Ball screw

Ball screw, having a spindle nut (2) which may be provided on a recirculating ball screw (1) and on the inner circumference of which turns (3) of ball grooves (4), helically wound around an axis, are provided in which balls (5) can roll, at least one return element (8), arranged on the outer circumference of the spindle nut (2) and provided with a return duct (9) for the balls (5), at its two ends connecting a start of a turn (3) to an end of a turn (3) in order to permit a continuous recirculation of the balls (5), the return element (8) between its two ends being supported on a support (15, 21), the return element (8) being resiliently supported by means of at least one spring element (14) on the support (15, 21).

FIELD OF THE INVENTION

The present invention relates to a ball screw. These mechanisms translate rotational movements into linear movements. In modern motor vehicles ball screws are being increasingly used in electromechanical power steering systems for rack-and-pinion steering. In such power steering systems the steering rack sometimes takes the form of a recirculating ball screw. An electric motor drives the spindle nut, so that a longitudinal displacement of the steering rack is assisted by electromotive means.

DE 100 56 275 A1, for example, disclosed a ball screw, having a spindle nut arranged on a recirculating ball screw, on the inner circumference of which nut turns of ball grooves, helically wound around an axis, are provided in which balls can roll. Return elements, each provided with a return duct for the balls, are arranged on the outer circumference of the spindle nut. With their two ends these return elements connect a start of a turn to an end of a turn, in order to permit a continuous recirculation of the balls. The return element is arranged in a recess in the fixing flange and is held in a radial direction. This fixing flange is firmly connected to the spindle nut. The recess in the fixing flange may be of somewhat larger design, in order to allow some radial play between the fixing flange and the return piece. This radial play may be necessary in order to take account of production tolerances. Unwanted rattling noises can furthermore manifest themselves when the return element strikes against the fixing flange. In the event of excessive radial play, correct guidance of the return piece outside the spindle nut is no longer assured. In the event of too little radial play the return piece may possibly jam, which can likewise adversely effect the correction functioning of the ball screw.

The object of the present invention is to specify a ball screw according to the features of the pre-characterizing part of Claim1, which is easy to manufacture and which functions reliably, ensuring a correct recirculation, especially in the return element.

According to the invention this object is achieved in that the return element is resiliently supported by means of at least one spring element on the support formed, for example, by a fixing flange. The arrangement of the spring element considerably reduces the influence of the radial play on the working and the operating reliability of the ball screw. The invention furthermore ensures that in the event of any radial play rattling is prevented, since the return element can no longer oscillate freely and strike against the inner wall of the fixing flange, for example, thereby causing unwanted rattling noises.

A spring travel of the spring element is preferably limited by stops provided on the return element and on the support. Limiting the spring travel ensures that undesirably large deflections of the return element are excluded. Within the spring travel, movements between the support and the return element are possible. These stops may simply be formed by the opposing walls of the support and the return element.

Multiple such spring elements may be arranged, preferably distributed symmetrically, between the ends of the return element. In a symmetrical arrangement of the spring elements the return element is uniformly supported in relation to the support, thereby ensuring correct positioning of the return element in relation to the support and in relation to the spindle nut.

As spring elements, separate springs may be provided, which are arranged between the return element and the support and which are supported on the support and on the return element.

It is particularly advantageous, however, if the return element is itself resilient and is resiliently supported on the support. The return element may accordingly be of elastic design in such a way that it can be accommodated even in the most unfavorable installation conditions, for example, in a recess of a fixing flange, without adversely affecting the working of the ball screw, whilst at the same time preventing unwanted rattling noises.

The return element may be resiliently formed on at least one spring point facing the support and may be sprung by this spring point against the support. Such spring points, for example, may be tongues which are integrally formed onto the return element and project radially from the contour of the return element, and are sprung against the support. These spring points, however, can also be webs standing proud of the contour of the return element, which have resilient characteristics. In these cases such return elements may be made from injection-moulded plastic, for example. The particular advantage is that no additional means are required to form a ball screw according to the invention.

The return element may have an external contour matched to the internal contour of the support, the return element being provided at a distance from its external contour with at least one recess, in such a way that the recess and the external contour define a resilient web forming the support. Such a return element can advantageously be formed from injection moulded plastic.

The support may be formed by a gear wheel arranged rotationally fixed on the spindle nut, a chamber to accommodate the return element being provided between the toothed rim of the gear wheel and the spindle nut. In this case an internal contour of the toothed rim and the external contour of the return element may be matched to one another, the spring element being sprung against the internal contour of the toothed rim. This development according to the invention is particularly compact, since the return element is accommodated inside the gear wheel.

The spring travel of the spring element referred to above may be defined by the distance between the internal contour of the support and the external contour of the return element.

The spring travel is designed so that even if the spring element should fail, the basic position of the return element in relation to the spindle nut is maintained. The small amount of play then occurring between the return element and the support would not adversely affect the basic working of the ball guide.

DETAILED DESCRIPTION OF THE DRAWING

The ball screw according to the invention depicted inFIGS. 1 and 2has a recirculating ball screw1, on which a spindle nut2is rotatably supported. On the inner circumference of the spindle nut2, turns3of ball grooves4, helically wound around the recirculating ball spindle1, are provided, in which balls5can roll. The recirculating ball spindle1is likewise provided with a helically wound ball groove6. The ball grooves4and6define a ball duct7, in which the balls5circulate.

The spindle nut2is further provided with multiple return elements8distributed over the circumference. The return elements8are arranged on the outer circumference of the spindle nut2. The ends of the return elements8connect a start of a turn3to an end of a turn3, so that a continuous ball duct7is formed for recirculation of the balls5.FIG. 2shows that an end of the return element8engages in a bore10provided on the spindle nut2, in order to join the return duct9to the ball groove4, that is to the ball duct7.

It can also be seen fromFIG. 1that an adapter11is arranged on the spindle nut. This adapter11is rotationally fixed to the spindle nut2. This adapter11serves for the accommodation and fixing of a gear wheel (not shown here) for driving the spindle nut2. One of the return elements8is arranged in each recess12of the adapter11. The return element8bears on the circumferential surface of the spindle nut2. The radially outer external contour of the return element8is of circular arc-shaped design. The recess12of the adapter11is defined radially outwards by a wall13likewise of circular arc-shaped design. A slight gap is formed between the return element8and the wall13of the adapter11. Spring elements14integrally formed onto the return element8stand proud of the external contour of the return element8(FIG. 2) and are sprung against the wall13of the adapter11. The adapter11here serves as support15for radially supporting the return element8. It can be seen fromFIG. 2that a spring element14is provided at each of the two peripheral ends of the return element8. In this symmetrical arrangement of the spring elements14a uniform radial support of the return element8on the support15is assured. The spring elements14are resilient spring points14aof the return element8.

FIGS. 3 and 4show a half of the return element8and an enlarged detail inFIG. 4.FIG. 3clearly shows the one half-length of the return duct9and the spring elements14, which stand proud of the circular arc-shaped external contour of the return element8. InFIG. 4the slight gap s between the circular arc-shaped external contour of the return element8and the wall of the support15, not depicted here, is identified by s. This gap s corresponds to a maximum spring travel by which the spring element14can be compressed. On reaching the maximum spring travel s, the circular arc-shaped contours of the return element8and the wall13touch one another, so that a further deflection of the return element8is excluded. The contours of the wall13and the return element8serve as stops for limiting the spring travel s.FIG. 4clearly shows the design of the spring element14as a web16. The web16is elastically deformable. The web16is radially defined by a recess16ain the return element8and by the external contour of the return element8. The recess16ais arranged at an interval from the external contour.

In a further ball screw according to the invention the adapter is removed and is replaced by a gear wheel17, which may be mounted directly onto the spindle nut2. Since this exemplary embodiment according toFIGS. 5-7is distinguished from that described above only by the gear wheel17provided here, the description of this exemplary embodiment will be confined solely to the design and function of the gear wheel17. The gear wheel17is likewise arranged, rotationally fixed on the spindle nut2. Chambers19, each designed to accommodate one of the return elements8, are formed between a toothed rim18of the gear wheel17and the spindle nut2. As in the preceding exemplary embodiment, the chamber19is here defined by a circular arc-shaped wall20of the toothed rim18. As in the exemplary embodiment described above, the return element8is supported by its spring elements14against the toothed rim18. The design of the chamber19can clearly be seen fromFIG. 6.FIG. 7clearly shows the circular arc-shaped design of the wall20. In this ball screw according to the invention also, the gear wheel17serves as support21for the return element8.

The ball screw according to the invention depicted inFIG. 8largely corresponds to the ball screw depicted inFIGS. 1 and 2. The recirculating ball screw has been omitted from the figure. In this figure an adapter22is provided for the rotationally fixed connection to the spindle nut2. This adapter22, like the adapter11, has a chamber23to accommodate the return element8. In this figure it is possible to discern the radial distance (=spring travel s) between the circular arc-shaped external contour of the return element8and the circular arc-shaped contour of the wall24of the adapter22defining the chamber23. It can be seen that the spring elements14arranged at the peripheral ends of the return element8are supported on, that is to say sprung against the wall24of the adapter22.

The return pieces depicted and described here are all made from injection moulded thermoplastic material. Other plastics may be used, depending on the application.

LIST OF REFERENCE NUMERALS