FR2867825A1 - Ball screw mechanism for electric power steering apparatus, has two ball circulating holes bored at midway at screw groove formed on inner circumference of ball nut fitted outside ball screw portion - Google Patents

Abstract

The mechanism has two ball circulating holes that are bored midway at a screw groove formed on an inner circumference of a ball nut (7) fitted outside of a ball screw portion (3b). The nut is rotated by driving a motor (4). A step is formed by forming an opening of the hole at the groove, such that an opening edge on the ball screw track side of one of the holes is deeper than an opening edge on outer side of the track.

Description

BALL SCREW MECHANISM FOR A STEERING APPARATUS

ELECTRICAL ASSISTED

  The present invention relates to a ball screw mechanism for a type of electric power steering apparatus which assists the movement of a rack shaft by an electric motor, and more particularly relates to a ball screw mechanism used for transmitting thrust. from the electric motor to the rack-and-pinion shaft as a steering shaft in the electric power steering apparatus.

  Conventionally, in an electric power steering apparatus so called tree-assisted type, the movement of the rack shaft in the axial direction is assisted by a thrust of an electric motor. In this type of electric power steering apparatus, a ball screw mechanism is constructed by forming a ball screw portion on the rack shaft and installing a ball nut on an outside of the ball screw portion, so that the torque generated by the electric motor is transmitted to the rack shaft through the ball screw mechanism.

  A screw groove is formed on the inner periphery of the spherical nut, and a path of the groove formed by a combination of the screw groove and the ball screw part of the rack shaft is filled with a plurality of balls. In such a construction, even rotation of the spherical nut is ensured, and the rack shaft is moved in the axial direction since the balls move in the path of the groove with the rotation of the spherical nut. Therefore, since the spherical nut can be rotated by the electric motor by attaching a large gear such as a bevel gear to the outer periphery of the spherical nut and engaging a small gear attached to the motor shaft of the spherical nut. electric motor with the large gear, it is possible to assist the steering by moving the rack shaft in the axial direction with the rotation of the ball nut.

  In addition, in the ball nut, to circulate the loaded balls in a portion called ball screw rail in the groove path, a thruster is attached to the outside of the ball nut. The thruster is a tube-shaped member for returning the balls discharged outside the ball nut from one end portion of the ball screw rail to the other end portion thereof in the nut. spherical by transferring the balls through the outside of the spherical nut. In other words, the groove path between two ball circulation holes for connecting the two end portions of the thruster to the groove portion within the ball nut becomes the ball screw rail.

  It should be noted that as a steering apparatus of a different type of the above-mentioned tree-assist electric power steering apparatus, for example, the Japanese patent application being inspected. No. 2,728,828 discloses an apparatus using a ball screw mechanism for transmitting thrust between a driving wheel shaft to which a steering wheel is attached and a transverse shaft is connected to a steering mechanism. In this conventional steering apparatus, the path of the groove, with the exception of the ball screw rail in which the balls actually flow from the ball nut of the ball screw mechanism incorporated in the apparatus, is not polished, thus preventing flow of the balls from the ball screw rail.

  Incidentally, as described above, the ball screw mechanism must circulate the balls in the range of the ball screw rail by the thruster. Thus, when mounting the ball screw mechanism occurs the problem that the balls sometimes flow to the path of the groove outside the scope of the ball screw rail. Since the balls passed to the path of the groove out of the ball screw rail may interfere with the rotation of the ball nut, the elongated balls must be removed from the ball nut.

  As a structure for evacuating the balls which are passed towards the path of the groove outside the ball screw rail from the ball screw mechanism, conventionally, the spherical nut is generally formed with a so-called open structure in its part. end. However, when the open structure is adopted, it is necessary to increase the outer diameter of the ball nut to ensure the rigidity thereof, and therefore the entire ball screw mechanism becomes larger and it occurs on the outside. problem that increasing the size of the ball screw mechanism does not meet the current situation where a reduction in the size of the device is required. In addition, in the ball screw mechanism integrated in the electric power steering apparatus, since the ball nut is rotated at a high speed driving the electric motor, when the outer diameter of the ball nut is increased, the moment of inertia is greatly increased. Thus, there is a problem of deterioration of the feeling of direction in an electric power steering apparatus incorporating such a ball screw mechanism.

  In addition, when the open structure is not adopted for the spherical nut, the balls passed to the path of the groove outside the ball screw rail will remain in the path of the groove outside the ball screw rail. . In this case, in a certain state of rotation of the spherical nut, there is a fear that the balls passed to the path of the groove outside the ball screw rail may push an end portion of the thruster and interfere with the regular circulation of the balls.

  The present invention has been created for the purpose of solving the above problems, and it is an object of the present invention to provide a ball screw mechanism for an electric power steering apparatus capable of preventing ball flow to a path of the groove out of a ball screw rail forming a pitch between a groove path of a screw groove corresponding to the ball screw rail of a spherical nut and the groove paths of both sides of it.

  Another object of the present invention is to provide a ball screw mechanism for an electric power steering apparatus in which the aforementioned step is formed by rendering the depth of the groove path of the screw groove corresponding to the guide rail. ball screw of the spherical nut deeper than that of a groove in another part, or forming an opening edge on the side of the ball screw rail of each hole opening for the circulation of balls by drilling into the two end portions of the ball screw rail to a greater depth than an opening edge of the opposite side.

  Yet another object of the present invention is to provide a ball screw mechanism for an electric power steering apparatus capable of discharging the spent balls to a path of the groove outside a ball screw rail by drilling a hole. discharging the balls in the path of the groove out of the ball screw rail and passing the balls through the ball discharge hole.

  A ball screw mechanism for an electric power steering apparatus according to the present invention, wherein two ball circulation holes each connected to each end portion of a thruster for circulating the balls through the balloon. outside of a spherical nut are drilled midway between a screw groove formed on an inner periphery of the ball nut installed on an outside of a ball screw portion formed on a steering shaft, and spherical nut is rotated by driving an electric motor, and is characterized in that a pitch is formed between a portion of the screw groove, which becomes a ball screw rail between the two ball circulation holes, and the two or one of the parts of the screw groove on the outside more distant than the two holes of circulation of the respective balls.

  In such a ball screw mechanism for an electric power steering apparatus of the present invention, the pitch is formed between a portion of the path of the groove of the ball nut between the two or one of the two circulation holes of the balls to which each end portion of the thruster is connected, which becomes the ball screw rail, and the two or one of the parts on both sides of the path of the groove between the ball holes.

  Since the pitch limits flow of the balls from the ball screw rail, it is possible to prevent flow of the balls from the ball screw rail.

  In addition, the ball screw mechanism for an electric power steering apparatus according to the present invention is characterized in that the pitch is formed by making a depth of the screw groove portion between the two ball holes more deep that a depth of the two or one of the parts of the screw groove on the outside more distant than the two holes of the respective ball circulation.

  In such a ball screw mechanism for an electric power steering apparatus of the present invention, the pitch formed by making a difference between the groove depth of a portion of the path of the groove corresponding to the ball screw rail of the present invention. screw groove of the spherical nut and the groove depth of another part by performing, for example, a polishing process only on the path portion of the groove corresponding to the portion of the ball screw rail.

  In addition, the ball screw mechanism for an electric power steering apparatus of the present invention is characterized in that the pitch is formed at the two or one of the two ball circulation holes forming an opening of ball circulation hole at the screw groove so that an opening edge on the side of the screw groove between two ball circulation holes is deeper than an opening edge on the outside more distant.

  In such a ball screw mechanism for an electric power steering apparatus of the present invention, the pitch is formed by forming a ball hole opening at the screw groove in which an opening edge on the side of the ball screw rail of one or both ball circulation holes is deeper than an opening edge on the outer side by performing, for example, a polishing method only on the part of the path the groove corresponding to the portion of the ball screw rail.

  In addition, a ball screw mechanism for an electric power steering apparatus of the present invention, wherein two ball circulation holes each connected to each end portion of a thruster to circulate the balls through from the outside of a spherical nut are drilled halfway from a screw groove formed on an inner periphery of the spherical nut installed on an outside of a ball screw portion formed on a steering shaft, and the spherical nut is rotated by driving an electric motor, and is characterized in that a ball circulation hole extending outwardly of the spherical nut and having a hole diameter larger than a diameter The outside of the ball is drilled at two or one of the two parts of the screw groove located on both sides of a portion of the screw groove between the two ball holes.

  In such a ball screw mechanism for an electric power steering apparatus of the present invention, since a ball discharge hole having a diameter greater than the outer diameter of the balls and connecting the inside and the outside of the the spherical nut is drilled at a portion of the groove path outside the ball screw rail, the balls out of the ball screw rail are discharged outside the ball nut through the hole of the ball screw evacuation of the balls. In addition, since it is only necessary to drill the aforementioned ball discharge hole into the spherical nut, an increase in the moment of inertia of the spherical nut due to a conventional open structure does not occur. and a good steering feel is maintained when the ball screw mechanism is integrated into an electric power steering apparatus. In addition, it is also possible to drill a plurality of ball discharge holes at the groove path outside the ball screw rail at appropriate intervals.

  The above objects and features and other objects and features will be more apparent from the detailed description which follows with the accompanying drawings.

  Fig. 1 is a schematic, partially exploded side view illustrating a schematic configuration of a complete electric power steering apparatus incorporating a ball screw mechanism for an electric power steering apparatus according to Embodiment 1 of the present invention; Fig. 2A is a cross-sectional view of the essential portion of a spherical nut, and Fig. 2B is a schematic cross-sectional view illustrating a portion of the cross section taken along the line A-A of Fig. 2A; ; Fig. 3 is a cross-sectional view of the essential portion of a ball screw mechanism for an electric power steering apparatus according to Embodiment 2 of the present invention; and FIG. 4 is a side view of a spherical nut according to a modified example of embodiment 2.

  The following description will explain the present invention, based on the drawings illustrating the embodiments thereof.

  Fig. 1 is a schematic, partially exploded side view illustrating a schematic configuration of a complete electric power steering apparatus incorporating a ball screw mechanism for an electric power steering apparatus according to Embodiment 1 of the present invention. An electric power steering apparatus 1 incorporating a ball screw mechanism of Embodiment 1 has a rack shaft 3 as a steering shaft and is supported within a long tubular housing 5 so that is movable in the left and right directions in Fig. 1. A rack unit 3a is formed on the right side of the rack shaft 3 in Fig. 1. The rack unit 3a is engaged with a pinion (not shown) provided on an end portion of a steering wheel shaft 6 to which a steering wheel 2 is attached. Thus, the rack shaft 3 moves in the left and right directions in FIG. 1 with a rotation of the steering wheel shaft 6 in a slave manner. It should be noted that, with the movement of the rack shaft 3 in the left and right directions, the wheels are directed through a hinge arm (not shown) connected to both end portions of the shaft. rack and pinion 3.

  In addition, a ball screw portion 3b is formed on the left side portion of the rack shaft 3 in Fig. 1. By installing a ball nut 7 on an outside of the ball screw portion 3b, a ball screw mechanism. A large conical gear 8 is fixed to the outer periphery 7a of the ball nut 7, and a small conical gear 9 attached to a drive shaft 4a of an electric servomotor 4 is engaged with the large bevel gear 8. It is necessary to note that the electric motor 4 is mounted in a motor support unit 5a formed to project from the housing 5.

  The electric motor 4 is driven and controlled by an electronic control unit, not shown, according to a rotation of the steering wheel shaft 6 when the steering wheel shaft 6 is rotated by a drive motor. By turning the spherical nut 7 by the transmission of the rotation of the motor shaft 4a of the electric motor 4 towards the large bevel gear 8 from the small bevel gear 9, the movement of the rack shaft 3 is assisted.

  FIG. 2a is a schematic cross-sectional view of an essential part of the spherical nut 7. In the spherical nut 7, a screw groove 12 is formed on a screwing portion 7d which is a span where an inner periphery 7b is formed with a smaller diameter. A first circulation hole of the balls 13a and a second circulation hole of the balls 13b penetrating the outer periphery 7a are drilled at two positions, respectively, separated by a required distance in the middle of the screw groove 12. As illustrated in FIG. 1, the two end portions of a thruster 11 for the circulation of balls fixed to the outer periphery 7a of the spherical nut 7 are connected to the first circulation hole of the balls 13a and to the second circulation hole of the 13b beads, respectively.

  By attaching the thruster 11 in this manner, a ball 20 (see Fig. 2B) flows through a portion (hereinafter referred to as the path of the intermediate groove 12a) of the screw groove 12 between the first circulation hole of the 13a and the second circulation hole of the balls 13b illustrated by the hatching in Figure 2A and the thruster 11.

  Therefore, the path of the intermediate groove 12a is a ball screw rail L1 through which the balls 20 flow. Moreover, the two paths of a first path of the groove 12b, which is a part in the screw groove 12 located further outside than the first groove of the balls 13a relative to the path of the intermediate groove 12a, and a second path of the intermediate groove 12c, which is a portion located further outside than the second circulation hole of the balls 13b with respect to the path of the intermediate groove 12a, are parts where the balls 20 can not move. It should be noted that the first path of the groove 12b and the second path of the groove 12c in which the balls 20 can not flow will hereinafter be referenced as the first outer rail L2 and the second outer rail L3, respectively.

  The screw groove 12 is formed by a cutting process so that it is completely uniform, but after the cutting process, the polishing process is further performed on the portion of the path of the intermediate groove 12a. As a result, the surface roughness of the inner peripheral surface of the path of the intermediate groove 12a is improved compared with that of the first path of the groove 12b and the second path of the groove 12c. Thus, the balls 20 can circulate more regularly in the ball screw rail L1 than in the other parts, that is to say, the first outer rail L2 and the second outer rail L3.

  By performing the polishing method as mentioned above, as illustrated in FIG. 2B, the groove depth D1 of the path of the intermediate groove 12a becomes deeper than the groove depth D2 of the second path of the groove 12c of the second outer rail L3 of the amount polished by the polishing method. Specifically, since the second path of the groove 12c is unpolished, a polishing rib 7c (illustrated by the hatching in Figure 2B), which has been removed by the polishing process in the path of the intermediate groove 12a, remains. Thus, the groove depth of the path of the intermediate groove 12a and that of the second path of the groove 12c differ from each other by an amount equal to the thickness of the polishing edge 7c, so that a step 14 is formed.

  Therefore, as shown in Fig. 2B, an opening edge 12d on the second side of the path of the groove 12c of the second ball circulation hole 13b is in the state in which the bottom of the groove is increased. an amount equal to the thickness of the polishing edge 7c and there is a step 14 when viewed from the side of the path of the intermediate groove 12a. Thus, by increasing the bottom of the groove of the second path of the groove 12c compared with that of the path of the intermediate groove 12a, it is possible to prevent an excess of balls 20 from the path of the intermediate groove 12a to the second path groove 12c. It should be noted that although the explanation above discusses the shape of the groove around the second circulation hole of the balls 13b illustrated in FIG. 2B, the shape of the groove around the first circulation hole of the balls 13b is formed. similarly, and thus a flow of the balls 20 from the path of the intermediate groove 12a to the first path of the groove 12b is prevented in the same manner as above.

  Processing the path of the intermediate groove 12a and the first path of the groove 12b (and likewise the second path of the groove 12c) so that their groove depths differ from each other as described above it is possible to prevent a flow of balls 20 loaded in the screw groove 12 to the first path of the groove 12b and the second path of the groove 12c which are parts other than the ball screw rail L1 by assembling a ball screw mechanism, and the balls 20 can flow smoothly through the path of the intermediate groove 12a and the thruster 11 into the assembled ball screw mechanism.

  It should be noted that in the embodiment 1 described above, although the groove depth of the path of the intermediate groove 12a is made deeper and thus made different from the other parts by the polishing process, it is also possible to obtain similar results by other processing methods such as precision cutting, rolling or the like, in place of the polishing process. Moreover, it is not necessary to make the entire range of the first path of the groove 12b and the second path of the groove 12c, which are parts other than the ball screw rail L1, less deep compared at the groove depth of the path of the intermediate groove 12a. More specifically, from the point of view of preventing flow of the balls from the ball screw rail L1, it is important to make at least the depth of the groove of the opening edge 12d on the second side of the path of the groove 12c of the second ball circulation hole 13b (and an opening edge on the first side of the path of the groove 12b of the first ball circulation hole 13a) shallower compared to an opening edge 12e thereof on the path side of the intermediate groove 12a.

  In addition, in a certain configuration of the ball nut 7, the balls 20 of the ball screw rail L1 can be discharged from an end portion of the ball nut 7. In such a case, the first ballast path the groove 12b or the second path of the groove 12c on the side capable of discharging the balls 20 can be treated in the same way as the path of the intermediate groove 12a so as not to make any difference in the depth of the groove. In this case, the specification load of the processing range in the finishing process (polishing process) of the screw groove 12 is reduced.

  Fig. 3 is a cross-sectional view of the essential portion of a ball screw mechanism for an electric power steering apparatus according to Embodiment 2 of the present invention, and illustrates the ball screw portion 3b formed on the rack shaft 3 and the ball nut 7 configuring the ball screw mechanism. The basic configuration of an electric power steering apparatus incorporating the ball screw mechanism of Embodiment 2 is the same as that of Embodiment 1 illustrated in FIG. 1. The configuration of Embodiment 2 differs from that of FIG. embodiment 1 in that the screw groove 12 formed on the inner periphery of the spherical nut 7 is uniformly polished over the entire span, and in that ball discharge holes 64a and 64b having a diameter of hole larger than the outer diameter of the ball 20 are drilled through the spherical nut 7 at the path of the groove of the first outer rail L2 and the path of the groove of the second rail L3, respectively, which are aligned outside. of ball screw rail L1.

  Unlike Embodiment 1, by polishing the screw groove 12 of Embodiment 2 so that it is completely uniform without dividing the span, the polishing range specification control becomes easier. It should be noted that by performing the polishing process in such a way, the groove depth of the screw groove 12 becomes completely uniform, and therefore the balls 20 flow easily to the first outer rail L2 and the second outer rail L3 from the ball screw rail L1 which is the path of the groove between the first ball circulation hole 13a and the second ball circulation hole 13b. However, the balls passed in such a manner are easily discharged from the bead drain holes 64a and 64b as illustrated by an arrow 21.

  Specifically, in the path of the groove corresponding to the first outer rail L2 located further outside than the first ball circulation hole 13a with respect to the ball screw rail L1, the ball discharge hole 64a having a diameter hole greater than the outer diameter of the ball 20 is pierced so that it penetrates the outer periphery 7a of the spherical nut 7. Similarly, in the path of the groove corresponding to the second outer rail L3 located over outside than the second circulation hole of the balls 13b with respect to the ball screw rail L1, the ball discharge hole 64b having a larger hole diameter than the outer diameter of the ball 20 is pierced so that it penetrates the outer periphery 7a of the spherical nut 7.

  Therefore, during the mounting of the ball screw mechanism, the balls 20 passed to the first outer rail L1 are vented out through the ball discharge hole 64a, while the balls 20 are passed to the second rail. outside L2 are evacuated outwardly through the ball discharge hole 64b. In particular, when the spherical nut 7 rotates, since the centrifugal force acts to push the balls 20 against the inner periphery 7a of the spherical nut 7, the balls 20 are very easily discharged from the ball discharge holes 64a and 64b by centrifugal force. In addition, the more the drilling position of the ball discharge holes 64a

  and 64b to the ball screw rail L1 is close, in other words, to the first ball circulation hole 13a and the second ball circulation hole 13b, plus the balls 20 to the outer rail L2 or L3. are evacuated quickly.

  Moreover, in a certain configuration of the spherical nut 7, the balls 20 of the ball screw rail L1 can be discharged from an end portion of the spherical nut 7. In such a case, the hole of Bead removal 64a or 64b may be drilled only at the path of the groove of the first outer rail L2 or the second outer rail L3 on the side from which the balls 20 can not be evacuated.

  It may also be possible to drill a plurality of ball discharge holes 64a, 64b at each of the outer rails L2 and L3. In such a case, from the point of view of effective evacuation of the balls, as illustrated by a modified example in FIG. 4, it is preferable to drill a first hole for evacuation of the balls 64x, a second hole of discharge of the balls 64y and a third hole of the ball 64z penetrating between the outer periphery 7a of the spherical nut 7 and the inner periphery 7b where the groove of the screw 12 is formed, at appropriate intervals, for example, equal angle intervals, in the peripheral direction.

  In addition, it may be possible to drill the ball discharge hole 64a at the path of the groove of the first outer rail L2, and to form the path of the groove on the second side of the outer rail L3 so that the depth groove thereof is shallower than the groove depth of the intermediate groove path corresponding to the ball screw rail L1 in the same manner as embodiment 1. By adopting such a configuration, the balls 20 passed to the first outer rail L2 can be discharged from the ball discharge hole 64a, and it is possible to prevent flow of the balls 20 to the second side of the outer rail L3 and to ensure even circulation of the spherical nut 7. It should be noted that it is possible, of course, to reverse the configuration described above in which the bead discharge hole 64a is drilled at the first outer rail L2 and the spoke depth. the path of the groove of the second outer rail L3 is made shallower.

  As described in the above description, according to the present invention, since it is possible to prevent a flow of the balls from a ball screw rail in which the balls are intended to circulate, it is possible to eliminate the fear. a flow of the balls to a path of the groove out of the range of the ball screw rail by mounting a ball screw mechanism, and to ensure even rotation of the ball nut even after mounting.

  In addition, according to the present invention, since the elongated balls of the ball screw rail are naturally discharged outside the ball nut through the ball discharge hole, it is possible to prevent the situation where part of the ball The end of the thruster is pushed and eliminate the need to increase the size of the spherical nut, and thus an electric power steering apparatus incorporating such a spherical nut ensures a good sense of direction.

  Of course, the invention is not limited to the embodiments described above and shown, from which we can provide other modes and other embodiments, without departing from the scope of the invention. .

Claims (1)

CLAIM   1. Ball screw mechanism for an electric power steering apparatus, wherein two ball circulation holes each connected to each end portion of a thruster for circulating the balls through the outside of the engine. a spherical nut are drilled midway between a screw groove formed on an inner periphery of said spherical nut installed on an outside of a ball screw portion formed on a steering shaft, and said spherical nut is rotated by driving an electric motor, characterized in that a circulation hole of the balls (64a or 64b) extending outwardly of a spherical nut (7) and having a hole diameter larger than an outside diameter of said ball (20) is drilled at two or one of the portions (12b and / or 12c) of said screw groove (12) on both sides of a portion (12a) of said screw groove ( 12) between said two holes of circulation of the balls (13a and 13b).