3 shows a control system according to the present invention which comprises selectively activatable by-pass means enabling the pressure modulating valves 23 to be by-passed, for unmodulated line pressure to be applied directly upon operation of the steering means. An additional cam plate 24 is fixed to the steer shaft 21, the second cam plate being arranged to depress plungers of by-pass operating switches in the form of micro-switches 25L and 25R. One terminal of each switch is connected to an electric energy source and the other terminal in each switch is connected to a solenoid-operated selection valve 26L or 26R. A preferred design for the selection valves 26L and 26R is shown by Figure 4, but alternative designs may be used. The valve is a solenoid-operated slide valve comprising an axially slidable plunger 27 movable within a valve body 28. The plunger is forced to the limit of a valve bore by a spring 29 and remains in this position when there is no current energising a solenoid 31. The plunger 27 is in direct contact with a central rod 30 of the solenoid and is moved axially against the spring pressure upon actuation of the solenoid. Three ports 32, 33 and 34 in the valve body 28 are arranged to co-operate with a circumferential groove 35 in the plunger 27 in the following manner:- a) In higher ratios of the vehicle gearbox no current passes to the solenoid 31 and the plunger 27 remains in the position shown by Figure 4. In this condition of the valve pressurised fluid passes from the steer controller 36 via the port 34 to the selected steer brake via port 32. This means that the controller functions to modulate line pressure (from A). b) Upon selection of low range gearing (which may be any preselected number of ratios, as required) power is directed to the micro-switches 25L or 25R by a gearbox selector control switch 37; the switch 37 forms an activating switch of the by-pass means which renders it operational when closed and disables it when opened upon deselection of the low range gearing. As a vehicle turn is initiated by the handwheel 20 the cam plate 24 causes the micro-switch (25L or 25R according to the direction of turn) to make contact and complete the electrical circuit to the appropriate solenoid. Energising the solenoid causes the rod 30 to move the plunger 27 so that the port 33 becomes opened to line pressure from A and port 34 becomes closed. This action results in hydraulic line pressure reaching the brake caliper 38 immediately a turn is initiated, by-passing the modulation of pressure normally given by the steer control unit 36. The effect is shown by curve D in Figure 5 in comparison with the modulated control shown by curves B and C. The improved system can reduce steer brake wear because the slip period is reduced by the application of high pressure at an early stage of brake application. This is feasible for lower speed ratios where lower vehicle speeds and small radius turns prevail. In higher ratios pressure modulation remains to provide a suitable brake application for the higher speed conditions.

Claims: A steering control system for a tracked vehicle in which a brake (38) can be applied hydraulically at one or the other of two transmission outputs (1, 13) on opposite sides of the vehicle to steer the vehicle, the control system comprising steering means (20, 21) whereby a driver can initiate application of the brakes, and can signal the tightness of a turn required, and pressure modulating means (23) which is responsive to operation of the steering means to control the level of hydraulic pressure applied according to the tightness of the turn called for, characterised in that the system comprises by-pass means (24, 25, 26, 37) which enables the pressure modulating means (23) to be by-passed when required for unmodulated line pressure to be applied directly upon operation of the steering means. A steering control system according to claim 1 in which the by-pass means becomes activated to respond to operation of the steering means, by-passing the pressure modulating means, upon selection of low range gearing of a change speed gear box of the vehicle drive line. A steering control system according to claim 2 in which selection of the low range gearing causes switching in an electrical circuit which renders the by-pass means operational in response to the steering means, the by-pass means becoming disabled upon deselection of the low range gearing. A steering control system according to any one of the preceding means in which the by-pass means comprises a selection valve (26) which is adjustable between a first condition in which modulated line pressure from the pressure modulating means is applied in response to operation of the steering means and a second condition in which unmodulated line pressure is applied. A steering control system according to claim 4 in which the selection valve (26) is a solenoid-operated slide valve. A steering control system according to claim 5 in which the by-pass means comprises an electrical circuit for operation of the selection valve (26), the circuit comprising a first switch (37) and a second switch (25) which becomes closed upon operation of the steering means (20, 21), closure of the second switch (25) causing adjustment of the valve (26)from its first condition to its second condition only when the first switch (37) is already closed. A steering control system according to claim 6 in which the steering means comprises a steering handwheel (20), a steering shaft (21) extending from the handwheel bearing a first cam arrangement (22) for operation of the pressure modulating means (23) and a second cam arrangement (24) for operation of said second switch (25). A steering control system according to any one of the preceding claims for a tracked vehicle having a three differential steer system.

Inventors: TURLEY, NORMAN ARTHUR, WEBSTER, DONALD JAMES
Applicants: SELF-CHANGING GEARS LIMITED
Application year: 1990
Main classification: B62D11
IPC codes: B62D11