FR2761038A1 - COUPLING MECHANISM OF A RACK AND ITS DRIVE GEAR - Google Patents

Abstract

The coupling mechanism of a rack (5) against a pinion (1) for translational drive of the rack (5) comprises a thrust spring (17) of the pinion (1) and of the rack (5) l 'against each other, spring arranged to exert its action according to at least two successive degrees of thrust and there is provided, between the rack (5) and the spring (17), a support (14) against which the rack rubs ( 5), and a guide (15) of the support (14).

Description

Coupling mechanism of a rack and its pinion

  drive. In a motor vehicle, the steering wheel controls the orientation of the steered wheels through a gear consisting of a pinion, integral with the steering column, and a rack cross bar, controlling the orientation of the shafts of the two hubs. affected wheel. To ensure correct coupling, the rack is pressed onto the pinion by a

coupling mechanism.

  Such a mechanism must maintain a coupling without play of the meshed teeth, despite the eccentricity defects of the pinion, its axial play and the wear of the teeth. In addition, it must withstand without destruction a percussion from a wheel striking an obstacle and which may correspond

almost to the mass of the vehicle.

  The coupling mechanism conventionally comprises a spring

  helical housed in a housing to push the rack against the pinion.

  The spring is compressed between a stopper stopper, closing its housing, and a sliding thrust piece on the rack bar, on which

slides this one.

  The end piece is guided in sliding by a skirt which extends it towards the plug and bears on the internal wall of the housing. An elastic stop washer is interposed between the free edge of the skirt and the plug

  to absorb the shocks transmitted by the wheels.

  In addition, washers and spring setting rings and washers are provided, respectively, to compensate for manufacturing tolerances and

  dilations of the different parts.

  Such a mechanism has many drawbacks. It is hardly economical in terms of parts and labor and, moreover, its effectiveness is limited, because the adjustment of tolerances, which must be adjusted manually during assembly, is never perfect. In particular, it remains noisy and the pushing force on the rack is likely to vary during the

  time, making the steering too soft or on the contrary too hard.

  The present invention aims to reduce the drawbacks indicated above.

  To this end, the invention relates to a mechanism for coupling a rack against a pinion for driving in translation of the rack, comprising means for pushing the pinion and the rack against each other, characterized by the fact that the pushing means are arranged

  to exert their action following at least two successive degrees of thrust.

  Thus, the damping of vibrations and shocks, which tend to disengage the rack from the pinion and produce noises, is increased since the second degree of thrust replaces the first degree as soon as

  vibrations or shocks exceed the possibilities of action of the first degree.

  There is thus an increased pushing force, better coupling the rack and the

  gable and therefore reducing noise.

  The invention will be better understood using the following description of two

  preferred embodiments of the mechanism of the invention, with reference to the accompanying drawing, in which: - Figures 1 and 3 are longitudinal half-sections of the mechanism of the invention according to the two respective variants, and - Figure 2 shows force / stroke characteristics of the mechanism's springs. The coupling mechanism 10 of FIG. 1, of which only the left half, relative to a longitudinal plane of symmetry 20 thereof, is shown for the sake of simplicity, serves to push back a rack rod 5, sliding on it perpendicular to the plane of the figure, against a pinion 1 for driving in translation of the rack 5 controlled by a column

direction 2.

  The mechanism 10 is mounted integral with a casing carried by the chassis of a motor vehicle, which supports the rack 5 as well as the pinion 1 and the

  steering column 2, shown uncut.

  In this example, the casing comprises a housing 11 receiving the mechanism 10. Although it is this housing 11 which is cited below, it will be understood that only the joining by the chassis between the mechanism 10 and the pinion 1 is necessary, forming counter-support for rack 5,

  to press these last two pieces on top of each other.

  The mechanism 10 here has a generally cylindrical shape extending along an axis 21 defined by the intersection of the plane 20 and the plane of Figure 1, substantially perpendicular to the direction of translation of the

rack 5.

  The housing 11 is presented as a well of corresponding cylindrical shape, leading to the rack 5. The mechanism 10 comprises a piston 13, a stopper plug 12, which can be fixed to the housing 11 to serve as a stop support for a spring 17, pushing piston 13 against

rack 5.

  The piston 13, here in one piece, consists of a portion 14, forming the pushing support of the rack 5, and of a portion 15 for guiding the sliding of the support 14. The portion 15, hollow cylindrical, is guided along the axis 21 by a wall, integral with the housing 11, precisely here longitudinal ribs equidistant angularly on the internal wall of the

housing 11.

  Two friction pads 141 are provided here, attached to the support 14, having a low coefficient of friction relative to the steel of the rack 5. Any self-lubricating material, for example polytetrafluoroethylene (PFTE), can be used. polyethylene (PE) or polyethylene terephthalate (PET), the latter can also be used to manufacture the piston 13 in its entirety. In this example, the section of the rack 5 is not circular but has two dishes cooperating with the support 14, arranged in V relative to each other, so that the latter blocks in rotation the

rack 5.

  Inside the cylindrical portion 15 is housed, in this example, a mass of vibration-absorbing material 22, in the zone situated on the back

platelets 141.

  The spring 17 is housed, with axial compression, between the plug 12 and the support 14. The spring 17 is here of a single piece, made of elastic material, here io thermoplastic rubber such as polyurethane or silicone, and comprises in this realization three portions, 171, 172, 173, which are here

united with each other.

  As explained below, the portions 171 and 172 push the rack 5 according to two successive degrees of thrust. In this example, the portions 171 and 172 are respectively in the form of a buffer and a

  skirt extending it by surrounding a central recess 175.

  The support 14 is interposed between the pad 171 and the rack 5, while

  the free end of the skirt 172 bears on the plug 12.

  The portion 173 is in the form of a shock absorbing collar attached to the free end of the skirt 172 and extending between the

  edge 16 of the guide portion 15 facing the plug 12 and the latter.

  A radial protuberance 174 of the flange 173 (shown without stress) is pressurized against the internal surface of the housing

11 for sealing.

  The plug 12, made of plastic, consists for example of polyamide (PA), polysulfone (PSU), polyacetal (POM) or even polyetherimide (PEI). It serves, as indicated, abutment support spring 17, by securing with the housing 11 and here more precisely the free end thereof. The plug 12, here housed almost entirely in the housing 11, comprises flexible lugs or tongues 121 angularly distributed and projecting radially, relative to the axis 21, to cooperate with corresponding recesses 111 of the housing 11 during the introduction of the plug 12 therein. Each lug 121 is integral with a tongue 122, remaining slightly projecting axially from the housing 11 once the plug 12 is mounted, to allow manual letting of the lug 122 radially and release it from the notch 111, in order to then withdraw without obstructs the plug 12. In addition to this purely mechanical stop function, the plug 12 also provides in this example

  the sealing by its bearing wall of the spring 17, which is full.

  The operation of the mechanism 10 will now be explained in more detail.

details.

  As has been understood, the spring 17, which does not require any adjustment, presses the rack 5 on the pinion 1 in order to compensate for the play due to the imperfections of the geared parts and to the shocks coming from the wheels through the bar.

rack 5.

  The spring 17 has, in the absence of any constraint, an elongation such that, once compressed by the plug 12, it exerts a bearing force F of at least 45 daN, that is to say greater than that of a mechanism

  conventional, and preferably 60 to 80 daN (Figure 2) or even more.

  The resistance force of friction of the rack 5 on the support 14 being for example fixed at 20 4 daN by the automobile manufacturers, a force F of support of 80 daN, chosen in this example, imposes to fix at 0,25 the

  coefficient of friction of the pads 141 relative to the rack 5.

  As shown schematically by the overflow of the bearing edge of the flange 173 on the edge 16, the flange 173 is at rest slightly crushed axially by the plug 12 to make up for any play and avoid separation of the parts, which would be a source of clicks. This also provides axial sealing and contributes slightly to the repulsion of the piston 13. Therefore, the three portions 171, 172, 173 of the spring act concurrently, but to varying degrees depending on the position of the piston 13. To determine the shape of the spring 17, unconstrained, making it possible to obtain, by nominal compression, the restoring force of 80 daN, we can first consider the pad 171 and the skirt 172 in series. We choose a section and an axial length of the recess 175 such that the skirt 172, of reduced section, the most flattened, is substantially (C1) at the end (high stroke side C) of the linear part R1 of its compression curve as a function of the force F applied, with the range Cd of dynamic variation necessary for taking up play (normal stroke of the piston 13, in the absence of shock). To be rigorous, the curve RI represented is in fact a mixed characteristic, taking all the more into account the value of stiffness of the skirt 172 that it is low compared to that of the pad 171. The flange 173 must remain in slight support on the edge 16 of the piston 13 and therefore exerts an additional return force to be taken

  into account. It could have been made up of a separate room.

  In the event of a shock causing the piston 13 to move back deeply towards the plug 12, the skirt 172 quickly reaches its maximum compression threshold and it is then the buffer 171, with greater stiffness (R2) which works fully to provide additional force opposing the recoil of the piston 13, therefore according to a characteristic R2 force / compression with higher slope. There are thus two successive degrees of thrust. The flange 173 has the same functionality (R3) as the buffer 171 but with a lesser stroke providing a third degree of thrust. It could have been provided that the flange 173 also comprises, a stack of two springs, therefore in series, having different stiffnesses, to maintain the contact, desired to avoid a percussion noise, between the flange 173 and the edge 16 while by limiting, by the lesser stiffness of one of the two springs above comprising it, its contribution to the restoring force as long as the piston 13 is

within its planned range.

  As shown in FIG. 2, in which the line segment R0 represents the characteristic of a spring of the coupling mechanism of the prior art, the slope at the origin of the curve R1 according to the invention is lower than that of the R0 segment. The resting compression point P1 (80 daN) therefore corresponds to a resting compression distance C1 greater than that, CO, of the point P0 of the curve R0. The point P0 here corresponds to a force F of 40 daN and therefore to a coefficient of friction of 0.5 to have the required friction force of 20 daN. Due to the reduced slope, the variation A F1 of force in the dynamic range

  of travel Cd from point P1 is less than that, A F0, for point P0.

  Expressed differently, this reduced slope would allow a dynamic travel range much greater than Cd, exceeding 1 mm in this example, without excessive variation in the force F applied. The firmness of the steering wheel is thus almost constant, especially since it has been provided that the pads 141 are made of abrasion-resistant material, for example self-lubricating plastic, so that the maximum travel take-up stroke Cd, which was in the prior art about 0.4 mm (0.2 mm of eccentricity defect or jumping of teeth and 0.2 mm of wear) is in fact reduced (not shown) to 0.2 mm

tooth jump.

  All the other parts of the mechanism 10 apart from the spring 17, are made of plastic material with good temperature resistance over a wide range (40, + 130 C), resistant to oils, greases and petroleum products and low

  absorption of liquids, in particular water repellent.

  In order to prevent collapse by aging of the portion 172 and to modify, if necessary, its mechanical characteristic, a helical spring can be provided in the recess 175, extending along the axis 21 of the mechanism. The piston 13 is guided in the housing 11 by the latter in this example. It could however have been expected that it is the plug which performs this function, since it is secured to the housing 11, therefore to the chassis. In this case, the plug would also have an appendage extending it axially (21) in the housing 11. This appendage may have the shape of a skirt, possibly reduced to a cage formed of simple longitudinal tongues, for receiving the guide portion. 15, guiding it axially. Guiding, internal to the spring 17, by axial rods of the plug passing through the portions 172 and 171 can also be envisaged, as can a single central rod, the recess 175 then preferably being eccentric. It will be understood that, in this case, the guide rod or rods have a length such that there remains a gap between their free end and the support 14 and that shock-absorbing pads can then be provided in this or these

  air gaps to replace, or cooperate with, the stop collar 173.

  Likewise, the cylindrical portion 15 of the piston 13 can itself have a function of taking up play and / or of a stop damper, that is to say that, exposed very schematically, its wall may include, on a certain axial length, a helical slot to form a spring, with possibly several portions of different stiffnesses, in order to obtain one or more of the curves of the genus R1, R2 and R3. It is thus possible to remove any stop spring, such as 173, the edge 16 then being applied in

permanently on the plug 12.

  0 It will be noted that the coupling mechanism of the invention can, as here, be used to push the rack against the pinion or, conversely, push the pinion or its shaft against it. More generally, and throughout the field of mechanics, the mechanism of the invention makes it possible to better control the coupling of two parts applied to one another, whether they are in simple support or else gears, by a rotary or linear gear. One can for example think of a use as shock absorber of a leaf spring supporting a variable load. By joining, for example by gluing, the damping spring (17) with the two parts of the mechanism (12, 13) which it couples, one can reverse the direction of the force which it exerts on the piston and, by adding to the latter of a hook, attract any desired piece. In this case, in order to limit the stretching constraint of the spring portion with low stiffness 172, it can be provided that the massive portion 171 is slightly reduced in diameter to form, at its junction with the portion 171, a radial shoulder facing the bearing 14. A corresponding reduction in the internal diameter of the housing 11, in its part close to the bearing 14, then makes it possible to create a shoulder

  turned towards the plug to limit the elongation of the weakened part 172.

  It is understood that, in general, the shape of the section of the spring 17 can be arbitrary and that it is the same for the relative position of the springs (171, 172) functionally stacked, whose constitution of each does not depend only specific cases. The spring 17 can therefore be made of a naturally elastic material or of a material, such as steel, which has been given a shape allowing sufficient elastic deformation,

like a propeller.

  In the variant of Figure 3, elements similar to those of Figure 1 bear the reference. Only the differences from the realization of the

  Figure 1 are set out below. There are two of these differences.

  They bear, on the one hand, on the support 14, then in one piece in matter

  plastic, therefore free of insert (141), as mentioned above where it is indicated that the piston 13 can be entirely made of PET.

  The support 14 in a single part is thus made of a material with a low coefficient of friction. On the other hand, the shape of the spring, referenced 37, is different. The spring 37 has a tubular shape 372 all along with, at the end, two opposite centering tubes 123 and 143 belonging respectively to the plug 32 and to the support 14. A flange 373, homologous to the flange 173, having at its periphery a portion of seal 374 (see 174), extends radially

  between the edge 16 and the opposite plug 32.

  As shown in FIG. 3, the tubular part 372 is, at rest, at a distance from the internal surface of the cylindrical portion 15 of the piston 13, and has

  in this example a curved external shape.

  Likewise, the flange 373 is not, at rest, pinched between the plug 32 and the edge 16 and is here at a distance from the latter. Furthermore, in this example, it has a variable section, this variation being illustrated in the direction

  radial for the clarity of the drawing, in the quantified form of two stages.

  The operation of the mechanism 30 is as follows.

  When the piston 13 moves back towards the plug 32, the spring 37 is axially crushed and the elastic material, free radially, tends to creep radially, therefore transversely to the detection of thrust of the spring 37. The cylindrical part 372 thus deforms in radially expanding until it reaches the internal surface of the cylindrical guide portion 15 of the piston 13. The domed portion here promotes buckling towards the outside, by radial expansion, of the spring 37. Compared to a simple compression without buckling, which could however have been provided, the compressive strength of the spring 37 is thus reduced as soon as the axial force crushing it causes buckling. As soon as the internal surface of the guide 15 is reached, the buckling can almost no longer take place and the resistance to axial compression of the spring 372 therefore increases (cf. portion R 2 of FIG. 2). Thus, the spring 37 comes by buckling in contact with the guide 15 which limits the buckling and the spring 37 bears on the guide 15 to push back the support 14, that is to say increase (R 2) the variation of the restoring force, when the latter compresses the spring 37 beyond a determined stroke. The

  guide 15 thus acts as a hoop for spring 37.

  It is understood that the spring 37 could have various other shapes, solid or hollow, possibly without symmetry of revolution, insofar as they allow it to expand radially up to a limiting surface thereof, located or not at the radial section of the spring undergoing

maximum radial expansion.

  Independently of this, the flange 373 comes to be pinched by the edge 16 which moves back and, having a variable section, it opposes the recoil a force which increases rapidly, on the one hand, as the compression of the volumes already concerned increases. and, on the other hand, with it, the percentage of compressed surface area of the flange 373, that is to say volumes of material adjacent to the preceding ones, until the entire mass of the flange 373 is stressed. thus acts of new slope breaks, (globally R 3) coming in

  more than that (R 2) due to the cylindrical part 372 of the spring.

  It could also have been provided, in addition or instead, an axial expansion, possibly by buckling, towards the hollow interior of the spring 37, up to an axial support rod integral with the plug 32 or the support 14 or even floating. It could even have been provided for the internal wall of the spring 37 to bear on itself during its expansion towards the axis 20, that is to say

  that this expansion throttles the internal channel of the spring 37.

he

Claims (15)

  1.- Coupling mechanism of a rack (5) against a pinion (1) driving in translation of the rack (5) comprising thrust means (17, 37) of the pinion (1) and the rack (5) one against the other, characterized in that the thrust means (17, 37) are arranged to exert their action according to at least two successive degrees (R1, R2) of thrust. 2. A coupling mechanism according to claim 1, in which support means (14) are provided, between the rack (5) and the thrust means (17), against which the rack rubs (5), and means (15) for guiding the support means (14).   3.- coupling mechanism according to one of claims 1 and 2, in   which the pushing means comprise a pad (171) of elastic material extended by a skirt (172) of the same material with degree of pushing   (RI) lower than that (R2) of the buffer (171).   4. A coupling mechanism according to claim 3, in which the   thrust means (171, 172) are in one piece.   5.- coupling mechanism according to one of claims 3 and 4, in   which the push skirt (172) comprises a damping collar (173).   6.- coupling mechanism according to claims 2 and 5, wherein   the damping collar (173) extends between the guide means (15) and stop means (12) for the pad (171) and the skirt (172) of thrust.   7.- Mechanism according to one of claims 2 to 6, wherein the means   support (14) and the guide means (15) are in one piece.   8.- Mechanism according to claim 7, wherein the guide means (15) are arranged in a housing (11) closed by a stopper plug (12)   for the push pad (171) and the skirt (172).   9. Mechanism according to claim 8, in which the stopper plug   (12) is extended by a skirt for receiving the guide means (15).   10.- Mechanism according to one of claims 2 to 9, wherein the means   support (14) carry a friction plate (141).   11.- Mechanism according to one of claims 2 to 9, wherein the means   support (14) are made in one part from a low-material coefficient of friction.   12.- Mechanism according to one of claims 1 to 11, wherein the   thrust means (37) are arranged to come, by expansion transverse to their thrust direction, into contact with means (15) for limiting   expanding and building on them to exert their push.   13.- Mechanism according to claim 12, wherein the means for   thrust (37) are arranged to deform by buckling.   14.- Mechanism according to claim 12, wherein the means for   guide (15) integrate the limiting means.   15.- Mechanism according to one of claims 12 and 13, wherein the   pushing means (37) are arranged to constitute the limiting means.