JP6365702B2 - Electric power steering device - Google Patents

Description

  The present invention relates to an electric power steering apparatus provided with a ball screw mechanism.

In a rack and pinion type steering device of a vehicle, a steering rotational force when a steering wheel is steered is transmitted to a pinion shaft, and is converted into a linear motion of the rack shaft by a rack and pinion gear meshing with the pinion shaft. It is transmitted to the steered wheels via tie rods connected to both ends of the rack shaft, and the steered wheels are steered.
By connecting the rack shaft to a ball screw mechanism driven by an electric motor, a rack assist type electric power steering device (EPS) can be configured. The rack assist type electric power steering device is referred to as a rack assist type EPS.

  In the rack assist type EPS ball screw mechanism, if any abnormality occurs, the vehicle may not be able to be steered. Therefore, reliability that does not lock in any case is required. In addition, vibration and sound generated by the ball screw mechanism are transmitted to the steering wheel via the rack shaft and the steering shaft, which may cause discomfort to the driver. Therefore, the rack assist type EPS has a high demand for operability and quietness.

  In particular, in an electric power steering device (hereinafter referred to as offset / rack assist type EPS) of a system in which a drive motor is offset with respect to a rack shaft and power is transmitted to a ball screw mechanism by a speed reduction mechanism such as a belt speed reducer. Therefore, the ball screw mechanism is often laid out on the outside of the vehicle, and the radial load of the rack shaft is received by the ball screw mechanism, so that there is a problem that noise from the ball screw mechanism is likely to occur.

For example, in a top-type ball screw, a single row circuit in which a circulating portion exists in one phase per rotation of a nut is arranged by arranging a plurality of windings while shifting the phase. In this case, the moment stiffness of the ball screw mechanism becomes low at a specific phase of the circuit located at the end of the nut (phase of the circulating portion), and the moment stiffness changes depending on the nut phase. As a result, the rack shaft bends as the ball screw mechanism is driven, and abnormal noise is generated.
As an offset / rack-assist type EPS that solves such a problem, a device is known in which one circuit of an external deflector type ball screw is arranged in a double row and the moment rigidity of the ball screw is designed not to depend on the nut phase ( For example, Patent Document 1).

  In addition, in order to obtain a rack assist type EPS having operability and quietness, a screw shaft having a spiral thread groove on the outer peripheral surface, and a thread groove corresponding to the screw groove on the inner peripheral surface, the axial direction A rolling element return passage that penetrates the screw shaft, and a nut that is loosely fitted to the screw shaft, and a rolling element circulation path that communicates between both screw grooves and the rolling element return passage, and is disposed on both end faces of the nut. In a ball screw mechanism including a circulation member, an apparatus including a circulation member that smoothly circulates the rolling element by scooping up the rolling element in a tangential direction of the spiral path of the screw (for example, Patent Document 2). In order to reduce the collision noise with the rolling elements, an apparatus using a circulating member as a resin material such as a polymer material (for example, Patent Document 3) is known.

Here, the ball screw mechanism of the offset / rack assist EPS is often mounted in the vicinity of a heat source such as an engine in an engine room, an exhaust pipe, a transmission, etc., and the ambient temperature is increased from −40 ° C. to 120 ° C. or more. The rolling element continuously collides with the circulating parts of the ball screw mechanism as it is steered.
Under such conditions, when the circulating component made of the resin material of Patent Document 3 is used, the impact strength of the resin material is reduced in a low-temperature atmosphere, and cracks and the like are likely to occur due to collision with the rolling elements, and the circulating component is damaged. The possibility of is increased. Moreover, when the circulation component which consists of a resin material of patent document 3 is used in a high temperature atmosphere, there exists a possibility that the malfunction of a scooping part etc. may arise, the tensile strength of a resin material falls.

Then, what consists of reinforced resin with which glass fiber or carbon fiber was filled is proposed as a circulation part of the ball screw mechanism which can be used even in a low temperature atmosphere or a high temperature atmosphere (for example, patent document 4).
However, if the surface of the circulated part made of reinforced resin is worn due to continuous collision with the rolling element, the filled fiber may be exposed on the surface of the circulated part and cause damage to the rolling element. In addition, there is a risk that abnormal sound and malfunction may be caused by the fallen fibers being bitten.

JP 2005-326209 A JP 2006-069518 A JP 2006-153216 A Japanese Patent Laid-Open No. 2004-100756

  The present invention has been made paying attention to the unsolved problems of the above-described conventional example, and an object thereof is to provide a highly reliable electric power steering device in which abnormal noise and malfunction are prevented.

In order to achieve the above object, an electric power steering apparatus according to an embodiment includes a screw shaft having a thread groove formed on an outer peripheral surface and a screw groove corresponding to the screw groove of the screw shaft formed on an inner peripheral surface. In addition, a nut formed with a rolling element return passage penetrating in the axial direction, a spiral track formed by the thread groove of the screw shaft and the thread groove of the nut, and a large number of members interposed in the rolling element return path A rolling plate , a pair of rolling element guide side plates, and a connecting plate portion that connects the pair of rolling element guide side plates, and is inserted into the threaded groove of the screw shaft in a non-contact state on the connecting plate portion. A tongue portion for scooping up the rolling element is formed, and the rolling element is scooped up from one end of the spiral track in the tangential direction of the spiral track and guided to the rolling element return passage and from the rolling element return passage to the spiral. Guide to the other end of the track A ball screw mechanism having a rolling element circulation member that forms a rolling element circulation track, wherein the screw shaft of the ball screw mechanism is coupled to a rack shaft that is screwed to a pinion shaft that constitutes a steering mechanism, and the nut In the electric power steering apparatus, the rolling element circulation track includes a first straight line extending in a tangential direction of the spiral track from the scooping point on the spiral track, and the rolling element return path. And a curve connecting the first and second lines, and the radius of the curve is set to 50% or more and 90% or less of the diameter of the rolling element. At the same time, the rolling element circulating member is formed of polyamide 46 when used in an atmospheric temperature of 125 ° C.

  According to an embodiment of the present invention, in the electric power steering apparatus according to claim 1, a BCD (Ball Circle Diameter) is set to φ25 mm to φ32 mm, and a diameter of the rolling element is set to φ3.968 mm to φ5.000 mm. The lead was 6.0 mm or more and 8.0 mm or less.

  As described above, according to the electric power steering apparatus according to the present invention, the rolling element circulation trajectory extends from the scooping point on the spiral trajectory to the tangential direction of the spiral trajectory, and the rolling element return passageway. A second straight line passing through the center and a curve connecting the first and second straight lines, the radius of the curve being set to 50% or more and 90% or less of the diameter of the rolling element; Is formed of polyamide 46 when used in an atmosphere of 125 ° C., nut inertia does not increase, end contact load acting on the ball screw mechanism is reduced, and steering feeling is improved. It is possible to provide an electric power steering device that can prevent noise and malfunction and has high reliability.

It is a front view which shows the steering gear mechanism which concerns on this invention. It is a front view which shows the rack shaft applied to the steering gear mechanism of FIG. It is principal part sectional drawing of the electric power steering apparatus of FIG. It is principal part sectional drawing which shows a ball screw mechanism. It is a perspective view which shows a rolling element circulation member. It is a figure which shows the inside of the ball screw nut of the ball screw mechanism which comprises an electric power steering apparatus. It is a figure which shows the rolling element circulation member mounting part formed in the ball screw nut. It is the figure which showed roughly the circulation path of a ball screw nut. It is the graph which showed the relationship between a sphere diameter ratio (ratio of the radius of curve R and the diameter of a ball), and inertia. It is a figure which sets the back surface shape of the tongue part of a rolling element circulation member. The shape after a ball screw endurance test of a plurality of kinds of polymer materials is shown. The shape after a ball screw endurance test of a plurality of kinds of polymer materials is shown.

DESCRIPTION OF EMBODIMENTS Hereinafter, modes for carrying out the present invention (hereinafter referred to as embodiments) will be described in detail with reference to the drawings.
(Electric power steering device)
FIG. 1 shows a steering gear mechanism 1 according to the present invention, and FIG. 2 is a front view showing a rack shaft.
The steering gear mechanism 1 in FIG. 1 includes a pinion mechanism 2 and a rack mechanism 3.

The pinion mechanism 2 includes a pinion shaft 5 that is rotatably supported by a pinion housing 4. The pinion shaft 5 is connected to a steering wheel via a steering shaft (not shown). The rack mechanism 3 includes a rack shaft 7 that is slidably supported by a rack housing 6. The pinion shaft 5 and the rack shaft 7 are engaged with each other, and the rotational force transmitted to the pinion shaft 5 is converted into the direct power of the rack shaft 7.
As shown in FIG. 2, the rack shaft 7 has a rack portion 8 with which the pinion shaft 5 is engaged, and a ball screw having a ball screw groove 9a having a helical Gothic arc cross-sectional shape formed on the right side of the rack portion 8. A shaft 9 is provided.

  A rack assist type electric power steering device (EPS: Electric Power Steering, hereinafter referred to as rack assist type EPS) 10 is disposed on the right end side of the rack shaft 7. The rack assist EPS 10 includes a ball screw mechanism 11, an electric motor 12 for assisting steering, and a speed reduction mechanism 13 that connects the ball screw mechanism 11 and the electric motor 12. The rack assist type EPS 10 is an offset type rack assist type EPS 10 in which a ball screw shaft 9 of a ball screw mechanism 11 described later and an axis of an electric motor 12 are arranged in parallel.

  As shown in FIG. 3, the electric motor 12 is fixedly supported on the pinion mechanism 2 side of the motor support portion 18 integrally formed radially outward of the bearing housing portion 16 of the rack housing 6, and its rotating shaft 12 a is It protrudes to the opposite side through a through hole 18 a formed in the motor support 18. As shown in FIG. 1, the steering assist electric motor 12 is rotationally driven by a motor control device 30. The motor control device 30 includes a torque sensor 30a that detects a steering torque input to a steering wheel (not shown) and a vehicle speed sensor 30b that detects a vehicle speed, and the steering torque and vehicle speed detected by the torque sensor 30a. A steering assist torque command value is calculated based on the vehicle speed detected by the sensor 30b, and a motor drive current for generating a necessary steering assist torque by the steering assist electric motor 12 is determined based on the calculated steering assist torque command value. This motor drive current is output to the steering assist electric motor 12.

Further, as shown in FIG. 3, the speed reduction mechanism 13 is integrated with a small-diameter pulley 19 attached to the distal end of the rotating shaft 12a of the steering assisting electric motor 12 and the aforementioned ball screw nut 15 in the radial direction. The formed large-diameter pulley 20 and a small-diameter pulley 19 and a timing belt 21 wound between the large-diameter pulley 20 are configured.
And the cover 22 is being fixed to the bearing accommodating part 16 and the motor support part 18 by bolting etc. so that the small diameter pulley 19, the large diameter pulley 20, and the ball screw groove 9a may be covered.

  As shown in FIG. 3, the ball screw mechanism 11 includes a ball screw nut 15 that is screwed into a ball screw groove 9 a of the ball screw shaft 9 of the rack shaft 7 via a rolling element 14. The ball screw nut 15 is rotatably supported by a rolling bearing 17 accommodated in a bearing accommodating portion 16 formed at the right end portion of the rack housing 6. The rolling bearing 17 includes an inner ring 17a in which an inner ring 17a is formed integrally with the ball screw nut 15, and an outer ring 17c connected to the inner ring 17a via a ball 17b and fixedly supported by the bearing housing portion 16. Yes.

  The rack assist EPS 10 rotates and rotates the rotating shaft 12a of the steering assisting electric motor 12, whereby the small-diameter pulley 19 and the large-diameter pulley 20 are rotationally driven by the timing belt 21, respectively. Since the nut 15 is rotationally driven, the ball screw shaft 9, that is, the rack shaft 7 is linearly driven. The left end portion of the rack portion 8 of the rack shaft 7 and the right end portion of the ball screw shaft 9 are connected to a steered wheel (not shown) via tie rods 23, respectively.

(Ball screw nut and rolling element circulation member)
As shown in FIG. 4, a ball screw groove 15 a having a Gothic arc sectional shape on which the rolling element 14 rolls is formed in a spiral shape on the inner peripheral surface of the ball screw nut 15 of the ball screw mechanism 11. A pair of rolling element circulation member mounting portions 31 </ b> A and 31 </ b> B for mounting the pair of rolling element circulation members 33 are formed at the left and right ends of the ball screw nut 15 in the axial direction. Further, at the left and right ends of the ball screw nut 15 in the axial direction, a pair of rolling elements are circulated between the ball screw groove 15a on the inner peripheral surface of the ball screw nut 15 and the outer peripheral surface in the axial direction. A rolling element return passage 32 that communicates between the member mounting portions 31A and 31B is formed.
As shown in FIG. 6 and FIG. 7B, the rolling element circulating member mounting portion 31A on the right end side is a notch recess opened in one end surface in the axial direction of the ball screw nut 15, and has a flat surface 31a and a curved surface. This is a member provided with surfaces 31b and 31c, a semi-cylindrical surface 31d, and an upper surface 31e.

  The flat surface 31a extends substantially continuously to the bottom of the ball screw groove 15a on one end surface in the axial direction of the ball screw nut 15 to the lower surface of the rolling element return passage 32 in the tangential direction. It extends parallel to. Further, the curved surfaces 31b and 31c are curved and extended to both sides of the ball screw groove 15a continuously from the front end side (front side in FIG. 6, left side in FIG. 7B) of the flat surface 31a. The semi-cylindrical surface 31d is continuous with the rear end side (the back side in FIG. 6 and the right side in FIG. 7B) of the flat surface 31a, and is formed with a diameter larger than the diameter of the rolling element return passage 32. Further, the upper surface 31e is formed to extend in parallel with the flat surface 31a from the upper end (upper side in FIG. 6) of the semi-cylindrical surface 31d to the inner peripheral surfaces on both sides of the ball screw groove 15a. A retaining recess 31f that is shallower than the semi-cylindrical surface 31d is formed in a direction along the ball screw groove 15a at a position close to the upper surface 31e of the semi-cylindrical surface 31d.

  Further, as shown in FIG. 6, the rolling element circulating member mounting portion 31B on the left end side is a notch recess opened on the other end surface in the axial direction of the ball screw nut 15, and is similar to the rolling element circulating member mounting portion 31A. A flat surface 31a, curved surfaces 31b and 31c, a semi-cylindrical surface 31d, and an upper surface 31e are provided. And the arrangement | positioning position of each site | part (the flat surface 31a, the curved surfaces 31b and 31c, the semi-cylindrical surface 31d, and the upper surface 31e) of the rolling element circulation member mounting part 31B seen from the left end side is the rolling element circulation member seen from the right end side. It is formed to be the same as the arrangement position of each part of the mounting portion 31A.

  Then, the rolling element circulating member 33 is mounted on a pair of rolling element circulating member mounting portions 31A and 31B provided at the left and right ends of the ball screw nut 15 in the axial direction, and the rolling element circulating member 33 shown in FIG. The tip position of the tongue 33e is set to be closer to the semi-cylindrical surface 31d than the boundary position between the flat surface 31a and the curved surfaces 31b and 31c of the rolling element circulation member mounting portions 31A and 31B. For this reason, the rolling element 14 scooped up by the tongue portion 33e is separated from the ball screw groove 15a and comes into contact with the flat surface 31a between the rolling element guide side plate portions 33a and 33b which is on the outer peripheral side from the bottom surface of the ball screw groove 15a. Thus, the tongue portion 33e can scoop up the rolling element 14.

As shown in FIGS. 5A and 5B, the rolling element circulating member 33 includes a pair of rolling element guide side plate portions 33a and 33b, and a connecting plate portion 33c that connects the rolling element guide side plate portions 33a and 33b. It has.
The side surface shape of the pair of rolling element guide side plate portions 33a and 33b (the shape seen from above in FIG. 5A) is a pair of rolling element circulation members formed as notched recesses on both end surfaces of the ball screw nut 15 in the axial direction. It is formed in substantially the same shape as the mounting portions 31A and 31B. Here, the rolling element guide side plate portion 33a is formed with a retaining protrusion 33d having the same shape as the retaining recess 31f of the pair of rolling element circulation member mounting portions 31A and 31B.

The connecting plate portion 33c connects the left side from the intermediate portion of the pair of rolling element guide side plate portions 33a and 33b when viewed from the right side of FIG. 5A, and forms the rolling element circulation member 33 in a gate shape.
The connecting plate portion 33c is provided with a tongue portion 33e that is inserted in a non-contact state into the ball screw groove 9a of the ball screw shaft 9 and scoops the rolling elements 14 in the tangential direction.
Then, it is surrounded by the tongue portion 33e and the rolling element guide side plates 33a and 33b, and is curved in a substantially inverted L shape when viewed from the plane guiding the rolling element 14 to the rolling element return passage 32, and the rolling element return passage 32 and A rolling element circulation path 33f that opens at an opposing position is formed.

Next, conditions necessary for determining a more specific shape of the rolling element circulation member 33 and determining a material will be described.
(Specifications of ball screw mechanism of rack assist type EPS)
The specifications of the ball screw mechanism 11 of the rack assist type EPS 10 (design requirements such as the durability of the ball screw, the outer diameter of the rack shaft, and the reduction ratio from the assist motor to the rack shaft) will be described.

In a standard passenger car, from the viewpoint of securing the bending strength of the rack shaft 7 and the meshing strength of the rack and pinion gear, the outer diameter of the rack shaft 7 is 25 mm or more and 32 mm or less, and the BCD ( Ball Circle Diameter) is approximately 25 mm or more and 32 mm or less, which is substantially the same as the outer diameter of the rack shaft 7.
As the rolling element 14, a rolling element having a diameter of φ3.968 mm to φ5.000 mm is used. If the diameter of the rolling element 14 is made smaller than φ3.968 mm, the rated load decreases. At the same time, the number of balls in the circulation path increases, and the rolling elements 14 compete with each other. As a result, durability is lowered and malfunctions are likely to occur. On the contrary, when the diameter of the rolling element 14 is larger than φ5.000 mm, it is necessary to increase the lead in addition to increasing the outer dimension of the ball screw mechanism 11, so that it becomes difficult to secure a reduction ratio described later.

  The lead is generally 6.0 mm or more and 8.0 mm or less. The reduction ratio from the assist motor to the rack shaft of a general electric power steering currently on the market is approximately 2.3 mm / rev (the amount of movement of the rack per motor rotation) and not more than 3.0 mm / rev. On the other hand, in order to ensure the same reduction ratio in the rack assist type EPS using the belt speed reducer of the present embodiment, the lead is set to 6.0 mm or more and 8.0 mm or less.

  The reason why the lead is set to 6.0 mm or more and 8.0 mm or less is that the reduction ratio of the belt speed reducer is designed to be approximately 3.0 or less. There are restrictions on the outer diameter of the drive pulley. When the driven pulley is enlarged, the outer dimension of the rack assist type EPS is also increased, and the vehicle mountability is lowered. On the other hand, if the drive pulley is made small, there is a problem that the number of meshing teeth is reduced and the durability is lowered. Therefore, the belt reduction ratio of the rack assist type EPS 10 is designed to be 3.0 or more. It becomes difficult.

Next, conditions for determining the shape and material of the rolling element circulation member 33 will be described.
(Conditions for collision between rolling elements and rolling element circulation member)
First, a description will be given of conditions under which the rolling element 14 and the rolling element circulating member 33 collide with each other in the above-described ball screw mechanism 11 when the driver steers.
In the description, specific specifications of the ball screw mechanism 11 are exemplified as follows.

<Specifications of ball screw mechanism>
BCD of rolling element 14: 30.0 mm
Diameter of rolling element 14: φ4.7625mm
Lead: 7.0
Contact angle: 45deg
Clearance: None

<Other conditions>
Rack & pinion specific stroke 50.0mm / rev (rack travel per handle rotation) Rack stroke 75.0mm (from neutral to stroke end)
Endurance cycle number 100,000cycle
Consider a case where steering is performed at a handle rotational speed of 360 deg / sec. When the rack and pinion ratio stroke is 50.0 mm / rev and the steering wheel is steered at 360 deg / sec, the rack shaft 7 moves straight at 50.0 mm / sec, and the ball screw nut 15 of the ball screw mechanism 11 is 7.14 rev. It rotates at / sec (about 430 rpm) (rack linear motion speed 50 mm / sec ÷ ball screw lead 7.0 mm / rev).

  In the above specifications, when the ball screw nut 15 rotates at 430 rpm, the revolution speed of the rolling element 14 becomes approximately 240 rpm. At this time, the rotation direction of the ball screw nut 15 and the revolution direction of the rolling element 14 are the same direction, and the relative speed of the ball screw nut 15 and the rolling element 14 is about 190 rpm. This is approximately 300 mm / sec when converted into the peripheral speed of the rolling element 14 in the BCD, and the rolling element 14 is guided to the rolling element circulation member 33 at this speed. In addition, as a durability test condition of the electric power steering, a turning speed of about 360 deg / sec is generally used, and when considering the fatigue strength of the rolling element circulation member 33, it is appropriate to calculate under this condition.

The number of repetitions when considering fatigue strength is determined from the rack stroke and the number of endurance cycles. The total number of rotations of the ball screw nut 15 is 4.3 × 10 ⁶ rev from the rack travel amount of 300 mm (75 mm × 4) per endurance cycle and the total travel amount of 30 km in 100,000 cycles. The number of collisions per rotation of the ball screw nut 15 is determined as about 8.4 times from the revolution of the rolling element 14 and the relative rotation of the ball screw nut 15.

The rolling element circulating member 33 scoops up the rolling element 14 only in one rotational direction, and at the time of reverse rotation, the rolling element 14 is guided from the rolling element circulating member 33 to the ball screw groove 15a. Therefore, the collision between the rolling element 14 and the rolling element circulating member 33 occurs when the total number of revolutions of the ball screw nut 15 is half of 4.3 × 10 ⁶ rev, and the number of collisions is about 1. It is found that it is 8 × 10 ⁷ times.
The earliest steering condition for a passenger car is generally about 800 deg / sec, which is a sudden steering operation when avoiding danger, and it is also required that the rolling element circulation member 33 is not damaged under this condition.

As mentioned above, although the calculation outline | summary in the item of the typical ball screw mechanism 11 was illustrated, when BCD of the rolling element 14 becomes large, the revolution peripheral speed of the rolling element 14 will become quick and a collision load will increase. Further, as the diameter of the rolling element 14 increases, the weight increases and the collision load increases. When the lead is reduced, the rotational speed of the ball screw nut 15 is increased and the total number of revolutions is increased, so that the collision load and the number of collisions are increased. In addition, the revolution speed of the rolling element 14 also changes due to changes in the contact angle and radial clearance.
Therefore, when determining the material of the rolling element circulation member 33 used in the ball screw mechanism 11 of the rack assist type EPS 10 of the present embodiment, the condition for the maximum collision energy is selected from the above range.

(Specific shape of rolling element circulation member)
Next, a specific shape of the rolling element circulation member 33 used in the ball screw mechanism 11 of the present embodiment will be described.
As shown in FIGS. 8A and 8B, the circular orbit shape of the rolling element circulation member 33 is Q when the scooping point on the spiral trajectory P of the ball screw mechanism 11 is Q. A straight line S extending from Q to the tangential direction of the spiral track P, a straight line T passing through the center of the rolling element return passage 32 provided in the ball screw nut 15, and a curve R connecting these straight lines S and T are constituted. . In order to smoothly circulate the rolling element 14 along this circulation path, the rolling element circulation member 33 is provided with a groove U having a groove width wider than the diameter of the rolling element 14.

The groove width of the groove U is preferably 115% or less of the diameter of the rolling element 14. More preferably, it is 110% or less. This is because when the groove width of the groove U exceeds 115%, the rolling elements 14 compete with each other and do not circulate smoothly.
The position of the straight line T is set by the positional relationship between the ball screw nut 15 and the ball screw groove 15a and the dimension of the curve R.

Further, the rolling element circulating member mounting portion 31A (31B) to which the rolling element circulating member 33 is mounted has a shape of a scooping point Q and the center of the opening of the rolling element return passage 32 provided in the ball screw nut 15. The distance L between them (see FIG. 7B) needs to be within a range that does not interfere with the groove bottom of the ball screw groove 15a.
7A, the contact surface Y of the rolling element circulation member mounting portion 31A with which the rolling element guide side plate portion 33b of the rolling element circulation member 33 contacts is adjacent to the adjacent ball screw groove 15a. Clearance CL is required. The end of the curve R connecting the above-described straight line S extending in the spiral tangential direction from the scooping point Q of the rolling element circulating member 33 and the straight line T passing through the center of the rolling element return passage 32 provided in the ball screw nut 15. Re is positioned below the contact surface Y.

If the dimension of the curve R is increased within the range of such restrictions, the position of the opening of the rolling element return passage 32 provided in the ball screw nut 15 is necessarily located on the outer peripheral side of the ball screw nut 15 ( 7 (b), the outer diameter of the ball screw nut 15 increases, and the inertia of the ball screw nut 15 increases.
Further, the radial mounting width V of the rolling element circulating member mounting portion 31 </ b> A shown in FIG. 7B varies depending on the strength of the rolling element circulating member 33. In order to secure the clearance CL described above, it is advantageous that the mounting width V is small. However, if the mounting width V is reduced, the rear surface of the tongue of the rolling element circulation member 33 becomes thin and the strength decreases. The width V is 150% to 200% of the diameter of the rolling element 14.

9 shows predetermined specifications of the ball screw mechanism 11 (BCD of the rolling element 14: 30.0 mm, diameter of the rolling element 14: φ4.7625 mm, lead: 7.0 mm), and the curve R shown in FIG. An example showing the inertia of the ball screw nut 15 and the result of converting the inertia into the mass of the rack shaft 7 when the dimensions (the ratio of the sphere diameter: the ratio of the radius of the curve R to the diameter of the rolling element 14) are changed. It is.
Since the axial dimension of the ball screw nut 15 differs depending on the number of turns of the ball screw, the absolute value fluctuates somewhat, but the correlation between the dimension of the curve R and the outer diameter of the ball screw nut 15 is constant. It can be seen that the nut inertia increases abruptly when the dimension of R is 90% or more of the spherical diameter ratio.

  Here, the outer diameter of the ball screw nut 15 will be specifically described. Since the diameter of the rolling element 14 is φ4.7625 mm with respect to BCD: 30.0 mm, the outermost diameter of the rolling element 14 rolling on the spiral trajectory is φ34.7625 mm. If the right angle shape of the ball screw groove 15a of the ball screw nut 15 is a Gothic groove having a contact angle of 45 ° and a groove curvature radius of 2.7 mm, the groove bottom diameter of the ball screw nut 15 is approximately φ35 mm. The hole diameter of the rolling element return passage 32 provided in the ball screw nut 15 is φ5.2 mm based on the same concept as the groove width of the groove U, and the minimum diameter between the hole diameter of the rolling element return passage 32 and the ball screw groove 15a. If the wall thickness is secured about 1 mm, the outer diameter of the ball screw nut 15 of the rolling element return passage 32 is required to be at least 50 mm.

Further, as in this embodiment, when the inner ring 17a of the rolling bearing 17 that supports the ball screw nut 15 is formed integrally with the outer diameter of the ball screw nut 15, or a power transmission structure such as a pulley is provided outside the ball screw nut 15. In the case of molding on the radial side, the outer diameter of the ball screw nut 15 is further increased. In addition, for example, as disclosed in Japanese Patent Application Laid-Open No. 2004-92855 or Japanese Patent Application Laid-Open No. 2006-125544, the radius of curvature of the curve R is set to be equal to or larger than the diameter of the rolling element 14 to smoothly move the rolling element 14, thereby improving operability. However, there is a prior art that increases inertia as shown in FIG. In the case of a ball screw for EPS, the specifications and environment of the ball screw mechanism 11 are limited as described above, and the ball screw is reduced in size without impairing operability by designing a circulation path suitable for it. This makes it possible to reduce the inertia.
Therefore, in this embodiment, it is preferable that the radius of the curve R is set to 50 to 90% of the diameter of the rolling element 14.

Next, the outer shape of the rolling element circulation member 33 that avoids interference with the ball screw shaft 9 will be described with reference to FIG.
The outer shape of the rolling element circulation member 33 is basically a shape that substantially follows the cross-sectional shape of the ball screw groove 9 a of the ball screw shaft 9.
The outer peripheral portion Rn of the rolling element circulating member 33 (the portion facing the ball screw shaft 9 of the rolling element guide side plate portions 33a and 33b) is matched with the inner diameter dimension of the ball screw nut 15 to the land portion of the ball screw shaft 9. On the other hand, it is formed so as to face a predetermined clearance.

  Of the contours of the tongue portion 33e of the rolling element circulation member 33, if the sectional view shape in the direction along the ball screw groove 9a is Rs and the sectional view shape perpendicular to the ball screw groove 9a is Rt, Rs and Rt are The clearance between the rear surface of the tongue portion 33e and the bottom of the ball screw groove 9a of the ball screw shaft 9 is 2% or more and 10% or less of the diameter of the rolling member 14 formed along the BCD of the rolling member 14. Is preferred.

This is because if the clearance between the back surface of the tongue portion 33e and the bottom portion of the ball screw groove 9a is less than 2%, the thickness of the tip of the tongue portion 33e becomes thin, and the rolling element circulation member 33 is likely to be damaged. . Further, if the clearance between the back surface of the tongue portion 33e and the bottom portion of the ball screw groove 9a exceeds 10%, the scooping point is shifted and the rolling element 14 is not smoothly circulated.
It should be noted that Rn and Rt are smoothly connected by a connecting portion Rf, and the connecting portion Rf is preferably set large so long as it does not interfere with the ball screw shaft 9. In FIG. 10, the shape of the back surface portion Rt is a single arc, but it may be Gothic or the like following the ball screw groove 9 a.

  When the rolling element 14 collides with the rolling element circulating member 33, as shown in FIGS. 5 and 8, the tip of the tongue portion 33 e of the rolling element circulating member 33 (the circled area indicated by the symbol A in FIG. 5A). In addition, the stress due to the collision increases at the base of the tongue portion 33e of the rolling element circulation member 33 (the circled region indicated by the symbol B in FIGS. 5A and 5B), and the rolling direction of the rolling element 14 changes. A load is also applied to the portion facing the moving body circulation path 33f (the circled region indicated by the symbol C in FIG. 5B).

(Material for rolling element circulation member)
Next, specific materials for the rolling element circulation member 33 used in the ball screw mechanism 11 of the present embodiment will be described.
The rolling element circulation member 33 of the present embodiment is formed of a polymer material having a tensile strength of 30 MPa or more at an atmospheric temperature of high temperature (125 ° C.). The value of the tensile strength of 30 MPa or more is a value obtained as a result of FEM structural analysis when the collision energy between the rolling element 14 and the rolling element circulation member 33 is maximized under the above-described conditions.

Further, the rolling element circulating member 33 of the present embodiment is formed of a polymer material having a Charpy impact strength of 3.3 kJ / m ² or more. The value of the Charpy impact strength of 3.3 kJ / m ² or more is also the result of FEM structural analysis in the case where the collision energy between the rolling element 14 and the rolling element circulation member 33 is maximized under the above-described conditions. This is the value obtained.

Moreover, the rolling element circulating member 33 of the present embodiment is formed of a polymer material that does not include reinforcing fibers such as glass fibers and carbon fibers. When the rolling element circulating member is made of a fiber reinforced polymer material, the fiber filled inside is exposed by continuous collision with the rolling element 14, and the damaged or dropped fibers are damaged by the ball screw mechanism 11. However, the rolling element circulation member 33 of the present embodiment that does not include reinforcing fibers such as glass fiber and carbon fiber prevents abnormal noise and malfunction of the ball screw mechanism 11. Can be planned.
In addition, a polymer material that does not contain reinforcing fibers, has a tensile strength of 30 MPa or more in an atmosphere of high temperature (125 ° C.), and has a characteristic of Charpy impact strength of 3.3 kJ / m ² or more is a thermal stabilizer. When added, heat resistance can be improved.

  Examples of the heat stabilizer include a copper heat stabilizer, a phenol heat stabilizer, a phosphorus heat stabilizer, a sulfur heat stabilizer, and an amine heat stabilizer alone or a mixture thereof. Can be mentioned. Copper heat stabilizers include inorganic substances such as copper chloride, copper bromide, copper iodide, copper sulfate, and copper phosphate, or organic substances such as copper acetate, copper stearate, copper myristate, copper naphthenate, and copper palmitate. Acid salts, as well as mixtures of two or more of these, and phenolic heat stabilizers include N, N′-hexamethylenebis (3,5-di-tert-butyl-4-hydroxy-hydrocinnamamide) 1,3,5-tris (4-t-butyl-3-hydroxy-2,6-dimethylbenzyl) isocyanate, triethylene glycol-bis [3- (3-t-butyl-5-methyl-4) -Hydroxyphenyl) propionate], 1-oxy-3-methyl-4-isopropylbenzene, or 2,6-di-t-butyl-4-ethylphenol. As the phosphorus-based heat stabilizer, diphenylisodecylphos Fah Ite, diphenylisooctyl phosphite, triphenyl phosphite, triisodecyl phosphite, tris (2,4-di-t-butylphenyl) phosphite, tetraphenyltetra (tridisyl) pentaerythritol tetraphosphite, cyclic neopentane There are phosphite compounds such as tetraylbis (2,4-di-t-butylphenyl phosphite) and distearyl pentaerythritol diphosphite, and disulfuryl thiodipropionate is used as a sulfur-based heat stabilizer. , Distearyl thiodipropionate, dimyristyl-3, 3′-thiodipropionate, ditridecyl-3, 3-thiodipropionate, or dilauryl thiodipropionate. Al, such as 1,4-dioctyldiphenylamine Le substituted diphenylamine or N, it can be exemplified p- phenylenediamine with a substituent such as N'- di-2-naphthyl -p- phenylenediamine.

Furthermore, you may mix | blend various additives in the range which does not impair the objective of this invention. For example, solid lubricants such as graphite, hexagonal boron nitride, fluorine mica, tetrafluoroethylene resin powder, tungsten disulfide, molybdenum disulfide, inorganic powder, organic powder, lubricating oil, plasticizer, rubber, resin, UV absorption Examples include agents, flame retardants, antistatic agents, pigments, and dyes.
And the method for obtaining the resin composition used when forming the rolling element circulation member 33 of the present embodiment is not particularly limited, but the base resin and additive are preliminarily tumbler, V blender, Henschel mixer, etc. Are mixed using a premixing machine, and then uniformly using a conventional kneading apparatus for resin such as a short or twin screw extruder, roll, pressure kneader, Banbury mixer, Brabender plastograph, etc. It is possible to knead, or to separately knead the raw material resin and additive which are base materials with a resin kneading apparatus.

  The kneading conditions are usually at a temperature above the softening point or melting point of the raw material resin, which is the base material, and below the temperature at which the raw material resin and various additives do not deteriorate. Moreover, the manufacturing method for making the said resin composition into a circulation component should just heat a resin composition, pressurizing in a metal mold | die, by well-known resin molding methods, such as compression molding, transfer molding, and injection molding. Can be manufactured.

(Function and effect)
Next, the effect of the rack assist type EPS 10 provided with the ball screw mechanism 11 of the present embodiment will be described below.

  As shown in FIGS. 8A and 8B, the circulation track of the rolling element circulation member 33 includes a straight line S extending from the scooping point Q on the spiral track P in the tangential direction of the spiral track P, and a ball screw. A straight line T passing through the center of the rolling element return passage 32 provided in the nut 15 and a curve R connecting the straight lines S and T are configured. The radius of the curve R is 50% of the diameter of the rolling element 14. It is set to 90% or less. Thus, by setting the radius of the curve R to 50% to 90% of the diameter of the rolling element 14, the nut inertia does not increase as shown in FIG. Since the applied load is reduced and the steering feeling is improved, the rack assist type EPS 10 provided with the ball screw mechanism 11 can provide a highly reliable device in which abnormal noise and malfunction are prevented. That is, when the rotation of the ball screw nut 15 stops momentarily by hitting the stroke end or hitting the curbstone, an impact load (end contact load) is applied to the ball screw mechanism 11 according to the inertia of the ball screw nut 15. Will occur. In general, the greater the inertia of the steering system, the poorer the response, and the worse the steering feeling. However, if the nut inertia does not increase as in the present embodiment, even if the end deflector-type ball screw mechanism 11 tends to have an increased outer diameter, the end contact load is reduced, and the steering feeling is reduced. Is improved.

  In the present embodiment, the BCD of the rolling element 14 of the ball screw mechanism 11 is set to φ25 mm or more and φ32 mm or less, so that the bending strength of the rack shaft 7 and the meshing strength of the rack and pinion gear can be ensured. Further, by setting the rolling element 14 of the ball screw mechanism 11 to φ3.968 mm or more and φ5.000 mm or less, durability of the rolling element 14 can be improved and a reduction ratio can be ensured. Furthermore, by setting the lead of the ball screw mechanism 11 to 6.0 mm or more and 8.0 mm or less, it is possible to secure a reduction ratio from the assist motor to the rack shaft of a general electric power steering currently on the market.

(Test results)
Next, the present invention will be described more specifically with reference to FIGS.
In FIG. 11, the rolling element circulation member 33 is formed of (A) PA46, (B) PPS, (C) POM, and (D) POM, and (A) 125 ° C, (B) 125 ° C, and (C) 80 ° C. (D) It is the result of having performed the durability test in the atmosphere of 125 degreeC. The durability test of the rolling element circulating member 33 refers to a test for observing the stroking durability of the ball screw mechanism 11 using the rolling element circulating member 33, and the test condition is generally an electric power steering as described above. As required by the equipment.

FIG. 12 shows the same durability test results as FIG. In FIG. 12, the rolling element circulation member 33 was molded with (A) POM, (B) PA 46, and (C) PPS, and the test was performed in an atmosphere at 125 ° C. Note that the rolling element circulating member 33 shown in FIG. 12 has a rear surface of the tongue portion 33e that faces the ball screw shaft 9 due to misalignment and dimensional variations among the members such as the ball screw shaft 9, the ball screw nut 15, and the rolling element circulating member 33. The ball screw groove 9a was operated in a state of interference.
The details of the test material are as follows.

[Test material]
POM (Polyacetal) (trade name “Duracon M90-44” Polyplastics Co., Ltd.)
PBT (Polybutylene terephthalate) (Brand name "Duranex 2002" Polyplastics Co., Ltd.)
PA6 (Polyamide 6) (trade name “Amilan CM1017” Toray Industries, Inc.)
PA46 (Polyamide 46) (trade name "Stanyl TW341" DSM Japan Engineering Plastics Co., Ltd.)
PA66 (polyamide 66) (trade name “Amilan CM3001-N” Toray Industries, Inc.)
PPS (Polyphenylene Sulfide) (trade name "Fortron 0220A9" Polyplastics Co., Ltd.)

[Test material molding conditions]
POM (resin melting temperature: 190-210 ° C, mold temperature: 60-80 ° C)
PBT (resin melting temperature: 250-270 ° C., mold temperature: 40-80 ° C.)
PA6 (resin melting temperature: 245-280 ° C, mold temperature: 80 ° C)
PA46 (resin melting temperature: 280-310 ° C, mold temperature: 80 ° C)
PA66 (resin melting temperature: 270-295 ° C, mold temperature: 80 ° C)
PPS (resin melting temperature: 290-310 ° C, mold temperature: 150 ° C)

  In the endurance test of the rolling element circulating member 33 in FIG. 11, in the (A) PA 46 and (B) PPS having an atmospheric temperature of 125 ° C. and (C) POM having an atmospheric temperature of 80 ° C., the rolling element circulating member 33 is damaged. unacceptable. On the other hand, in the (D) POM having an atmospheric temperature of 125 ° C., the rolling element circulating member 33 is damaged, and the damaged portion is also noticeably at the tip portion and the root portion of the tongue portion 33e. This is consistent with the FEM structural analysis result described above.

From the above, the rolling element circulation member 33 in the ball screw mechanism 11 used in the rack assist type EPS 10 is formed of PA46 or PPS when used in an atmospheric temperature of 125 ° C., and is in an atmospheric temperature of 80 ° C. It is understood that it is necessary to be molded with POM.
The durability test of the rolling element circulating member 33 in FIG. 12 is performed in a state where the back surface of the tongue portion 33e of the rolling element circulating member 33 and the gothic groove of the ball screw shaft 9 are in contact with each other. As a result, in (A) POM and (B) PA 46 having an atmospheric temperature of 125 ° C., wear on the back surface of the tongue portion 33e was observed.

  On the other hand, in (C) PPS having an atmospheric temperature of 125 ° C., chipping was observed on the back surface of the tongue portion 33e. In the case of a non-fiber reinforced polymer material, such a chipping does not greatly impair the function, but it is preferable to avoid such chipping.

From the above, it can be seen that the rolling element circulating member 33 in the ball screw mechanism 11 used in the rack assist type EPS 10 needs to be molded with PA 46 when used in an atmospheric temperature of 125 ° C.
The screw shaft of the present invention corresponds to the ball screw shaft 9, the screw groove formed on the outer peripheral surface of the screw shaft of the present invention corresponds to the ball screw groove 9 a, and the nut of the present invention corresponds to the ball screw nut 15. The thread groove formed on the inner peripheral surface of the nut corresponds to the ball screw groove 15a, the rolling element of the present invention corresponds to the rolling element 14, and the first straight line of the present invention corresponds to the straight line S. The straight line 2 corresponds to the straight line T, and the polyamide 46 of the present invention corresponds to the PA 46.

  The material of the present invention can be applied not only to the end deflector type ball screw mechanism 11 as in the present embodiment, but also to a rolling element circulation member having a tongue portion such as an end cap type or a tube type. However, it is preferable that the rolling element circulation member be an end deflector when used in an electric power steering device as a configuration capable of ensuring sufficient moment rigidity regardless of the phase of the ball screw nut.

  DESCRIPTION OF SYMBOLS 1 ... Steering gear mechanism, 2 ... Pinion mechanism, 3 ... Rack mechanism, 4 ... Pinion housing, 5 ... Pinion shaft, 6 ... Rack housing, 7 ... Rack shaft, 8 ... Rack part, 9 ... Ball screw shaft, 9a ... Ball Screw groove, 10 ... Electric power steering device, 11 ... Ball screw mechanism, 12 ... Electric motor for assisting steering, 12a ... Rotating shaft, 13 ... Reduction mechanism, 14 ... Rolling element, 15 ... Ball screw nut, 15a ... Ball screw Groove, 16 ... bearing housing part, 17 ... rolling bearing, 17a ... inner ring, 17b ... ball, 17c ... outer ring, 18 ... motor support, 18a ... through hole, 19 ... small diameter pulley, 20 ... large diameter pulley, 21 ... timing Belt, 22 ... cover, 23 ... tie rod, 30 ... motor control device, 30a ... torque sensor, 30b ... vehicle speed sensor, 31A ... rolling element circulating member mounting portion, 31B Rolling member circulation member mounting portion, 31a ... flat surface, 31b, 31c ... curved surface, 31d ... semi-cylindrical surface, 31e ... upper surface, 31f ... retaining recess, 32 ... rolling member return passage, 33 ... rolling member circulation member, 33a 33b ... rolling element guide side plate part, 33c ... coupling plate part, 33d ... retaining projection, 33e ... tongue part, 33f ... rolling element circulation path

Claims (2)

A screw shaft having a thread groove on the outer peripheral surface;
A screw groove corresponding to the screw groove of the screw shaft is formed on the inner peripheral surface, and a nut that forms a rolling element return passage penetrating in the axial direction;
A number of rolling elements interposed in the spiral raceway formed by the thread groove of the screw shaft and the thread groove of the nut and the rolling element return passage;
A connecting plate portion connecting the pair of rolling element guide side plate portions and the pair of rolling element guide side plate portions; and inserting the rolling element into the connecting plate portion in a non-contact state in the screw groove of the screw shaft. A scooping tongue is formed, and the rolling element is scooped up from one end of the spiral track in the tangential direction of the spiral track and guided to the rolling member return passage, and the other end of the spiral track from the rolling member return passage. A ball screw mechanism having a rolling element circulation member that forms a rolling element circulation track that guides to
In the electric power steering device in which the screw shaft of the ball screw mechanism is connected to a rack shaft that is screwed to a pinion shaft constituting the steering mechanism, and the nut is driven to rotate by an electric motor.
The rolling element circulation trajectory is:
A first straight line extending in a tangential direction of the spiral orbit from a scooping point on the spiral orbit;
A second straight line passing through the center of the rolling element return passage;
It is comprised with the curve which connects these 1st and 2nd straight lines,
The radius of the curve is set to 50% or more and 90% or less of the diameter of the rolling element,
The rolling element circulating member is made of polyamide 46 when used in an atmospheric temperature of 125 ° C.   2. The BCD (Ball Circle Diameter) is set to φ25 mm to φ32 mm, the diameter of the rolling element is set to φ3.968 mm to φ5.000 mm, and the lead is set to 6.0 mm or more and 8.0 mm or less. Electric power steering device.

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