FOURWHEEL STEERING FOR MOTOR VEHICLE

Abstract: Hydraulic steering device for steering the wheels (6) on a rear axle (5) of a truck relative to its front wheels (2). The wheels on the rear axle are steered by a servo motor (20) in a high-pressure circuit (19). The servo motor is controlled by a control valve (22), which is operated by a slave cylinder (14) in a separate low-pressure circuit (12). The displacement of the slave cylinder follows the displacement of a master cylinder (13) coupled into the low-pressure circuit, said master cylinder being coupled to the steering mechanism (8, 30) of the front wheels in order to provide a piston displacement dependent on the degree of turning of the front wheels.

Description: Fourwheel steering for motor vehicle The present invention relates to a hydraulic steering mechanism for motor vehicles, for steering wheels on a first wheel axle relative to the wheels on a second wheel axle, comprising a pressure fluid activated servo motor coupled to steering mechanism for the first wheel axle, and which is connected via control valve means to a pressure source, and means for actuating the control valve means dependent on the steering movement of the wheels on the second axle. Vehicles with four-wheel steering, i.e. with means for steering the wheels on two parallel wheel axles, have been known for many years. Originally they were primarily directed to steering a rear pair of wheels on a heavy vehicle relative to the front wheels, the rear pair of wheels being steered via a system of rigid linking arms from the steered front wheels. In recent years, however, very advanced hydraulic or electrohydraulic systems for four-wheel steering have been developed especially for passenger cars. Such an advanced hydraulic system is shown for example in EP 0 284 022. In this case a pressure which is dependent on the speed of the vehicle is used to determine whether the rear wheels of the vehicle are to be turned oppositely to or with the front wheels. When driving at low speed, hydraulic pressure in the front wheel servo system is led to a servo motor for the rear wheels, and this servo motor then turns the rear wheels in the opposite direction to the front wheels. The pressure in the front wheel servo system is counterbalanced by the speed- dependent pressure in a control valve, and when the latter pressure at a certain speed exceeds the servo pressure, the control valve is switched over so that the speed-dependent pressure thereafter produces turning of the rear wheels in the same direction as the front wheels. The purpose of the present invention is, starting from the known hydraulic system for four-wheel steering, to provide a simple and reliable hydraulic mechanism, especially for trucks, of the type described by way of introduction, which can replace the linkage systems used up to now to thereby make it possible to reduce the effective load-carrying surface of the vehicle and at the same time eliminate demanding assembly work. This is achieved according to the invention by virtue of the fact that said means for actuating the control valves have a hydraulic proportioning device coupled to a steering mechanism for steering the wheels on the second axle, said proportioning device being disposed to provide a volume of fluid dependent on the turning angle of said wheels to a second servo motor which is coupled between the first servo motor and the control valve means in such a manner that fluid supply to the second servo motor with accompanying activation thereof results in the control valve means opening and activating the first servo motor, which in turn in its activated state causes the second servo motor to close the control valve means. A steering mechanism of this type can be designed to be very simple. The proportioning device and the second servo motor can consist of a pair of double-acting hydraulic cylinders, of which the first is a master cylinder, which follows the steering angle of the front wheels, and the other is a slave cylinder, the stroke of which is proportional to that of the first. These can be connected in a separate low pressure circuit separate from the hydraulic circuit in which the servo motor effects the actual steering of the rear wheels. This reduces stresses on the components and increases reliability and useful life. The invention will be described more in detail below with reference to an example shown in the accompanying drawing. The Figure shows schematically a three-axle truck with a simplified coupling scheme of a steering mechanism according to the invention. In the Figure, 1 designates the front axle of the vehicle with steerable front wheels 2. 3 designates the rear axle with rear wheels 4, and 5 designates an intermediate axle, a so-called "pusher axle" with steerable wheels 6. The front wheels 2 are coupled via a conventional linkage system 7 to a steering gear with a servo motor 8, coupled into a hydraulic 5 circuit 9 with a pump 10 and a tank 11. The front wheel servo circuit is of conventional design and is hydraulically separated from a low pressure circuit (generally designated 12) containing first and second hydraulic piston-cylinder devices 13 and 14, respectively, of which the first forms a 0 master cylinder and the second a slave cylinder. The circuit 14 is connected via check valves 15,16, safety valves 17 and magnetic valves 18 to the tank 11. A third hydraulic circuit 19 separate from the hydraulic 5 circuits 9 and 12, contains a servo motor in the form of a hydraulic piston-cylinder device 20 and a pump 21 which provides substantially higher pressure in the circuit 19 than the pressure in the circuit 12. A control valve 22 coupled to the pump 21 and the tank 11 regulates the flow to and from o the respective cylinder chambers of the hydraulic cylinder 20. The control valve 22 can be of any known type which is used for steering the piston stroke of a double-acting piston- 5 cylinder device and need not be described in more detail here. The cylinder 23 of the servo motor 20 is hinged 24 to the intermediate axle 5 while its piston rod 25 is coupled to a 0 steering arm 26 belonging to the steering linkage system of the wheels 6. The piston rod 25 is also joined to the piston rod 27 of the slave cylinder 14 in such a manner that the piston rods 25,27 move synchronously. The cylinder housing 28 of the slave cylinder 14 is limitedly displaceable in the 5 axial direction of the cylinder and is operatively connected in a manner not shown in more detail here to a valve element biased towards a neutral position in the control valve 22. In the neutral position the connections between the cylinder chambers on either side of the servo motor piston and the pump 21 and tank 11, respectively, are broken. The piston rod 29 of the master cylinder 13 is joined to the steering arm 30 of the steering gear 8 while its cylinder is axially fixed. The steering mechanism described here thus functions as follows: A certain movement of the steering arm 30 turns the front wheels 2 of the vehicle and in dependence thereon displaces the piston rod 29 of the master cylinder 13. A volume of a fluid dependent on the piston movement and the cylinder diameter will thus be supplied to one cylinder chamber of the slave cylinder 14, and a corresponding volume will be drained from its other cylinder chamber. Since the servo motor 20 in its initial stage is blocked by the control valve 22, the cylinder housing 28 of the slave cylinder 14 will be displaced axially and this results in the control valve opening so that a pressure medium is supplied via the pump 21 to one cylinder chamber of the servo motor and is drained from its other chamber with resulting turning of the wheels 6 on the intermediate axle 5. As the piston rod 25 of the servo motor 20 begins to move, the slave cylinder 14 is moved towards its neutral position by virtue of the coupling between the piston rods 25 and 27. Thus, the control valve 22 is also returned to its neutral position and the supply of pressure medium to the servo motor 20 is cut off. The pressure in the low pressure circuit 12 is limited to a certain predetermined maximum pressure by the safety valves 17, which, when the maximum pressure is exceeded, drain the excess fluid to the tank. In a practical embodiment, the pressure can be limited to about 25 bar. If, for some reason, there should be a fluid shortage in the circuit 12, the check valves 15,16 will then open (since there will be a slight underpressure in the system) . This can result in the master and slave cylinders 13 and 14, respectively, getting out of phase with each other. Such an out-of-phase state can be measured with position sensors (not shown) coupled to each cylinder, which sensors control magnetic valves 18 to open them for draining the circuit 12 to the tank on either side of the respective piston. When driving straight ahead, the wheels 6 of the intermediate axle 5 are automatically returned to the straight ahead-position by the self-aligning torque acting on the wheels. As soon as the position sensors sense this position, the magnetic valves are closed.

Claims: CLAIMS 1. Hydraulic steering device for motor vehicles, for steering wheels on a first wheel axle relative to the wheels on a second wheel axle, comprising a pressure-fluid activated servo motor coupled to a steering mechanism for the first wheel axle, and which is connected via control valve means to a pressure source, and means for actuating the control valve means dependent on the steering movement of the wheels on the second axle, c h a r a c t e r i z e d in that said means for actuating the control valves (22) have a hydraulic proportioning device (13) coupled to a steering mechanism (8,30) for steering the wheels (2) on the second axle (1), said proportioning device being disposed to provide a volume of fluid dependent on the turning angle of said wheels to a second servo motor (14) which is coupled between the first servo motor (20) and the control valve means (22) in such a manner that fluid supply to the second servo motor (14) with accompanying activation thereof results in the control valve means opening and activating the first servo motor (20) , which in turn in its activated state causes the second servo motor to close the control valve means. 2. Steering device according to Claim 1, characterized in that the proportioning device (13) and the second servo motor (14) are coupled together in a separate second hydraulic circuit (12) , separated from said pressure source (21) and the hydraulic circuit (19) of the first servo motor (20) , in which second hydraulic circuit there is a lower pressure than in the first mentioned circuit. 3. Steering device according to Claim 1 or 2, characterized in that the proportioning device (13) and the second servo motor (14) are formed of a pair of hydraulic piston-cylinder devices of which the first serves as a master cylinder and the second as a slave cylinder, the displacement of which is proportional to the displacement of the master cylinder. 4. Steering device according to Claim 3, characterized in that either the cylinder housing (28) or the piston rod (27) of the slave cylinder (14) cooperates with the control valve means (22) while the other of said components is connected to an element (25) for turning the wheels on the first wheel axle and belonging to the first servo motor (20) , and that the cylinder housing (28) of the slave cylinder is limitedly displaceable in the axial direction relative to the control valve means. 5. Steering device according to one of Claims 1 - 4, characterized in that the proportioning device (13) is coupled to the steering mechanism (8,30) to the front wheels (2) on a truck and that the first servo motor (20) is coupled to a steering mechanism (26) to an additional steered axle (5) on the vehicle.

Inventors: WENDEBERG STAFFAN, WENDEBERG, STAFFAN
Applicants: VOLVO AB, AB VOLVO
Application year: 1991
Main classification: B62D7
IPC codes: B62D7