Electromechanical power steering for a motor vehicle

Abstract

The present invention relates to an electromechanical power steering system for a motor vehicle, comprising a servomotor (1) and a ball screw drive (2), wherein the ball screw drive (2) has a belt pulley (3), a ball nut (4), and a threaded spindle (5), wherein the belt pulley (3) is arranged concentrically to the ball nut (4) and surrounds it, wherein the ball nut (4) engages with the threaded spindle (5), wherein the belt pulley (3) can be driven by the servomotor (1) via a drive belt (6), wherein the belt pulley (3) has, on a first half of its inner circumference, two radially inwardly extending projections (7, 8), which are positively connected to the ball nut (4) in the circumferential direction, wherein the belt pulley (3) has, on a second half of its inner circumference, a third projection (9), which extends radially inwardly and via which the belt pulley (3) is supported on the outer circumference (10) of the ball nut (4).In order to further develop such an electromechanical power steering system in such a way that, on the one hand, there is great design freedom with regard to the design of the components of the ball screw drive, and, on the other hand, that the pushing-through force, i.e. the force required to move the rack, has a force-displacement curve which has little ripple without large force peaks, with vibrations and noise development being as low as possible, it is proposed that the belt pulley (3) has compensating material (12a, 12b) on its inner circumference (11) on both sides of the third projection (9), by means of which compensating material any imbalance of the ball screw drive (2) is compensated.

Description

[0001] The invention relates to an electromechanical power steering system for motor vehicles having the features of patent claim 1.

[0002] From DE 10 2016 007 542 A1 an electromechanical power steering system according to the preamble of patent claim 1 is known. As in particular the Fig. 10 and the description of DE 10 2016 007 542 A1, the ball screw drive of this known power steering system comprises a ball nut, which is connected to a pulley in a rotationally fixed manner on its outer circumferential surface. The rotationally fixed connection is achieved via a positive connection effective in the circumferential direction. To form the positive connection, the ball nut has grooves on its outer circumferential surface and the pulley has grooves on its inner circumferential surface, in which positive-locking elements designed as webs 31, 32, 33, 34 are arranged. The webs are formed on a deflection body, which is positively received between the pulley and the ball nut.

[0003] As is particularly evident from Fig. 10 of DE 10 2016 007 542 A1, the pulley 14 has a total of three projections on its inner circumference facing the ball nut 13, which projections extend radially inwards towards the ball nut 13. Two of these projections are arranged on the upper half of the inner circumferential surface with respect to an imaginary horizontal plane which divides the pulley into two halves. The third projection is arranged on the lower half of the inner circumferential surface. The pulley 14 is supported on the outer surface of the ball nut via the third projection. The two projections arranged on the upper half of the inner circumference are also arranged on different sides of an imaginary vertical plane which divides the pulley into two halves, at equal distances from this vertical plane in the circumferential direction (i.e. approximately at the "10 o'clock position" and at the "2 o'clock position").The third projection is divided in half by this imaginary vertical plane and is therefore positioned at the “6 o’clock position”.

[0004] In practice, it has been found that this known design of the ball screw can produce undesirable vibrations during operation, and that this can lead to undesirable noise and increased wear.

[0005] It is also known from the state of the art that unwanted vibrations, noise and wear in ball screws can result from imbalances in the ball nut or pulley.

[0006] DE 10 2010 034 698 A1 describes, for example, a ball screw drive of an electromechanical power steering system for motor vehicles, in which one of the toothed belt wheels of the belt drive (e.g. the belt pulley, as in Fig. 8 of DE 10 2010 034 698 A1) is mounted in such a way that it can be moved from an off-axis position, in which the toothed belt is loose, to a position with respect to its rotational axis, in which the toothed belt is tensioned. The aim is to be able to tension the toothed belt without additional components in the form of belt tensioners and the like (see paragraph

[0042] ).

[0007] To prevent pulley imbalance, DE 10 2010 034 698 A1 proposes arranging three radially inwardly extending projections evenly distributed around the inner circumference of the pulley (i.e., with a 120-degree pitch). These three projections interact with three radially inwardly recessed recesses on the outer circumference of the ball nut to allow the pulley to be moved into an off-axis position, in which the timing belt is loose. These recesses in the ball nut are also evenly distributed around the circumference (i.e., also with a 120-degree pitch).

[0008] A disadvantage of the ball screw drive known from DE 10 2010 034 698 A1 is that the force required to move the rack (the so-called "push-through force") is high and exhibits a force-displacement curve with considerable undulation. This means that in a diagram in which the push-through force is plotted against the path of the rack movement, the push-through force exhibits a wave-like curve with large amplitudes, i.e., high force peaks. This force curve and the high force peaks lead to undesirable noise and increased wear.

[0009] DE 10 2008 002 627 A1 discloses a very special design of an electromechanical power steering system for motor vehicles with a ball nut-spindle transmission. In this special design, the ball nut has exactly one single ball return channel in its wall. This single ball return channel causes the ball nut to exhibit an imbalance that could be avoided by providing several ball return channels evenly distributed around the circumference. However, according to DE 10 2008 002 627 A1, providing multiple ball return channels increases manufacturing costs and weight, so only a single ball return channel should be provided.

[0010] In order to minimize disturbing noises and vibrations in such a ball nut with only a single ball return channel, DE 10 2008 002 627 A1 proposes that the ball nut have compensating means that counteract any imbalance caused by the one-sided ball return and are arranged and dimensioned such that a main axis of inertia of the ball nut at least approximately coincides with a rotational axis of the spindle section of the ball nut-spindle transmission. As a preferred embodiment, DE 10 2008 002 627 A1 proposes that the compensating means be designed as recesses in the ball nut.

[0011] A disadvantage of the electromechanical power steering system known from DE 10 2008 002 627 A1 is that the restriction to ball nuts with only a single ball return channel results in a very large design limitation and that imbalances introduced into the ball screw drive by the pulley are not compensated.

[0012] German patent application DE 10 2011 082 514 A1 describes a ball screw drive comprising a nut with a ball groove formed on the inner circumference, a traction wheel, and at least one deflection with a return channel for transferring balls running in the ball groove. The deflection is formed in two parts, with a first deflection section formed integrally on the inner circumference of the traction wheel and a separate second deflection section connected to it in the assembled position. The problem of individual imbalances of individual components of the ball screw drive and the problem of a resulting overall imbalance of the entire ball screw assembly is not mentioned, and no measures for compensating for imbalances are described.

[0013] The German patent application DE 22 39 523 A1 relates to a device for quickly and reliably detecting imbalance in the wheel of a motor vehicle and for restoring the balance.

[0014] The object of the invention is to develop an electromechanical power steering system for motor vehicles according to the preamble of the patent claim in such a way that, on the one hand, a great deal of design freedom is given with regard to the configuration of the components of the ball screw drive, and, on the other hand, that the push-through force, i.e. the force required to move the rack, has a force-displacement curve that has little ripple without large force peaks, while vibrations and noise development are kept to a minimum. The object of the invention is also to provide a cost-effectively manufactured electromechanical power steering system with a ball screw drive, in which the entire assembly of the ball screw drive is designed to be unbalance-optimized and therefore only slight vibrations and noise developments occur during operation, lying below a predeterminable tolerance threshold.

[0015] This object is achieved by an electromechanical power steering system according to claim 1. Advantageous further developments emerge from the subclaims, the following description and the drawings.

[0016] The electromechanical power steering system according to the invention for a motor vehicle has a servo motor and a ball screw drive, wherein the ball screw drive has a belt pulley, a ball nut and a threaded spindle, wherein the belt pulley is arranged concentrically to the ball nut and surrounds it, wherein the ball nut engages with the threaded spindle, wherein the belt pulley can be driven by the servo motor via a drive belt, wherein the belt pulley has, on a first half of its inner circumference, two radially inwardly extending projections which are positively connected to the ball nut in the circumferential direction, wherein the belt pulley has, on a second half of its inner circumference, a third projection extending radially inwardly, via which third projection the belt pulley is supported on the outer circumference of the ball nut.

[0017] According to the invention, the pulley has compensating material on its inner circumference on both sides of the third projection, by means of which any imbalance of the ball screw is compensated. The compensating material thus compensates for the imbalance of components rotating in the ball screw, in particular the pulley, balls of a ball return and the ball return itself. In the context of the present invention, "compensated" means that the compensating material not only ideally completely compensates for any imbalance of the ball screw resulting from the shape of the pulley and in particular from the three projections of the pulley, but that it also ideally completely compensates for, i.e. neutralises, any imbalance resulting from the ball return and the balls as well as the ball return itself.However, even reducing the effects of the resulting imbalance to a predefined level falls under the term "compensating for the imbalance of the ball screw," without completely neutralizing the effects. According to the invention, the effects of a resulting imbalance of the ball screw during operation of the power steering system are reduced at least to such an extent that vibrations and noise caused by the imbalance remain below acceptable limits for the operation of the ball screw within the power steering system.

[0018] The solution according to the invention also ensures that the push-through force exhibits a force-displacement curve with only minimal ripple. Large amplitudes, i.e., large force peaks of the push-through force, do not occur in the force-displacement curve. This minimizes wear on the ball screw drive.

[0019] A further advantage achieved by the present invention is that the entire ball screw is balanced as a whole, and not just individual components of the ball screw. The comparatively small mass of the compensating material compensates for the imbalance of the entire assembly and optimizes the function of the ball screw. A comparatively small mass of imbalance is preferably understood to mean that an additional weight of <=1g is provided on the pulley. In percentage terms, it preferably makes up <0.3% of the weight of the pulley. However, the small mass of imbalance is adapted to the specific product, so this must be designed anew for each ball screw. As a result, the weight specifications can vary and consequently the small mass on the pulley can also make up >1g or, in percentage terms, >0.3%.In the invention, all rotating parts in the ball screw are taken into account during balancing by the compensation material, including the ball chain acting between the ball nut and the threaded spindle. The rotational form-fitting fixation between the pulley and ball nut allows the invention to achieve balance on the pulley. This is more cost-effective than the balancing of the ball nut known from the prior art cited above.

[0020] According to one embodiment of the invention, each of the two projections has a first groove and the ball nut has two second grooves on its outer circumference, wherein the first and second grooves together define two receiving spaces, wherein a form-locking element is arranged in each receiving space. This design of the form-locking connection between the pulley and the ball nut achieves a secure, rotationally fixed connection between the pulley and the ball nut, which is independent of the pretensioning force of the belt via which the pulley is connected to the servo motor. Even with a comparatively low pretensioning force of the belt, the pulley is driven and the rotation of the pulley is transmitted to the ball nut.This represents a significant difference from the solution known from DE 10 2010 034 698 A1, which requires a comparatively high belt pretension to transmit the rotation of the pulley to the ball nut via the frictional connection between the pulley and the ball nut. If the belt pretension is too low, the pulley can slide or slip relative to the ball nut, preventing the rotation of the pulley from being reliably transmitted to the ball nut.

[0021] According to one embodiment of the invention, the compensating material is an integral component of the pulley. During the pulley manufacturing process, the compensating material is arranged circumferentially on both sides of the third projection in such a way that any imbalance in the ball screw resulting from the three projections is compensated for. In this way, the pulley is balanced immediately after its manufacture. Subsequent operational balancing of the installed pulley can therefore be avoided. In principle, the compensating material could also be formed by compensating masses that are designed separately from the pulley and, for example, are integrally bonded to the pulley at its inner circumference.

[0022] According to one embodiment of the invention, the two projections are circumferentially spaced at the same distance from a reference plane that bisects the first half and the second half of the inner circumference of the pulley, with a rotational axis of the pulley lying in the reference plane, and with one half of the third projection extending circumferentially on one side of the reference plane and another half of the third projection extending on the other side of the reference plane. This arrangement of the three projections of the pulley on its inner circumference facilitates the compensation of the resulting imbalance of the ball screw drive by means of compensating material arranged on both sides next to the third projection. This ensures that the compensating material can be arranged symmetrically in the circumferential direction on both sides of the third projection of the pulley. That is,Due to this arrangement of the three projections on the inner circumference of the pulley, the same amount of balancing material can be provided on both sides of the third projection with the same spatial distribution. This enables easy integration of the balancing process into the pulley manufacturing process. A separate balancing process following pulley production can thus be avoided.

[0023] According to one embodiment of the invention, the compensating material has the same mass and the same spatial distribution in the circumferential direction on both sides adjacent to the third projection. In particular, the compensating material can be directly adjacent to the third projection, so that there is no gap between the third projection and the compensating material arranged on the respective side adjacent to the third projection. This results in a particularly robust design of the pulley because the compensating material has a supporting or stabilizing effect on the third projection. The extension of the third projection in the circumferential direction can therefore be designed or dimensioned for its actual function, which is to radially support the pulley relative to the ball nut. The third projection can therefore be minimized with regard to its extension in the circumferential direction.It can have a circumferential extension just sufficient to support the pulley against the ball nut without exceeding the permissible surface pressure of the third projection surface in contact with the ball nut. The compensating material, on the other hand, compensates for any imbalance resulting from the dimensioning of the three projections of the pulley. Pulley production is also facilitated if the compensating material is directly adjacent to the third projection.

[0024] The invention is explained in more detail below with reference to the figures, each of which shows schematically Fig. 1 a ball screw of an electromechanical power steering in an axial half-section; Fig. 2 a ball screw drive according to the invention in radial half section; Fig. 3 shows the arrangement of a ball screw drive according to the invention on a rack of a steering gear with a threaded spindle formed on the rack; Fig. 4 is a perspective view of a pulley constructed according to the invention; Fig. 5 a side view of a pulley designed according to the invention.

[0025] In Fig. Figure 1 shows a ball screw drive of an electromechanical power steering system in axial section. The ball nut 4 engages with the threaded spindle 5. The ball nut 4 is rotatably mounted in a housing 21 via roller bearings 20. A pulley 3 is arranged concentrically to the ball nut 4 and surrounds the ball nut 4. The pulley 3 is connected to the ball nut 4 in a rotationally fixed manner. The ball nut 4 is driven by a drive belt 6 (see Figure 1). Fig. 2) the pulley 3 is provided with a Fig. 1 servo motor (not shown). The drive belt 6 transmits the rotation of a belt driven by the servo motor 1 (see Fig. 2) driven drive pulley 22 onto the pulley 3, which in turn drives the ball nut 4 and sets it in rotation. A rotation of the ball nut 4 in one direction causes an axial displacement of the threaded spindle 5 along its longitudinal axis 23 in a first direction. A rotation of the ball nut 4 in the other direction causes an axial displacement of the threaded spindle 5 along its longitudinal axis 23 in a second direction opposite to the first direction. The axial displacement of the threaded spindle 5 is in Fig. 1 represented by the double arrow 24.

[0026] Fig. Figure 2 shows the ball screw drive according to the invention in a radial half-section. The pulley 3 is arranged concentrically to the ball nut 4 and surrounds it. A servomotor 1 drives a drive pulley 22. The drive pulley 22 is connected to the pulley 3 via a drive belt 6. The surface of the drive belt 6 facing the drive pulley 22 and the pulley 3 has a profile that positively engages a corresponding profile on the outer circumference of the drive pulley 22 and the outer circumference of the pulley 3. The pulley 3 is connected to the ball nut 4 in a rotationally fixed manner. The rotationally fixed connection is achieved by a positive connection effective in the circumferential direction.

[0027] The pulley 3 has two projections 7, 8 on its inner circumference 11, which are arranged on one half of the inner circumference 11 of the pulley 3 and extend radially inward towards the ball nut 4. Each of the projections 7, 8 has a groove 13 extending in the axial direction (cf. Fig. 4). The ball nut 4 also has two grooves 14 extending in the axial direction on its outer circumference 10. The grooves 13 in the projections 7, 8 and the grooves 14 on the outer circumference of the ball nut 4 form two receiving spaces, in each of which a form-locking element 15 is arranged. The form-locking elements 15 establish the circumferentially effective, form-locking connection between the pulley 3 and the ball nut 4.

[0028] In principle, the form-locking elements 15 can be separate elements or components. In the Fig. In the embodiment shown in Figure 2, the positive-locking elements 15 are designed as webs of a deflecting body 28. The deflecting body 28 is held in a form-fitting manner between the pulley 3 and the ball nut 4. The webs are formed integrally with the deflecting body 28.

[0029] On the half of the inner circumference 11 of the pulley 3 opposite the projections 7, 8, a third projection 9 is arranged, which also extends radially inwards towards the ball nut 4. Thus, the two projections 7, 8 are arranged on one half of the inner circumference 11 of the pulley 3 and the third projection 9 is arranged on the other half of the inner circumference of the pulley 3. In relation to an imaginary reference plane E (see Fig. 5), which bisects the pulley 3 and in which the axis of rotation of the pulley 3 lies, the projections 7, 8 are arranged on different sides of the reference plane E and are equidistant from the reference plane E in the circumferential direction. At the same time, the reference plane E divides the third projection 9 of the pulley 3 into two halves, with one half of the third projection 9 extending on the side of the reference plane E on which one projection 7 of the two projections 7, 8 is arranged, and with the other half of the third projection 9 extending on the side of the reference plane E on which the other projection 8 of the two projections 7, 8 is arranged.

[0030] The pulley 3 is supported on the outer circumference 10 of the ball nut 4 via the third projection 9. The pulley 3 is also supported on the outer circumference 10 of the ball nut 4 via the projections 7, 8.

[0031] Due to the shape of the pulley 3 with the two projections 7, 8 arranged on one half of the inner circumference 11 and the third projection 9 arranged on the other half of the inner circumference 11, the ball screw drive 2 has a resulting imbalance. To compensate for this imbalance, the pulley 3 has compensating material 12a, 12b on its inner circumference 11 in the area of ​​the third projection 9.

[0032] In the Fig. In the embodiment shown in Figure 2, the compensating material 12a, 12b is an integral component of the pulley 3, i.e., formed in one piece with the pulley 3. In principle, the compensating material 12a, 12b could also be formed by compensating masses that are formed separately from the pulley 3 and, for example, are integrally connected to the pulley 3 at its inner circumference 11.

[0033] In the illustrated embodiment, the compensating material 12a, 12b directly adjoins the third projection 9 in the circumferential direction, i.e., there is no gap between the third projection 9 and the compensating material 12a, 12b. The pulley 3 is thus particularly robust in the region of the third projection 9.

[0034] Fig. 3 shows the arrangement of a ball screw drive according to the invention on a rack 25 of a steering gear with a threaded spindle 5 formed on the rack 25. The belt pulley 3 surrounds the (therefore not visible) ball nut 4. On its outer circumference, the belt pulley 3 has a profile which corresponds to a profile of the drive belt 6 (not shown) (cf. Fig. 2). A rotation of the pulley 3 causes a rotation of the ball nut 4, which in turn rotates via the balls (cf. Fig. 2) acts on the threaded spindle 5 and leads to a displacement of the rack 25 in the axial direction along the double arrow 24. The toothing 26 of the rack 25 interacts with other components of a steering gear (not shown).

[0035] The Fig. 4 shows a perspective view and the Fig. 5 shows a side view of a pulley 3 designed according to the invention. The profiling can be seen on the outer circumference, which, during operation, corresponds to the profiling of the drive belt 6 (cf. Fig. 2). On its inner circumference 11, the pulley 3 has the two projections 7, 8, which are arranged on the upper half of the inner circumference 11. The projection 7 is arranged approximately at the "10 o'clock position" and the projection 8 is arranged approximately at the "2 o'clock position". The two projections 7, 8 have the same shape, ie they are formed in a manner that is congruent with one another with respect to a reference plane E ( Fig. 5), which halves the pulley 3 in the “12 o’clock position” and the “6 o’clock position”.

[0036] The projections 7, 8 have grooves 13 extending in the axial direction of the pulley 3. The grooves 13 serve to positively receive a part of a form-locking element 15 in the circumferential direction (see Fig. 2). Another part of each form-locking element 15 is, in the assembled state of the ball screw drive, positively received in a groove 14 provided on the outer circumference 10 of the ball nut 4 (cf. Fig. 2). The two projections 7, 8 are circumferentially equidistant from an imaginary reference plane E ( Fig. 5) which axially bisects the upper and lower halves of the pulley 3. For reasons of clarity, the reference plane E is only shown in Fig. 5 is represented by a dotted line. The reference plane E extends perpendicular to the drawing plane.

[0037] Between the two projections 7, 8, the pulley 3 has a recess 27 (cf. Fig. 5), which serves to positively accommodate a deflection body 28 (cf. Fig. 2). In the installed state, the deflecting body 28 sits in the recess 27 in a form-fitting manner in the circumferential direction. The form-fitting elements 15, which are received by the grooves 13, can be designed, for example, as webs in one piece or as a single piece with the deflecting body 28.

[0038] The third projection 9 is arranged on the lower half of the inner circumference 11 in the "6 o'clock position." The projection 9 has a support surface 29, via which the pulley 3 is supported on the outer circumference 10 of the ball nut 4. An imaginary reference plane E intersecting the pulley 3 in the "12 o'clock position" and the "6 o'clock position" divides the third projection 9 into two halves.

[0039] Compensating material 12a, 12b is arranged circumferentially on both sides next to the projection 9. This compensating material 12a, 12b compensates for an imbalance of the pulley 3 resulting from the projections 7, 8, and 9 arranged on the inner circumference 11. The specific mass and spatial distribution of the compensating material 12a, 12b depends on the resulting imbalance, which in turn depends on the shape of the projections 7, 8, and 9.

[0040] The present invention balances the entire ball screw as a whole, not just individual components. Due to the comparatively small mass of the balancing material 12, 12b, the imbalance of the entire assembly is compensated and the function of the ball screw is optimized. All rotating parts in the ball screw are taken into account during balancing by the balancing material, including the ball chain acting between the ball nut 4 and the threaded spindle 5. The rotational form-fitting fixation allows the invention to achieve balance on the pulley 3. This is more cost-effective than the balancing of the ball nut 4 known from the prior art cited at the beginning.

[0041] The pulley 3 can be made of metal, for example. Aluminum or steel are suitable materials for the pulley 3. The production of the pulley 3 with the compensating material 12a, 12b integrally formed thereon can be carried out, for example, by machining. Pulleys 3 according to the invention can also be produced by forging, stamping or casting. Alternatively, the pulley 3 can be produced by a sintering process (powder metal compacting). The pulley 3 is then a sintered component. Alternatively, the pulley can also be produced by plastic injection molding. In this case, the pulley 3 is a plastic component and can be lighter than a metal component, which contributes to a reduction in the overall weight of the ball screw drive.

[0042] A ball screw drive of an electromechanical power steering system for motor vehicles, equipped with a balanced pulley 3 according to the invention, exhibits a push-through force with a force-displacement curve that exhibits low ripple and no large force peaks. This minimizes vibrations and noise. The push-through force is the force required to move the rack. In a force-displacement diagram, the magnitude of the push-through force is plotted against the distance of the rack movement. List of reference symbols 1 servo motor 2 ball screw 3 pulley 4 ball nut 5 threaded spindle 6 drive belts 7 lead 8 lead 9 lead 10 Outer circumference of the ball nut 11 Inner circumference of the pulley 12a Leveling material 12b Leveling material 13 grooves 14 grooves 15 Form-locking element 16 Rotation axis 20 rolling bearings 21 housings 22 Drive pulley 23 Longitudinal axis 24 double arrow 25 rack 26 Gearing 27 Recess 28 deflection bodies 29 Support surface E Reference plane

Claims

[1] An electromechanical power steering system for a motor vehicle, comprising a servomotor (1) and a ball screw drive (2), wherein the ball screw drive (2) comprises a pulley (3), a ball nut (4), and a threaded spindle (5), wherein the pulley (3) is arranged concentrically to the ball nut (4) and surrounds it, wherein the ball nut (4) engages with the threaded spindle (5), wherein the pulley (3) is drivable by the servomotor (1) via a drive belt (6), wherein the pulley (3) has, on a first half of its inner circumference, two radially inwardly extending projections (7, 8) which are positively connected to the ball nut (4) in the circumferential direction, wherein the pulley (3) has, on a second half of its inner circumference, a third projection (9) extending radially inwardly, via which the pulley (3) is supported on the outer circumference (10) of the ball nut (4), characterized bythat the pulley (3) has compensating material (12a, 12b) on its inner circumference (11) on both sides of the third projection (9), by means of which an imbalance of the ball screw drive (2) is compensated. [2] Electromechanical power steering according to claim 1, characterized by that each of the two projections (7, 8) has a first groove (13) and the ball nut (4) has two second grooves (14) on its outer circumference, wherein the first (13) and second grooves (14) together define two receiving spaces, wherein a form-fitting element (15) is arranged in each receiving space. [3] Electromechanical power steering according to one of the preceding claims, characterized by that the compensating material (12a, 12b) is an integral part of the pulley (3). [4] Electromechanical power steering according to one of the preceding claims, characterized bythat the two projections (7, 8) have the same distance in the circumferential direction from a reference plane (E) which bisects the first half and the second half of the inner circumference (11) of the pulley (3), wherein an axis of rotation (16) of the pulley (3) lies in the reference plane (E), and wherein one half of the third projection (9) extends in the circumferential direction on one side of the reference plane (E) and another half of the third projection (9) extends on the other side of the reference plane (E). [5] Electromechanical power steering according to one of the preceding claims, characterized by that the compensating material (12a, 12b) has the same mass and the same spatial distribution in the circumferential direction on both sides next to the third projection (9).

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