JP2014101898A - Ball screw device and motor-driven power steering device including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ball screw device capable of reducing variation of axial force generated when fastening a ball screw nut to a fitting object, and provide a motor-driven power steering device including the ball screw device.SOLUTION: A ball screw device includes an annular intermediate member 61 held between a shaft seat surface 31 of a motor shaft 26 to be a fitting object and a nut seat surface 47 of a ball screw nut 42. The intermediate member 61 includes: a plurality of projections 63 that can be inserted into a notch 32 formed in the shaft seat surface 31 so as to project to the side of the motor shaft 26 while keeping a gap in the circumferential direction of the motor shaft 26; and a flat intermediate seat surface 64 that contacts with the nut seat surface 47.

Description

  The present invention relates to a ball screw device and an electric power steering device including the ball screw device.

  2. Description of the Related Art Conventionally, a so-called hollow shaft that is inserted through a rack shaft and rotated by a motor is provided, and the rotation of the hollow shaft is converted into axial movement of the rack shaft by a ball screw device, thereby providing an assist force to the steering system. There are rack assist type electric power steering devices (for example, Patent Documents 1 and 2). In general, such a ball screw device has a ball screw nut that rotates integrally with a hollow shaft, and the screw groove formed on the inner periphery of the ball screw nut and the screw groove formed on the outer periphery of the rack shaft are opposed to each other. A plurality of balls are arranged in a spiral ball trajectory.

  As a structure for fixing the ball screw nut to the hollow shaft, for example, as described in Patent Document 1, a hollow cylindrical male screw portion is formed on the ball screw nut, and the male screw portion is formed on the hollow shaft. It is known that the ball screw nut is fastened to the hollow shaft by being screwed to the portion. Specifically, in this fixing structure, the ball screw nut has an end surface (nut seating surface) on the hollow shaft side of the ball screw nut that is hollow due to an axial force generated when the male screw portion is screwed into the female screw portion of the hollow shaft. It is fastened while pressed against the shaft.

JP 2011-256901 A JP 2011-80574 A

  By the way, in order to restrict the rotation of the hollow shaft when fastening the ball screw nut, a notch is formed in the end surface (shaft seating surface) of the hollow shaft on the ball screw nut side, and a fixing jig is formed in this notch. It is conceivable to engage. However, when the notch is formed in the shaft seat surface, stress concentrates in the vicinity of the notch when the ball screw nut is pressed against the hollow shaft, so that the shaft seat surface is easily deformed. When the friction coefficient changes due to the deformation of the shaft seat surface, the friction force acting between the shaft seat surface and the nut seat surface changes.

  Here, since the deformation of the shaft bearing surface is not uniform and varies depending on the manufactured individual, the change in the frictional force varies from individual to individual. On the other hand, the axial force for fastening the ball screw nut to the hollow shaft further tightens the ball screw nut from the state where the shaft seat surface and the nut seat surface are in contact with each other and causes the male screw portion to advance to elastically deform the ball screw nut. Caused by Therefore, if the friction coefficient (friction force) changes due to the deformation of the shaft seating surface, even if the ball screw nut is tightened with a constant tightening torque, the tightening amount (rotation amount) of the ball screw nut varies from individual to individual. . As a result, the amount of elastic deformation of the ball screw nut varies, causing a problem that the axial force varies. Then, due to such variations in axial force, for example, there is a possibility that abnormal noise may be generated or durability may be lowered, and there is still room for improvement in this respect.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a ball screw device and a ball screw device that can reduce variations in axial force generated when a ball screw nut is fastened to a mounting target. Another object is to provide an electric power steering apparatus.

  A ball screw device that solves the above problems includes a screw shaft having a screw groove formed on an outer periphery, a ball screw nut having a screw groove formed on an inner periphery, a screw groove of the screw shaft, and a screw groove of the ball screw nut. And a plurality of balls disposed in a spiral ball track that is opposed to each other, and the ball screw nut is a male screw portion that is screwed into a female screw portion formed on the mounting target The target seating surface, which is the end surface of the mounting target on the side of the ball screw nut, is formed with at least a notch that opens to the outer side in the radial direction of the target seating surface and the mounting target, An annular intermediate member sandwiched between the target seating surface and a nut seating surface that is an end surface on the mounting target side of the ball screw nut is provided, and the intermediate member protrudes toward the mounting target side and protrudes toward the mounting target side. The mounting in the notch And insertable projections away 該切 leaving a gap in the circumferential direction of the elephants, and summarized in that the flat intermediate seat surface for contact with the nut bearing surface is formed.

  An electric power steering apparatus that solves the above problems includes a rack shaft that is reciprocally movable in an axial direction, a hollow shaft that is inserted through the rack shaft and that is rotated by a motor, and the rack shaft that is a screw shaft. And the above-described ball screw device for mounting the hollow shaft.

  According to each said structure, when the protrusion of an intermediate member engages with the notch of attachment object (hollow shaft), relative rotation with these attachment object and intermediate member is controlled. Therefore, when the ball screw nut is fastened, the ball screw nut and the intermediate member rotate relative to each other, and a frictional force acts between the nut seat surface and the intermediate seat surface. And since the intermediate seat surface which contact | abuts a nut seat surface is made into the flat shape (plane), stress does not concentrate like the case where the notch is formed, but it is hard to deform | transform. Therefore, when the ball screw nut is fastened, the friction coefficient of the intermediate seat surface (friction force acting between the nut seat surface and the intermediate seat surface) is difficult to change, and the tightening amount (rotation amount) of the ball screw nut is individual. Since the variation is reduced every time, the variation in the axial force for fastening the ball screw nut can be reduced. Thereby, generation | occurrence | production of the abnormal noise resulting from the dispersion | variation in axial force, for example, and the fall of durability can be suppressed.

  According to the present invention, it is possible to reduce variations in the axial force generated when the ball screw nut is fastened to the attachment target.

A sectional view showing a schematic structure of an electric power steering device of one embodiment. The fragmentary sectional view of the ball screw device vicinity of one Embodiment. The partial side view which looked at the motor shaft and ball screw nut of one Embodiment from the radial direction outer side. (A) is the side view which looked at the intermediate member of one Embodiment from the motor shaft side, (b) is the side view similarly seen from the ball screw nut side. The partial side view which looked at the motor shaft and ball screw nut of another example from the radial direction outer side.

Hereinafter, one embodiment of a ball screw device and an electric power steering device (EPS) provided with the same will be described with reference to the drawings.
As shown in FIG. 1, the EPS 1 includes a pinion shaft 2 that rotates by a steering operation, and a rack shaft that reciprocates in the axial direction according to the rotation of the pinion shaft 2 to change the steering angle of a steered wheel (not shown). 3 is provided. The EPS 1 includes a substantially cylindrical rack housing 5 through which the rack shaft 3 is inserted. The rack housing 5 includes a center housing 6 formed in a substantially cylindrical shape, a gear housing 7 fixed to one axial end side (left side in FIG. 1) of the center housing 6, and the axial other end side of the center housing 6. And an end housing 8 fixed to the right side in FIG.

  The rack shaft 3 is supported by a rack guide 11 provided in the gear housing 7 and a bush (slide bearing) (not shown) provided in the end housing 8 so as to be capable of reciprocating along the axial direction. In the rack housing 5, the pinion shaft 2 is rotatably supported in an oblique manner with the rack shaft 3, and the rack shaft 3 is engaged with the pinion shaft 2 by being urged by the rack guide 11. doing. The pinion shaft 2 is connected to a steering shaft, and a steering wheel (both not shown) is fixed to the tip thereof. Then, the pinion shaft 2 rotates in accordance with the steering operation, and the rotation is converted into the axial movement of the rack shaft 3, whereby the steered angle of the steered wheels, that is, the traveling direction of the vehicle is changed.

  The EPS 1 also includes a motor 21 serving as a driving source thereof, and a ball screw device 22 that converts the rotation of the motor 21 into the axial movement of the rack shaft 3. That is, the EPS 1 of the present embodiment is configured as a so-called rack assist type EPS.

First, the configuration of the motor will be described.
The motor 21 is configured as a brushless motor including a stator 24 fixed to the inner periphery of the center housing 6 and a rotor 25 that is rotatably provided inside the stator 24. The rotor 25 includes a hollow shaft formed in a hollow cylindrical shape, a motor shaft 26 as an attachment target, and a magnet 27 fixed to the outer periphery of the motor shaft 26. The motor shaft 26 is made of a soft magnetic material and functions as a magnetic path of the magnet 27. An opening end portion 26a on the gear housing 7 side of the motor shaft 26 is rotatably supported by a first bearing 28a. The motor 21 is arranged coaxially with the rack shaft 3 by inserting the rack shaft 3 into the motor shaft 26. In the motor 21 configured as described above, the motor shaft 26 (rotor 25) is generated by a magnetic attractive force and a repulsive force generated between a magnetic field formed by supplying driving power to the stator 24 and the magnet 27. Is designed to rotate.

  As shown in FIGS. 2 and 3, the shaft seat surface 31, which is the axial end surface of the opening end portion 26 b on the side to which a ball screw nut 42, which will be described later, is connected, is provided from the shaft seat surface 31. A plurality (four in this embodiment) of notches 32 extending in the axial direction and penetrating the inner and outer peripheries of the motor shaft 26 are formed. That is, in this embodiment, the shaft seat surface 31 corresponds to the target seat surface. For convenience of explanation, in FIG. 2, the upper half of the axis of the motor shaft 26 shows a cross section passing through the notch 32, and the lower half shows a cross section not passing through the notch 32.

  Specifically, the cutout 32 penetrates the motor shaft 26 in the radial direction, and is formed in a groove shape having a substantially constant circumferential width when viewed from the shaft seating surface 31 side. The notches 32 are provided at equiangular intervals in the circumferential direction. When the motor screw 26 fastens the ball screw nut 42 of the ball screw device 22, the fixing jig J is engaged with each notch 32 as shown by a two-dot chain line in FIG. 3. Rotation is regulated. In the present embodiment, the width along the circumferential direction of the fixing jig J is set to approximately half of the width along the circumferential direction of the notch 32, and along the axial direction of the fixing jig J. The length is set slightly smaller than the depth of the notch 32 along the axial direction. Further, as shown in FIG. 2, a female thread portion 34 is formed on the inner peripheral surface of the motor shaft 26 from a position spaced from the shaft seat surface 31 to the back side (left side in FIG. 2) and further to the back side. It is formed towards.

Next, the configuration of the ball screw device will be described.
As shown in FIG. 1, the rack shaft 3 is formed with a screw groove 41 in a part of the outer periphery thereof. That is, in this embodiment, the rack shaft 3 corresponds to a screw shaft. The ball screw device 22 has a ball screw nut 42 that rotates integrally with the motor shaft 26, and is configured by screwing the ball screw nut 42 onto the rack shaft 3 via a plurality of balls 43. . The ball screw nut 42 is rotatably supported by a second bearing 28b provided on the inner peripheral surface of the center housing 6.

  More specifically, as shown in FIG. 2, the ball screw nut 42 is formed in a substantially cylindrical shape, and a main body 44 disposed outside the motor shaft 26 and an opening on the end housing 8 side in the motor shaft 26. It has the engaging part 45 inserted in the inside from the edge part 26b. The ball screw nut 42 is made of a material having higher strength than the motor shaft 26. On the inner periphery of the ball screw nut 42, a spiral screw groove 46 is formed that extends over a range including the entire main body 44 and a part of the engaging portion 45. The outer diameter of the main body 44 is formed substantially equal to the outer diameter of the motor shaft 26, and the nut seat surface 47, which is the axial end surface of the main body 44 on the motor shaft 26 side, is a shaft seat surface 31 of the motor shaft 26. Are provided at positions facing each other in the axial direction.

  The engaging portion 45 is formed continuously from the main body portion 44 in the axial direction, and an inlay portion 48 fitted to an inner surface of the opening end portion 26b of the motor shaft 26 where the internal thread portion 34 is not formed. A hollow cylindrical male screw portion 49 that extends in the axial direction from the tip of the portion 48 and is screwed with the female screw portion 34 of the motor shaft 26 is provided. The ball screw nut 42 is fastened to the motor shaft 26 so that the screw groove 46 faces the screw groove 41 of the rack shaft 3 in the radial direction by screwing the male screw portion 49 with the female screw portion 34. . Specifically, the ball screw nut 42 is fastened in a state where the nut seat surface 47 is pressed against the shaft seat surface 31 by an axial force generated when the male screw portion 49 is screwed into the female screw portion 34. Thus, a spiral ball track R1 is formed by the screw groove 41 of the rack shaft 3 and the screw groove 46 of the ball screw nut 42.

  A plurality of balls 43 are disposed in the ball track R1, and each ball 43 is sandwiched between a screw groove 41 of the rack shaft 3 and a screw groove 46 of the ball screw nut 42. The ball screw nut 42 is formed with a circulation path R2 that short-circuits two points in the screw groove 46. The circulation path R2 of the present embodiment is a circulation member having a function of scooping up the ball 43 from the ball raceway R1 and a function of discharging the ball 43 to the ball raceway R1 with respect to the mounting hole 51 of the ball screw nut 42. It is formed by mounting a deflector 52.

  Accordingly, in the ball screw device 22, each ball 43 receives a load (frictional force) from the rack shaft 3 and the ball screw nut 42 when the ball screw nut 42 rotates relative to the rack shaft 3. By rolling inside, the torque of the ball screw nut 42 is transmitted to the rack shaft 3, and the rack shaft 3 is moved in the axial direction with respect to the ball screw nut 42. Each ball 43 that rolls in the ball track R1 and reaches one end of the ball track R1 passes through the circulation path R2 formed in the ball screw nut 42 and is discharged to the other end of the ball track R1. Then, the ball trajectory R1 moves from the downstream side to the upstream side in the ball flow direction. That is, the ball screw device 22 converts the rotation of the ball screw nut 42 into the axial movement of the rack shaft 3 by each ball 43 rolling on the ball track R1 circulating infinitely via the circulation path R2. Is possible. The two points short-circuited by the circulation path R2 are set at positions where a screw groove 46 of several turns is sandwiched in the axial direction of the ball screw nut 42, and one system is formed by the ball track R1 and the circulation path R2. The distribution channel is formed. The EPS 1 rotates the ball screw nut 42 using the motor 21 and transmits the torque to the rack shaft 3 as an axial pressing force, thereby assisting the steering system in assisting the steering operation. It is a configuration to grant.

  Here, as shown in FIGS. 2 and 3, the ball screw device 22 includes an annular intermediate member 61 that is sandwiched between the shaft seat surface 31 and the nut seat surface 47. Note that the intermediate member 61 of the present embodiment is made of a material having higher strength than the ball screw nut 42 (motor shaft 26).

  Specifically, as shown in FIGS. 2 to 4, the intermediate member 61 includes an annular ring body 62 and a plurality (four in this embodiment) of protrusions 63 protruding from the ring body 62 toward the motor shaft 26. And have. The inner diameter of the ring body 62 is set to be approximately equal to the outer diameter of the spigot portion 48 of the ball screw nut 42, and the outer diameter of the ring body 62 is set to be approximately equal to the outer diameter of the main body portion 44 of the ball screw nut 42. ing. The intermediate member 61 is sandwiched between the shaft seat surface 31 and the nut seat surface 47 in a state where the ring body 62 is fitted to the outer periphery of the spigot portion 48. Thus, the notch 32 of the motor shaft 26 is covered with the intermediate member 61 when viewed from the ball screw nut 42 side.

  As shown in FIG. 4A, the protrusions 63 are provided at equal angular intervals in the circumferential direction, and are inserted into the notches 32 of the motor shaft 26, respectively. As shown in FIGS. 2 and 3, the protrusion amount of the protrusion 63 along the axial direction is set slightly smaller than the depth of the notch 32. The width of the protrusion 63 along the circumferential direction is set slightly smaller than half the width of the notch 32. That is, the protrusion 63 is formed in the notch 32 so as to be inserted into the notch 32 leaving a gap in the circumferential direction of the motor shaft 26. Since the fixing jig J is set to substantially half the circumferential width of the notch 32 as described above, the protrusion 63 can be inserted into the notch 32 together with the fixing jig J. It has become. As shown in FIGS. 3 and 4B, the intermediate seat surface 64 facing the nut seat surface 47 of the ring body 62 (intermediate member 61) in the axial direction has a flat shape. That is, the intermediate seat surface 64 is formed in a planar shape that is parallel to the nut seat surface 47 in the assembled state.

  Further, a process (Toruker process) for stabilizing the friction coefficient of the intermediate member 61 by applying a water-dispersed processing liquid mainly composed of a polymer resin and a surfactant to the intermediate seating surface 64. It has been subjected. In the present embodiment, the processing liquid is applied to the intermediate seat surface 64 by immersing the entire intermediate member 61 in a container (not shown) in which the processing liquid is stored.

Next, the fastening (action) of the ball screw nut to the motor shaft will be described.
The ball screw nut 42 is tightened with a predetermined tightening torque set in advance with the fixing jig J and the protrusion 63 of the intermediate member 61 inserted into the notch 32 of the motor shaft 26, whereby the motor shaft 26 To be concluded. At this time, the protrusion 63 is engaged with the notch 32, so that relative rotation between the motor shaft 26 and the intermediate member 61 is restricted. Therefore, when the ball screw nut 42 is fastened, the ball screw nut 42 and the intermediate member 61 rotate relative to each other, and a frictional force acts between the nut seat surface 47 and the intermediate seat surface 64. Here, since the notch 32 of the motor shaft 26 is covered with the intermediate member 61 as described above, the notch 32 is not exposed to the nut seat surface 47. Further, since the intermediate seat surface 64 that comes into contact with the nut seat surface 47 has a flat shape, the stress does not concentrate as in the case where the notch is formed, and is difficult to deform. Accordingly, when the ball screw nut 42 is fastened, the friction coefficient of the intermediate seat surface 64 (friction force acting between the nut seat surface 47 and the intermediate seat surface 64) is difficult to change. Compared to the case where a frictional force acts between the surface 47 and the ball screw nut 42, the amount of tightening (rotation amount) of the ball screw nut 42 varies from individual to individual.

Next, the effect of this embodiment will be described.
(1) The friction coefficient of the intermediate seat surface 64 is formed as described above by forming the intermediate seat surface 64 that contacts the nut seat surface 47 of the intermediate member 61 in a planar shape parallel to the nut seat surface 47 in the assembled state. Is less likely to vary, and variations in the tightening amount of the ball screw nut 42 from one individual to another are reduced, and variations in axial force generated when the ball screw nut 42 is fastened to the motor shaft 26 can be reduced. Thereby, generation | occurrence | production of the abnormal noise resulting from the dispersion | variation in axial force, for example, and the fall of durability can be suppressed.

  (2) Since a treatment liquid that stabilizes the friction coefficient is applied to the intermediate seat surface 64 of the intermediate member 61, the amount of tightening of the ball screw nut 42 is further reduced from individual to individual, and variations in axial force can be achieved. It can be further reduced. Here, the intermediate member 61 is a separate member from the ball screw nut 42, and even if a liquid agent or the like is applied to the whole member, it does not affect the shape accuracy of the ball track R1, so that the friction coefficient is stabilized. Can be easily applied to the intermediate seating surface 64 by dipping or the like.

In addition, the said embodiment can also be implemented in the following aspects which changed this suitably.
In the above embodiment, the intermediate member 61 may be made of a material having lower strength than the motor shaft 26 and the ball screw nut 42. However, the strength of the intermediate member 61 is preferably as high as possible.

In the above embodiment, the intermediate member 61 may not be subjected to the process of stabilizing the friction coefficient of the intermediate seat surface 64.
In the above embodiment, the width of the protrusion 63 is set to be slightly smaller than half the width of the notch 32. However, the present invention is not limited to this. For example, as shown in FIG. It is good also as a shape with small protrusion amount. In short, the shape of the protrusion 63 can be inserted into the notch 32 leaving a gap in the notch 32 in the circumferential direction of the motor shaft 26 and inserted into the notch 32 together with the fixing jig J. If possible, it can be changed as appropriate. Further, the shape of the notch 32 can be appropriately changed as long as it opens at least radially outward of the shaft seat surface 31 and the motor shaft 26. Needless to say, the numbers of the notches 32 and the protrusions 63 can be changed as appropriate.

  In the above embodiment, the EPS 1 is a rack assist type in which the motor 21 and the motor shaft 26 as a hollow shaft are arranged coaxially with the rack shaft 3, but the invention is not limited thereto, so-called rack cross type, rack parallel type, etc. A hollow shaft through which the rack shaft is inserted may be driven by a motor provided outside the housing.

  In the above embodiment, the ball screw device 22 may be used for purposes other than EPS.

  DESCRIPTION OF SYMBOLS 1 ... Electric power steering device (EPS), 3 ... Rack shaft, 21 ... Motor, 22 ... Ball screw device, 26 ... Motor shaft, 31 ... Shaft seat surface, 32 ... Notch, 34 ... Female thread part, 41, 46 ... Screw groove, 42 ... Ball screw nut, 43 ... Ball, 47 ... Nut seat surface, 49 ... Male screw part, 61 ... Intermediate member, 62 ... Ring body, 63 ... Projection, 64 ... Intermediate seat surface, J ... Fixing jig , R1 ... ball track, R2 ... circulation path.

Claims (2)

A screw shaft having a screw groove formed on the outer periphery;
A ball screw nut having a screw groove formed on the inner periphery;
A ball screw device comprising: a plurality of balls disposed in a spiral ball raceway in which a screw groove of the screw shaft and a screw groove of the ball screw nut are opposed to each other;
The ball screw nut is formed with a male screw portion that is screwed into a female screw portion that is formed on the mounting target,
The target seating surface that is the end surface of the mounting target on the side of the ball screw nut is formed with at least a notch that opens to the outer side in the radial direction of the target seating surface and the mounting target,
An annular intermediate member that is sandwiched between the target seating surface and a nut seating surface that is an end surface on the mounting target side of the ball screw nut;
In the intermediate member,
A protrusion that protrudes toward the attachment object side and is inserted into the notch, leaving a gap in the circumferential direction of the attachment object; and
A ball screw device, characterized in that a flat intermediate seat surface abutting on the nut seat surface is formed.   A rack shaft provided so as to be capable of reciprocating in an axial direction, a hollow shaft through which the rack shaft is inserted and rotated by a motor drive, the rack shaft as a screw shaft and the hollow shaft as an attachment target. An electric power steering device comprising the ball screw device according to claim 1.