IMPROVEMENTS RELATING TO ELECTRIC POWER STEERING SYSTEMS

Abstract: An electric power steering system for vehicles of the kind is disclosed in which torque from an electric motor drives a steering column through a gearbox, in operation, electric signals passed to a control unit from a torque sensor mounted on the steering column are used to control the drive of the electric motor. The gearbox comprises meshing members (1, 2), and the members are resiliently biased into engagement with each other to remove backlash for at least the lower end of the operating torque range.

Description: IMPROVEMENTS RELATING TO ELECTRIC POWER STEERING SYSTEMS This invention relates to improvements in electric power steering systems for vehicles of the kind in which torque from an electric motor drives a steering column through a gearbox. In operation, electric signals passed to a control unit from a torque sensor mounted on the steering column are used to control the drive of the electric motor. In most electric power steering systems (EPAS) the gearbox comprises a single reduction worm and wheel set, with a ratio typically between 12: 1 and 20: 1 in order to achieve a compact motor. In order to achieve optimum mechanical layout, it is also advantageous to use a right-angle gearbox in most applications. To obtain satisfactory efficiency and, in particular, to be able to back-drive the motor from a shaft from the steering column in the event of a motor failure, it is necessary to employ a plastics material in the construction of the gear wheel, such as acetal and poly amide. This is because a relatively low sliding friction, 6 to 7% with lubrication, is achieved when such a gearwheel co-operates with a steel worm. In such gearboxes, even when using the best design and manufacturing practices, an amount of backlash, namely gear tooth to worm tooth clearance, of up to 150 microns can arise. This is roughly attributable, in equal measures of 50 microns each, to (1) component manufacturing variations, (2) different thermal expansions of the various materials being used, and (3) wear of the plastics gearwheel during the life of the gearbox. Manual adjustments to the meshing of the worm and wheel would allow only for correction of item (1) and are, in any case, difficult to design into a gearbox of this type at an economic cost. It should be noted that backlash levels in excess of 5 microns can give rise to significant audible rattle on those road surfaces which cause torque vibrations to be fed back to the steering column from the wheels of the vehicle. This occurs mainly when steering at or near to the "straight ahead" position. The aim therefore must be to achieve substantially zero backlash, at least whilst low torque levels are being produced in the system. This is when the rattle problem mainly rises. Because of a predominately sliding nature of the gearing action, the use of heavy forced meshing, for example by spring loading the worm and the wheel relatively towards each other, as a method of removing backlash, would produce large levels of friction. Such friction would be present at all torque levels and result in unacceptable steering feel. The preferred strategy is to use forced meshing but only up to a level sufficient to eliminate backlash during the transmission between substantially zero to substantially 10% of the maximum torque rating of the unit. Hence the meshing force needed, and its associated quiescent friction, will only be substantially 10% of that which would arise if the backlash were forced to zero for all torque values. We are also aware of another type of gearing which, in a single pair of members, can produce the required reduction ratio together with the desired efficiencies. Such gearing is known as a spiroidal gear set, comprising a special gearwheel with teeth on one face, plus a pinion. According to one aspect of our invention in an electric power steering system of the kind set forth the gearbox comprises meshing members, and the members are resiliently biased into engagement with each other to remove backlash for at least the lower end of the operating torque range. Arranging for the resilient biasing to be effective only at the lower end of the operating torque range reduces or substantially eliminates rattle without any substantial increase in friction throughout the total operating torque range. When mass production techniques are to be employed the meshing members can be produced by insert moulding and sintering. The resilient means may comprise a thin flexible ring attached to the gear wheel. The annular portion defining the supplementary gear may occupy an annular space inside the inner diameter of the said remainder of the gear wheel. In modifications the supplementary gear may be situated at the outside of the outer diameter of the said remainder of the gear wheel, or it may be housed in an annular groove intermediate the inner and outer diameters of the gear wheel. Torque generated at the supplementary gear is transmitted to a hub, and hence to a shaft, through the flexible ring. Preferably the flexible ring is clamped to the hub using the same screws as are used for securing the gear wheel to it. Mounting the supplementary gear in such a manner gives it the necessary freedom of movement in an axial direction whilst restraining it rotationally in a manner which is free of back lash. When the gearbox in of the worm and wheel type the supplementary gear includes a flexible web which provides resilient biasing for the lower end of the operating torque range thereby defining the resilient means. The web may be of any convenient construction in order to provide the necessary degree of inherent flexibility. For example the web may be of a reduced wall thickness in comparison with the remainder of the gear. Although it may be of planar outline, in another construction it may be curved to increase flexibility. In either construction the web may also be formed with a plurality of angularly spaced radial slots or openings to lend additional flexibility to the web. According to a second aspect of our invention in an electric power steering system of the kind set forth the gearbox comprises meshing members which are resiliently biased into engagement with each other, and one of the members comprises a gear wheel in turn comprising a gear, a hub, and a clamping member, the hub and clamping member being clamped against opposite faces of the gear by clamping devices incorporating resilient means so constructed and arranged as to allow a given clearance between the gear, the hub and the clamping member. The gear may include a radial flange formed with the plurality of angularly spaced openings, and each resilient means may comprise a washer of which the peripheral edge co-operates with the edges of a complementary opening. Preferably spacers are housed between the hub and the clamping member. In one construction each opening has the profile of a pentagon, with curved sides. In operation the meshing members are resiliently biased into engagement with each other for at least the lower end of the operating torque range. Some embodiments of our invention are illustrated in the accompanying drawings in which :- Figure 1 is a view of a known spiroid/spiradrive gear set for an electrical power assisted steering (EPAS) system; Figure 2 is a section on the line 2-2 of Figure 1 ; Figure 3 is a plan of a gear wheel for a gearbox of the worm and wheel type; Figure 4 is an inverted plan of the gear wheel; Figure 5 is a section through the gear wheel; Figure 6 is a view on an enlarged scale showing the installation of the wheel in a gearbox; Figure 7 is a plan of another gear wheel; Figure 8 is an inverted plan of the gear wheel; Figure 9 is a section through the gear wheel; Figure 10 is a view on an enlarged scale showing the installation of the gear wheel in a gearbox; Figure 11 is a plan of yet another gear wheel; Figure 12 is an inverted plan of the gear wheel; Figure 13 is a section through the gear wheel; Figure 14 is a view on an enlarged scale showing the installation of the wheel in a gearbox; Figure 15 is a plan of yet another gear wheel; and Figure 16 is an exploded view of the gear wheel of Figure 15. The known spiroid/spiradrive gearset illustrated in Figures 1 and 2 of the accompanying drawings comprises a gear wheel 1 of plastics material, and a steel pinion 2 having meshing teeth of spiral formation. The pinion 2 is arranged chordally with respect to the gear wheel 1 and with its main axis normal to that of the gear wheel 1. The gear wheel 1 is mounted on a metal hub 3 and is secured to a flange 4 projecting radially outwards from the hub 3 by means of a plurality of angularly spaced screws 5 which are located on a pitch circle of constant diameter and are screwed into tapped holes 6 extending through the flange. The gear wheel 1 is clamped between the flange 4 and an annular clamping ring 7 provided with counterbored recesses 8 to receive the heads of the screws 5. The hub 3 is keyed to a shaft 9 leading to a steering column, and the hub 3 and the shaft 9 are journalled for rotation in spaced bearings 10 and 11 housed in spaced walls of a housing of a gear box. The pinion 2 is similarly journalled for rotation in a bearing 12. In operation the pinion 2 driven from an electric motor imparts a drive to the shaft 9 through the gear wheel 1. In the embodiment illustrated in Figures 3-6 of the accompanying drawings the gear wheel 1 comprises a supplementary gear wheel 20 in the form of a one-piece plastics moulding for a gearbox of the worm and wheel type. The supplementary gear 20 comprises a web 30 of reduced thickness which integrally connects a spirably toothed peripheral rim 31 for meshing with the worm 2 to an inner annular radial flange 32. Angularly spaced openings 33 in the flange 32 receive the screws 5 for clamping the flange 32 against the flange 4 of the hub 3. The web 30 is of sinusoided outline to facilitate installation in a limited space and yet provide the necessary inherent resilience or flexibility to achieve the necessary freedom of movement in an axial direction whilst restraining it rotationally in a manner which is free of backlash. In the embodiment illustrated in Figures 7-10 of the accompanying drawings the web 30 of reduced thickness is planar in outline and is formed with a plurality of angularly spaced radial slots or openings 36 of generally segmental outline to achieve the desired degree of flexibility and inherent resilience. Regions 37 between adjacent pairs of slots 36 act as fingers which flex to enable the rim 31 to deflect with respect to the flange 32. In the embodiment illustrated in Figures 11-14 of the accompanying drawings the web 30 is of arcuate outline facing away from the gear wheel 1 , and is formed with a plurality of slots or openings 38, similar to the slots or openings 36. In the embodiment of Figures 15 and 16 the gear wheel 1 comprises an annular peripherally toothed gear 40 of synthetic plastics material, suitably acetal, having an inwardly directed radial flange 41 surrounding a central aperture 42, an internally splined hub 43, and an annular clamping member 44. The gear 40 may comprise a one-piece moulding. The flange 41 is formed with a plurality of circumferentially spaced openings 45, each having a profile of a pentagon with inwardly curved sides 46, and with lands 47 in the form of teeth between adjacent openings 45 lying in a circle concentric with the hub 43. As illustrated the flange 41 is clamped between the hub 43 and the clamping member 44 by clamping devices comprising circumferentially spaced screws 48 which are passed through openings 49 in the clamping member 44 and screwed into tapped complementary openings 50 in the hub 43. A spacer 51 on each screw 47 clamped between the clamping member 44 and the hub 43, is surrounded by a circular washer 52 of elastomeric material and each member is received in a respective one of the openings 45 in the flange 41. The spacers 51 are so arranged as to allow a clearance, substantially in the order of 1000 microns, between the gear 40, the hub 43, and the clamping member 44. This therefore allows the gear 40 to essentially "float" albeit constrained by the elastic properties of the washers 52 as they engage the curved sides 46 of the respective openings 45. Utilising openings 45 of pentagonal outline to house the washers 52 ensures substantially that radial compliance is uniformly low whilst maintaining fairly stiff torsional compliance. In a modification mechanical steps may have to be included to prevent overtravel in the torsional direction.

Claims: CLAIMS 1. An electric power steering system for vehicles in which torque from an electric motor drives a steering column through a gearbox, characterised in that the gearbox comprises meshing members (2, 20) , and the members are resiliently biased into engagement with each other to remove backlash for at least the lower end of the operating torque range. 2. A system according to claim 1 , in which the resilient means comprises a thin flexible ring (30) attached to the gear wheel (20). 3. A system according to claim 1 or claim 2, in which an annular portion defining the supplementary gear (20) occupies an annular space inside the inner diameter of the remainder of the gear wheel. 4. A system according to claim 1 or claim 2, in which the supplementary gear is situated at the outside of the outer diameter of the remainder of the gear wheel. 5. A system according to claim 1 or claim 2, in which the supplementary gear is housed in an annular groove intermediate the inner and outer diameters of the gear wheel. 6. A system according to any preceding claim, in which the torque generated at the supplementary gear (20) is transmitted to a hub, and hence to a shaft, through the flexible ring (30). 7. A system according to claim 6, in which the flexible ring is clamped to the hub using the same screws as are used for securing the gear wheel to it. 8. A system according to any preceding claim, in which the gearbox is of the worm and wheel type and the supplementary gear includes includes a flexible web (30) which provides resilient biasing for the lower end of the operating torque range thereby defining the resilient means. 9. A system according to claim 8, in which the web is of a reduced wall thickness in comparison with the remainder of the gear. 10. A system according to claim 9, in which the web is of planar outline. 11. A system according to claim 9, in which the web is curved to increase flexibility. 12. A system according to claim 10 or claim 11 , in which the web is also formed with a plurality of angularly spaced slots or openings to lend additional flexibility to the web. 13. An electic power steering system for vehicles in which torque from an electric motor drives a steering column through a gearbox, characterised in that the gearbox comprises meshing members which are resiliently biased into engagement with each other, and one of the members comprises a gear wheel (1) in turn comprising a gear (40), a hub (43), and a clamping member (44) , in which the hub and clamping member are clamped against opposite faces of the gear by clamping devices incorporating resilient means so constructed and arranged as to allow a given clearance between the gear, the hub and the clamping member. 14. A system according to claim 13, in which the gear includes a radial flange (41) formed with the plurality of angularly spaced openings (50) , and each resilient means comprises a washer (52) of which the peripheral edge co-operates with the edges of a complementary opening. 15. A system according to claim 14, in which spacers are housed between the hub and the clamping member. 16. A system according to claim 14 or claim 15, in which each opening has the profile of a pentagon, with curved sides. 17. A system according to any one of claims 13 to 16, in which the meshing members are resiliently biased into engagement with each other for at least the lower end of the operating torque range.

Inventors: DUROCHER DANIEL JOHN, DUROCHER, DANIEL, JOHN
Applicants: DUROCHER DANIEL JOHN, TRW LUCASVARITY ELECTRIC STEER, DUROCHER, DANIEL, JOHN, TRW LUCASVARITY ELECTRIC STEERING LTD.
Application year: 1999
Main classification: B62D5
IPC codes: B62D5, F16H55