The present invention concerns the field of electric power steering for motor vehicles, and more particularly the speed reducer making it possible to transmit the torque produced by an electric assistance motor to the mechanical steering link connecting the steering wheel of the vehicle to the steered wheels.
An electric power steering system for a motor vehicle generally includes a mechanical part comprising a steering wheel linked in rotation to a steering column, the end of which that is remote from the steering wheel carries a steering pinion engaged with a rack, slidably mounted in a steering case. The two opposite ends of the rack are respectively linked, via rods, to the right and left steering wheels of the vehicle. To assist the manual effort exerted by the driver of the vehicle on the steering wheel, such a steering system comprises an electric assistance motor with two rotational directions, the output shaft of which is coupled, by means of a speed reducer to the mechanical steering link between the steering column and the steered wheels of the vehicle, so as to transmit a motor torque (possibly also a resistive torque) for assistance to the steering column. The electric assistance motor is controlled by an on-board electronic computer, which receives and processes various signals, coming from sensors including in particular a sensor of torque exerted on the steering column by the driver of the vehicle.
There are various known speed reducer devices, in particular provided with worm and worm wheel. Japanese patent application No. JP2006-117049 describes a worm speed reducer including a gap compensation spring that includes elastic blades. The spring is shaped to clips onto a bearing of the worm located on the side opposite to the motor and moves with it, inside an oblong housing machined in the case accommodating the speed reducer. The housing is closed with a waterproof plug. The elastic blades are arranged to hold the worm against the worm wheel, and thus compensate the gap between the worm and the worm wheel. This gap is due in particular to geometric dispersions inherent in the manufacture of mechanical components, to temperature variations, and to normal operating wear. The portion of the case accommodating the spring has a complex, oblong shape with a cavity to accommodate the elastic blades. This complex shape can only be obtained by an additional machining operation that is complex, long, and requiring very high precision of shape and positioning, compared to the other machining operations necessary to form the housing of the speed reducer within the case.
It is therefore desirable to be able to provide a worm speed reducer that can reduce the noise and vibrations of the shock and rattle type (“backlash/rattle noise”) generated when driving on uneven ground (cobblestones, uneven road, road joints, etc.) or during a reversal steering. It is also desirable to be able to reduce the number of parts, the complexity of the machining, and the size of such a speed reducer, while increasing its service life.
Embodiments relate to a speed reducer comprising: a case, a worm disposed in a housing of the case and including a proximal portion coupled to an input shaft, a worm wheel coupled to an output shaft and arranged so to be driven in rotation by the worm, a proximal bearing holding the proximal portion of the worm in the housing, a distal bearing holding a distal portion of the worm, the distal bearing being disposed in a cylindrical distal portion of the housing, a spring held fixed in the distal portion of the housing around the distal bearing, the spring comprising at least one elastic blade disposed and shaped to rest on the case and to exert forces on the distal bearing in a direction towards the worm wheel.
Thanks to these arrangements, the spring is fixed in the case and the distal bearing moves in the spring. Thus, the friction and the wear of the housing due to the displacements of the spring or of the bearing in the housing, generally made of aluminum, are reduced. Friction occurs mainly between the spring and the distal bearing which are generally made of steel, the coefficient of friction between two steel parts being relatively lower than the coefficient of friction of a steel part on an aluminum part. In addition, it is possible to eliminate the side gap between the spring and the distal bearing, by varying the shape and the flexibility of the spring.
Furthermore, the distal portion of the housing on the spring side has a cylindrical shape, which is therefore easy to manufacture. Consequently, the machining of the distal portion of the housing can be carried out in the same operation as the machining of the housing of the proximal bearing on the motor side, and therefore without having to open the distal portion of the housing to carry out this machining. It can thus be ensured that the two bearings are perfectly coaxial. Such non-through machining makes it possible to save a plug, a possible joint and assembly operations thereof.
If the space between the spring and the case is small, the two blades are caused to deform only slightly without exceeding their elastic limit.
In the event that the speed reducer or the input and output shafts may be subject to shocks, these arrangements make it possible to reduce the noise or vibrations of the type of shock and rattling noises generated in the speed reducer, when driving on uneven ground or during a reversal steering.
According to one embodiment, the spring comprises a protrusion provided so as to be engaged in a recess formed in the distal portion of the housing, in order to block the spring in rotation in the housing.
Thus, the locking in rotation of the spring in the case can be achieved simply, without requiring complex machining.
According to one embodiment, the protrusion has a U-shape extending radially outwardly of the spring.
Thus, the direction of action of the blades can be easily changed, by changing only the position of the protrusion on the spring. Indeed, the change of this direction can be useful to carry out adaptations of the reducer according to geometrical reducer characteristics, such as the intersecting angle, the helix angle, and the pressure angle.
According to one embodiment, the spring comprises flat side portions disposed and shaped to guide the distal bearing in the direction towards the worm wheel and in an opposite direction, and eliminate a side gap between the spring and the distal bearing. In this way, the worm can be held precisely in a median plane of the worm wheel and thus prevent an error in the intersection angle which would be detrimental to the quality of the mesh. In addition, the elimination of such a gap helps to reduce the noise likely to be generated in the event of shocks caused to the reducer and the input and output shafts.
According to one embodiment, the flat side portions are extended radially inwardly of the spring by tabs cooperating with the distal bearing to block the spring axially in a distal direction.
Thus, the axial retaining tabs of the spring on the distal bearing can be formed by bending of protrusions extending the flat areas of the annular portion of the spring, without affecting the cylindrical shape of the major portion of the spring. The presence of the tabs extending the flat portions also makes it possible to stiffen these latter.
According to one embodiment, each elastic blade has a curvature and a variable width between its fixed end and its free end, adjusted so as to obtain a curve of variation of the force exerted by the blade on the distal bearing as a function of a position of the distal bearing in the spring.
Thus, impact noise can be reduced by adjusting the shape and curvature of each elastic blade.
According to one embodiment, the curve of variation of the force exerted by each elastic blade on the distal bearing as a function of a position of the distal bearing in the spring is linear with a relatively low slope, then more rapidly increasing in the vicinity of an end of stroke of the distal bearing in the direction towards the worm wheel.
According to one embodiment, the spring comprises an annular portion extending over an angular sector comprised between 240° and 300°, each elastic blade having a free end and a fixed end secured to the annular portion.
According to one embodiment, the spring comprises two elastic blades having a width lower than a height of the spring and arranged so as to intersect in an area diametrically opposite to a contact area between the worm and the worm wheel.
In this way, the contact forces exerted by the spring on the distal bearing are balanced and oriented in the direction of the worm wheel.
Embodiments may also relate to a power steering for a motor vehicle, the power steering comprising a speed reducer coupled between an assistance motor and a rotary member of a steering system of a motor vehicle, the speed reducer being as previously defined.
According to one embodiment, the worm wheel of the speed reducer is secured to a steering column of the steering system.
According to one embodiment, the worm wheel of the speed reducer is secured to a pinion shaft coupled to a rack pinion of the steering system.
According to one embodiment, the worm wheel of the speed reducer is secured to a pinion shaft coupled to an additional rack pinion of the steering system.
According to one embodiment, the worm wheel of the speed reducer is secured to a force feedback steering column of a steering system without any mechanical link between a steering wheel and steered wheels of the motor vehicle.
According to one embodiment, the power steering comprises another worm speed reducer and worm wheel, the worm wheels of the speed reducer and of the other speed reducer being coupled respectively to rack pinions of the steering system.
The present invention will be better understood through the following description with reference to the appended figures, in which identical reference signs correspond to structurally and/or functionally identical or similar elements.
To assist the manual effort exerted by the driver of the vehicle on the steering wheel 2, each of the power steering devices 1a, 1b, 1c comprises a motor system SM comprising an electric assistance motor M with two rotational directions and a speed reducer 17. The output shaft of the motor M is coupled, by means of the speed reducer 17 to the power steering of the vehicle, so as to transmit a motor torque (possibly also a resistive torque) to the steering. The motor system SM also comprises a control unit (on-board electronic computer) ECU, which receives and processes various signals from sensors and in particular a torque sensor 13 and supplies control signals to a control circuit DC of the engine M. The speed reducer 17 may be of the type comprising a worm formed by a worm shaft 18 and a worm wheel 19 secured to the power steering of the vehicle. The torque sensor 13 comprises for example a torsion bar 12 interconnecting an upstream portion to a downstream portion of a vehicle steering shaft. A motion detector 13 coupled to the torsion bar 12 provides a measurement of the relative rotational movement between the upstream and downstream portions of the steering shaft on either side of the torsion bar 12.
In a power steering device of the C-EPS type (
In a power steering device of the DP-EPS type (
In a power steering device of the P-EPS type (
The device 1e (
However, the housing of the reducer in the case 117a of oblong shape along an axis defined by the direction X1 and with the cavity 136, is machined in an additional, long operation (contouring), requiring very high precision of shape and of positioning compared to another type of machining of the case 117a.
After assembly, the housing of the reducer in the case 17a must be obturated, and in a tightly sealed manner, in particular for systems that must be mounted under the cover, such as the P-EPS and DP-EPS systems.
The annular portion 31 comprises a proximal annular edge 38, a distal annular edge 39 and side ends 37 facing each other. Each side end 37 is partly extended by an elastic curved blade 32, 33. The blades 32, 33 have a width lower than half the height of the spring and extend over a length lower than the distance between the longitudinal edges 37, so as to close the cylindrical shape and to intersect for example at mid-distance between the longitudinal edges 37. Thus, the spring 30 has a shape symmetrical with respect to a plane XZ passing through a longitudinal axis Z of the worm 8 and perpendicular to an axis of rotation of the wheel 19.
The distal edge 39 of the spring 30 is extended by tabs 34, 34a extending radially inwardly of the annular portion 31. The tabs 34 are provided to block the spring 30 axially on the distal bearing 23 in the proximal direction. In the distal direction, the spring 30 is blocked by the bottom of the housing 17b or a shoulder formed near thereto. Furthermore, the bearing 23 can be retained axially in the proximal direction by an annular shoulder 18a provided at the distal end of the worm 18. The bearing 23 is also blocked axially in the distal direction by its press-fitting on the worm 18. The blades 32, 33 are arranged and shaped to exert a force on the distal bearing 23 in the direction X1 towards the worm wheel 19, by bearing on the inside of the case 17a.
The tabs 34, 34a each extend a flat portion 35, 35a of the annular portion 31. The flat side parts 35 are disposed and shaped to laterally block the spring 30 in the case 17a in order to eliminate any side gap of the bearing 23 and of the worm 18, and to guide the bearing 23 in its movements along the direction X1 and the opposite direction. The annular portion 31 comprises a protrusion intended to be engaged in a recess 36 formed in the case 17a, in order to block the spring 30 in rotation (around the Z axis) in the case 17a.
According to one embodiment, this protrusion is formed by a U-folding of the strip forming the annular portion 31, so as to move the flat portion 35a radially outwardly of the annular portion (
In this way, the spring 30 is fixed relative to the case 17a. The flat portion 35a is for example located in a position radially opposite to the intersection area of the blades 32, 33. The flat portion 35a also forms an index making it possible to define the direction of the forces exerted by the blades 32, 33 on the bearing 23, relative to the case 17a. The direction of the forces exerted by the blades 32, 33 can be finely adjusted by adjusting the position, relative to the direction X1, of the shape 35a on the annular portion 31 of the spring 30, which can be manufactured for example by stamping and/or or bending of a spring blade. It should be noted that the flat portion 35a is not in contact with the case, and only the side portions connecting the flat portion 35a to the remainder of the spring define the angular position of the spring 30 around the Z-axis.
According to one embodiment, the flat portions 35 are stiffened by the formation of folds. However, it should be noted that the extension of the flat portions 35, 35a by bent tabs contributes to stiffening these portions.
Under assistance torque, the elastic blades 32, 33 retract unfoldingly by pressing against the case 17a as the worm 18 moves in the opposite direction to the wheel 19. Thanks to this unfolding, the local deformations of the elastic blades 32, 33 are very low and therefore the risks of plastic deformation and fatigue failure of the elastic blades are minimized.
The curvature of the elastic blades 32, 33, towards the bearing 23, and their profiles with varying section along the blades are defined so as to exert forces whose intensity is defined according to the stroke. According to one embodiment, the curvature and the shape of the blades 32, 33 are defined so that the value of the exerted forces varies according to the stroke, first linearly with a slight slope, then increases more rapidly at the end of stroke. Thus the value of the exerted forces increases rapidly on approaching the abutment position of the bearing 23 in the case 17a, due to the unfolding of the blades (shortening of the lever arm). Thus, the shock noise likely to occur due to a brutal contact of the bearing 23 at the end of stroke against the case 17a is reduced.
In abutment, the radii of curvature of the outside of the bearing 23, the unfolded blades 32, 33 and of the housing in the case 17a are close. This results in contacts with well-distributed pressures, which avoids pressure stress concentrations that are too localized, which could be harmful to the service life of the spring 30 and in particular of the blades 32, 33.
According to an embodiment illustrated by
According to an embodiment illustrated in
It will clearly appear to those skilled in the art that the present invention is subject to various embodiments and various applications. In particular, the invention is not limited to the shape of the spring 30 previously described. Indeed, the spring 30 may include only one elastic blade arranged to exert on the distal bearing 23 a force in the direction of the worm wheel 19.
In addition, the spring can be held in the case by a means other than a protrusion engaged in a recess in the case. Thus, for example, the recess can be formed in the spring and the protrusion can be formed in the case.
Furthermore, the speed reducer can be used in other mechanical systems than a motor vehicle power steering system.
Number | Date | Country | Kind |
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2207095 | Jul 2022 | FR | national |