Electromechanical power steering system with play compensation for the worm gear mechanism

The invention relates to an electromechanical steering system having an electric servomotor (1) which drives a worm shaft (2) which meshes with a worm gear (7) which is arranged on a steering shaft (8), wherein the worm gear (7) is operatively connected to an input shaft of a steering gear, and wherein the worm shaft (2) and the steering shaft (8) are mounted rotatably in a common gearbox casing (9), in which the worm shaft (2) has a free end (12) which is remote from the motor and is mounted in a rolling bearing (13) with an inner ring (14), rolling bodies (15) and an intermediate ring (16), wherein the intermediate ring has an inner running surface for the rolling bodies (15) and an outer running surface for outer rolling bodies (17), and wherein the inner running surface and the outer running surface of the intermediate ring (16) are arranged eccentrically with respect to one another.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a National Stage of PCT International Application No. PCT/EP2012/001276, filed on Mar. 23, 2012, and claims priority of German Patent Application No. 10 2011015883.9, filed on Apr. 4, 2011. The disclosures of the aforementioned applications are incorporated herein in their entirety by reference.

The present system relates to an electromechanical power steering system.

When mounting shafts, it is conceivable and possible to use plain bearings, having an outer ring with an outer peripheral surface that is arranged eccentrically to the bearing surface. Such a bearing is only suitable for rotating shafts, however. It is disadvantageous when a shaft often comes to a standstill because plain bearings have a high breakaway torque. They require more fundamentally continuous lubrication and monitoring during operation. Therefore, plain bearings are not used for the mounting of drive shafts of electromechanical power steering systems.

European Patent EP 1 727 723 B1 discloses a ball bearing in an electromechanical power steering system with an eccentric outer ring, with which the position of a shaft mounted in the bearing can be adjusted. Following adjustment, the outer bearing shell is fixed in the bearing seat. Play compensation during operation, which may be necessary due to wear, is not provided for, however.

Document U.S. Pat. No. 6,357,313 B1 discloses an electromechanical power steering system comprising a worm gear mechanism, wherein the free end of the worm shaft is mounted in a ball bearing with concentric outer bearing ring. The ball bearing itself is arranged in a cam which is rotatably arranged in the steering housing about an axis which is spaced from the axis of rotation of the rolling bearing. In this way, the position of the rolling bearing and thus the engagement of the worm shaft in the assigned worm gear are adjustable by rotation of the cam in the housing. In one embodiment, the cam is acted upon by a spring force, so that the rolling bearing may be pretensioned with the shaft against the meshing engagement. The cam is in this case mounted in respect of the gearbox casing in a sliding bearing.

The latter embodiment of the prior art, which is considered as generic, is indeed capable of compensating for a change of the gear engagement between the worm shaft and the worm gear during operation. The required forces or torques that must ultimately be applied to the meshing engagement, are quite high, however, since the sliding bearing of the cam has a high breakaway torque in the gearbox casing. Accordingly, the burden on the transmission components is high when said worm shaft and the worm gear are in close engagement.

It is therefore an object of the present invention to provide a self-adjusting bearing of a worm shaft in the gear mechanism of an electromechanical power steering system in which the forces required for automatic adjustment are smaller.

This object is achieved by a device having the features of claim1or6.

Since an electromechanical power steering system having an electric servomotor which drives a worm shaft which meshes with a worm gear which is arranged on a steering shaft, wherein the worm gear is operatively connected to an input shaft of a steering gear, and wherein the worm shaft and the steering shaft are mounted rotatably in a common gearbox casing, worm shaft has a free end which is remote from the motor and is mounted in a rolling bearing with an inner ring, rolling bodies and an intermediate ring, wherein the intermediate ring has an inner running surface for the rolling bodies and an outer running surface for outer rolling bodies, and wherein the inner running surface and the outer running surface of the intermediate ring are arranged eccentrically with respect to one another, the worm shaft when under load or if there is a change in engagement due to the effects of temperature can rapidly move out of the way. The rolling bearing of the cam in the gearbox casing allows for quick evasive action due to the low breakaway torque that must be overcome for the evasive action.

If the intermediate ring together with the rolling bodies and an outer ring form a rolling bearing that is eccentric to the rolling bearing, whose outer ring is seated in a bearing seat in the gearbox casing, the function is further improved. Preferably, the intermediate ring is pretensioned by spring means so that the worm shaft is forced into engagement with the worm gear.

The arrangement is particularly compact if the rolling bearing at the free end of the worm shaft is a needle bearing.

A particularly smooth adjustment is achieved when the rolling bearing supporting the intermediate ring in the gearbox casing is a ball bearing.

The object is also achieved in that in an electromechanical power steering system with an electric servo motor which drives a worm shaft which meshes with a worm gear arranged on a steering shaft, wherein the worm gear is in operative connection with an input shaft of a steering gear and the worm shaft and the steering shaft are rotatably supported in a common gearbox casing, the worm shaft having a free end remote from the motor, which is mounted in a rolling bearing with an inner ring, rolling bodies and an outer ring, wherein the rolling bearing is located in a cam lever which is mounted in the gearbox casing so that it can pivot about a pivot axis lying outside the rolling bearing.

Advantageously, the cam lever is pretensioned by a helical spring such that the worm shaft is forced into engagement with the worm gear.

It may also be provided that an electromechanical actuating element is arranged in the gearbox casing such that by the operation of the intermediate ring or the cam lever it can set or regulate the position of the worm shaft relative to the worm gear as a function of a controller. In particular, it can be provided that the control or regulation occurs in response to a torque applied by the servomotor. Thus, the engagement can be controlled as a function of the load.

FIG. 1shows in a longitudinal section the gear mechanism of an electromechanical power steering system and the longitudinal section running along an axis of rotation1of a worm shaft2, which is driven by an electric motor3. The electric motor3has a motor shaft4which is coupled via a flexible coupling5non-rotatably with the worm shaft2. The worm shaft2meshes via a worm gearing6with a worm gear7. The worm gear7is in turn non-rotatably connected to a steering shaft8, which extends between a steering wheel (not shown), and the actual steering gear of the vehicle.

The stated components are mounted in a common gearbox casing9.

The mounting of the worm shaft2in the casing9is at a motor-side end10of the worm shaft2in a conventional rolling bearing11in the form of a ball bearing. The ball bearing11is designed such that the worm shaft2can perform small axial movements and minor modifications to the axis of rotation1with respect to the casing9.

The worm shaft2also has an end12remote from the motor, which is similarly mounted in a rolling bearing13. The rolling bearing13comprises an inner ring14, rolling bodies15, and an intermediate ring16. In turn, the intermediate ring16is itself provided on its outside with a running groove for balls17. The balls17run in an outer ring18, which is finally secured in a bearing seat19of the casing9.

The intermediate ring16is finally provided with a pin20which is secured on the side of the intermediate ring16facing away from the casing9.

The intermediate ring16is designed such that on its inner side a running surface for the rolling bodies15of the inner bearing13is formed. This running surface has a substantially cylindrical form, as the rolling bodies15are provided as pins in this exemplary embodiment. On the outer peripheral surface the intermediate ring16is provided with a ball running surface for the externally running balls17, wherein the outer running surface is not positioned concentrically with the inner running surface. Rotation of the worm shaft2causes the intermediate ring16to define the position of the axis of rotation1, while the inner rolling bearing13brings about the easy and play-free rotation of the worm shaft2relative to the intermediate ring16. A rotation of the intermediate ring16, however, causes a displacement of the axis of rotation1of the worm shaft2, and thus a variation of the engagement of the worm6with the worm gear7. In this manner, feeding of the worm shaft2to the worm gear7can, in particular, be effected in order to achieve a play-free meshing engagement.

In this arrangement, the intermediate ring16is likewise mounted relative to the casing9via the rolling bodies17. In particular, the intermediate ring16itself forms a part of this outer bearing, which is formed from the intermediate ring16, the rolling bodies17and the bearing outer ring18. This arrangement allows for a very smooth adjustment of the intermediate ring16, even if it is under load. A particularly fine and responsive adjustment of the position of the worm shaft2, more specifically the axis of rotation1of the worm shaft2in relation to the worm gear7is in this way possible.

FIG. 2shows the worm shaft with the rolling bearings and the worm gear7meshing with the worm shaft in a perspective view, wherein the components of the casing and the electric motor have been omitted. Identical components bear the same reference numbers. Here the intermediate ring16is provided with two actuating elements20. These actuating elements20may serve as contact points for springs for elastic pretensioning, as described below regardingFIG. 3. They can also serve as contact points for an electric actuator, which operates the intermediate ring16in response to a control or regulation.

FIG. 2shows how the axis of rotation of the worm shaft2is arranged concentrically to the inner ring14and to the inner running surface of the intermediate ring16, but is positioned eccentrically in relation to the outer running surface of the intermediate ring16and the outer ring18. Accordingly, rotation of the intermediate ring16, causes a displacement of the axis of rotation1with respect to the worm gear7. The centre of rotation of the intermediate ring16, that is to say the point about which the intermediate ring16can rotate relative to the casing9, is positioned in the centre of the outer ring18. InFIG. 2it can be seen that this pivot point is located within the inner rolling bearing13, which is formed of the inner ring14, the rolling bodies15and the inner running surface of the intermediate ring16. The spatial distance between these two centres of rotation can be referred to as the eccentricity of the intermediate ring16and in the present case this eccentricity is less than the radius of the inner running surface of the intermediate ring16. Such a low eccentricity is preferred in this exemplary embodiment because it permits a particularly fine adjustment of the position of the worm shaft2.

FIG. 3shows the exemplary embodiment ofFIG. 2with two helical springs21which act on the actuating elements20. Here the actuating elements20take the form of pins which are arranged axially parallel to the end face of the intermediate ring16. The helical springs21work on strain. They force the intermediate ring16in the exemplary embodiment according toFIG. 3into an anticlockwise rotation. Since the pivot point of the worm shaft2is located on the left of the point of rotation of the intermediate ring16, the worm shaft2is forced by the springs against the worm gear7.

FIG. 4shows an arrangement in which the worm shaft2is mounted rotatably at its free end12in a conventional rolling bearing22. The rolling bearing22is seated with its outer ring in a cam lever23having a corresponding bearing seat. The cam lever23is mounted in a pivot axis24in the casing9(not shown). A helical spring25, which in turn works on strain, engages with a hook-shaped end26of the cam lever23, which is located opposite the pivot axis24. The bearing22is arranged between the pivot axis24and the hook26. The tension spring25acts downwards inFIG. 4, thereby pulling the cam lever23and thus the worm shaft towards the worm gear7. In this way also an elastic pretensioning of the worm shaft2against the worm gear7is obtained. As inFIG. 3, in this way a play-free engagement of the worm shaft2in the worm gear7is achieved.

In contrast to the embodiments according toFIG. 2andFIG. 3, in the embodiment according toFIG. 4the bearing22and thus the axis of rotation of the worm shaft2is moved in a considerably larger radius, since the pivot axis24of the cam lever23is spaced further from the axis of rotation1of the worm shaft than inFIGS. 2 and 3. In particular here the eccentricity, that is to say the distance of the axis of rotation1from the pivot axis24is selected to be between one and three times the diameter of the rolling bearing22.

In operation, these exemplary embodiments provide the advantage that the position of the worm shaft2with respect to the worm gear7is adjustable. In the embodiment according toFIG. 2adjustment is by means of an actuator, while in the embodiment ofFIGS. 3 and 4it is by means of spring pretension. The mounting of the cam is in all cases designed such that the feeding of the worm shaft2to the worm gear7takes place at particularly low friction and low breakaway torque. In this way it is possible to compensate for small changes in the dimensions and in the relative position of components to one another, which may occur for example due to thermal influences. The cam mounting is easily movable so that no adverse forces arise in the area of the meshing engagement or in the area of the bearing.

REFERENCES