JP2010285000A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rack assist type electric power steering device capable of improving a degree of freedom of design of the reduction gear ratio, and capable of high output and miniaturization. <P>SOLUTION: The rack assist type electric power steering device includes a reduction gear 32 arranged between an electric motor 34 and a ball screw mechanism 33 and incorporated coaxially with a rack shaft 31, the electric motor 34 the ball screw mechanism 33. The reduction gear 32 is a ball reduction gear constituted of two pairs of trochoidal waveform grooves, and incorporated into a motor housing 30B for storing the electric motor 34. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a rack assist type electric power steering apparatus, and more particularly to a rack assist type electric power steering apparatus that assists a steering force of a vehicle such as an automobile with an electric motor.

  2. Description of the Related Art In an automobile steering system, an electric power steering device (Electric Power Steering, hereinafter also referred to as EPS) that performs steering assist using an electric motor as a power source is widely used. EPS uses an in-vehicle battery as a power source for the electric motor, so there is no direct engine drive loss, and since the electric motor is started only at the steering assist time, a reduction in running fuel consumption can be suppressed, and electronic control can be performed. It has the feature that it can be done very easily.

  On the other hand, as a steering gear for a passenger car, the rack and pinion type is currently the mainstream because of its high rigidity and light weight. And for EPS for rack and pinion type steering gears, the rack shaft is driven by an electric ball screw mechanism in addition to a column assist type in which an electric motor is arranged on the side of the column to drive the steering shaft and pinion itself. A rack assist type is also used. In the rack assist type EPS, since the assist force does not act on the meshing surface between the pinion and the rack, the contact surface pressure between both members, which causes wear and deformation, is relatively small.

  In the rack assist type EPS, the male screw groove of the ball screw shaft formed on the rack shaft and the female screw groove formed on the ball nut are engaged via a large number of circulating balls (steel balls). The ball nut is rotationally driven by an electric motor arranged coaxially or separately from the shaft, and thereby the rack shaft moves in the axial direction. Further, as a power transmission method between the electric motor and the ball nut in the separate-axis rack assist type EPS, a gear type, a timing belt type, and the like are generally used.

  In the rack assist type EPS in which the electric motor is arranged coaxially with the rack shaft, when the electric motor directly drives the ball nut without providing a reduction gear, the motor needs to be a high torque type, and belt reduction or the like is used. There is a problem that the electric motor is increased in size and cost as compared with the rack-assisted EPS of another shaft type.

  For this reason, the coaxial rack assist EPS includes a rack shaft, an electric motor for assisting steering, a ball screw mechanism that converts the rotational motion of the ball nut into a linear motion and transmits the linear motion to the rack shaft, and an electric motor A rolling ball type differential reduction gear that transmits rotational force to a ball nut is coaxially arranged to reduce the protrusion of the electric motor and to increase the transmission torque (for example, (See Patent Document 1).

  As shown in FIGS. 9 and 10, the EPS 100 described in Patent Document 1 includes a rack shaft 102 on which rack teeth 101 are formed, an electric motor 103 that is coaxially disposed around the rack shaft 102, and a rack shaft. A power conversion mechanism 104 that is coaxially disposed around the motor 102 and converts the rotational force of the electric motor 103 into a moving force in the axial direction of the rack shaft 102, and the rack shaft 102 between the electric motor 103 and the power conversion mechanism 104. And a rolling ball type differential speed reducer 105 disposed coaxially therearound.

  The power conversion mechanism 104 holds the balls 107 with a spiral groove 106 provided on the outer peripheral surface of the rack shaft 102, a plurality of balls 107 that engage with the spiral grooves 106, and rotation and axial movement are restricted. This is a ball screw mechanism including a holding ring 108. The rolling ball type differential speed reducer 105 is disposed so as to be opposed to an input ring 112 that is supported by an eccentric cylinder 111 formed on the rotor 109 of the electric motor 103 so as to revolve, and one side surface of the input ring 112. An output member 113 connected to the holding ring 108 of the ball screw mechanism and a stationary member 114 arranged to face the other side surface of the input ring 112 are provided.

  A hypocycloid curve guide groove 115 and an epicycloid curve guide groove 116 are formed on the surfaces of the input ring 112 and the output member 113 facing each other, and a plurality of rolling balls are formed between the guide grooves 115 and 116. 117 is held. In addition, annular recesses (not shown) and a plurality of recesses 118 are formed on the surfaces of the input ring 112 and the stationary member 114 facing each other, and the plurality of rolling balls 119 engage with the recesses 118. Thus, a rolling ball type differential speed reducer 105 is configured.

  A pair of cycloidal wave-shaped guide grooves 115 and 116 formed in the input ring 112 and the output member 113 generate a rotation force in the revolving input ring 112 and a radial due to the revolution of the input ring 112 by the rolling ball 117. The displacement in the direction is absorbed, and the rotation force of the input ring 112 is transmitted to the output member 113.

Japanese Patent Laid-Open No. 2003-261048

  However, according to the rolling ball type differential reduction gear 105 of EPS100 disclosed in Patent Document 1, there is a problem that it is difficult to obtain a degree of design freedom because the reduction ratio is restricted. For example, in order to set the reduction ratio small, it is necessary to reduce the number of rolling balls inserted between the wave-shaped grooves. For this purpose, the pitch circle diameter of the rolling balls to be engaged is reduced or the rolling balls are rolled. The balls need to be spaced apart. However, when the rolling ball type differential speed reducer is used in the rack assist type EPS, the rack shaft penetrates the center of the rolling ball type differential speed reducer, so that the pitch circle diameter of the rolling ball to be engaged is reduced. It is difficult. Moreover, in order to ensure the space | interval of a rolling ball, a holder | retainer is needed and it becomes disadvantageous in cost. Further, by reducing the number of rolling balls, there is a risk that the surface pressure increases and the durability decreases accordingly.

  Conversely, in order to set a large reduction gear ratio, it is necessary to increase the number of rolling balls, so that the pitch circle diameter of the rolling balls to be engaged becomes large. Therefore, the outer shape of the rack assist type EPS is increased, and there is a problem that the vehicle mountability is lowered, and there is room for improvement.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a rack-assist type electric power steering apparatus that can improve the degree of freedom in design of the reduction ratio, and can increase the output and reduce the size. There is to do.

The above object of the present invention can be achieved by the following constitution.
(1) A rack shaft provided with a male screw groove, a ball nut provided with a female screw groove and disposed coaxially with the rack shaft, and a ball provided between the male screw groove and the female screw groove A ball screw mechanism configured; an electric motor disposed coaxially with the rack shaft; and a speed reducer disposed coaxially with the electric motor and the rack shaft. In a rack assist type electric power steering apparatus that transmits the machine through the machine and moves the rack shaft in the axial direction by rotationally driving the ball nut constituting the ball screw mechanism,
The speed reducer is a ball speed reducer with two pairs of trochoidal wave grooves,
The rack assist type electric power steering apparatus, wherein the ball speed reducer is assembled in a motor housing that houses the electric motor.
(2) The ball speed reducer includes a stationary member fitted and fixed to the motor housing, an input rotating member supported rotatably on an eccentric cylindrical portion provided on a rotor of the electric motor, and the motor housing. An output rotating member supported so as to be capable of rotating,
The rack assist type electric power steering apparatus according to (1), wherein an inner diameter of the stationary member is smaller than an outer diameter of a first bearing supporting the rotor.
(3) The ball speed reducer via a second bearing that supports the output rotating member so as to be capable of rotating with respect to the motor housing by a preload nut that is screwed into a female thread portion provided on an inner peripheral surface of the motor housing. The rack assist type electric power steering apparatus according to (2), wherein a preload is applied to the rack.
(4) The motor housing has a steel cylindrical housing fitted therein.
The rack assist type electric power steering apparatus according to any one of (1) to (3), wherein a female screw for screwing a preload nut is provided on an inner peripheral surface of the cylindrical housing.
(5) a rack shaft provided with a male screw groove, a ball nut provided with a female screw groove and coaxially disposed on the rack shaft, and a ball provided between the male screw groove and the female screw groove A ball screw mechanism configured; an electric motor disposed coaxially with the rack shaft; and a speed reducer disposed coaxially with the electric motor and the rack shaft. In a rack assist type electric power steering apparatus that transmits the machine through the machine and moves the rack shaft in the axial direction by rotationally driving the ball nut constituting the ball screw mechanism,
A rack assist type electric power steering apparatus, wherein an output portion of the ball reducer and a ball nut are connected so that torque can be transmitted and relative displacement is possible in an axial direction.
(6) The rack according to any one of (1) to (4), characterized in that an output portion of the ball reducer and a ball nut are connected so as to be capable of transmitting torque and relatively displaced in the axial direction. Assist type electric power steering device.
(7) The ball nut is provided with a torque transmitting portion integrally or separately,
Torque is transmitted between the inner peripheral surface of the output rotating member and the outer peripheral surface of the torque transmitting portion,
The rack assist type electric power according to (5) or (6), wherein the torque transmission region on the inner peripheral surface of the output rotating member and the bearing fitting region on the outer peripheral surface overlap in the axial direction. Steering device.
(8) The rack-assisted electric power according to any one of (5) to (7), wherein an elastic body is interposed between the output portion of the ball reducer and the connection portion of the ball nut. Steering device.
(9) The ball nut is supported by a double row angular contact ball bearing so as to be capable of rotating,
The rack assist type electric power steering according to any one of (5) to (8), characterized in that an elastic body is interposed between both end faces in the axial direction of the inner ring or outer ring of the double row angular ball bearing. apparatus.

  According to the rack assist type electric power steering apparatus according to (1) of the present invention, the reduction gear disposed coaxially with the electric motor and the rack shaft is configured by a combination of two pairs of trochoidal wave grooves. Since it is a ball reducer, the degree of freedom in setting the reduction ratio is improved, and the output of EPS can be increased and the size can be reduced. For example, by setting a large reduction ratio of the ball speed reducer, the electric motor can be downsized with a high rotation and low torque type.

  Further, in the conventional rack assist type EPS, since a speed reducer is not used between the electric motor and the ball screw mechanism, in order to increase the reduction ratio as a unit, it is common to set the lead of the ball screw to about 5 to 7 mm. It is. On the other hand, in the EPS of the present invention, the reduction ratio of the ball reducer can be arbitrarily set, so that the lead of the ball screw can be increased. Thereby, the rotation speed of the ball nut at an arbitrary steering speed can be suppressed, and noise reduction can be achieved. In addition, since it is possible to change the reduction ratio without greatly changing the dimensions of the ball reducer, it is possible to change the specifications of the rack assist type EPS without changing the specifications of the electric motor and the ball screw. It is possible to make many parts common and to reduce costs.

  In addition, since the ball speed reducer is assembled in a motor housing that houses the electric motor, in the assembly process of the electric power steering apparatus, the electric motor is set in the state of the subunit in which the ball speed reducer is assembled in the motor housing. By measuring the electric power required for driving at the rotational speed of the optimum speed reducer preload amount can be adjusted. Further, by connecting a load device with a torque meter to the output part of the ball speed reducer, it is possible to check the speed reducer performance.

  According to the rack assist type electric power steering device described in (2) of the present invention, the ball speed reducer can revolve to the stationary cylindrical member fitted and fixed to the housing and the eccentric cylindrical portion provided in the rotor of the electric motor. Since it is configured to include an input rotating member that is supported and an output rotating member that is rotatably supported by the motor housing, the ball speed reducer can be compactly arranged coaxially with the rack shaft, and the EPS can be downsized. This makes it possible to improve vehicle mountability.

  In addition, since the inner diameter of the stationary member is smaller than the outer diameter of the first bearing that supports the rotor, the first bearing can be pressed by the stationary member, thereby preventing the first bearing from falling off.

  According to the rack assist type electric power steering device according to (3) of the present invention, the preload amount of the ball speed reducer is controlled by the tightening amount of the preload nut fitted to the female screw portion provided on the inner peripheral surface of the motor housing. Adjustment is made, but by applying the preload via the outer ring of the second bearing, the output rotating member is not rotated by tightening the preload nut, and the preload can be adjusted easily and reliably.

  According to the rack assist type electric power steering apparatus described in (4) of the present invention, the motor housing has a steel cylindrical housing fitted therein, and a female screw for screwing a preload nut on the inner peripheral surface of the cylindrical housing. Therefore, it is possible to suppress a change in the preload of the ball reducer due to a difference in linear expansion coefficient between the motor housing and the ball reducer, and to stabilize the performance.

  According to the rack assist type electric power steering apparatus according to (5) of the present invention, the speed reducer disposed coaxially with the electric motor and the rack shaft is configured by a combination of two pairs of trochoidal wave grooves. Since it is a ball reducer, the degree of freedom in setting the reduction ratio is improved, and the output of EPS can be increased and the size can be reduced. For example, by setting a large reduction ratio of the ball speed reducer, the electric motor can be downsized with a high rotation and low torque type.

  Further, in the conventional rack assist type EPS, since a speed reducer is not used between the electric motor and the ball screw mechanism, in order to increase the reduction ratio as a unit, it is common to set the lead of the ball screw to about 5 to 7 mm. It is. On the other hand, in the EPS of the present invention, the reduction ratio of the ball reducer can be arbitrarily set, so that the lead of the ball screw can be increased. Thereby, the rotation speed of the ball nut at an arbitrary steering speed can be suppressed, and noise reduction can be achieved. In addition, since it is possible to change the reduction ratio without greatly changing the dimensions of the ball reducer, it is possible to change the specifications of the rack assist type EPS without changing the specifications of the electric motor and the ball screw. It is possible to make many parts common and to reduce costs.

  In addition, since the output part of the ball reducer and the ball nut are connected so as to be able to transmit torque and be displaced relative to each other in the axial direction, the minute displacement generated in the ball nut due to thrust assist by the electric power steering device is reduced. Therefore, the preload of the ball speed reducer is stabilized and a predetermined performance can be ensured.

  According to the rack assist type electric power steering device described in (6) of the present invention, the minute displacement generated in the ball nut due to the thrust assist by the electric power steering device is not transmitted to the output part of the ball reducer. The preload is stable and a predetermined performance can be ensured.

  According to the rack assist type electric power steering device described in (7) of the present invention, the torque transmission region on the inner peripheral surface of the output rotating member and the bearing fitting region on the outer peripheral surface overlap in the axial direction. Further, the ball nut can be prevented from swinging due to the thrust assist by the electric power steering device, and the engagement of the ball reducer can be kept good.

According to the rack assist type electric power steering device described in (8) of the present invention, since the elastic body is interposed between the output portion of the ball reducer and the connection portion of the ball nut, the inclination of the ball nut is absorbed. And the attitude | position of the output part of a ball reducer can be stabilized.
In addition, the impact torque input to the ball nut when abutting against the stroke end is absorbed to prevent the ball screw mechanism from being damaged, and the hitting sound caused by reversal or vibration input can be suppressed.

  According to the rack assist type electric power steering device according to (9) of the present invention, the elastic body is interposed between the axial end surfaces of the inner ring or the outer ring of the double row angular contact ball bearing that supports the ball nut. The rattle noise generated from the ball screw during reversal can be reduced without affecting the preload of the ball reducer.

It is a longitudinal cross-sectional view of the rack assist type EPS of this embodiment. It is a principal part enlarged view of the rack assist type EPS shown in FIG. It is a fragmentary perspective view which shows the structure of a connection part. It is a fragmentary perspective view which shows the state which interposed the elastic body in the connection part shown in FIG. It is a disassembled perspective view of the ball reducer shown in FIG. It is sectional drawing which shows the subunit which assembled | attached the ball reducer to the 2nd housing. It is a principal part enlarged view of the rack assist type EPS which concerns on a modification. It is a principal part enlarged view of the rack assist type EPS which concerns on a modification. It is principal part sectional drawing of conventional EPS. FIG. 10 is an exploded perspective view of the rolling ball type differential reducer shown in FIG. 9.

  DESCRIPTION OF EMBODIMENTS Hereinafter, an embodiment of a rack assist type electric power steering device (rack assist type EPS) according to the present invention will be described in detail based on the drawings.

  As shown in FIG. 1, the rack assist type EPS 10 includes a housing 30, a rack shaft 31, a ball reducer 32, a ball screw mechanism 33, an electric motor 34, and a pinion part 35. The housing 30 includes a first housing 30A that houses the ball screw mechanism 33, and a second housing (motor housing) 30B that houses the electric motor 34 and the ball speed reducer 32, and the rack shaft 31 can be moved in the axial direction. And it supports in the state which blocked | prevented rotation. A ball speed reducer 32, a ball screw mechanism 33, and an electric motor 34 are coaxially arranged on the rack shaft 31, and tie rods (not shown) are connected to both ends protruding from both sides of the housing 30. .

  The rack shaft 31 is provided with a rack tooth 38 constituting a rack and pinion mechanism built in the pinion part 35 and a male screw part 39 constituting a ball screw mechanism 33. The pinion unit 35 includes an input shaft 36 to which a steering force from a steering wheel (not shown) is input, a pinion (not shown), and a torque detection device 37, and a torsion bar (not shown) of the torque detection device 37. The input shaft 36 and the pinion are coupled via a not-shown). The pinion meshes with the rack teeth 38 provided on the rack shaft 31.

  As shown in FIG. 2, the ball screw mechanism 33 is arranged around a male thread portion 39 formed on the outer peripheral surface of the rack shaft 31 and the rack shaft 31, and is a double-row angular ball bearing (hereinafter simply referred to as a bearing) 40. The ball nut 41 is held by the first housing 30A with its axial movement restricted and the male screw portion 39 and the female screw portion 42 formed on the inner peripheral surface of the ball nut 41. A plurality of balls 43 are provided, and are arranged coaxially with the rack shaft 31. A separate torque transmission portion 80 is fixed to the right end portion of the ball nut 41 by fitting, welding, or the like, and the left end portion of the torque transmission portion 80 also serves to prevent the end deflector type ball 43 from coming off. In addition, the torque transmission part 80 may be integrally formed with the ball nut 41. The inner ring 44 of the bearing 40 is fixed to the ball nut 41 by a nut 45 screwed to the ball nut 41, and the outer ring 46 of the bearing 40 is fixed to the first housing 30A by a nut 47 screwed to the first housing 30A. Has been.

  The electric motor 34 includes a stator 50 fixed in the second housing 30 </ b> B, and a rotor 51 that is rotatably supported between the inner peripheral surface of the stator 50 and the outer peripheral surface of the rack shaft 31. And a brushless motor arranged coaxially. Both ends of the rotor 51 protruding in the axial direction from the stator 50 are rotatably supported by the second housing 30B by bearings 52 and 53 (see FIG. 1).

  The ball speed reducer 32 is arranged coaxially with the rack shaft 31 and the electric motor 34 between the electric motor 34 and the ball screw mechanism 33. The ball speed reducer 32 has an eccentric cylindrical portion 61 formed eccentrically with respect to the axis of the rotor 51 at one end of the rotor 51, and an input rotation supported by the eccentric cylindrical portion 61 via a bearing 62 so as to be able to rotate. A member 63, an annular output rotating member 64 arranged to face one side of the input rotating member 63 so as to be able to transmit torque to the ball nut 41, and the other side of the input rotating member 63 fixed to the second housing 30B. And an annular stationary member 65 arranged.

  The stationary member 65 opposes a bearing (first bearing) 52 that supports the rotor 51 on the side opposite to the side facing the input rotating member 63, and the bearing 52 that supports the rotor 51 has an inner diameter of the stationary member 65. It is set to be smaller than the outer diameter. The stationary member 65 is in contact with the outer ring 52a of the bearing 52, and a notch 65a is formed inward from a position larger than the outer diameter of the inner ring 52b so as not to contact the inner ring 52b. Prevents falling off.

  The output rotation member 64 has a cylindrical output portion 82 at the left end, and a bearing 83 (second bearing) that supports the output rotation member 64 on the second housing 30B so as to be capable of rotating on the outer peripheral surface of the output portion 82. ) And the torque transmission portion 80 of the ball nut 41 is engaged with the inner peripheral surface of the output portion 82. The bearing fitting region on the outer peripheral surface of the output portion 82 and the torque transmission region on the inner peripheral surface overlap in the axial direction. A connecting portion 85 between the output portion 82 of the output rotating member 64 and the torque transmitting portion 80 of the ball nut 41 is coupled with an involute spline or involute serration, and can transmit torque, and slide in the axial direction even during torque transmission. (Relative displacement) is set to a possible friction coefficient. FIG. 3A illustrates a case where the output portion 82 of the output rotating member 64 and the connecting portion 85 of the torque transmitting portion 80 of the ball nut 41 are involute spline coupled.

  In the connecting portion 85, a resin coat layer may be provided between the output portion 82 of the output rotating member 64 and the torque transmitting portion 80 of the ball nut 41. Thereby, the contact between metals can be avoided and the hitting sound at the time of inversion or input of vibration can be suppressed. Further, in the connecting portion 85, as shown in FIG. 3B, a groove 86 is formed in the output portion 82 of the output rotating member 64, and the protruding portion 87 formed in the torque transmitting portion 80 of the ball nut 41 is engaged. May be. Since the side surface of the groove 86 and the protrusion 87 are in line contact, the torque transmission area is small and the sliding force is small. Further, the fitting length in the axial direction is short, and it is possible to make it difficult to transmit the inclination of the ball nut 41 to the output rotating member 64. Further, as shown in FIG. 3C, a groove 88 may be formed between the torque transmitting portion 80 of the ball nut 41 and the output portion 82 of the output rotating member 64 to interpose the ball 89. Since axial displacement is allowed not by sliding but by rolling, smoother axial displacement is possible. In addition, the connection may be made by concave / convex fitting or a key.

  4A to 4C, an elastic body 81 may be interposed between the output portion 82 of the output rotating member 64 and the torque transmission portion 80 of the ball nut 41. The elastic body 81 can absorb the inclination of the ball nut 41 and stabilize the posture of the output rotation member 64. If the relative displacement is minute, axial displacement can be allowed by elastic deformation of the elastic body 81. Furthermore, the impact torque input to the ball nut 41 when it is abutted against the stroke end can be absorbed to prevent the ball screw mechanism 33 from being damaged, and the hitting sound caused by reversal or vibration input can be suppressed. 4A corresponds to the connecting portion 85 in FIG. 3A, FIG. 4B corresponds to the connecting portion 85 in FIG. 3B, and FIG. 4C corresponds to FIG. This corresponds to the connecting portion 85 of FIG.

  Returning to FIG. 2, the bearing 83 is a ball bearing or an angular ball bearing, the inner ring 83 a is fitted and fixed to the outer peripheral surface of the output rotating member 64, and the outer ring 83 b is spaced from the second housing 30 </ b> B so as to be axially displaceable. It is fitted. A preload nut 90 is provided on the left end surface of the bearing 83 so as to be screwed into a female thread portion provided on the inner peripheral surface of the second housing 30B. Preload is applied to the outer ring 83b by tightening the preload nut 90. In addition, a preload is applied to the ball reducer 32. Note that the preload application method is not limited to this, and it is sufficient that a fixed position preload or a constant pressure preload is applied.

  As shown in FIGS. 2 and 5, a wave-shaped groove 67 (see FIG. 2) having an epitrochoidal curve is formed on one side surface (on the output rotating member 64 side) of the input rotating member 63. On the side surface on the input rotating member 63 side, a corrugated groove 68 having a hypotrochoidal curve is formed. These two wave-shaped grooves 67 and 68 are arranged at the same radial direction position corresponding to the pitch circle diameter D1 of the rolling ball 71 to be engaged.

  The wave number of the wave shape groove 67 of the epitrochoid is two less than the wave number of the wave shape groove 68 of the hypotrochoid curve. In the case of this embodiment, the wave number of the wave-shaped groove 68 of the output rotating member 64 having a hypotrochoidal shape is N, and the wave number of the wave-shaped groove 67 on one side of the input rotating member 63 having an epitrochoidal shape is “N”. -2 ". Further, “N−1” rolling balls 71 are arranged between the wave-shaped groove 67 of the input rotating member 63 and the wave-shaped groove 68 of the output rotating member 64.

  Further, a corrugated groove 69 having a hypotrochoidal curve is formed on the other side surface (the stationary member 65 side) of the input rotating member 63, and an epitrochoidal curve waveform is formed on the side surface of the stationary member 65 on the input rotating member 63 side. A groove 70 is formed. Both wave-shaped grooves 69 and 70 are disposed at the same radial position corresponding to the pitch circle diameter D2 of the rolling balls 72 to be engaged.

  The wave number of the corrugated groove 70 of epitrochoid is two less than the wave number of the corrugated groove 69 of the hypotrochoid curve. In the case of this embodiment, the wave number of the wave-shaped groove 69 on the other side surface of the input rotating member 63 having a hypotrochoid shape is M, and the wave number of the wave-shaped groove 70 of the stationary member 65 having an epitrochoid shape is “M−”. 2 ”. Then, “M−1” rolling balls 72 are arranged between the wave-shaped groove 69 of the input rotating member 63 and the wave-shaped groove 70 of the stationary member 65.

  The epitrochoid curve is a curve drawn by one point of a small-diameter circle when the large-diameter circle having a predetermined diameter is rolled with the small-diameter circle circumscribed. Further, the hypotrochoid curve is a curve drawn by one point of a small diameter circle when the large diameter circle of a predetermined diameter is rolled with the small diameter circle inscribed therein.

  Note that the cross-sectional shapes of the two pairs of corrugated grooves 67, 68, 69, and 70 are Gothic arch shapes, respectively, and the contact portions between the corrugated grooves 67, 68 and the rolling ball 71, and the corrugated grooves 69, 70. The sliding element of the contact portion between the rolling ball 72 and the rolling ball 72 may be set large.

  Preload is applied between the stationary member 65, the input rotation member 63, and the output rotation member 64 through the bearing 83 by the preload nut 90 as described above. As a result, the rolling contact portion between the wave-shaped grooves 67 and 68 and the rolling ball 71 and the rolling contact portion between the wave-shaped grooves 69 and 70 and the rolling ball 72 are not rattled.

  In the ball speed reducer 32 configured as described above, the engagement between the wave-shaped grooves 67 and 68 and the rolling balls 71 and the wave-shaped grooves 69 and 70 and the rolling balls 72 as the input rotation member 63 revolves. By changing the position, the output rotation member 64 performs a reduced-speed rotation. Since the reduction ratio at this time is determined by the wave numbers of the wave-shaped grooves 67 and 68 and the wave-shaped grooves 69 and 70, the reduction ratio can be arbitrarily set.

  The shape of the wave-shaped grooves 67, 68, 69, and 70 formed in one side surface of the input rotation member 63 and the output rotation member 64, and the other side surface of the input rotation member 63 and the stationary member 65 (epitrochoid, hypotrochoid) ) May be opposite to the above shape.

  The wave number N of the wave-shaped groove 67 and the wave number M of the wave-shaped groove 69 of the input rotating member 63 are set to N <M, whereby the pitch circle of the rolling ball 71 engaged with the wave-shaped grooves 67 and 68 is set. The diameter D1 <the pitch circle diameter D2 of the rolling balls 72 engaged with the wave-shaped grooves 69 and 70. In this way, the pitch circle diameters D1 and D2 of the rolling balls 71 and 72 engaged with the two pairs of corrugated grooves 67 and 68 and 69 and 70 are made different (N ≠ M), and an arbitrary reduction ratio is set. Obtainable.

FIG. 6 is a cross-sectional view showing the subunit 20 in which the electric motor 34 and the ball speed reducer 32 are assembled to the second housing 30B. The subunit 20 is accessible to the output portion 82 of the output rotating member 64 from one end face of the second housing 30B. Therefore, in the assembly process of the rack assist type EPS 10, the optimum reduction gear preload amount can be adjusted by measuring the electric power required to drive the electric motor 34 at a predetermined rotational speed. Further, the reduction gear performance can be confirmed by connecting a load device with a torque meter to the output portion 82 of the output rotating member 64.
The operation of this embodiment will be described. As shown in FIG. 1, when a steering wheel (not shown) is operated, the rotation is transmitted to the input shaft 36, and the pinion is rotated through this while twisting the torsion bar. The rotation of the pinion is transmitted to the rack shaft 31 with which the rack teeth 38 mesh, and moves the rack shaft 31 in the axial direction.

  On the other hand, the amount of torsion of the torsion bar is detected by the torque detection device 37, an output signal corresponding to the amount of torsion is input to a control device (not shown), and the electric motor 34 rotates based on a rotation command from the control device. The rotation of the electric motor 34 (rotor 51) revolves the input rotation member 63 supported by the eccentric cylindrical portion 61 of the ball speed reducer 32 so as to be able to rotate, and the wave-shaped groove is generated along with the rotation and rotation of the input rotation member 63. 67, 68 and the rolling ball 71, and the engagement positions of the corrugated grooves 69 and 70 and the rolling ball 72 are changed, so that the output rotating member 64 performs a reduced rotational motion.

  The rotation of the output rotating member 64 causes the ball nut 41 to rotate by the coupling of the output portion 82 of the output rotating member 64 and the torque transmitting portion 80 of the ball nut 41, so that the rack shaft 31 is moved in the axial direction by the action of the ball screw mechanism 33. Moving. The movement direction of the rack shaft 31 is the same direction as the direction driven by the pinion, thereby assisting the force of the pinion to move the rack shaft 31.

  As described above, according to the rack assist type EPS 10 of the present embodiment, the reduction gears disposed coaxially with the electric motor 34 and the rack shaft 31 have two pairs of trochoidal wave grooves 67, 68, and 69, Since the ball speed reducer 32 is composed of 70 combinations, the degree of freedom in setting the reduction ratio is improved, and the output of the EPS 10 can be increased and the size can be reduced. Further, since the reduction ratio of the ball speed reducer 32 can be arbitrarily set, the lead of the ball screw mechanism 33 is enlarged to suppress the rotation speed of the ball nut 41, thereby reducing the noise. In addition, since the reduction ratio can be changed without greatly changing the external dimensions of the ball reducer 32, the rack assist type EPS can be changed without changing the specifications of the electric motor 34 and the ball screw mechanism 33. It is possible to change the specifications, and it is possible to share many parts, thereby reducing the cost.

  Further, since the ball speed reducer 32 is assembled in the second housing 30B that houses the electric motor 34, the subunit 20 in which the ball speed reducer 32 is assembled to the second housing 30B in the assembly process of the rack assist type EPS 10. In this state, by measuring the electric power required to drive the electric motor 34 at a predetermined rotational speed, the optimum reduction gear preload amount can be adjusted. Further, by connecting a load device with a torque meter to the output unit 82 of the ball reducer 32, it is possible to check the reducer performance.

  Further, the ball speed reducer 32 is fitted and fixed to the second housing 30B, a stationary member 65, an input rotating member 63 supported by an eccentric cylindrical portion 61 provided in the rotor 51 of the electric motor 34, and a first rotating member 63. 2 Since the output rotation member 64 is rotatably supported by the housing 30B, the ball speed reducer 32 can be compactly arranged coaxially with the rack shaft 31, and the EPS can be downsized and mounted on the vehicle. Improves.

  Further, since the inner diameter of the stationary member 65 is smaller than the outer diameter of the bearing 52 that supports the rotor 51, the bearing 52 can be pressed by the stationary member 65, thereby preventing the bearing 52 from falling off.

  Further, the preload amount of the ball reducer 32 is adjusted by the tightening amount of the preload nut 90 fitted to the female thread portion provided on the inner peripheral surface of the second housing 30B, but via the outer ring 83b of the bearing 83. By applying the preload, the output rotation member 64 is not rotated by the tightening of the preload nut 90, and the preload can be easily and reliably adjusted.

  Further, since the output portion 82 of the ball reducer 32 and the ball nut 41 are connected so as to be able to transmit torque and to be relatively displaced in the axial direction, a minute displacement generated in the ball nut 41 with thrust assist by the rack assist type EPS 10. Is not transmitted to the output unit 82 of the ball reducer 32, the preload of the ball reducer 32 is stabilized, and a predetermined performance can be ensured.

  Further, since the torque transmission region on the inner peripheral surface of the output rotating member 64 and the bearing fitting region on the outer peripheral surface overlap in the axial direction, the swing of the ball nut 41 accompanying the thrust assist by the rack assist type EPS 10 is prevented. And the meshing of the ball reducer 32 can be kept good.

  Further, since the elastic body 81 is interposed between the output portion 82 of the ball reducer 32 and the connection portion 85 of the ball nut 41, the inclination of the ball nut 41 is absorbed, and the attitude of the output portion 82 of the ball reducer 32. Can be stabilized. Further, when the ball speed reducer 32 is connected coaxially with the electric motor 34, the inertia (inertia) increases, and the impact torque input to the ball nut 41 tends to increase when it abuts against the stroke end. However, it is possible to absorb the impact torque by the elastic body 81 and prevent the ball screw mechanism 33 from being damaged, and to suppress the hitting sound at the time of reversal or vibration input.

In addition, this invention is not limited to embodiment mentioned above, A deformation | transformation, improvement, etc. are possible suitably.
For example, as shown in FIG. 7, a steel cylindrical housing 91 may be further provided in the second housing 30B, and the ball speed reducer 32 may be assembled to the inner periphery thereof. As a result, the stationary member 65 is fitted and fixed to the cylindrical housing 91, the output rotating member 64 is rotatably supported by the cylindrical housing 91, and the preload nut 90 is provided on the inner peripheral surface of the cylindrical housing 91. By being screwed into the portion, a preload is applied to the outer ring 83b and a preload is applied to the ball reducer 32. Normally, the housing 30 is made of an aluminum material, and the ball speed reducer 32 that requires hardness on the surface of the corrugated groove is made of a steel material. However, since the operating temperature range of the rack assist EPS 10 is wide, the linear expansion coefficient is Although the preload amount in the ball reducer 32 may change due to the difference, by making the steel cylindrical housing 91 made of steel, the change in the preload of the ball reducer 32 is suppressed and the performance of the rack assist type EPS 10 is improved. Can be stabilized.

  Further, as shown in FIG. 8, an annular elastic body 92 may be interposed on both axial end surfaces of the outer ring 46 of the bearing 40 that supports the ball nut 41. As a result, the ball nut 41 can be displaced in the axial direction by a certain distance, so that the rattle sound generated from the ball screw mechanism 33 at the time of inputting vibration from the road surface without affecting the preload of the ball reducer 32. Can be reduced. In this modification, the elastic body 92 is provided on the outer ring 46 of the bearing 40. However, the elastic body 92 may be provided on both end surfaces in the axial direction of the inner ring 44 of the bearing 40.

  In the present embodiment, the wave number N of the wave-shaped groove 67 and the wave number M of the wave-shaped groove 69 of the input rotating member 63 have been described as N <M. However, the wave numbers N and M are not limited to this and are arbitrary. (N ≧ M) can be set, whereby the reduction ratio can also be set arbitrarily.

10 Rack Assist Electric Power Steering Device (Rack Assist EPS)
30 Housing 30A First housing 30B Second housing (motor housing)
31 Rack shaft 32 Ball reducer 33 Ball screw mechanism 34 Electric motor 39 Male thread portion 40 Bearing (double row angular ball bearing)
41 Ball nut 42 Female thread portion 43 Ball 52 Bearing (first bearing)
61 Eccentric cylindrical portion 63 Input rotating member 64 Output rotating member 65 Stationary members 67, 68, 69, 70 Trochoidal wave groove 80 Torque transmitting portion 81 Elastic body 82 Output portion 83 Bearing (second bearing)
85 Connecting portion 90 Preload nut 91 Cylindrical housing 92 Elastic body D1 Pitch circle diameter of rolling ball engaged with trochoidal wave groove D2 Pitch circle diameter of rolling ball engaged with trochoidal wave groove

Claims (9)

A rack shaft provided with a male screw groove, a ball nut provided with a female screw groove and disposed coaxially with the rack shaft, and a ball provided between the male screw groove and the female screw groove. A ball screw mechanism; an electric motor disposed coaxially with the rack shaft; and a speed reducer disposed coaxially with the electric motor and the rack shaft, and driving force of the motor via the speed reducer In the rack assist type electric power steering device that moves the rack shaft in the axial direction by rotationally driving the ball nut constituting the ball screw mechanism,
The speed reducer is a ball speed reducer with two pairs of trochoidal wave grooves,
The rack assist type electric power steering apparatus, wherein the ball speed reducer is assembled in a motor housing that houses the electric motor. The ball speed reducer is fitted to and fixed to the motor housing, an input rotating member supported rotatably on an eccentric cylindrical portion provided on a rotor of the electric motor, and rotatable on the motor housing. A supported output rotating member,
The rack assist type electric power steering device according to claim 1, wherein an inner diameter of the stationary member is smaller than an outer diameter of a first bearing supporting the rotor.   Preload is applied to the ball speed reducer via a second bearing that supports the output rotating member so as to be able to rotate with respect to the motor housing by a preload nut screwed into a female thread portion provided on an inner peripheral surface of the motor housing. The rack assist type electric power steering device according to claim 2, wherein the rack assist type electric power steering device is provided. The motor housing is fitted with a steel cylindrical housing,
The rack assist type electric power steering apparatus according to any one of claims 1 to 3, wherein a female screw for screwing a preload nut is provided on an inner peripheral surface of the cylindrical housing. A rack shaft provided with a male screw groove, a ball nut provided with a female screw groove and disposed coaxially with the rack shaft, and a ball provided between the male screw groove and the female screw groove. A ball screw mechanism; an electric motor disposed coaxially with the rack shaft; and a speed reducer disposed coaxially with the electric motor and the rack shaft, and driving force of the motor via the speed reducer In the rack assist type electric power steering device that moves the rack shaft in the axial direction by rotationally driving the ball nut constituting the ball screw mechanism,
A rack assist type electric power steering apparatus, wherein an output portion of the ball reducer and a ball nut are connected so that torque can be transmitted and relative displacement is possible in an axial direction.   The rack-assist type electric machine according to any one of claims 1 to 4, wherein an output part of the ball reducer and a ball nut are connected so as to be able to transmit torque and to be relatively displaced in an axial direction. Power steering device. The ball nut is provided with a torque transmission part integrally or separately,
Torque is transmitted between the inner peripheral surface of the output rotating member and the outer peripheral surface of the torque transmitting portion,
The rack assist type electric power steering apparatus according to claim 5 or 6, wherein a torque transmission region on the inner peripheral surface of the output rotating member and a bearing fitting region on the outer peripheral surface overlap in the axial direction. .   The rack assist type electric power steering device according to any one of claims 5 to 7, wherein an elastic body is interposed between an output portion of the ball reducer and a connection portion of the ball nut. The ball nut is supported by a double row angular ball bearing so as to be capable of rotating,
The rack assist type electric power steering according to any one of claims 5 to 8, wherein an elastic body is interposed between both end faces in the axial direction of the inner ring or the outer ring of the double row angular ball bearing. apparatus.

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