RACK AND PINION STEERING GEAR

Abstract: A rack bearing system for supporting a rack (1) in mesh with a pinion (3). The rack (1) can be of "Y" cross-section or it can be cylindrical in shape. Rack (1) is supported by means of pads (12, 13) bearing on inclined surfaces (10, 11) of the rack (1). The pads (12, 13) are supported by means of screws (14, 15) adjustable in the housing (2) and locked by lock nuts (16, 17). Typically one pad (13) will bear on a spherical seat (18) of screw (17) in a manner which allows rocking. Similarly pad (12) will bear on leaf spring (20) which bears on a spherical seat of screw (14). By allowing movement of the rack (1) towards the pinion (3), the leaf spring (20) ensures slack-free engagement between the rack (1) and pinion (3). Alternatively, the pads are provided with convex spherical recesses in the upper ends of the screws. A coil spring is provided within the screws to force the pads into engagement with the rack. Each pad (12, 13) can pivot or tilt when movement or bending of the rack (1) occurs. In the case of the alternative construction the pads can swivel. The pads can be adjusted and so can accommodate manufacturing defects in the rack and so provides substantially uniform contact pressure between the pads and the rack.

Description: - 1 - RACK AND PINION STEERING GEAR TECHNICAL FIELD The present invention relates to rack and pinion steering gear and in particular to the sliding bearing which supports the rack in mesh with the pinion at the pinion end of such steering gear. BACKGROUND ART In conventional gears of this type ' the rack bar is cylindrical and the bearing referred to comprises a pad having a near semi-circular concave surface engaging the back of the rack on the side thereof opposite the teeth. This pad is itself generally cylindrical and slides in a bore in the housing positioned at right angles to the longitudinal axis of the rack at a point along that axis opposite the pinion. The pad is spring loaded so as to urge the rack into slack-free engagement with the pinion and the movement of the pad and hence the rack under high load is limited by an adjustable stop so as not to exceed say 0.1 mm. Now it is undesirable in a rack and pinion steering gear that there be any slack or lost motion in normal straight- ahead driving, and because a helical pinion is always used, slack will occur notwithstanding that the rack is forced into tight engagement with the pinion if side to side movement of the rack can occur . There are many patents covering improvements in the design of rack pads and one which shows several typical features is the specification of United States Patent 3,623,379. Here the bearing surface of the rack pad is not strictly semi-circular in section but comprises two arcuate faces of radii larger than the back of the rack, disposed at 45° each side of the rack centre-line. Such an arrangement allows for poor fit of the curvature of the rack to that of the pad to occur without causing slack in the steering, such a poor fit being the result of either manufacturing tolerances or because of wear of the pad as a result of the - 2 - side to side thrust of the helical pinion. Note that in these typical rack pad arrangements, the necessary clearance between the pad and the bore of the housing in which it slides, will still allow side to side movement of the rack 5 relative to the pinion, and hence slack in the steering. However the principle disadvantage of the above widely used rack pad geometry is that the contact between the rack and pad is localised into two narrow bands and, under high-load conditions as when parking, very high intensity of 0 load will occur. Furthermore, under high load bending deflection of the rack also occurs and, as the rack pad slides on a fixed axis in the housing, further localisation of load will occur at one or other end of the rack pad. This occurs because the mounting of the pad does not permit it to 5 move in such a manner as to maintain substantially uniform surface contact with the rack over its whole area when bending deflection occurs as a result, for example, of the bending of the rack in a direction at right angles to a plane containing the tips of the teeth of the rack. A similar 0 situation can arise by reason of manufacturing defects in the rack. Now it is desirable in manual rack and pinion steering gear to achieve a high efficiency at high loads as when parking, and the necessity of resorting to the use of power steering in a given model will depend on whether maximum 5 parking efforts can be kept within acceptable limits. To this end the use of low friction coefficient materials such as PTFE is desirable, but such materials cannot tolerate localised high intensities of pressure. The above-named specification claims that such materials can be used with the 0 rack pad geometry shown, but, for the reason just given, it has been found that such materials are unsuited. Other inventions have been directed at providing low-friction in rack and pinion steering gear, for example Australian Patent 515,182. In the specification of this 5 patent a two mode rack support system is provided by having - 3 - the inclined faces of the rack carried by rolling bearings so supported that they yield and move away, under high load, so that the rack moves into contact with plain bearing surfaces. This arrangement however fails to provide the highest efficiency when loads are high, as when parking, and it has been found that this latter aspect is far more important than providing high efficiency at low loads. DISCLOSURE OF THE INVENTION These short comings of the prior art are overcome using a rack support system made according to the invention, which employs two separate rack pads or support bearings, each separately supported in the housing. This arrangement eliminates the source of steering slack attributable to side to side movement of conventional rack pads referred to earlier. The support of each pad in the housing is such that the pad may align itself with the back of the rack in the area of the pad avoiding localisation of load irrespective of manufacturing errors, distortions due to load or wear of the rack, pinion and housing. In addition the pad contact areas with the rack may be of greater length along the rack axis and rectangular rather than "D" shaped as are those used in the prior art, so greatly increasing the available contact area. These improvements make possible the practical application of low friction coefficient materials for the surface of the pads. Generally, one rack pad will be spring-loaded to eliminate slack due to wear while the other pad will be mounted without compliance thus introducing an asymmetry in the rack support system not present in the known art devices. This asymmetry may be used to advantage in reducing rack rattle etc. by changing the direction of compliance of the rack support system to best resist disturbing forces transmitted to the rack by the tie rods and front suspension of the vehicle. The invention which is the subject of the present application may be most advantageously applied to racks having in the toothed area of the rack, the special "Y" section described in the specification of Australian Patent 498,666. According to the invention described in that specification the conventional concave support pad is replaced by a pad having, in section, two slightly convex faces substantially at right angles to each other which bear on the flat inclined faces of the "Y" section of the rack. This arrangement allows slight rolling of the rack to occur about the axis determined by the geometry of the "Y" section, whereas in conventional round rack bars such rolling axis is necessarily the central axis. The advantage of this arrangement is that the rolling axis of the rack may be moved towards the pinion into a plane lying within the rack teeth, so reducing the tendency of the rack to roll under load. This arrangement has proved to be successful in improving distribution of the loads on the teeth of both rack and pinion particularly when the pinion has a high helix, but results in localised rather than surface contact between the back of the rack and the rack support. Such contact is unsuited to the use of low friction materials. The application of the two pad system which is the subject of the present invention to such "Y" form racks results in the same benefits and gains referred to earlier in respect of conventional cylindrical racks. However when applied to "Y" section racks the dual pad system is very compact, as will be seen from the accompanying drawings. The pads may readily be faced with a suitable mesh reinforced low friction material, for example material known as "PAMPUS METALOPLAST" . Other objectives of the invention relate to the avoidance of slack inherent in the conventional pad system and certain detailed advantages to be described in avoiding rack rattle. These various objectives are achieved in a design of rack support systems which are lower in cost, more compact and lighter than conventional designs. The present invention consists in a rack bearing system for a rack and pinion steering gear for supporting the rack in mesh with the pinion comprising a pair of rack supporting pads each having a surface in contact with a surface of the rack opposite the pinion, each pad being supported in a housing enclosing the pinion and at least a part of the rack, characterised in that each pad is supported by mounting means such that the pad is moveable in such a manner that it will align itself with that part of the rack with which it is in contact irrespective of limited rotation of the rack about its longitudinal axis and about the two principal axes at right angles thereto thereby providing substantially uniform surface contact pressure between the pad and the rack irrespective of such rotations. It is preferred that two such pads be provided symmetrically disposed on either side of the axis of symmetry of the rack. It is further preferred that one of the pad mountings incorporate a spring, acting to force the pad against the rack and hence forcing the rack into slack-free engagement with the pinion. The surfaces of the pads can be rectangular in shape to provide maximum area of engagement with the surface or surfaces of the rack, and may be surfaced with low friction bearing material. BRIEF DESCRIPTION OF DRAWINGS In order that the nature of the invention may be better understood a preferred form thereof is hereinafter described, by way of example, with reference to the accompanying drawings in which :- Fig. 1 shows a typical arrangement of a rack and pinion steering gear in plan view, Fig. 2 is a section on line A-A of Fig. 1 of a preferred embodiment of the invention, Fig. 3 is a section on line B-B of Fig. 2 with the rack removed, Fig. 4 is a section on line A-A of Fig. 1 of the invention applied to rack and pinion steering gear employing a round sectioned rack, Fig. 5 is a scrap section on line C-C of Fig. 2 , Fig. 6 is a scrap section on line D-D of Fig. 2 , Fig. 7 is a scrap section as in Fig. 2 with the rack rolled Fig. 8 is a view of the spring 20 of Fig. 6 and Fig. 9 is a view similar to Fig. 2 of an alternative form of construction. MODES FOR CARRYING OUT THE INVENTION Fig. 1 shows rack 1 arranged to slide about its axis 7 in housing 2 which also carries the journals of pinion 3. The rack is connected by tie rods 4 to the steering arms (not shown) of the front axles and pinion 3 is connected through flexible joints to the steering wheel. It will be seen that pinion 3 has its axis 5 inclined in some small angle to the normal to the rack axis 7. Rack 1 is slideable in housing 2 in journal 6 at one end and, at the other, by the bearing arrangement shown in Fig. 2. It will be here seen that the rack is of "Y" shape in section in the toothed area, and may be either of the same section where it passes through journal 6 or may be a cylinder concentric with axis 7. Pinion 3 is carried typically by ball bearing 8 at the upper end and plain bearing 9 at the low end. The helical teeth of the pinion engage inclined teeth of the rack in the manner well known in the art. Rack 1 has, at the side opposite the teeth, flat sliding surfaces 10 and 11 which engage in surface contact rack pads 12 and 13. The surfaces of these rack pads may be either plain or covered with anti friction material for example, a fine mesh of stainless steel impregnated with PTFE (polytetrafluoroethylene) material shown in Figs. 2-7 as double cross hatched. Many other bearing surfaces may be used. As mentioned earlier rack 1 may, under load roll slightly and it is highly desirable that it do so, as outlined in the specification of Australian Patent 498,666. In the present construction the rolling of the rack is not accommodated at the journal surfaces 10 and 11 but instead pads 12 and 13 are themselves permitted to roll. To this end these pads are supported by screws 14 and 15 adjustable in housing 2 and locked, for example, by lock nuts 16 and 17. Typically, one pad e.g. 13 will bear on spherical seat 18 of screw 15 in a manner which allows slight rocking. Similarly, pad 12 ' will bear on leaf spring 20 which bears on a similar spherical seat of screw 14. The function of this leaf spring is to allow slight movement of the rack toward or from the pinion in a manner analogous to that widely used in the prior art to ensure slack-free engagement between the rack and pinion. This slight rocking of the rack is more clearly shown in Fig. 7 where the rack 1 has rotated anti-clockwise through some small angle 19. High localized pressure may occur between pad 13 and screw 15 but this will not matter as both parts may be hardened. In a like manner if, due to manufacturing tolerances or due to bending of the rack under load, as illustrated in Fig. 5, that part of the rack axis 7 which is adjacent the pad is inclined as at 7b, rack pad 13 may also become slightly inclined as at 13b in order to preserve surface contact between the pad and rack. The lower face of pad 13 may have a local spherical depression formed therein to provide a better seat between it and screw 15. Spring 20 is such that it exerts a force of say 20 Kg when nearly fully deflected - for example, so that the space between it and the pad, as at 21, is about .1 mm. If excessive loads are transmitted to the rack or the rack is caused to roll the separating forces generated between the teeth of pinion 3 and rack 1 will cause the rack to move away from the pinion, but as screw 15 permits no movement such separating forces will cause the rack to move in direction 22 Fig. 2 , thus compressing spring 20. Even under such conditions the part-spherical ends of screws 14 and 15 assure that the load between the pads and the rack faces will be distributed uniformly. An alternative construction of the invention is illustrated in Fig . 9. Here pads 32 and 33 are provided with convex spherical abutments 36 and 37 which engage concave spherical recesses or seats in the upper ends of screws 24 and 25. The necessary swivelling motions of the pads to allow conformance with misalignments of the sliding faces 30 and 31 of rack 41 may not take place without any locallizing of contact forces between the pads and corresponding screws as may occur in the earlier described construction. Such an arrangement is not suited to the use of a flat spring as in 20, Fig. 6, and hence a coil spring 39 is provided within screw 24 which acts against plunger 38 to force pad 32 into pre-coated engagement with rack 41. The lower end of spring 39 abuts retainer 40 and is so designed that it exerts a force of about 20 Kg against the pad 32. Screw 24 is adjusted at assembly of the steering gear in the same manner as previously described in relation to the setting of screw 14, and a small clearance will normally occur between abutment 36 and the seat of screw 24 (not illustrated here) corresponding to the clearance 21 of the earlier described design. In the construction described previously included angle 22 between sliding surfaces 10 and 11 of the rack (Fig. 7) is about 110°. However a lesser sngle, say 90° may be desired to be used, as indicated at 23 in Fig. 9. The exact angle chosen in a particular steering gear will depend on the geometry of the teeth of the pinion and the rack, and may be as small as, say 70°. If screws 24 and 25 had axes 34 and 35 normal to faces 30 and 31 of the rack (as in the earlier described design) screw 24 would interfere with ball race 42. The spherical seating between the pads and screws of this design allow such an angular disposition of the parts. Fig. 4 shows the invention applied to conventional rack and pinion steering gear employing a round rack bar la having axis 7a. Here screws 14a and 15a are arranged with their axes converging on axis 7a rather than axis 21 of Fig. 2 and the subtended angle between the axes of the screws may be somewhat larger than illustrated in Fig. 2. However all of the advantages described in the preferred embodiment of the invention employing a "Y" section rack will also apply in the case of a cylindrical rack. Surfaces 10a and 11a will in this case, be part of the same surface and pads 12a and 13a will be concave in section as shown. The retention of pads 12 and 13 may be accomplished at low cost as follows. Housing 2, according to current practice, is typically of die cast aluminium and hence can be provided with two slots whose edges 24 and 25 provide a guide for the pads 12 and 13 respectively. In addition screws 14 and 15 are provided with extensions 23 and 27 engaging corresponding holes in pads 12 and 13 so restraining the pads from end movement. The additional bearing surface area provided by the arrangement described as compared with accepted practice may be judged from Fig. 3. Here in circle 28 the hatched areas show the extent of the surface provided by the cylindrical rack pad made according to prior practice. The larger surface area provided by the invention is apparent. Furthermore the cylindrical rack pad conventionally used is incapable of accommodating an inclination of the rack axis from 7 to 7a as in Fig. 5. The leaf spring which is shown on the left hand side which is in practice the uppermost side of the rack in Fig. 2 may alternatively be positioned on the right hand or low side of the rack. It is sometimes hard to predict in which direction the combined forces acting on the rack from the engagement of the pinion teeth as well as from the tie-rods will cause the rack to rattle; the vehicle designer however has two possible directions of compliance either direction 22 or direction 26 (Fig. 2) . In practice it is contemplated that screw 15 will be set and permanently locked at original assembly so as to provide ideal meshing between the teeth of the rack and the pinion. Thus, if screw 14 is advanced and screw 15 is retracted by the same amount, the rack will move to the right and provide a sligntly different mesh between pinion 3 and rack 1. The optimum mesh will, in general, be provided when the least centre distance is achieved. By this means wider tolerances may be given to the various components which determine the perfect meshing relationship between the rack and pinion.

Claims: - 11 - Claims:- 1. A rack bearing system for a rack and pinion steering gear for supporting the rack in mesh with the pinion comprising a pair of rack supporting pads each having a surface in contact with a surface of the rack opposite the pinion, each pad being supported in a housing enclosing the pinion and at least a part of the rack, characterised in that each pad is supported by mounting means such that the pad is moveable in such a manner that it will align itself with that part of the rack with which it is in contact irrespective of limited rotation of the rack about its longitudinal axis and about the two principal axes at right angles thereto thereby providing substantially uniform surface contact pressure between the pad and the rack irrespective of such rotations. 2. A rack bearing system as claimed in claim 1 wherein two separate pads are provided symmetrically disposed on either side of the axis of symmetry of the rack and the mounting means for at least one pad includes means whereby the pad can be moved towards or away from the surface of the rack. 3. A rack bearing system as claimed in claim 2 wherein the mounting means for both pads include means whereby each pad can be moved independently towards or away from the surface of the rack. 4. A rack bearing system as claimed in claim 2 wherein the mounting means for at least one pad includes spring means urging the pad into contact with the surface of the rack. 5. A rack bearing system as claimed in any one of claims 2, 3 or 4 wherein the pads bear on separate inclined surfaces arranged on the back of the rack parallel to and symmetrically on either side of the longitudinal axis thereof, each pad having a flat surface engaged with one surface of the rack. 6. A rack bearing system as claimed in any one of claims 1, 2, 3 or 4 wherein each pad is provided on a surface remote from the surface of the rack on which it bears with a convex - 12 - or concave part-spherical portion which is received into a corresponding concave or convex portion on the mounting means to form a ball and socket type joint between the pad and the mounting. 7. A rack bearing system as claimed in any one of claims 1, 2, 3 or 4 wherein the surface of each pad contacting a surface of the rack is covered with an anti-friction material

Inventors: BISHOP ARTHUR ERNEST, ROESKE KLAUS JUERGEN, BISHOP, ARTHUR, ERNEST, ROESKE, KLAUS, JUERGEN
Applicants: BISHOP ARTHUR E, BISHOP, ARTHUR, ERNEST
Application year: 1983
Main classification: F16C25
IPC codes: B62D3, F16C29, F16H19, F16H55, F16H57