Electromechanical motor vehicle steering system comprising housing parts that are interconnected by means of tapered interference fit

An electromechanical motor vehicle steering system includes an electric motor and an electronic control unit. The electromechanical motor vehicle steering system is at least partially surrounded by housing parts. At least two of the housing parts are connected to one another by interference fit, wherein the interference fit is at least partly a tapered interference fit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Entry of International Patent Application Serial Number PCT/EP2019/064471, filed Jun. 4, 2019, which claims priority to German Patent Application No. DE 10 2018 113 302.2, filed Jun. 5, 2018, the entire contents of both of which are incorporated herein by reference.

FIELD

The present disclosure generally relates to an electromechanical motor vehicle steering system.

BACKGROUND

Electromechanical motor vehicle power steering systems have servo units which may be arranged as a power assistance device on a pinion or a rack of the steering gear. In this case the servo unit has, in addition to an electric motor, an electronic control unit (ECU) for calculating the steering assistance. The housing parts of the components must have a sufficiently stable connection which ideally permits a closed ECU. Additionally it is desirable that the power pack, comprising the motor and the ECU, is cylindrical so that it is able to be used in a manner which is as variable as possible.

From the prior art, for example in the published patent application US 2016 065 027 A1, it is known to connect a multipart electric motor housing, which is connected to an ECU housing, mechanically via a plurality of bolts. This solution proves to be a drawback since contaminants and water may penetrate via unsealed regions and may damage the components. Moreover, significant constructional space is required for such a connection.

From the prior art it is additionally known to introduce the ECU housing into the electric motor housing by means of a cylindrical press connection. When joining the motor housing and the ECU housing very high process forces are present, preventing other joining processes (for example of plug contacts for the electrical connection) from being able to be reliably carried out and monitored at the same time. Additionally, the length of the press connection has to be selected to be sufficiently great that the desired stability of the connection may be achieved.

Thus, a need exists for a method for producing an improved mechanical connection between the housing parts of an electromechanical motor vehicle power steering system which permits a stable, compact and sealed connection which is simple in terms of its production and which is able to be monitored in a simple manner.

DETAILED DESCRIPTION

The present invention relates to an electromechanical motor vehicle steering system and a method for connecting housing parts of an electromechanical motor vehicle steering system.

Accordingly an electromechanical motor vehicle steering system comprising an electric motor and an electronic control unit is provided, wherein the electromechanical motor vehicle steering system is at least partially surrounded by housing parts and at least two of the housing parts are connected to one another by interference fit, wherein the interference fit is at least partly a tapered interference fit. The interference fit is preferably a longitudinal interference fit.

The tapered interference fit permits a stable, compact and sealed connection of the two housing parts. The process forces which are present are significantly lower than in the case of a purely cylindrical press connection. This has the advantage that other joining processes may be carried out and monitored more reliably. Additionally, insensitive parts of the ECU may be placed in the space produced by the press connection, whereby a compact design of the power pack may be achieved.

Preferably, the tapered interference fit is formed by means of two corresponding tapered joining surfaces which adopt a setting angle of greater than 0°, which preferably ranges from 0.5°-5°, in particular ranges from 1°-3°. The setting angle of the two joining surfaces preferably coincides. It is advantageous if the interference fit has an oversize. The oversize is dependent on the joining distance and the setting angle.

In a preferred embodiment, the interference fit is exclusively a tapered interference fit having a joining distance comprising a joining distance without force and a push-on distance, wherein the joining distance without force is greater than the push-on distance. The joining distance describes the entire axial penetration depth of the one housing part into the respective other housing part. This has the advantage that the joining force only increases at the end of the joining distance. Other joining processes may be carried out, in particular, in a controlled manner over the joining distance without force. The oversize may be preferably set by means of the push-on distance, wherein the push-on distance describes the last or final part of the axial joining distance which the second housing part located loosely on the first housing part has to cover in order to reach the position of the interference fit.

The axial relative displacement (pushing-on) of the housing parts to be joined leads to transverse strains and thus to the creation of a corresponding joining pressure in the active surfaces. Due to the oversize, therefore, one of the housing parts is widened in the region of an opening. As a result, a surface pressure is produced in the friction surfaces.

The length of the joining distance without force depends on the choice of setting angle. The greater the angle, the greater the joining distance without force. However, with an increasing setting angle the joining force also rises, whilst the releasing force drops. An optimal compromise is desired with a releasing force which is as high as possible, a joining force which is as low as possible and a joining distance without force which is as great as possible.

Preferably the joining distance without force takes up more than 60% of the entire joining distance, further preferably more than 70% of the entire joining distance. Preferably the push-on distance takes up more than 8% and less than 30% of the entire joining distance.

It is advantageous if an internal part of the housing parts connected by means of the interference fit has a peripheral shoulder which delimits the joining distance.

In a further advantageous embodiment, in the longitudinal direction or axial direction the interference fit is a combination of a cylindrical press connection and a tapered interference fit. The joining surfaces to be joined together may be produced in a simpler manner. Additionally, in this case an annular groove with an inserted O-ring may be provided in the region of the cylindrical press connection for sealing between the housing parts. Preferably, the cylindrical region is between 5% to 15%, wherein the remaining region is of tapered configuration.

Moreover, a method for connecting housing parts of an electromechanical motor vehicle steering system is provided, comprising an electric motor and an electronic control unit, wherein the method comprises the following steps:providing two housing parts as an internal part and an external part, wherein the external part has an opening with an at least partially tapered internal surface which tapers when it passes into the opening, and wherein the internal part has a corresponding taper with a tapered seat surface,positioning the external part on the internal part and pressing the external part onto the internal part with a defined axial joining force.

Preferably the tapered interference fit is formed by means of two corresponding tapered joining surfaces which form a setting angle which is greater than 0°, which preferably ranges from 0.5°-5°, in particular ranges from 1°-3°. It is advantageous if the interference fit has an oversize.

In a preferred embodiment of the method, the interference fit is exclusively a tapered interference fit having a joining distance comprising a joining distance without force and a push-on distance, wherein the joining distance without force is greater than the push-on distance. Preferably the joining distance without force takes up more than 60% of the entire joining distance, further preferably more than 70% and particularly preferably 90% of the entire joining distance. Preferably the push-on distance takes up more than 8% and less than 30% of the entire joining distance.

It is advantageous if the internal part of the housing parts has a peripheral shoulder which delimits the joining distance.

In a further embodiment, the interference fit in the longitudinal direction is a combination of a cylindrical press connection and a tapered interference fit.

In this case, an annular groove with an inserted O-ring may be provided in the region of the cylindrical press connection for sealing between the joining partners.

Preferably generally the housing parts connected by means of the interference fits described above are a motor housing surrounding the electric motor and a housing part surrounding the electronic control unit. However, for example, the parts of a multipart steering gear housing, a worm housing and the motor housing or a sensor housing and a pinion housing may also be connected by the interference fits.

An electromechanical motor vehicle power steering system1with a steering wheel2, which is coupled fixedly in terms of rotation to a steering shaft3, is shown schematically inFIG.1. The driver introduces a corresponding torque via the steering wheel2as a steering command into the steering shaft3. The torque is then transmitted via the steering shaft3to a steering pinion5. The pinion5meshes in the known manner with a toothed segment of a rack6. The steering pinion5forms together with the rack6a steering gear40.

The steering shaft3has on the input side an input shaft30connected to the steering wheel2and on the output side an output shaft31connected to the rack6via the steering pinion5. The input shaft30and the output shaft31are connected together in a torsionally flexible manner via a torsion bar, not shown inFIG.1. A torque introduced by the driver via the steering wheel2into the input shaft30leads to a relative rotation of the input shaft30with respect to the output shaft31. This relative rotation between the input shaft30and the output shaft31may be determined by a rotary angle sensor.

The steering shaft3according toFIG.1further comprises one or more universal joints32, the path of the steering shaft3in the motor vehicle being able to be adapted thereby to the spatial conditions. The intermediate steering shaft of the steering shaft3, which in the example shown is arranged between two universal joints32and which connects the output shaft31to the steering pinion5of the steering gear40, is configured as a steering shaft3which may be altered in terms of length.

The rack6is displaceably mounted in a steering housing60in the direction of its longitudinal axis. At its free end the rack6is connected to track rods7via ball joints, not shown. The track rods7in turn are connected in the known manner via steering knuckles to one respective steered wheel8of the motor vehicle. A rotation of the steering wheel2leads, via the connection of the steering shaft3and the pinion5, to a longitudinal displacement of the rack6and thus to a pivoting of the steered wheels8. Via a road80the steered wheels8are subjected to a reaction which counteracts the steering movement. As a result, a force which requires a corresponding torque on the steering wheel2is needed for pivoting the wheels8. A servo unit10consisting of an electric motor9and an electronic control unit13is provided in order to assist the driver with this steering movement. The servo unit10in this case may be coupled either to a steering shaft3, the steering pinion5or the rack6. The respective power assistance system introduces an assistance torque into the steering shaft3, the steering pinion5and/or into the rack6, whereby the driver is assisted with the steering operation. The three different power assistance systems10,100,101shown inFIG.1show alternative positions for the arrangement thereof. Generally only one of the positions shown is provided with a power assistance system. In this case the servo unit may be arranged as superimposed steering on the steering column or as a power assistance device on the pinion5or the rack6.

InFIG.2an electromechanical motor vehicle steering system1is shown with an electric motor9which acts on a ball nut of a ball screw drive11. InFIG.2only the housing of the ball screw drive and the steering drive40is shown. The ball nut is in engagement via circulating balls with a ball screw which is arranged on the outer periphery of the rack6. A rotation of the ball nut effects an axial displacement of the rack6, whereby a steering movement of the driver is assisted. Preferably, the ball screw drive11is coupled to the electric motor9via a toothed belt.

FIG.3shows the electric motor9with the motor shaft12and an electronic control unit (ECU)13connected to the electric motor9. The electric motor9is received in a motor housing90. The motor housing90of the electric motor9is connected to a housing part130of the electronic control unit13by means of a press connection.

As shown inFIG.4the motor housing90of the electric motor9has an opening14on a front face15, the housing part130of the ECU13being pressed therein. The housing part130has a taper16with a peripheral seat surface20which is delimited by a peripheral shoulder17at the end remote from the motor. The shoulder17serves as a stop in the joining process. The shoulder17thus delimits the joining distance. In the pressed-in state the shoulder17bears with the lower face18against the front face15of the motor housing90.

FIGS.5and6show schematically a joining process between the motor housing90and the housing part130of the ECU13. The motor housing90has in the upper front face15in the vicinity of the electronic control unit13the opening14into which the housing part130surrounding the electronic control unit may be inserted. The opening14is of tapered design and widens in the direction of the front face15. In other words, the internal surface19is a tapered joining and active surface with a setting angle β (half cone angle). The housing part130of the electronic control unit has a corresponding tapered seat surface20for producing a tapered press connection. The annular shoulder17, which serves as a stop during the joining process, adjoins the tapered seat surface20. The tapered seat surface20tapers away from the shoulder17. The setting angle β of the internal surface19and the tapered seat surface20in this case are identical apart from production-related differences, so that during the joining process a bearing of the two surfaces is produced over the entire surface.

As shown inFIG.5, initially the joining process takes place without force until the tapered seat surface20bears over the entire surface against the inner face19of the opening14. In this case, in the longitudinal direction50the housing of the ECU130has already been introduced into the motor housing90, preferably by 90% of the joining distance, wherein the joining distance d describes the entire axial penetration depth of the ECU housing130into the motor housing90. The joining of the two housing parts130,90is carried out by an interference connection (seeFIG.6).

The oversize may be set by means of the push-on distance a, wherein the push-on distance a describes the last or final part of the axial joining distance d which the motor housing90located loosely on the ECU housing130has to cover in order to reach the position of the interference fit.

The axial relative displacement (pushing-on) of the parts130,90to be joined, leads to transverse strains and thus to the creation of a corresponding jointing pressure in the active surfaces. As a result of the oversize, therefore, the motor housing90is widened in the region of the opening14. Consequently, a surface pressure is generated in the friction surfaces.

The length of the joining distance without force b depends on the choice of setting angle β. The greater the angle β, the greater the joining distance without force b. However, with an increasing setting angle β the joining force also rises, whilst the releasing force drops. An optimal compromise is desired, with a releasing force which is as high as possible, a joining force which is as low as possible and a joining distance without force which is as great as possible.

The setting angle β preferably ranges from 0.5°-5°, in particular ranges from 1°-3° and is preferably approximately 1°. The joining distance without force b is additionally preferably greater than the push-on distance a so that the required joining force only rises at the end of the joining process. Preferably the joining distance without force b takes up more than 60% of the entire joining distance d, further preferably more than 70% of the entire joining distance d. Preferably the push-on distance a takes up more than 8% and less than 30% of the entire joining distance d. The advantage of this delayed rise in the joining force is that other joining processes with lower joining forces carried out in parallel may be reliably monitored. Thus, for example, electrical plug contacts may be inserted during the joining process and the plug-in process may be assessed using the prevailing plug-in forces.

Since the joining distance d is delimited by the shoulder17of the ECU housing130, other dimensions which are dependent on the joining process may be constant, for example the insertion depth of electrical plug contacts.

However, it is not possible for the joining to take place until a desired joining force is reached, due to component tolerances. The press connection is thus less secure than in a nominal case. This reduction of the force due to the tolerances of the joining partners has to be taken into consideration in the design.

FIG.7shows a second possible embodiment in which the ECU housing130has a tapered opening14and the motor housing90has a corresponding tapered seat surface20.

In the embodiments, a limit to the joining distance may be dispensed with. The joining process is only terminated when a fixed joining force is reached, whereby a mechanical connection which may be reproduced very effectively may be achieved.

It is also possible, instead of the purely tapered connection, to use a combination of a cylindrical and tapered press connection in the axial direction. In this case, the internal surface of the opening has a cylindrical and a tapered partial region, wherein the cylindrical partial region extends inwardly starting from the front face and the tapered region adjoins thereto in the axial direction. Preferably, the cylindrical region is between 5% to 15%, wherein the remaining region is of tapered configuration. Correspondingly, the tapered seat surface to be inserted into the opening also has a cylindrical partial region and a tapered partial region, wherein the tapered partial region adjoins the cylindrical partial region at the end in the vicinity of the joining partners. This combined press connection has the advantage that the joining partners may be manufactured in a simpler manner and the manufacturing is able to be monitored in a simpler manner in terms of measuring technology. Additionally, an annular groove for receiving an O-ring, which improves the seal between the joining partners, may be incorporated in the cylindrical region. During the joining process, a joining distance without force is not present due to the cylindrical region. The force required for the joining rises abruptly at the end as soon as the push-on distance is reached, when the oversize comes into effect. The cylindrical region thus also constitutes a releasing force, whereby the connection becomes more secure. The joining distance is delimited by a stop of the two components to be joined together.