Abstract:
A power-steering drive for a steering system in a motor vehicle includes a motor driving a motor shaft and a transmission situated between the motor shaft and an output shaft. The transmission takes the form of a combined worm-belt transmission. Furthermore, in a method for adjusting the transmission of a power-steering drive, the adjustment is performed by solely adjusting the intermediate shaft relative to the motor shaft and/or the output shaft.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a power-steering drive and a method for adjusting the transmission of a power-steering drive. 
       BACKGROUND INFORMATION 
       [0002]    Certain power-steering drives having a single-stage or two-stage belt transmission are conventional. In two-stage belt transmissions, the belt pulleys may have a smaller diameter as compared to a power-steering drive having a single-stage belt transmission, because the overall transmission ratio results from a product of the individual transmission ratios. A disadvantage in conventional power-steering drives is the limited, maximally possible transmission ratio since the available space is limited. In very large transmission ratios, the driven belt pulley must be selected to be so large that there is hardly a chance for installing it in a motor vehicle. A large transmission ratio is desirable, however, if one wants to achieve high servo torques using relatively small electric motors. 
         [0003]    It is furthermore conventional to use power-steering drives having worm transmissions. In this instance, the worm is coupled by a dog clutch to the motor shaft for example. The worm normally drives a worm wheel connected in a rotatably fixed manner to the steering shaft. Often, the worm is adjusted to the worm wheel to prevent bothersome noises. The worm transmission is likewise limited in terms of transmission ratio since otherwise the efficiencies for turning in and/or turning back become too unfavorable. 
       SUMMARY 
       [0004]    Example embodiments of the present invention provide a power-steering drive that makes it possible to achieve large transmission ratios in a small space. Furthermore, example embodiments of the present invention provide a method for adjusting the transmission of such a power-steering drive. 
         [0005]    Example embodiments of the present invention provide the transmission between the motor shaft and the output shaft as a combined worm-belt transmission, the overall transmission ratio being obtained by multiplying the transmission ratio of the worm transmission segment by the transmission ratio of the belt transmission segment. The belt transmission segment may be arranged such that it has a transmission ratio of at least 1:2, e.g., between approximately 1:2.5 and 1:4. The worm transmission segment may have a transmission ratio of at least 1:8, e.g., in the range between 1:10 and 1:15. The overall transmission ratio is at least 1:25 and is, e.g., in a range between 1:30 and 1:40. Using example embodiments of the present invention it is possible to achieve large transmission ratios in an extremely compact construction. Small and therefore cost-effective electric motors may be used. Furthermore, it is possible to adjust the belt tension and the relative position between worm and worm wheel in a simple and cost-effective manner. The output shaft of the power-steering drive may include a steering shaft of the steering system or the input shaft of a steering gear, particularly a rack-and-pinion steering. 
         [0006]    Example embodiments provide for the transmission of the power-steering drive to have, in addition to the motor shaft and the output shaft, an intermediate shaft, which takes the form of a transmission worm. 
         [0007]    According to example embodiments of the present invention, the intermediate shaft is coupled to the motor shaft via a belt, the belt being mounted onto a first belt pulley that is situated in a rotatably fixed manner on the motor shaft and on a second belt pulley that is situated in a rotatably fixed manner on the intermediate shaft. 
         [0008]    In further example embodiments of the present invention, the intermediate shaft or the worm segment of the intermediate shaft mates with a worm wheel situated in a rotatably fixed manner on the output shaft, in particular the steering shaft. 
         [0009]    There may be a provision for the intermediate shaft to be situated in parallel to the motor shaft and for the output shaft to extend transversely with respect to the intermediate shaft and the motor shaft. 
         [0010]    Example embodiments of the present invention provide for the intermediate shaft to be, e.g., exclusively adjustable in its relative position with respect to the motor shaft and/or to the output shaft. After releasing a fixing element of the intermediate shaft it is possible to adjust the intermediate shaft, whereby the desired belt tension is applied and/or the desired relative position between worm and worm wheel is adjustable, in particular in one work step. The entire transmission may be adjusted by shifting only one element, in this case the intermediate shaft, e.g., using an external shaft shifting tool, in particular by applying a defined adjusting force in a defined direction of force. If the adjustment is performed using an external shaft shifting tool, adjusting aids integrated into the power-steering drive may be omitted. It is also possible to provide such adjusting aids such as adjusting screws and/or spring force assisted tension rollers for example. 
         [0011]    If the intermediate shaft is shiftable after releasing a fixing mechanism, then, in example embodiments of the present invention, the motor shaft and the output shaft may be situated in a fixed, non-adjustable position relative to each other in the power-steering drive, which overall substantially decreases the required structural measures since respective adjusting devices and fixing devices may be omitted. 
         [0012]    The intermediate shaft that has been brought into the exact position may be fixed in the desired position, for example, by welding, crimping sheet metal parts, clamping or bolting. The intermediate shaft, however, may be fixed exclusively by an axial bracing against at least one component, in particular a component of the power-steering drive, e.g., against a housing. A direct support in the radial direction may be omitted. For this purpose, the intermediate shaft or the component bearing the intermediate shaft is received, e.g., with radial play around its entire periphery, in at least one receiving opening or is guided through the latter. 
         [0013]    There are various options for supporting the intermediate shaft in a rotatable manner. A first alternative provides for the intermediate shaft to be supported in a rotatable manner on a spindle. In particular, this spindle is braced axially against a component using the fixing arrangement. For this purpose, the spindle may extend through at least one opening with radial play around its entire periphery such that, when the axial bracing is released, the spindle and thus the intermediate shaft is shiftable in radial directions within the limits predetermined by the edge of the opening. It is possible to brace the spindle only on one side or on both sides against a component. 
         [0014]    According to one alternative, the intermediate shaft is received radially within a shaft bearing. The shaft bearing in turn or a spindle connected to the shaft bearing housing in a rotatably fixed manner may be braced in the axial direction against one component or against several components. For this purpose, a radial play may be provided around the shaft bearing or the spindle in order to be able to adjust the intermediate shaft with the shaft bearing in a radial plane when the fixing device(s) are released and thus to be able to pretension the at least one belt and/or adjust the relative position between the worm and the worm wheel. 
         [0015]    Furthermore, example embodiments of the present invention provide a method for installing or adjusting the transmission of a power-steering drive for a motor vehicle, the power-steering drive having a motor shaft, an intermediate shaft in the form of a worm and an output shaft. Example embodiments of the present invention provide for the adjustment of the transmission, that is, the pretensioning of the belt transmission segment and/or the modification of the relative position between the worm and the worm wheel to be performed solely by adjusting the intermediate shaft in the form of a worm. 
         [0016]    According to a first alternative, the adjustment of the intermediate shaft occurs by a defined, predetermined adjusting force in a defined direction of force. For this purpose, the currently applied adjusting force may be measured by at least one sensor during the adjusting process. For example, the adjustment is performed by an independent shaft shifting device, which is not part of the power-steering drive and which engages, e.g., on two contact segments, though at least on one contact segment, on the intermediate shaft or a component rigidly connected to the intermediate shaft and may be operated electromotively. In this case, the adjustment of the belt tension occurs not by adjusting a predetermined travel, but by applying a defined adjusting force. If the intermediate shaft to be adjusted has a defined adjusting force applied to it, then this results in a defined pretensioning force of the belt(s) connected to the intermediate shaft. Measuring the belt tension during or after the adjustment is not required. For example, only the applied adjusting force in the desired direction of force is measured. 
         [0017]    According to a second alternative, the adjustment of the belt tension and/or of the relative position between the intermediate shaft having the worm segment and the worm wheel occurs via the adjustment by a travel. If the belt tension is adjusted via a travel adjustment, then the belt tension must be measured during the adjusting process. The adjustment by a travel is particularly suitable for setting the relative position between the intermediate shaft having the worm segment and the worm wheel. For this purpose, the intermediate shaft having the worm segment is adjusted relative to the worm wheel, for example in the direction of the worm wheel, until the force rises in the adjusting direction. The rise of the force is an indicator that the worm segment of the intermediate shaft is contacting the worm wheel. After the contact to the worm wheel has been established, the intermediate shaft may be adjusted back by a certain travel in order to obtain a defined play between the intermediate shaft or worm segment and the worm wheel. 
         [0018]    Further features and aspects of example embodiments of the present invention are described in more detail below with reference to the appended Figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic view of a power-steering drive having an intermediate shaft in the form of a worm. 
           [0020]      FIG. 2  illustrates the power-steering drive rotated by 90° and sectioned along the sectional line A-A as shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In the figures, the same components and components having the same function are designated by the same reference numerals. 
         [0022]      FIG. 1  shows a power-steering drive  1  for a motor vehicle steering system that is not shown in greater detail. The steering force applied by the driver may be supported by power-steering drive  1 . For this purpose, the torque provided by an electric motor  2  is transmitted via a transmission  3  onto an output shaft  4 , that is, a steering shaft. 
         [0023]    Transmission  3  includes a worm transmission segment  5  and a belt transmission segment  6 . Belt transmission segment  6  includes a first, small belt pulley  8  mounted in a rotatably fixed manner on a motor shaft  7 , which is coupled via a belt  9 , e.g., a toothed belt, to a second, large belt pulley  10 . Second belt pulley  10  is mounted in a rotatably fixed manner on an intermediate shaft  11 . 
         [0024]    At an axial distance from second belt pulley  10 , intermediate shaft  11  takes the form of a worm, that is, it has a worm segment  12 . Worm segment  12  mates with a worm wheel  13 . Worm wheel  13  is mounted in a rotatably fixed manner on the output shaft and drives the latter. The torque of electric motor  2  is accordingly transmitted via motor shaft  7  having first belt pulley  8  onto intermediate shaft  11  or onto second belt pulley  10  connected in a rotatably fixed manner to intermediate shaft  11  and from worm segment  12  of intermediate shaft  11  onto worm wheel  13  connected in a rotatably fixed manner to output shaft  4  and thus onto the output shaft, in particular the steering shaft. 
         [0025]    The left end of intermediate shaft  11  in  FIG. 1  is supported in a rotatable manner by a roller bearing  14 . The right end of intermediate shaft  11  is supported radially and axially by a ball bearing  15 . Roller bearing  14  has a rigid first prolongation  17  situated coaxially with respect to longitudinal axis  16  of intermediate shaft  11 . Analogously, ball bearing  15  has a second prolongation  18  situated coaxially to longitudinal axis  16  and pointing in a direction opposite to first prolongation  17 . First and second prolongation  17 ,  18  are used for fixing roller bearing  14  or ball bearing  15  and thus intermediate shaft  11 . First prolongation  17  extends, with radial play over its entire circumference, through an opening  19  in a first housing part  20  of power-steering drive  1 . Analogously, second prolongation  18  extends, with radial play over its entire circumference, through a second opening  21  in a second housing part  22 . Both prolongations  17 ,  18  are provided on their free end with respectively one outer thread  23 ,  24 . Roller bearing  14  abuts with a radial section  25  on the inner side of first housing part  20 . A disk  26  is situated on the outside of first housing part  20 , which is slid onto first prolongation  17  and abuts upon housing part  20  from outside. A first nut  27  is screwed onto outer thread  23  of first prolongation  17 , which nut axially braces roller bearing  14  of intermediate shaft  11  against first housing part  20 . Analogously, roller bearing  15  is braced axially against second housing part  22 . Second prolongation  18  extends through second opening  21  of second housing part  22 . A second disk  28  is slid on from the outside in the axial direction. The axial bracing occurs via a second nut  29 , which is screwed onto outer thread  24  of second prolongation  18  which braces ball bearing  18  axially against housing part  22 . 
         [0026]    After unscrewing the two nuts  27 ,  29 , intermediate shaft  11  may be shifted along radial directions  30 ,  31  illustrated in  FIG. 2 , which changes distance  32  between intermediate shaft  11  and motor shaft  7  and thus the pretension of belt  9 . At the same time, intermediate shaft  11  is adjusted relative to worm wheel  13 , which makes it possible to optimize the interaction between worm segment  12  and worm wheel  13 , in particular with a view to a low noise generation. 
         [0027]    Intermediate shaft  11  is shifted by a shaft shifting device  33 , shown schematically, having two adjusting arms  34 ,  35  set apart in the axial direction. Adjusting arms  34 ,  35  engage laterally and axially by conical points in centering bore holes of prolongations  17 ,  18 . Each adjusting arm  34 ,  35  has sensors assigned to it for measuring the respective adjusting force. The adjusting forces are determined prior to the adjustment on the basis of the setpoint belt forces and the torques acting on the intermediate shaft as a result of these belt forces. The sensors may be placed directly adjacent to spindle  11 . Shaft shifting device  33  or adjusting arms  34 ,  35  are adjusted until the previously ascertained adjusting forces or the belt forces are applied on adjusting arms  34 ,  35 . Adjusting arms  34 ,  35  may be adjusted independently of each other using at least one electric motor. According to an alternative it is possible that the two adjusting arms  34 ,  35  are only jointly adjustable. It is possible that adjusting arms  34 ,  35  do not engage axially with prolongations  17 ,  18 , but engage laterally with intermediate shaft  11  or with a component connected to the intermediate shaft. It is possible to provide only one single adjusting arm, which holds intermediate shaft  11  when making the adjustment such that intermediate shaft  11  cannot tilt in the radial direction. After reaching the desired adjusting position, that is, after the desired adjusting force is applied in the predetermined direction of force, nuts  27 ,  29  are tightened and the adjusting position thus found is fixed. Alternatively, fixing in place may also be achieved by welding, deforming, etc.