Device for Pressing a Gear Rack Against a Pinion

A device for pressing a rack against a pinion comprises a housing, a thrust piece which is shiftably guided in the housing along a pressing axis, a bearing element which can axially be fixed at the housing, and radially pressurized wedge members which each rest on the thrust piece and on the bearing element and axially urge the thrust piece away from the bearing element. A truncated coned is formed at least on one of the thrust piece on a side facing the bearing element and the bearing element on a side facing the thrust piece. At least three wedge members are provided which are uniformly distributed in a circumferential direction.

BACKGROUND OF THE INVENTION

This invention relates to a device for pressing a rack against a pinion, in particular for use in a steering gear for a vehicle.

Rack-and-pinion steering systems for vehicles are known from the prior art in various configurations. Due to their operating principle, all rack-and-pinion steering systems include a steering gear with a rack and a pinion, wherein the pinion meshes with a toothed region of the rack. A rotational force applied onto the steering shaft and the pinion via the steering wheel is converted into a rack normal force and passed on to steerable wheels of a vehicle. Usually, rack-and-pinion steering systems nowadays are formed as hydraulic, electrohydraulic or electric power-assisted steering systems which assist a vehicle operator in the steering operation.

Since considerable forces occasionally occur in the steering gear, it has already been recognized quite early that particular measures must be taken for keeping the rack in engagement with the pinion with as little backlash as possible. Otherwise, there is a risk that under load the rack moves away from the pinion by being deformed transversely to the longitudinal direction of the rack. There would at least occur an undesired increase of the backlash in the steering system, in the extreme case even slipping through of the steering system.

In order to prevent this, a thrust piece usually is employed in the region of the pinion, which urges the rack against the pinion with a rather constant pressing force. Adjusting the desired pressing force, taking account of wear phenomena as a result of the sliding friction between thrust piece and rack, which occurs during the steering operation, and avoiding disturbing rattling noise during the vehicle operation constitute the greatest challenges for pressing devices for rack-and-pinion steering systems.

U.S. Pat. No. 7,654,166 B2 already describes a pressing device for rack-and-pinion steering systems, which in operation of the vehicle operates largely free from backlash and hence particularly quietly and in addition allows an adjustment of the pressing force of the thrust piece. For pressing the rack against the pinion, this document shows a device which comprises a housing, a thrust piece which is shiftably guided in the housing along a pressing axis, a bearing element which can axially be fixed at the housing, and radially pressurized wedge members which each rest on the thrust piece and on the bearing element and axially urge the thrust piece away from the bearing element.

To keep the pressing device largely free from backlash, two separate wedge members are provided, but the assembly of the device, in particular the radial alignment and centering of the wedge members relative to the thrust piece, as well as the exact pressurization of the thrust piece in axial direction via the two inclined wedge surfaces turns out to be expensive. An off-center or not exactly axially aligned pressurization of the thrust piece can lead to jamming of the pressing device and hence to an undesired “jerking” of the steering wheel during the steering operation.

BRIEF SUMMARY OF THE INVENTION

It is a feature of the invention to create a pressing device which with particularly little assembly effort ensures an exact and uniform pressurization of the thrust piece in axial direction.

For solving this feature, the invention provides a device for pressing a rack against a pinion comprises a housing, a thrust piece which is shiftably guided in the housing along a pressing axis, a bearing element which can axially be fixed at the housing, and radially pressurized wedge members which each rest on the thrust piece and on the bearing element and axially urge the thrust piece away from the bearing element. A truncated coned is formed at least on one of the thrust piece on a side facing the bearing element and the bearing element on a side facing the thrust piece. At least three wedge members are provided which are uniformly distributed in a circumferential direction. As a result of the frustoconical formation of the thrust piece and/or the bearing element at an axial end and of the at least three uniformly distributed wedge members, the wedge members are radially centered with respect to the pressing axis. The thrust piece on the one hand is reliably centered in radial direction towards the pressing axis, wherein the radial force components cancel each other out in the centered position, and on the other hand is uniformly urged against the rack in axial direction. The frustoconical side in particular is formed as “straight” truncated cone, i.e. as truncated cone in which the base area and the top area are arranged in parallel and concentrically.

Preferably, the wedge members of the pressing device are made of plastics. Since the occurring loads can easily be absorbed by choosing a suitable plastic material, the plastic version offers advantages with respect to weight, manufacturing costs and adaptable shape.

In one embodiment of the device for pressing a rack against a pinion, the wedge members are movable relative to each other and preferably connected with each other by flexible coupling elements. Due to the connection of the wedge members, the number of individual components is reduced and the assembly effort for the pressing device is reduced considerably.

In this embodiment, in particular two wedge members adjacent in circumferential direction can each be connected by a flexible coupling element. This represents a simple possibility for positioning all wedge members relative to each other and yet maintain an individual, radial movability.

Particularly preferably, the wedge members are designed integrally with the coupling elements and form a wedge member unit. This wedge member unit in particular can be fabricated of plastics with little expenditure and in addition requires no preassembly in which individual wedge members must be connected with each other via separate coupling elements.

In another embodiment of the pressing device, an element elastic in axial direction, in particular a disk spring or a rubber plate, is provided axially between the bearing element and the thrust piece. In this way, for example manufacturing tolerances in the components of the rack-and-pinion steering system can be compensated without a movement of the rack being undesirably impeded by the thrust piece during a steering maneuver.

In addition, there is preferably provided a spring element which radially pressurizes the wedge members with respect to the pressing axis. By this spring element, the self-centering of the thrust piece with respect to the pressing axis can easily be realized on the one hand, and on the other hand the occurring thrust piece wear can be compensated by radially shifting the wedge members.

For example, the spring element can surround, in particular enclose the wedge members and urge the same radially to the inside, i.e. towards each other. This provides for an easy manufacture of the wedge members or the wedge member unit and for an uncomplicated assembly of the spring element on the wedge members.

In a special design variant the spring element axially protrudes beyond the wedge members, is formed axially elastic and axially rests on the bearing element. The spring element, preferably an O-ring made of rubber or a similar elastic material, hence ensures both a radial pressurization of the wedge members and a backlash-free axial elasticity within the pressing device.

In a further embodiment of the pressing device, the wedge members flare in axial direction in a wedge-shaped manner, as seen radially from the inside to the outside. To reduce the surface pressures, the wedge members also can flare in circumferential direction, as seen radially from the inside to the outside, and form segment-shaped wedge members.

The invention moreover also comprises a rack-and-pinion steering system for motor vehicles, comprising a housing, a rack shiftably mounted in the housing, a pinion which engages into the rack, and an above-described device which urges the rack against the pinion.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2show a section of a rack-and-pinion steering system10for motor vehicles, comprising a housing12, a rack14longitudinally shiftably mounted in the housing12, a pinion16which engages into the rack14, and a pressing device18which urges the rack14against the pinion16. The pressing device18according toFIG. 1is shown in an assembled state and according toFIG. 2in a state of use.

In the present case, the housing12of the pressing device18is designed integrally with the housing12of the rack-and-pinion steering system10. Alternatively, however, the pressing device18also can include a separate housing which then is attached to a housing of the rack-and-pinion steering system10.

The device18for pressing the rack14against the pinion16comprises the housing12, a thrust piece20which is shiftably guided in the housing12along a pressing axis A, a bearing element22which can axially be fixed at the housing12, as well as radially pressurized wedge members24which each rest on the thrust piece20and on the bearing element22and axially urge the thrust piece20away from the bearing element22in direction of the rack14. On a side facing the bearing element22the thrust piece20is formed as truncated cone, concretely as “straight” truncated cone, in which the circular base area is arranged in parallel and concentrically relative to the circular top area. The wedge members24rest against a shell surface of the frustoconical thrust piece portion, wherein in the present exemplary embodiment there are provided six wedge members24uniformly distributed in circumferential direction26(cf.FIGS. 3 and 4).

The pressing axis A in essence extends vertically, i.e. radially to a rack axis Z. In addition, the pinion16and the thrust piece20are arranged on opposite sides of the rack14such that an axis of rotation R of the pinion16and the pressing axis A of the pressing device18intersect each other. In alternative embodiments, however, the pressing axis A and the rack axis Z also can be offset to each other.

FIGS. 3 and 4each show a section A-A through the pressing device18according toFIG. 2. It can clearly be seen that the individual wedge members24are connected with each other by flexible coupling elements28, but nevertheless are movable relative to each other. Concretely, the coupling elements28each connect two wedge members24adjacent in circumferential direction26.

In the present case, the wedge members24and the coupling elements28are made of plastics and formed integrally as wedge member unit30.

This integrally formed wedge member unit30considerably simplifies the assembly of the pressing device18, since the wedge members24need not be positioned individually in the housing12.

The pressing device18furthermore comprises a spring element32which urges the wedge members24radially to the inside with respect to the pressing axis A. In the exemplary embodiment according toFIGS. 1 to 4, the spring element32is a circlip made of metal, in particular spring steel, which has two windings and encloses the wedge members24.

The circlip can of course also be formed as C-spring. However, in order to durably provide a large enough and largely constant radial force, there are preferably used circlips with two or more windings. As an alternative to a circlip, there can also be used a hose spring.

With reference toFIGS. 1 to 4, the mode of operation and the advantages of the illustrated pressing device18will be described below:

In the assembled state of the pressing device18according toFIG. 1, the thrust piece20and the wedge members24or the wedge member unit30are arranged in an opening33of the housing12, wherein at least the thrust piece20is accommodated in the housing opening33with a precise fit, but axially shiftably in radial direction with respect to the pressing axis A.

During insertion of the wedge member unit30, the spring element32already is mounted and urges the wedge members24radially to the inside. To initially prevent, however, a radial displacement of the individual wedge members24during assembly of the pressing device18, an assembly pin34is provided, which extends axially through the wedge member unit30, so that the wedge members24radially rest on the assembly pin34.

The assembly pin34also extends into a recess36of the otherwise frustoconical end face of the thrust piece20and thereby ensures an arrangement of the thrust piece20and the wedge member unit30which in the assembled state is concentric with respect to the pressing axis A.

Finally, the bearing element22is inserted into the housing opening33and axially fixed at the housing12. Optionally, the bearing element22can be fixed such that it already exerts a certain axial pretension, so that the rack14is forced against the pinion16via the wedge member unit30and the thrust piece20.

In the illustrated exemplary embodiment, the bearing element22is a bearing cap, wherein an external thread of the bearing cap engages into an internal thread of the housing opening33, in order to axially fix the bearing element22at the housing12. A desired axial positioning is easily adjustable in this case.

According toFIG. 1, the bearing element22includes an assembly opening38through which the assembly pin34extends axially to outside of the housing12. Finally, the assembly pin34is axially withdrawn from the pressing device18via the assembly opening38, in order to activate the pressing device18, i.e. transfer the same from the assembled state into its state of use according toFIG. 2.

After removing the assembly pin34, the wedge members24move radially to the inside due to the spring force of the spring element32, so that a circumferential gap40is formed, whose radial dimension is designated with d inFIG. 2. Since there are provided at least three wedge members24uniformly distributed over the circumference, a radial centering between the wedge member unit30and the thrust piece20takes place automatically. At the same time, a predeterminable axial pressing force Fpressureis obtained via the shell surface of the truncated cone and the inclined surfaces of the wedge members24resting against the same. This pressing force Fpressurecan be adjusted for example via an angle of the shell surface and the inclined surfaces relative to the pressing axis A, a radial spring force of the spring element32, and the friction values between wedge members24and thrust piece20or between wedge members24and bearing element22.

To prevent the ingress of dirt into the pressing device18, an assembly plug42, for example a rubber plug, is clipped into the assembly opening38after removing the assembly pin34, in order to tightly close the assembly opening38in essence.

FIG. 3shows a cross-section A-A through the pressing device18in the state of use at the beginning of the useful life. The circumferential gap40ensures that during a steering maneuver a movement of the rack14along its rack axis Z is not impeded by the pressing device18. If during the steering maneuver, for example as a result of manufacturing tolerances in the components of the rack-and-pinion steering system10, the rack14exerts a force FZSon the pressing device18in direction of the pressing axis A, which exceeds the pressing force Fpressureof the pressing device18, the thrust piece20can move axially in direction of the bearing element22by pushing the wedge members24radially to the outside against the spring force of the spring element32. The radial dimension d of the circumferential gap40represents a maximum path of displacement of the wedge members24. In other words, the housing12forms a stop which limits a displacement of the wedge members24radially to the outside. Particularly preferably, the spring element32is accommodated in a circumferential groove44of the wedge members24or the wedge member unit30, so that the wedge members24or the wedge member unit30radially extend(s) further to the outside than the spring element32. The spring element32thereby is axially fixed, and there is produced less impact noise during the radial movement against the housing12.

As soon as the force FZSproduced by the rack14in direction of the pressing axis A falls below the pressing force Fpressure, the spring element32again moves the wedge members24into their position according toFIG. 3.

FIG. 4shows a cross-section A-A through the pressing device18in the state of use towards the end of the useful life. At this time, an axial dimension of the thrust piece20has been reduced due to the wear occurring during the useful life. To ensure that the desired pressing force Fpressureis maintained and no axial backlash is produced in the pressing device18, the wedge members24were more and more urged towards each other, i.e. radially to the inside, by the spring element32with increasing wear.

As seen radially from the inside to the outside, the wedge members24conically flare in axial direction (seeFIGS. 1 and 2). In addition, as seen radially from the inside to the outside, the wedge members24however also flare in circumferential direction26and form segment-shaped wedge members24according toFIGS. 3 and 4, in order to reduce the surface pressures and hence also the material stress.

FIGS. 5 to 8show alternative embodiments of the pressing device18. However, since in terms of their basic construction and general mode of operation these design variants substantially correspond to the pressing device18according toFIGS. 1 to 4, reference is made to the above description and in the following merely the differences of the embodiments will be discussed. Individual features, which were only explained with reference to a special embodiment, can of course also expediently be combined with other embodiments.

FIG. 5shows a section of the rack-and-pinion steering system10with a pressing device18which differs from the embodiment according toFIG. 2merely in that not the thrust piece20is formed on a side facing the bearing element22, but the bearing element22is formed as truncated cone on a side facing the thrust piece20. The wedge member unit30correspondingly is reversed.

It is of course also conceivable to combine the embodiments according toFIGS. 2 and 5. In this case, both the thrust piece20would be formed on a side facing the bearing element22and the bearing element22would be formed as truncated cone on a side facing the thrust piece20. The wedge members24then would each have complementary, inclined wedge surfaces on both axial sides.

FIG. 6shows a section of the rack-and-pinion steering system10with an alternative pressing device18. In contrast to the aforementioned embodiments, the spring element32of the pressing device18here is formed as O-ring, wherein the O-ring for example is made of a plastic material.

In addition, axially between the bearing element22and the thrust piece20, concretely between the bearing element22and the wedge members24, an element46elastic in axial direction is provided. According toFIG. 6, this elastic element46is a rubber plate which increases the friction between the wedge members24and the bearing element22. In the case of an overload, i.e. when the force FZSincreases in direction of the pressing axis A beyond the desired pressing force Fpressure, a movement of the thrust piece20in direction of the bearing element22first of all is achieved by an axial compression of the elastic element46. A displacement of the wedge members24relative to the bearing element22radially to the outside only is possible to a small extent by a corresponding deformation of the elastic element46.

A section of the rack-and-pinion steering system10with another alternative pressing device18is shown inFIG. 7. The pressing device18according toFIG. 7differs from the embodiment according toFIG. 6merely in that the elastic element46is formed as disk spring. By this disk spring the friction between the thrust pieces20and the bearing element22is not increased or only to a small extent as compared to the embodiment according toFIG. 2. In a case of overload, an axial movement of the thrust piece20in direction of the bearing element22hence can occur both due to an axial deformation of the disk spring and due to a radial displacement of the wedge members24relative to the bearing element22.

FIG. 8shows a section of the rack-and-pinion steering system10with another alternative pressing device18. Analogous to the embodiment according toFIGS. 6 and 7, the spring element32is manufactured as O-ring made of plastics, in particular rubber, wherein the O-ring in this case however axially protrudes beyond the wedge members24, is axially elastic and axially rests on the bearing element22.

Thus, the spring element32formed as O-ring urges the wedge members24radially to the inside analogous to the remaining embodiments, in order to provide a largely constant pressing force Fpressureand a wear compensation. Since the spring element32according toFIG. 8, however, is axially elastic and axially protrudes beyond the wedge members24, it also assumes the function of the elastic element46of the pressing device18(cf.FIGS. 6 and 7). Consequently, such elastic element46can be omitted.

Analogous toFIGS. 3 and 4,FIGS. 9 and 10show cross-sections A-A through the pressing device18, wherein the spring element32inFIGS. 9 and 10, however, is formed as O-ring made of plastics and not as circlip made of metal.