GB2235424A - Vehicle four-wheel steering system - Google Patents

Abstract

A front wheel steering unit drives a shaft (6) of a rear wheel steering unit 5. Carried for rotation with the shaft (6) is a gear segment (15) meshing with a further gear segment (18) which is pivotally mounted by a pin (19) on a housing (17) which is also pivotally mounted on the shaft (6). The gear segment (18) provides a lever from which tie-rods (22) extend to the rear wheels. During initial rotation of the shaft (6), the gear segment (15) moves (22) against biasing springs (23) and moves tie-rods (22) in a first direction via the gear segment (18). On continued rotation, segment (15) abuts the housing (17) and rotates in unison therewith so that the gear segment (18) is displaced with the housing (17) to move the tie-rods (22) in the opposite direction, i.e the steering applied to the rear wheels is reversed. Pivotal displacement of the housing (17) with the gear segment (15) is resisted by biasing springs (24) which react between the carrier and arms (25) on afixed frame (13). <IMAGE>

Description

TITLE "A vehicle steering system TECHNICAL FIELD & BACKGROUND ART The present invention relates to a vehicle steering system of the kind comprising a first steering unit (which may be power assisted) that is responsive to a steering input torque to effect in steering displacement of steerable road wheels in a first set and a second steering unit having a rotational input which is responsive to the steering input torque to effect in steering displacement of steerable road wheels in a second set remote from those in the first set.Systems of the kind mentioned have gained popularity in four wheeled vehicles, typically cars, in which steering displacement of one pair of wheels, usually the front pair, is controlled by the first steering unit and steering displacement of the second, usually the rear, set of wheels is controlled by the second steering unit.

The rotational input to the second steering unit can be provided by a rotational shaft which is driven from the first steering unit, for example where the first steering unit is of the rack and pinion type in which the pinion is rotatable in response to the steering input torque, the rotational drive may be derived from rotation of the pinion.

With so-called four wheel steering systems as aforementioned it is usual for the steering displacement which occurs in the rear pair of wheels under control of the second steering unit to be considerably less than the steering displacement which occurs, for a given steering input, in the front wheels under control of the first steering unit. Furthermore, it is considered advantageous with four wheel steering for the front and rear pairs of wheels to be steered in the same sense of direction during a steering manoevure at high vehicle speeds and in relatively opposite senses of direction when the vehicle is at low speed, for example during parking manoeuvres.To achieve these latter advantages relatively expensive and complicated systems have been proposed which include a sensor to be responsive to the speed of the vehicle whereby the steering characteristics change as appropriate between the front and rear wheels steering in the same or in the opposite senses of direction dependinq upon vehicle speed.

Usually, the features in a four wheel steering system which are necessary to provide the speed responsive characteristics contribute considerably to the expense and complexity of the system and it is an object of the present invention to provide a steering system of the kind specified which is not speed responsive but by which steering characteristics similar to those achieved in speed responsive systems can be provided.

STATEMENT OF INVENTION & ADVANTAGES According to the present invention there is provided a vehicle steering system comprising a first steering unit responsive to a steering input torque to effect in steering displacement of steerable raoad wheels in a first set; a second steering unit having a rotational input which is responsive to the steering input torque to effect in steerinq displacement of steerable road wheels in a second set remote from those in the first set, and wherein the second steering unit in response to an initial said rotational input initially responds from a neutral condition thereof to effect in steering displacement of the wheels in the second set in one direction and subseguently responds with further rotational input in the same sense to effect in steering displacement of the wheels in the second set in the opposite direction.

Usually the steering system of the present invention will be applied to a four wheel vehicle having a pair of front wheels steering displacement of which is controlled by the first steerinq unit and a pair of rear wheels steerinq displacement of which is controlled by the second steering unit - for convenience the invention will hereinafter be discussed with reference to such a four wheel steering system. Typically each of the first and second steering units (and the wheels steered thereby) will have a neutral condition in which there is zero steering displacement of the front and rear wheels.By the present invention it is envisaged that when a steering input torque is applied, usually by rotation of a steering column, to effect a steering manoeuvre for the front wheels, a rotational input will simultaneously be provided to effect in steering displacement (to a lesser extent) of the rear wheels. The neutral condition of the two steering units will generally correspond with the four wheels being directed straight ahead of the vehicle in a path parallel to the fore-and-aft centre line of the vehicle.The rotational input which is initially applied to the second steering unit may be arranged to actuate that unit for it to respond so that the rear wheels are steered in the same sense or in the opposite sense of direction to the direction in which the front wheels are steered; however, if continued steering input torque is applied to the first steering unit so that further rotational input is applied to the rear steering unit the latter unit responds so that there is, in effect, a reversal in the sense of steering displacement of the rear wheels whereby the rear wheels are displaced in the opposite sense or in the same sense of direction respectively to that previously mentioned.

Following the aforementioned reversal in the direction of steering for the rear wheels it will be apparent that the rear wheels may be displaced through their neutral condition. Preferably during an initial steering input and initial rotational input at the commencement of a steering manoeuvre when both steering units are displaced from their respective neutral conditions, the second steering unit effects in steering displacement of the rear wheels in the same sense of direction as the direction in which the front wheels are steered and upon continued or further steering input torque being applied to increase the steering angle of the front wheels, the continued or further rotational input to the second steering unit effects in reverse of the steering displacement of the rear wheels so that the latter steer in the opposite sense of direction to the front wheels.It will be appreciated that the amount of steering input torque and thereby rotational input which has to be applied to the system to cause a reversal in the direction of steering of the rear wheels will be determined as appropriate for the steering characteristics required of the vehicle. However the system is advantageous in so far as durinq the initial steering input torque at the commencement of a steering manoeuvre both the rear and front wheels can be arranged to steer in a similar direction which is considered to be appropriate for vehicle manoeuvres at high speed (where, other than for hazardous conditions, small input torques are normally applied to the system as during overtaking or steering lon shallow curves) while it is not unusual when effecting low speed manoeuvres (such as parkins a vehicle) for large steering input torques to be applied to the system and a condition can be attained where the front and rear wheels of the vehicle are steered in opposite senses as is considered appropriate for such low speed manoeuvres.

Although during low speed manoeuvring the front and rear wheels may initially be steered in similar senses at the commencement of a steering manoeuvre this is unlikely to adversely affect the steerinq characteristics for the relatively short period during which such circumstances are likely to exist. Similarly if sufficient steering input torque is applied during high speed manoeuvring to cause reversal in the direction in which the rear wheels are steered it is likely that such occurance will be in hazardous circumstances where the relatively small steering displacement of the rear wheels (as compared to the displacement of the front wheels) is unlikely to materially affect the situation).

Preferably the rotational input is derived from the first steering unit, for example by a drive shaft which is rotated during actuation of the first steering unit.

Alternatively the rotational input can be provided by an electrical steppinq motor which is actuated by switch means that is responsive to the steering input torque.

Preferably the second steering unit comprises a gear system which, during the initial rotational input at the commencement of a steering manoeuvre, is driven to displace an output member of the gear system between predetermined limits; displacement of the output member by the aforementioned drive through the gear system provides the steering displacement of the wheels in the second set in the one direction. At the predetermined limits of the output member the gear system is arranged to be displaceable by the continued or further rotational input to provide the steering displacement of the wheels in the second set in the opposite direction. The gear system may comprise a first toothed gear rotatable with the rotational input while the output member comprises a second toothed.

gear which is driven from the first toothed gear and pivotally mounted on a displaceable carrier. The carrier has associated therewith spring biasing means by which it is biased to a predetermined position of the second steering unit. During the initial rotational input the first toothed gear drives the second toothed gear to pivot on the carrier while the carrier is maintained substantially in its predetermined position and the pivotal displacement of the second toothed gear provides the steering displacement of the wheels in the second set in the one direction.At the aforementioned predetermined limits of displacement of the second toothed gear, continued or further rotational input to the first toothed gear displaces the carrier (together with the second toothed gear mounted thereon) against its spring biasing to provide the steering displacement of the wheels in the second set in the opposite direction. The spring biasing means may bias the carrier, substantially, to the neutral condition of the second steering unit. However, it is to be appreciated that the spring biasing means for the carrier may permit limited displacement of the carrier from the neutral condition prior to that spring biasing becoming effective.Second spring biasing means may be provided between the carrier and the gear system to bias the gear system to the neutral condition of the second steering unit and such second biasing means should be weaker than the spring biasing means which biases the carrier to the neutral condition. Preferably the first toothed gear is a first toothed segment mounted for angular displacement with a drive shaft which is rotatable by the rotational input and the carrier is pivotally mounted on the drive shaft while the second toothed gear is a second toothed segment pivotally mounted on the carrier in driving engagement with the first toothed segment. The carrier conveniently provides a housing for the first toothed segment and iS pivotally mounted concentric with an axis about which the first toothed segment is rotatable.

DRAWINGS One embodiment of a vehicle steering system donstructed in accordance with the present invention and applied to.a four wheel vehicle will now be described, by way of example only, with reference to the accompanying illustrative drawings in which: Figure 1 is a general diagrammatic view of the system Figure 2 diagrammatically illustrates a rear steering unit of the system with the unit in its neutral condition; Figure 3 is a section of the unit shown in Figure 2 taken on the line IIIw Figure 4 shows the steering unit of Figure 2 displaced from its neutral condition and to a predetermined limit in response to an initial steering input torque being applied to the system, and Figure 5 shows the steering unit of Figure 4 and displaced therefrom in response to further or continued steerinq input torque being applied to the system.

DETAILED DESCRIPTION OF DRAWINGS The steering system shown in Figure 1 has a pair of front steerable road wheels 1, steering displacement of which is controlled by a front steering unit 2 in response to a steering input torque provided by rotation of a steering column 3. The system also includes a pair of rear steerable road wheels 4 steering displacement of which is controlled by a rear steering unit 5 in response to a rotational input applied to a drive shaft 6 thereof.

The front steering unit 2 is, for convenience of description, a simple form of rack and pinion gear in which the teeth of a pinion 7 rotatable with the steerinq column 3 are in driving engagement with a rack bar 8 longitudinally displaceable through a rack bar housing 9 to effect steering of the wheels 1. Conventional steering linkages are provided between the ends of the rack bar and the wheels 1. An additional 'pinion 7a is in driving engagement with the rack bar 8 to be rotated thereby and is coupled through a transmission shaft 10, universal joints 11 and a step-down gear unit 14 to the drive shaft 6 of the rear steering unit 5 so that the steering column 3, pinions 7 and 7a and drive shaft 6 are rotated in response to the application of a rotational steering input torque being applied to the steering column to effect a steering manoeuvre.

The drive shaft 6 for the rear steering unit 5 (which is shown in Figures 2 to 5) is mounted for rotation about its axis 12 in a fixed frame part 13. The maximum steering displacement which is to be provided for the rear wheels 4 will be considerably less than the lock-to-lock displacement which is provided for the front wheels 1, say in the ratio of 4:1, and this is achieved by the step down gear unit 14 in the rotary transmission to the drive shaft 6. The angular rotation which is intended to be imparted to the shaft 6 will likely be considerably less than 3600 typically such angular rotation will be in the range of 250 to 450 during adjustment of the front steering unit 2 from lock-to-lock of the front wheels.

Secured to the shaft 6 for partial rotation therewith is a gear segment 15 having an annular array of gear teeth 16 which are concentric with the axis 12 and disposed in a plane which is perpendicular to the axis 12. The gear segment 15 is housed within a carrier 17 that is pivotally mounted on the shaft 6 to be capable of rotation on the shaft (and relative to the gear segment 15) about the axis 12. A second gear segment i8 is pivotally mounted on the carrier 17 by a pin 19 so that an annular array of gear teeth 20 on the gear segment 19 are positioned in the same plane as the gear teeth 16 and enaage therewith. The pin 19 on which the gear segment 18 is rotatable extends parallel to the axis 12 and it will-be apparent from Figure 2 that when the gear segment 15 is displaced arcuately about its axis 12 and relative to the carrier & the gear teeth 16 will drive the gear segment 18 through its teeth 20 to pivot the gear segment 18 about the pin 19. The gear segment 18 extends from the carrier to provide a lever or drop arm having a ball peg 21 from which is taken the steering output of the rear steering unit. In Figures 1 and 2 the ball peg 21 is shdwn coupled to rods 22 which form part of conventional steering linkages to the rear wheels 4.

Mounted in the carrier 17 are opposed biasing springs 23 for abutment with opposed side' faces of the gear segment 15 so that the reaction of the springs 23 between the gear segment 15 and carrier 17 biases the gear segments to be positioned centrally with respect to the carrier and symmetrically about a radial line from the axis 12 as shown in Figure 2.

Substantial pivotal displacement of the carrier 17 about the axis 12 is restrained by a pair of opposed biasing springs 24 which are located on opposed sides of the carrier to react one each between a pair of arms 25 which extend from the fixed frame 13 and a flange 26 which forms a projection of the carrier 17. Thrust plates 27 for the springs 24 are provided on the respective free ends of the arms 25. The biasing force provided by each spring 23 is considerably less than that provided by each spring 24. The arms 25 are pivotally mounted by pins 28 on the fixed frame 13 to be capable of pivotal movement in a plane which extends perpendicular to the axis 12. Pivotal movement of the arms 25 is limited by studs 29 on the frame 13 engaging in slots 30 in the arms 25 which slots are of arcuate form concentric with the pins 28.

The rear steering unit 5 as shown in Figure 2 is in its neutral condition corresponding to zero displacement of the rear wheels 4 (as would apply-when the front steering unit 2 is also in its neutral condition and zero steering displacement is provided for the wheels 1). With the units 2 and 5 in their neutral conditions the four wheels would normally be in a neutral condition and directed to steer along a path parallel to the fore-and-aft centre line of the vehicle.It will be seen from Figure 2 that with the rear steering unit in its neutral condition the gear segments are disposed centrally and symmetrically within the carrier 17 as aforementioned, the arms 25 (through the reaction of the biasing springs 24) are pivotted to a position with the studs 29 disposed centrally within the arcuate extent of their slots 30,'and the pin 19 and peg 21 are positioned in a plane which includes the axis 12 and about which plane the gear unit is substantially symmetrical. With such a symmetrical structure it will be apparent that the reaction of the rear steering unit in response to a rotational input drive being imparted to the shaft 6 will be the same but in relatively opposite senses for when the drive shaft 6 is driven clockwise or anticlockwise in Figure 2.For convenience therefore operation of the rear steering unit will be considered only when the drive shaft 6 is rotated anti-clockwise (as indicated by the arrow 31 in Figure 2) and which results from a steering input torque being applied to actuate the rack and pinion unit 2 to effect steering displacement of the front wheels 1 in one sense of direction.During the initial application of such a steering input torque to the system and resultant initial partial rotation of the drive shaft 6 at the commencement of a steering manoeuvre from the neutral conditions of the wheels 1 and 4, the gear segment 15 rotates anti-clockwise in Figure 2 in unison with the drive shaft to drive the gear segment 18 causing the latter to pivot about the pin 19 in a clockwise sense as shown in Figure 4. Spch pivotal displacement of the gear segment 18 displaces the steering rods 26 longitudinally and to the left in Figure 2 to impart steering displacement to the rear wheels 4 in, a similar sense of direction to the steering provided for the front wheels 1.In this initial stage of drive shaft rotation the gear segments 15 and 18 are both rotationally displaced relative to the carrier 17 and the segment 15 reacts against and compresses one of the biasing springs (as shown in Figure 4) until a limit is reached where the gear segment 15 abuts against the carrier 17 (as shown in Figure 4). During this initial response by the unit 5, the carrier 17 and arms 25 may be maintained in their centralised position by the drag'reaction or resistance to displacement which is applied to the carrier 17 (from the rear steering linkage and rods 22'through the gear segment 18).

Upon the gear segments 15 and 18 reaching their predetermined limit of angular displacement with respect to the carrier 17 (by abutment of the segment 15 with the carrier), further or continued rotational displacement of the drive shaft 6 in the direction of arrow 31 causes the gear segment 15 to pivot the carrier 17 from its central position and the carrier to be angularly displaced in the direction of arrow 31 in unison therewith. Such rotational displacement of the carrier 17 displaces the gear segmentl8 rightwardly from the position shown in Figure 4 to that shown in Figure 5 so that the steering rods 22 are similarly displaced rightwardly to effect a reversal in the steering displacement of the rear wheels 4 whereby they are steered in an opposite direction relative to the front wheels 1.Because the studs 29 are initially located centrally within the slots 30, immediately following abutment of the gear segment 15 with the carrier 17 the pivotal displacement of the carrier is not resisted by the biasing springs 24 (since the arms 25 can pivot on the pins 28 in unison with the carrier 17) until the studs 29 abut an end of their respective slots as shown in Figure 5. Consequently the initial rightward displacement of the rods 22 following reversal as aforementioned is not restrained by the biasing springs 24 as the rear wheels 4 return towards (and possibly move through) their neutral condition.If the steering input torque which is applied to the system is sufficient to rotate the drive shaft 6 in the direction of arrow 31 beyond a condition where the arms 25 are restrained from pivoting by abutment of the studs 29 with the ends of their respective slots 30, the continued displacement of the carrier 17 will be resisted by the biasing effect of the springs 24 as the steering rods 22 continue to be displaced rightwardly and effect further steering displacement of the rear wheels in the opposite sense of direction to the steering which is applied to the front wheels.

Following a steering manoeuvre the rear steering unit 5 will follow a reverse path and steering characteristics to that described as the drive shaft 6 is partially rotated in a clockwise direction to revert the steering units to their neutral condition.

During normal steering manoeuvres at high vehicle speeds (as when overtaking or steering on long shallow curves) relatively small rotational input torques are applied to the steering column 3 and under these conditions it is to be expected that the angular rotation of the drive shaft 6 will not be sufficient to displace the gear segment 15 from the neutral condition to a position where the carrier 17 is pivotally displaced in unison with the gear segment; consequently the rear wheels 4 will normally be steered in a similar sense of direction to the front wheels 1 as is considered advantageous for manoeuvring at high speed. However, during low speed manoeuvres, particularly when parking, large rotational input torques are frequently applied to the steering column which would rotate the drive shaft 6 sufficiently to displace the gear segment 15 beyond the limit where it abuts the carrier 17 (so that the carrier 17 is displaced in unison with the gear segment and against the biasing of springs 24) causing the rear wheels 4 to reverse and be steered in an opposite sense of direction to the front wheels 1 as is considered advantageous for low speed manoeuvring (especially when the front wheels may be at or near their full lock).

Claims (10)

1. A vehicle steering system comprising a first steering unit responsive to a steering input torque to effect in steering displacement of steerable road wheels in a first set; a second steering unit having a rotational input which is responsive to the steering input torque to effect in steering displacement of steerable road wheels in a second set remote from those in the first set, and wherein the second steering unit in response to an initial said rotational input initially responds from a neutral condition thereof to effect in steering displacement of the wheels in the second set in one direction and subsequently responds with further rotational input in the same sense to effect in steering displacement of the wheels in the second set in the opposite direction.

2. A system as claimed in claim 1 in which the second steering unit comprises a gear system which during said initial rotational input is driven to displace an output member thereof between predetermined limits, displacement of the output member by drive through the gear system providing the steering displacement of the wheels in the second set in said one direction and wherein at said predetermined limits of displacement of the output member the gear system is displaceable by said further rotational input to provide the steering displacement of the wheels in the second set in said opposite direction.

3. A system as claimed in claim 2 in which the gear system comprises a first toothed gear rotatable by the rotational input and the output member comprises a second toothed gear driven from the first toothed gear and pivotally mounted on a displaceable carrier, said carrier having spring biasing means by which it is biased to a predetermined position in the second steering unit, and wherein during said initial rotational input the first toothed gear drives the second toothed gear to pivot on the carrier and the pivotal displacement of the second toothed gear provides the steering displacement of the wheels in the second set in said one direction and at a said predetermined limit of pivotal displacement of the second toothed gear, the further rotational input to the first toothed gear displaces the carrier and the second toothed gear mounted thereon to provide the steering displacement of the wheels in the second set in the opposite direction.

4. A system as claimed in claim 3 in which the spring biasing means permits limited displacement of the carrier from its predetermined position prior to that spring biasing becoming effective.

5. A system as claimed in either claim 3 or claim 4 in which second spring biasing means is provided between the carrier and the gear system to bias the oear system to a predetermined position relative to the carrier and wherein said second spring biasing means is weaker than the spring biasing means which biases the carrier to its predetermined position.

6. A system as claimed in any one of claims 3 to 5 in which the first toothed gear is a first toothed segment mounted for angular displacement with a drive shaft rotatable by said rotational input; the carrier is pivotally mounted on said drive shaft and the second toothed gear is a second toothed segment pivotally mounted on the carrier in driving engagement with the first toothed segment.

7. A system as claimed in claim 6 in which the carrier provides a housing for the first toothed segment and is pivotally mounted concentric with an axis about which the first toothed segment is angularly displaceable.

8. A system as claimed in any one of claims 3 to 7 in which the said predetermined limits are determined by abutment of the first toothed gear with the carrier.

9. A system as claimed in any one of the preceding claims in which the rotational input is derived from the first steering unit and by way of a gear unit which provides a step down ratio.

10. A vehicle steering system substantially as herein described with reference to the accompanying illustrative drawings.