BALL SCREW DRIVE OF AN ELECTROMECHANICAL POWER STEERING DEVICE WITH DEFLECTING BODIES FOR A BALL RETURN
An electromechanical power steering device may include a servomotor that drives an axially movable component via a ball nut mounted in a bearing such that the ball nut can be rotated about a longitudinal axis in a housing. The ball nut may be engaged with a threaded spindle configured on the axially movable component. The ball nut may include a ball screw on its inner side for balls to roll on, and an external ball return means that connects a start of the ball screw to an end of the ball screw to make endless circulation of the balls possible. A return channel of the ball return means may be formed by way of a deflecting body and a pulley wheel.
- 1. -15. (canceled)
- 16. An electromechanical power steering device for a motor vehicle comprising:
a servomotor that drives an axially movable component via a ball nut that is mounted in a bearing such that the ball nut is rotatable about a longitudinal axis in a housing, wherein the ball nut is engaged with a threaded spindle configured on the axially movable component, wherein an inner side of the ball nut includes a ball screw for balls to roll on, wherein an external ball return means of the ball nut connects a start of the ball screw to an end of the ball screw to enable endless circulation of the balls; a pulley wheel; and a deflecting body, wherein a return channel of the external ball return means is formed by way of the deflecting body and the pulley wheel.
- View Dependent Claims (17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30)
The present invention relates to an electromechanical power steering device having the features of the preamble of claim 1.
In electromechanical power steering devices, a torque is generated by an electric motor, which torque is transmitted to a gear mechanism, and the steering torque which is introduced by the driver is superimposed therein.
An electromechanical power steering device of the generic type has a servomotor which acts on a ball nut of a ball screw drive. The ball nut is in engagement via circulating balls with a ball screw which is arranged on the outer circumference of a rack which is part of a rack and pinion steering system. A rotation of the ball nut brings about an axial movement of the rack, as a result of which a steering movement of the driver is assisted. The ball screw drive is preferably coupled via a toothed belt to the electric motor.
EP 1 659 312 B1 discloses a ball screw drive, in the case of which a deflecting body is disclosed which conveys the balls via a ball return means back into the ball nut. The deflecting body is manufactured from plastic and is plugged from the outside into a main body of the ball nut. Here, the deflecting body projects beyond the outer circumference of the metallic main body of the ball nut. Recesses are provided in the interior of the toothed belt pulley in order to fasten the deflecting body in the ball nut.
Patent specification EP 2 713 078 B1 has disclosed a ball screw drive with a ball return means. On the inner side, the toothed belt pulley has a recess for receiving the ball return means and two webs as an anti-rotation safeguard.
It is an object of the present invention to specify an electromechanical power steering device with a ball screw drive, in the case of which the ball return means is configured to be as durable and inexpensive as possible.
Said object is achieved by an electromechanical power steering device having the features of claim 1. Further advantageous embodiments of the invention can be gathered from the subclaims.
Accordingly, an electromechanical power steering device is provided, for a motor vehicle, with a servomotor which drives an axially movable component via a ball nut which is mounted in a bearing such that it can be rotated about the longitudinal axis in a housing, the ball nut being in engagement with a threaded spindle which is configured on the component, and having a ball screw on its inner side for balls to roll on, and with an external ball return means which connects the start of the ball screw to the end of the ball screw, in order to make an endless circulation of the balls possible, and with a deflecting body, a return channel of the ball return means being formed by way of the deflecting body and a pulley wheel.
Said arrangement is particularly compact and inexpensive, since the pulley wheel forms a part of the return channel. In addition, the pulley wheel preferably consists of a sintered pulley wheel body, as a result of which the wear of the ball return means is reduced unlike in the case of plastic parts.
It is preferred if the deflecting body has at least two webs on its outer side, which at least two webs engage into the pulley wheel in such a way that a torque can be transmitted from the pulley wheel to the ball nut.
Furthermore, it is preferred that the deflecting body has at least two webs on its inner side, which at least two webs engage in operation into corresponding recesses of the ball nut and therefore ensure the transmission of the torque.
The webs preferably extend in the longitudinal direction. It can be provided here that the webs of the one side and the webs of the other side are arranged offset with respect to one another in the longitudinal direction.
In one preferred embodiment, the webs of the underside are snap-action elements and enter into a snap-action connection with the respective recess of the ball nut.
The webs are preferably configured in one piece from a single component, with preference are configured integrally with the deflecting body in a manner which is formed from a single material.
It can be provided, furthermore, that the deflecting body has pins for positioning the deflecting body on the ball nut. Said pins preferably engage into recesses of the ball screw which are provided for the entry and exit of balls for the external return to the opposite end of the ball screw. In particular, the pins can have a pin inlet, at which the balls to be deflected come into contact first of all with the deflecting body.
The bearing is preferably a double-row angular contact ball bearing with at least one bearing inner ring and two bearing outer rings, the pulley wheel and the deflecting body being arranged between the bearing outer rings. This results in a particularly compact arrangement.
In order to configure the bearing to be resistant to tilting, it can be provided that the contact angles of the double-row angular contact ball bearing are selected in such a way that a supporting spacing of greater than zero is configured.
Furthermore, it is advantageous if the two bearing outer rings are received in a sleeve which is arranged in a bearing seat of the housing. Here, said sleeve is preferably configured in such a way that it can compensate for thermal expansions between the mechanism housing and the ball nut.
In one preferred embodiment, the component is a rack of a rack and pinion steering mechanism.
In the following text, one exemplary embodiment of the present invention will be described using the drawings. Identical components or components with identical functions have identical designations. In the drawings:
A rotation of the nut sets the threaded spindle of the ball screw drive 12, which threaded spindle is part of the rack 6, in an axial movement which ultimately brings about a steering movement for the motor vehicle.
Even if an electromechanical power steering device with a mechanical coupling between the steering wheel 2 and the steering pinion 5 is shown here in the example, the invention can also be applied to motor vehicle steering devices, in which there is no mechanical coupling. Steering systems of this type are known under the term steer-by-wire.
Furthermore, the bearing 15 has two separate outer rings 18. The outer rings 18 are received in each case in a separate sleeve 19 which is arranged in a bearing seat 20 of the housing 21. The pulley wheel 14 of the toothed belt drive 11 is fastened in a torque-proof manner on the ball nut 13. The sleeve 19 is preferably formed from a material which has a greater thermal expansion than aluminum and steel. In particular, the sleeve 19 is preferably formed from a plastic, particularly preferably from PA66GF30 (polyamide 66 with glass fiber reinforcement with a 30% volume share). It is preferably manufactured from plastic and compensates for thermal expansions between the mechanism housing 21 and the ball nut drive 12. The sleeve preferably comprises a circular-cylindrical circumferential wall 191 which encloses the bearing 15 and the bearing axis 24, and a circular-cylindrical bottom region 192 which extends radially inward in the direction of the bearing axis 24 and has a circular-cylindrical opening 193 which encloses the bearing axis 24. Here, the two separate sleeves 19 are preferably arranged in such a way that the two bearings 15 are arranged between the two bottom regions 192. The bottom regions 192 are preferably of planar configuration with a preferably constant thickness. It is also conceivable and possible, however, to provide the bottom regions in a targeted manner with grooves, engravings or ribs or an undulating shape, in order, for example, to influence the lubrication and/or the thermal properties in a targeted manner.
For further improvement of the compensation properties, the sleeve can have recesses in its circumferential wall 191, preferably slots 194 which extend in the direction of the bearing axis 24. Said slots preferably run as far as to that open end of the circumferential wall 191 which is directed away from the bottom region 192. In other words, the slots 194 are open in the direction of the pulley wheel 14.
The sleeve 19 is preferably formed in one piece from a single component, is preferably formed integrally from a single material, and is particularly preferably formed in an injection molding method.
As shown in
Depending on the application, however, said corrugated spring 22 can be replaced by way of a cup spring or by way of a combination of a cup spring and a corrugated spring.
The balls 100 of the angular contact ball bearing 15 are guided in a ball cage 101.
The raceways of the double-row angular contact ball bearing 15 are configured in such a way that the connecting lines 23, 23′, 23″, 23′″ of the contact points between the ball and the raceways intersect the bearing axis 24 so as to lie between the outer rings 18. A predefined supporting spacing X is formed between the two intersection points with the bearing axis 24. The bearing 15 becomes particularly resistant to tilting as a result of the great supporting spacing X. For a particularly high tilting resistance, the supporting spacing X preferably lies in an interval between one time and three times the diameter of the balls 100 of the angular contact bearing. A supporting distance which corresponds to twice the diameter of the balls 100 of the angular contact ball bearing is to be particularly preferred. The contact area of the ball 100 on the raceway face 17 and an inner face of the sleeve preferably corresponds to a quarter of a ball circumferential area. An undercut which is not contacted by the ball preferably remains both on the raceway face and on the inner face of the sleeve. The angle which connecting line of the two contact points between the ball 100 and the raceways encloses with the radial plane and at which the loading is transmitted from one raceway to the other is called the contact angle α. The contact angle is preferably of equal magnitude for both rows of the bearing 15. The optimum tilting resistance of the bearing 15 can be set at a defined contact angle α by way of a predefined value of the supporting spacing X.
The pins 30 are preferably oriented at an angle β, with the result that the pins 30 are inserted with prestress into the recesses 27, the prestress prestressing the pin inlet 301 into the recess 27. As a result, the transition of the balls 28 into the return channel 25′ is improved. In particular, the requirements of the manufacturing tolerances for the deflecting body can be lowered as a result.
The deflecting body can be configured or penetrated with honeycomb-like cutouts or depressions on the upper side and/or on the underside.
The assembly of the pulley wheel and the ball nut can take place with a play-free seat without an interference fit as a result of the arrangement of the webs. This has the advantage that the ball nut is not disadvantageously deformed by way of the joining operation.
The bearing 15 of the ball nut 13 is configured in such a way that the deflecting body 26 can be arranged between the ball nut and the pulley wheel. The ball return means and/or the deflecting body therefore have/has space within the double-row bearing, as a result of which the arrangement becomes particularly compact.