Abstract:
A thrust device, in particular for a roller cam mechanism in a steering system, includes a first mechanism member which is held on a frame in engagement with a second mechanism member. The thrust device comprises a pressure piece which is mounted on the frame so as to be displaceable along an axis and is acted on by a spring element for pressing the first and second mechanism members against one another. In order to construct the thrust device for compensating for wear and thermal expansion of the engagement of the mechanism members and to configure it to be inexpensive and quiet during operation, there is provision for the spring element to be formed from rolling bodies which are spring loaded approximately transversely with respect to the axis of the pressure piece, the rolling bodies being supported on oblique surfaces between the pressure piece and a frame part which is configured as a setting screw or cover. The pressure piece can additionally be spring loaded axially by a compression spring.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a thrust device, in particular  5  for a roller cam mechanism in a steering system.  
         [0002]     DE 100 51 306 A1 describes a gear mechanism, in particular for a vehicle steering system, having a pinion which is configured as a first mechanism member and is connected fixedly to a drive shaft of a servomotor so as to rotate with it. The pinion is in engagement with a second mechanism member which is configured as a gear wheel. The drive shaft is held pivotably in a locating bearing and a floating bearing. The floating bearing is held radially displaceably in a frame which is configured as a mechanism casing. The floating bearing is pressed onto the gear wheel by a spring element in the radial direction via a supporting ring which serves as a pressure piece. The supporting ring is held radially displaceably in a slot in the mechanism casing and is secured against rotation via the spring element.  
         [0003]     During operation of the gear mechanism, the development of noise by the thrust device cannot be ruled out, as the pressure piece comes into contact with the mechanism casing when the drive shaft is pivoted to its maximum, structurally predefined extent.  
         [0004]     DE 38 35 947 A1 describes a steering system which is configured as a rack and pinion steering system for a motor vehicle and has a rack which is guided axially displaceably in a frame which is configured as a casing. A pinion which is connected fixedly to a steering handle so as to rotate with it is inserted into the casing, the pinion meshing with toothing of the rack. The pinion forms a first mechanism member and the rack forms a second mechanism member. A pressure piece which is arranged in the casing so as to be displaceable along an axis, is acted on by a spring element and acts on the rear side of the rack ensures a lack of play between the pinion and the toothing of the rack.  
         [0005]     In addition, an adjusting element is arranged in series with the spring element and produces a lack of play in the arrangement of the pressure piece and the spring element. In this way, a thrust device is represented which avoids rattling noise in the casing during operation of the steering system. The thrust device has a multiplicity of components and the installation space of the thrust device is not minimized.  
       SUMMARY OF THE INVENTION  
       [0006]     The invention is based on the object of providing a thrust device for mechanism members which makes low-noise operation of the mechanism members possible with a simple construction and a thrust force which can be varied in a narrow range.  
         [0007]     The thrust device has a spring element which makes it possible to act on the pressure piece permanently with a spring force which varies in a narrow range and also ensures constant contact of fixed components between the pressure piece and the frame, as viewed in the radial direction of the mechanism members, as a result of which impact between the components of the thrust device and the development of noise is avoided. Instead of providing a thrust device having a plurality of devices for adjusting the pressure piece and for action by a spring force, the thrust device has a spring element which both makes possible permanent application of a spring force which varies in a narrow range to the pressure piece and also has a self-adjusting function.  
         [0008]     For this purpose, as viewed in the axial direction of the axis of the pressure piece, the spring element is arranged between the pressure piece and a frame part on which the first and second mechanism members are held. The spring element comprises rolling bodies which are spring loaded approximately transversely with respect to the axis of the pressure piece and are supported in each case at least on an oblique surface. Here, the oblique surface is configured in each case obliquely with respect to the axis of the pressure piece at an angle which opens radially toward the axis or at an angle which opens obliquely away from the axis. Accordingly, the rolling bodies are spring loaded with a spring in the radial direction away from or toward the axis, and are pressed in each case against the oblique surfaces. In order, in particular, to minimize hysteresis of the spring element, a further spring which is configured as a compression spring can be arranged between the pressure piece and the frame part and act on the pressure piece and/or the spring which loads the rolling bodies.  
         [0009]     Preferred refinements of the invention emerge from the subclaims.  
         [0010]     If the oblique surfaces are arranged at an angle which opens radially toward the axis of the pressure piece, it is expedient to arrange the spring in such a way that it is supported on rolling bodies which in each case lie diametrically opposite the axis and acts on the latter with a spring force.  
         [0011]     The oblique surfaces merge in each case with an arcuate segment into a flat surface which extends approximately perpendicularly with respect to the axis of the pressure piece. The arcuate segment has a larger radius than the rolling bodies which slide and/or roll on the oblique surfaces and the flat surface. The oblique surfaces can be arranged on the pressure piece on its end surface which faces the frame part. It can also be expedient to arrange the oblique surfaces and the flat surface in the frame part.  
         [0012]     The respective component (pressure piece or frame part) which, in the axial direction of the axis of the pressure piece, lies opposite the component which bears the oblique surfaces preferably has a flat end surface on which the rolling bodies slide and/or roll. Radial stops for the rolling bodies can be arranged on the end surface. The arcuate segments form radial stops for the rolling bodies with the opposite surface and therefore a stop for the pressure piece.  
         [0013]     In order to minimize costs, the rolling bodies are preferably configured as spherical components or balls. The compression spring which acts on the pressure piece or the spring in the axial direction can have a greater spring force than the spring which loads the rolling bodies. The spring force of the compression spring is preferably as much as approximately 20 N. The frame part on which the spring element is supported is preferably a setting screw which can be adjusted in the axial direction with respect to the pressure piece. It can be expedient to provide a cover or a crossmember instead of the setting screw and to position the cover fixedly in the frame with a form-fitting connection, preferably by calking. In order to ventilate the space between the pressure piece and the cover or the setting screw, a hole is provided in the pressure piece which penetrates the pressure piece in the axial direction. The frame in which the first and second mechanism members are arranged is expediently configured as a preferably closed mechanism casing.  
         [0014]     It can be expedient to provide a cover or a crossmember instead of the setting screw and to position the cover fixedly in the frame with a form-fitting connection, preferably by calking. The frame in which the first and second mechanism members are arranged is expediently configured as a preferably closed mechanism casing.  
         [0015]     It is expedient to allow the pressure piece to act on a 30 radially displaceably mounted bearing of the first or second mechanism member.  
         [0016]     The spring element according to the invention brings about a permanent contact between solid, metallic components, as a result of which impact noise is avoided during operation of the thrust device. A thrust force which varies within narrow limits and can be predefined is made possible by the thrust device, with the simultaneous possibility of automatic adjustment for the purpose of compensating for tolerances as a result of wear or thermal expansion.  
         [0017]     One exemplary embodiment is shown in the following text using the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  shows a partial cross section through a rack and pinion steering system in the region of the thrust device,  
         [0019]      FIG. 2  shows a partial cross section through a worm gear mechanism having a thrust device,  
         [0020]      FIG. 3  shows a longitudinal section through the thrust device from  FIG. 2 , and  
         [0021]      FIG. 4  shows a longitudinal section through a thrust device having a compression spring. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]      FIG. 1  partially shows a cross section of a steering system which is configured as a rack and pinion steering system for a motor vehicle. Here, a frame  2  which is configured as a mechanism casing  26  is shown in the axial direction with respect to a rack  28  in the region of a thrust device  1 . The rack  28  bears toothing  18  which extends over the rack  28  in the axial direction and is in engagement with a pinion  27  which is connected fixedly to the end of a steering shaft so as to rotate with said steering shaft.  
         [0023]     The pinion  27  is mounted rotatably in the mechanism casing  26  in a manner which is not shown in greater detail and forms a first mechanism member  3 . The rack  28  is displaced in the axial direction as a function of a rotation of the pinion  27  at a steering handle on the steering shaft. The rack  28  forms a second mechanism member  4  and is connected to steerable wheels of the motor vehicle using steering tie rods and wheel steering levers.  
         [0024]     The toothing  18  of the rack  28  is pressed against the pinion  27  with the aid of a pressure piece  5  which is arranged in the mechanism casing  26  so as to be axially displaceable along an axis  6  which is arranged perpendicularly with respect to the longitudinal axis  19  of the rack  28 . The pressure piece  5  can be cylindrical or cuboid and is inserted in a hole  20  in the mechanism casing  26 . In its axial end which faces the rack  28 , the hole  20  has an opening  21  through which a pressure piece end  22  protrudes which is in sliding contact with the rack  28 .  
         [0025]     A spring element  7  which is inserted in the hole  20  is supported on a base  23  of the pressure piece  5  which is configured as a flat surface  14 . The spring element  7  is supported in the axial direction of the axis  6  on a frame part  10  which is configured as a setting screw  16 .  
         [0026]     The spring element  7  is formed from two rolling bodies  8 ,  8 ′ which lie opposite one another diametrically with respect to the axis  6  and are configured as balls  15 ,  15 ′. A spring  11  which is configured as a helical compression spring is inserted between the rolling bodies  8 ,  8 ′, the radius of the helical compression spring and the radius R w  of the rolling bodies  8 ,  8 ′ being approximately identical.  
         [0027]     A frustoconical turned groove is made in an end surface  24  of the setting screw  16  which lies opposite the base  23  of the pressure piece  5  at the spacing s. In the cross section shown, the turned groove has oblique surfaces  9 ,  9 ′ which are configured in each case with an angle α which opens radially toward the axis  6 .  
         [0028]     As viewed in the radial direction with respect to the axis  6 , the oblique surfaces  9 ,  9 ′ each merge with an arcuate segment  12  into a surface  13  which extends perpendicularly with respect to the axis  6  of the pressure piece  5 . Here, the radius R s  of the arcuate segments  12  is larger than the radius R w  of the rolling bodies  8 ,  8 ′ or balls  15 ,  15 ′.  
         [0029]     The depth of the turned groove is selected such that it is smaller than the diameter of the rolling bodies  8 ,  8 ′. This brings about the situation where the pressure piece  5  is supported axially on the setting screw  16  exclusively via the rolling bodies  8 ,  8 ′, irrespective of the respective operating path covered during operation of the steering system.  
         [0030]     During operation of the steering system, the rack  28  and the pressure piece  5  are pressed away from the pinion  27  by the toothing engagement angle as a result of a steering force being introduced onto the pinion  27  and/or restoring forces from the steerable vehicle wheels, which restoring forces act on the rack  28 .  
         [0031]     The balls  15 ,  15 ′ are pressed radially with respect to the axis  6  as a result, counter to the spring force F f  of the spring  11 . On account of the selected geometric variables of the opening angle α of the oblique surfaces  9 ,  9 ′ and the radius R s  of the arcuate segments  12 , the ratio of the thrust force F a  of the pressure piece  5  to the spring force F f  changes as a function of the respective point of contact of the rolling bodies  8 ,  8 ′ on the flanks of the setting screw turned groove, the thrust force F a  of the pressure piece  5  rising with increasing compression of the spring  11 .  
         [0032]     The spring element  7  makes permanent, jolt-free contact possible for the pressure piece  5  on the mechanism casing  26  via the rolling bodies  8 ,  8 ′, as a result of which rattling noise is avoided. As a result, the adjusting function and spring function are united in a single spring element. The spring element is simple to install, of small overall size and has only few parts.  
         [0033]     Further possible refinements of the thrust device  1  are feasible, for instance with an oblique surface angle α which opens radially away from the axis  6  of the pressure piece  5 , and with rolling bodies which are spring loaded in the direction of the axis  6 . It can also be expedient to arrange the oblique surfaces or the frustoconical turned groove in the pressure piece instead of in the setting screw or a frame part.  
         [0034]     The thrust device can also be used for other purposes, such as for pressing a first mechanism member  3  which is configured as a worm  29  against a second mechanism member  4  which is configured as a worm gear  30 , and vice versa (cf.  FIG. 2 ). FIG.  2  shows a worm gear mechanism in a cross section along an axis  32  of a worm  29 , which worm gear mechanism is preferably used for force transmission of a servomotor in a steering column drive, a pinion drive or double pinion drive of an electric power steering system.  
         [0035]     The worm  29  which is configured as a first mechanism member  3  is held in a radially displaceable bearing  33  in a frame  2  which is configured as a mechanism casing  26 . The bearing  33  is arranged at the axial end  34  of the worm  29 . The worm  29  is held in the mechanism casing  26 , in a further bearing  36 , at its end  35  which lies opposite the axial end  34 . The bearing  36  is configured as a pivotable locating bearing. The worm  29  is connected fixedly to a servomotor (not shown) so as to rotate with it via a clutch and is in engagement with a worm gear  30 . As a second mechanism member  4 , the worm gear  30  forms the output side of the worm gear mechanism. In the exemplary embodiment shown, the axis of the worm gear  30  is arranged in the perpendicular direction with respect to the axis  32  of the worm  29 . In order to ensure the absence of play in the toothing engagement of the worm  29  and worm gear  30  during operation of the worm gear mechanism irrespective of wear, of thermal expansion and the like, a thrust device  1  is arranged in the mechanism casing  26 .  
         [0036]      FIG. 3  shows an enlarged illustration of the thrust device  1  in a longitudinal section along an axis  6  of a pressure piece  5 . The same designations as in  FIG. 2  apply for identical components. The pressure piece  5  is of cylindrical form and is inserted in a hole in the mechanism casing  26 . The pressure piece  5  is held in the hole so as to be displaceable along the axis  6  which is oriented in the perpendicular direction with respect to the axis  32  of the worm  29  and in the perpendicular direction with regard to the axis of the worm gear  30 . In its axial end which faces the worm  29 , the hole has an opening  21  through which a pressure piece end  22  protrudes. The pressure piece end  22  and the opening  21  have a smaller diameter or free width than the pressure piece  5  itself. The pressure piece end  22  can act on a bearing  31  of the first or second mechanism member and, in the exemplary embodiment shown, acts on the bearing  33  and presses the worm  29  in the radial direction of its axis  32  toward the worm gear  30 . For this purpose, a spring element  7  is provided which is arranged in the hole. The spring element  7  is supported on a base  23  of the pressure piece  5 , which base  23  is configured as a flat surface  14  or, as shown in  FIGS. 2 and 3 , is configured as a cylindrical surface, as viewed transversely with respect to the axis  6 . As viewed in the axial direction of the axis  6 , the spring element  7  is supported on a frame part  10  which is configured as a cover  17 .  
         [0037]     The spring element  7  is formed from two rolling bodies  8 ,  8 ′ which lie opposite one another diametrically with respect to the axis  6  and are configured as balls  15 ,  15 ′. A spring  11  which is configured as a helical compression spring is inserted between the rolling bodies  8 ,  8 ′, the radius of the spring being smaller than the radius R w  of the rolling bodies  8 ,  8 ′.  
         [0038]     The spring  11  and the balls  15 ,  15 ′ are guided in a half-open hole. A frustoconical turned groove is made in an end surface  24  of the cover  17 . In the cross section shown, the turned groove has oblique surfaces  9 ,  9 ′ which are configured in each case with an angle α which opens radially toward the axis  6 . As viewed in the radial direction with respect to the axis  6  of the pressure piece  5 , the oblique surfaces  9 ,  9 ′ each merge with an arcuate segment  12  into a surface  13  which extends perpendicularly with respect to the axis  6  of the pressure piece  5 . Here, the radius R s  of the arcuate segments  12  is larger than the radius R w  of the rolling bodies  8 ,  8 ′ or balls  15 ,  15 ′.  
         [0039]     The parts are dimensioned in such a way that the pressure piece  5  is supported axially on the cover  17  exclusively via the rolling bodies  8 ,  8 ′, irrespective of the respective operating path covered during operation of the thrust device  1 . In order to reduce friction between the rolling bodies  8 ,  8 ′, the cover  17  and the pressure piece  5  and the associated hysteresis in the movement sequence of the pressure piece  5 , a compression spring  43  can be provided which acts on the pressure piece  5 . As  FIG. 4  shows, the compression spring is arranged in a blind hole  45  of the cover  17  in the region of its surface  13 . The compression spring  43  which is formed as a cylindrical helical compression spring has a spring force which is greater than that of the spring  11 . The compression spring  43  thus brings about the majority of the thrust force F a  of the pressure piece  5 .  
         [0040]     During operation of the steering system, the worm  29  and the pressure piece  5  are pressed away from the worm gear  30  by the toothing engagement angle as a result of a motor torque being introduced onto the worm  29  and/or restoring forces from the steerable vehicle wheels, which restoring forces act on the worm gear  30 .  
         [0041]     The balls  15 ,  15 ′ are pressed radially with respect to the axis  6  as a result, counter to the spring force F f  of the spring  11 . On account of the selected geometric variables of the opening angle α of the oblique surfaces  9 ,  9 ′ and the radius R s  of the arcuate segments  12 , the ratio of the thrust force F a  of the pressure piece  5  to the spring force F f  changes as a function of the respective point of contact of the rolling bodies  8 ,  8 ′ on the flanks of the cover turned groove, the thrust force F a  of the pressure piece  5  rising with increasing compression of the spring  11 . In the region of the arcuate segment  12 , however, the thrust force F a  rises rapidly without a jump and free of jolts.  
         [0042]     The spring element  7  makes permanent, jolt-free contact possible for the pressure piece  5  on the mechanism casing  26  via the rolling bodies  8 ,  8 ′, as a result of which rattling noise is avoided. As a result, the adjusting function and spring function are united in a single spring element. The spring element is simple to install, of small overall size and has only few parts.  
         [0043]     In the exemplary embodiment shown, the cover  17  is calked to a casing  37  which combines the pressure piece  5 , the spring element  7  and the cover  17  to form a unit, a pressure piece assembly  38 . The cover  17  is sealed against the casing  37  via an O-ring  40  which is inserted in a peripheral groove  39  of the cover  17 . The pressure piece  5  which is held axially displaceably in the casing  37  is optionally sealed against the casing  37  with an O-ring  42  which is inserted into a peripheral groove  41  in the pressure piece  5  and also serves for damping. As shown in  FIG. 4 , a hole  44  is guided axially through the pressure piece. The hole  44  serves for ventilation of the space between the cover  16  and the pressure piece  5 .  
         [0044]     In the exemplary embodiment shown in  FIGS. 2 and 3 , the casing  37  is of cylindrical configuration and is screwed into the mechanism casing in a sealing manner. It can also be expedient to allow the pressure piece to act on a bearing of the worm gear  30 . Instead of the application of the thrust device  1  according to the invention for pressing mechanism members which are engaged with one another with a form-fitting connection, the thrust device can also be used in the case of mechanism members which are in frictional engagement.