JP2012228953A - Electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric power steering device capable of securing durability, without enlarging an outer diameter of a circulating part of a ball screw.SOLUTION: This electric power steering device includes an electric motor, a rack shaft 7 connected between a steering mechanism and a tie rod, and a power transmission mechanism for transmitting power from the electric motor to the rack shaft 7. The power transmission mechanism is constituted of: a screw shaft 9 connected to or integrated with the rack shaft 7 and having a male screw groove; and a ball screw mechanism having a nut arranged around the screw shaft and having a female screw groove, and a plurality of rolling elements rollable between the male screw groove and the female screw groove. In at least one in a stroke end of the rack shaft 7, a load bearing power reducing area LA of the nut is set to be positioned in an area except for a phase position φp and φp' in the rack shaft circumferential direction defined by a plane formed of the rack shaft 7 and the tie rod connected to the rack shaft.

Description

  The present invention relates to an electric power steering device mounted on a rack shaft of a steering gear mechanism.

In a rack and pinion type steering device for a vehicle, a steering rotational force when a steering wheel is steered is transmitted to a pinion shaft, converted into a linear motion by a rack shaft screwed to the pinion shaft, and connected to both ends of the rack shaft. Is transmitted to the steered wheels via the tie rods, and the steered wheels are steered.
Generally, since the tie rod is attached at an angle with respect to the rack shaft, a radial load and a moment load are applied to the rack shaft in addition to the axial load as a steering reaction force.

  A ball screw groove is formed in the rack shaft in series with the rack portion, a ball screw nut is screwed into the ball screw groove via a ball, and the ball screw nut is rotated by an electric motor. In the case of a power steering device, since the radial load and moment load described above are applied to the ball screw, the reduction in efficiency and durability becomes a problem.

  In order to solve this problem, conventionally, a rotating cylinder supported by a rack housing is screwed through a rolling element to a screw shaft integrated with a rack that meshes with a pinion that rotates by steering, and the rotating cylinder is driven by a motor. The entire rolling element is disposed between a rack support member that supports the rack at a position where the rack is engaged with the pinion and a support body that is disposed in the moving range of the screw shaft and can support the outer periphery of the screw shaft, and the screw The opening edge of the raceway groove is chamfered to prevent wear of the support due to contact with the opening edge of the spiral raceway groove on the outer periphery of the shaft and to prevent a reduction in transmission efficiency due to rattling of the screw shaft. And the load acting on the rack from the road surface side is received by the support and the rack support member. Steering mechanism of a vehicle as is located (for example, see Patent Document 1).

  As another conventional example, in a ball screw drive device for a spindle, a spindle nut is disposed on the inner peripheral surface along a spiral, and a ball groove defining a load-bearing channel for the ball and the spindle An external deflector with a return channel for the ball, disposed on the outer peripheral surface of the nut, the external deflector having two ball openings, and the two ball openings Each is connected to one end of the load bearing channel, deflects the ball from one end of the load bearing channel to the other end of the load bearing channel, and connects the external deflector to the two ball openings. Extends from one side to the other of the two ball openings over at least one complete turn around the axis of rotation of the spindle nut. Ball screw drive has been proposed a (e.g., see Patent Document 2).

Japanese Patent No. 37817648 JP 2005-326209 A

However, in the conventional example described in Patent Document 1, a loss due to friction between the outer periphery of the ball screw and the support occurs, and the wear of the support is caused by sliding with the uneven screw outer periphery. Occurs, and there is an unresolved problem that it leads to functional loss of the ball screw.
Further, in the conventional example described in Patent Document 2, the load capacity fluctuation due to the circulating portion of the ball screw can be eliminated, but the outer diameter is increased because the ball is circulated substantially once around the nut. There are unresolved issues.
Accordingly, the present invention has been made paying attention to the unsolved problems of the above-described conventional example, and an electric power steering device that can ensure durability without increasing the outer diameter of the circulating portion of the ball screw. It is intended to provide.

  In order to solve the above-described object, an electric power steering apparatus according to an aspect of the present invention transmits a rack shaft connected between a steering mechanism and a tie rod, and power from the electric motor to the rack shaft. A power transmission mechanism, wherein the power transmission mechanism is connected to or integrated with the rack shaft and has a male screw groove, and is arranged around the screw shaft and has a female screw groove. An electric power steering device comprising a nut and a ball screw mechanism having a plurality of rolling elements capable of rolling in a rolling path formed between the male screw groove and the female screw groove, the stroke of the rack shaft A phase in the circumferential direction of the rack axis defined by a plane formed by the rack shaft and a tie rod connected to the rack shaft, in which at least one of the ends has a load bearing force reduction region of the nut. It is characterized in that set to be located outside the region.

Moreover, the electric power steering apparatus according to another aspect of the present invention is characterized in that the load supporting force reduction region is a region in which the number of balls in the load sphere receiving the load is minimized.
Furthermore, in the electric power steering device according to another aspect of the present invention, the load supporting force reduction region has a minimum number of balls in a load zone that receives the radial load with respect to the radial load input to the ball screw. It is the area which becomes.
Further, in the electric power steering device according to another aspect of the present invention, the load supporting force reduction region has a minimum number of balls in a load zone that receives the moment load with respect to the moment load input to the ball screw. It is the area which becomes.

  According to the present invention, the power transmission mechanism includes a screw shaft that is connected to or integrated with the rack shaft and includes a male screw groove, and a ball screw nut that is screwed to the screw shaft via a rolling element. In at least one of the stroke ends of the rack shaft, the load supporting force reduction region of the nut is located in a region other than the phase in the rack shaft circumferential direction defined by the plane formed by the rack shaft and the tie rod connected to the rack shaft. Therefore, it is possible to reduce the contact surface pressure generated in the ball screw and to increase the life of the ball screw.

1 is a front view showing a first embodiment of an electric power steering apparatus according to the present invention. It is a front view which shows a rack axis | shaft. It is an expanded sectional view showing the important section of an electric power steering device. It is explanatory drawing which shows the steering reaction force of the rack side stroke end. It is explanatory drawing which shows the steering reaction force of a ball screw side stroke end. It is explanatory drawing of the state which has the load support force reduction | decrease area | region at the time of radial load input in the case of employ | adopting the top method as a circulation system of a ball screw nut in a load action phase position. It is explanatory drawing of the state which has the load support force reduction | decrease area | region at the time of the moment load input in the case of employ | adopting the top method as a circulation system of a ball screw nut in a load action phase position. It is explanatory drawing of the state in which the load supporting force reduction | decrease area | region at the time of radial load input and moment load input exists outside a load action phase position at the time of employ | adopting a top method as a circulation system of a ball screw nut. It is explanatory drawing of the state which has the load supporting force reduction | decrease area | region at the time of radial load input in the case of employ | adopting a tube system as a circulation system of a ball screw nut. It is explanatory drawing of the state which has the load supporting force reduction | decrease area | region at the time of the moment load input in the case of employ | adopting a tube system as a circulation system of a ball screw nut in a load action phase position. It is explanatory drawing of the state in which the load supporting force reduction | decrease area | region at the time of radial load input and moment load input exists outside a load action phase position at the time of adopting a tube system as a circulation system of a ball screw nut. It is explanatory drawing of the state which has the load supporting force reduction | decrease area | region at the time of radial load input in the case of employ | adopting an end deflector system as a circulation system of a ball screw nut in a load action phase position. It is explanatory drawing of the state which has the load supporting force reduction | decrease area | region at the time of the moment load input in the case of employ | adopting an end deflector system as a circulation system of a ball screw nut in a load action phase position. It is explanatory drawing of the state in which the load supporting force reduction | decrease area | region at the time of radial load input and moment load input is outside a load action phase position at the time of employ | adopting an end deflector system as a circulation system of a ball screw nut.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a front view showing a main part of an electric power steering apparatus according to the present invention, and FIG. 2 is a front view showing a rack shaft.
In the figure, reference numeral 1 denotes a steering gear mechanism. The steering gear mechanism 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 the pinion housing 4, and the rack mechanism 3 includes a rack shaft 7 that is slidably supported by the 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 a linear motion of the rack shaft 7 by the pinion mechanism 2.
As shown in FIG. 2, the rack shaft 7 includes a rack portion 8 with which the pinion shaft 5 is engaged, and a ball screw portion 9 having a ball screw groove 9a integrally formed on the right side of the rack portion 8. ing.

An electric power steering device 10 is disposed on the right end side of the rack shaft 7. The electric power steering apparatus 10 includes a ball screw mechanism 11, a steering assist electric motor 12, and a speed reduction mechanism 13 that connects the ball screw mechanism 11 and the electric motor 12.
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 rack shaft 7 via a ball 14 as a rolling element. 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 17 a formed integrally with the ball screw nut 15 and an outer ring 17 c connected to the inner ring 17 a via a ball 17 b and fixedly supported by the bearing housing 16.

Further, the steering assist 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 supported by the motor. It protrudes to the opposite side through a through hole 18 a formed in the portion 18.
Furthermore, the speed reduction mechanism 13 can rotate integrally with the ball screw nut 15 radially outward of the small-diameter pulley 19 attached to the tip of the rotating shaft 12a of the steering assist electric motor 12 and the ball screw nut 15 described above. A supported 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 bolted to the bearing accommodating part 16 and the motor support part 18, for example so that the small diameter pulley 19, the large diameter pulley 20, and the ball screw part 9 may be covered.
In this electric power steering apparatus 10, the small-diameter pulley 19 and the large-diameter pulley 20 are respectively driven by the timing belt 21 by rotationally driving the rotating shaft 12 a of the steering assisting electric motor 12 clockwise as viewed from the left in FIG. 3. Since the ball screw nut 15 is driven to rotate clockwise in response to the clockwise rotation, the ball screw portion 9, that is, the rack shaft 7 is moved to the left, and conversely the rotation of the steering assist electric motor 12. The rack shaft 7 is moved to the right by rotationally driving the shaft 12a counterclockwise when viewed from the left.

Then, the left end portion of the rack portion 8 of the rack shaft 7 and the right end portion of the ball screw portion 9 are respectively connected to the steered wheels via tie rods 30 as shown by chain lines in FIG. 1, and these steered wheels are steered more than necessary. An end stopper (not shown) is provided to prevent cornering.
Therefore, as described above, by rotating the rotating shaft 12a of the steering assisting electric motor 12 in the clockwise direction when viewed from the left, the rack shaft 7 is moved to the rack side stroke end as shown in FIG. Consider the case of moving to. At this time, the steering reaction force input from the steered wheels (not shown) is generated on the plane formed by the tie rod 30 and the rack shaft 7. Here, on the left end side of the rack portion 8, a compression reaction force F <b> 1 is generated to steer while pushing the steered wheel, and on the right end side of the ball screw groove 9, a tensile reaction force F <b> 2 to steer while pulling the steered wheel and Become.

  At the rack side stroke end shown in FIG. 4, the length between the ball screw portion 9 and the load point of the tensile reaction force F2 is the shortest. The moment load and radial load input as the component force of the compression reaction force F1 are supported by the engagement of the pinion mechanism 2 and the rack mechanism 3, and therefore the moment load and radial load input to the ball screw portion 9 are supported. Is dominated by the tensile reaction force F2.

On the contrary, by rotating the rotating shaft 12a of the steering assisting electric motor 12 counterclockwise when viewed from the left, the rack shaft 7 is moved rightward to the ball screw side stroke end as shown in FIG. Consider the case. Also at this time, the steering reaction force input from the steered wheels (not shown) is generated on the plane formed by the tie rod 30 and the rack shaft 7. Here, a tensile reaction force F1 for turning while pulling the steered wheel is generated on the left end side of the rack portion 8, and a compression reaction force F2 for turning while pushing the steered wheel on the right end side of the ball screw portion 9. Become.
Therefore, the direction of the steering reaction force generated in the left turning and the right turning is in the reverse direction. The steering reaction force increases at the rack side stroke end and the ball screw side stroke end, and the tie rod angle increases. Therefore, the radial load input to the ball screw portion 9 as the component force becomes maximum at both stroke ends.

On the other hand, the moment load is input to the ball screw portion 9 when the length of the ball screw portion 9 and the load point of the steering reaction force F2 becomes maximum at the ball screw portion side stroke end shown in FIG. The moment load is maximized.
On the other hand, in the region where the ball circulation path of the ball screw mechanism 11 is formed, it becomes a load supporting force decreasing region LA in which the number of balls supporting the load is reduced. For this reason, the load bearing force reduction area LA has the rack shaft 3 and the tie rod on which the road surface reaction forces F1 and F2 act as shown in FIGS. 8A to 8D at the rack shaft side stroke end and the ball screw side stroke end. The ball screw nut 15 so that a region other than the load bearing force reduction region LA exists at the load action phase position Pp so as not to be located at the load action phase position φp in the rack shaft circumferential direction defined by a plane formed by 30 Adjust the circumferential phase position of.

  That is, in the ball screw nut 15 adopting the above-described ball screw mechanism 11 as a circulation method of the ball screw nut 15 and adopting a top method (deflector method) in which one ball circulation portion is present in one winding, the number of balls to be arranged is In the case of four rows, as shown in FIG. 6 (a), when the ball circulation path is sequentially made BR1, BR2, BR3 and BR4 from the left side, the ball circulation path BR1 is about 135 as seen in FIG. 6 (b). The ball circulation path BR2 exists in the range of about 225 ° to about 315 °, the ball circulation path BR3 exists in the range of about 315 to about 45 °, It is assumed that the circulation path BR4 exists in the range of about 45 ° to about 135 °. In this case, when a radial load Fr is applied to the ball screw portion 9 from below to above, the load application phase position φp at which the radial load Fr is applied is, as shown in FIG. The position is about 90 ° passing through the center of 9. For this reason, the number of balls existing at this load action phase position φp is three rows surrounded by the upper rectangular frame shown in FIG. 6A, and the balls supporting the load compared to the entire 4.5 rows. The number is minimized.

  Similarly, when the ball screw side stroke end is reached and the steering reaction forces F1 and F2 in FIG. 5 described above are applied to the ball screw portion 9 as shown in FIG. 7A, the load phase position is as shown in FIG. As shown in FIG. 7B, the load phase position φp is the same as that in the case of the radial load Fr described above on the right end side, but the load phase position φp ′ is 180 ° different on the left end side. For this reason, the number of balls receiving the load at the load phase position φp is two rows surrounded by an upper rectangular frame as shown in FIG. 7A, and the number of balls receiving the load at the load phase position φp ′ is lower. There is only one row surrounded by a square frame.

  However, as shown in FIGS. 8A to 8D, the load acting phase is decreased by moving the load supporting force decreasing region LA in the circumferential direction, for example, about 135 ° clockwise as viewed in FIG. 6B, for example. As shown in FIG. 8 (e), when the area other than the load supporting force reduction area LA is removed from the positions φp and φp ′ and the area other than the load supporting force reduction area LA is present at the load action phase positions φp and φp ′, The four rows of balls receive the radial load Fr, and the contact surface pressure generated in the ball screw mechanism 11 decreases. Similarly, for the moment loads F1 and F2, as shown in FIG. 8 (f), at the left end side, two rows surrounded by a left-side square frame on the lower side exist at the load action phase position φp ′ of the moment load F1. In the right end side, the two rows surrounded by the right-side rectangular frame on the upper side are present at the load action phase position φp of the moment load F2 and receive the load. The generated contact surface pressure can be reduced.

Therefore, the circumferential direction phase control of the ball screw groove 9 is performed so that the load bearing force reduction region LA is deviated from the load action phase positions φp and φp ′ of the radial load Fr and the moment loads F1 and F2 at the stroke end.
For this purpose, three processing methods can be applied, in which processing is performed based on the reference surface, processing is performed based on the thread groove, and processing is performed based on the rack portion.

(When machining based on the reference surface)
A reference surface (for example, a notch, a protrusion, a hole, or the like at the shaft end) is provided on a blank in which the rack portion 8 and the ball screw groove 9 are not formed, and the thread groove is processed based on the reference surface. After the thread groove machining step, similarly, the rack portion 8 is machined after phase-shifting the reference surface. At this time, whichever of the machining orders may be performed first. Here, a processing method such as rolling, cutting, and grinding can be applied to the formation of the thread groove, and a processing direction such as forging, cutting, and grinding can be applied to the formation of the rack portion 8.

(When machining based on thread groove)
First, the groove is machined on the blank, and the rack portion 8 is machined based on the thread groove. For example, the shape of the thread groove is measured and managed by a non-contact measuring machine, or the groove is fitted to the groove. A method of clamping the screw portion 8 with a clamp jig having a protrusion is conceivable. In addition, the processing method of a thread groove and the processing method of the rack part 8 are the same as the case where it processes on the basis of a reference plane.

(When processing with the rack as a reference)
The rack portion 8 is first processed on the blank, and the thread groove is processed based on the rack portion 8 (for example, a rack tooth tip surface).
In addition, when the rack part 8 is processed first, the distortion accompanying the processing of the rack part 8 may affect the accuracy of the ball screw groove 9, and the screw shaft part of the screw shaft part is compared with the case where the screw groove is processed first. The accuracy is inferior. Therefore, when accuracy is required, it is preferable to process the thread groove first.

  As described above, in the first embodiment, the top method is adopted as the circulation method of the ball screw nut 15, and when there is one ball circulation path per roll, the load where the ball circulation path exists is supported. Since the load bearing force reduction area LA with a small number of balls is set to deviate from the load action phase positions φp, φp ′ of radial load and moment load at the stroke ends at both ends, the load can be applied without adopting a special structure. It is possible to reduce the contact surface pressure generated in the ball screw mechanism 11 at the stroke end where the maximum is, and to extend the life of the ball screw mechanism 11.

  In the first embodiment, the case where the ball circulation path BR4 is shifted from the load acting phase position φa by about 135 ° has been described. However, the present invention is not limited to this and corresponds to the ball circulation paths BR1 to BR4. It is only necessary to adjust the phase so that the load bearing force reduction region LA to be moved deviates from the load action phase positions φp and φp ′. Further, the number of balls to be arranged can be arbitrarily set.

Next, a second embodiment of the present invention will be described with reference to FIGS.
In the second embodiment, a tube method is adopted as a ball screw nut circulation method.
That is, in the second embodiment, as shown in FIG. 9, as a circulation method of the ball screw nut 15, a tube method in which the balls are returned from the front row to the last row with a tube bent in a U shape is adopted. .

  In this tube system, as shown in FIGS. 9A and 9B, when the tube 31 is positioned at the upper end at the stroke ends at both ends, a radial load Fr from the lower side to the upper side is applied to the rack shaft 7. As shown in FIG. 9B, the load action phase position φp is about 90 ° above, and the tube 31 is located at the load action phase position φp, so that the tube 31 is connected. The ball scooping-back portion of the ball does not exist at the load action phase position φp, and the balls that receive the radial load Fr are only two rows surrounded by the upper rectangular frame, and the number of balls supporting the load is small. The supporting force decrease region LA exists at the load action phase position φp.

  Similarly, as shown in FIGS. 10A and 10B, when the moment loads F1 and F2 are received, the load action phase position φp ′ is lowered downward on the left end side as in the first embodiment described above. And a load action phase position φp is formed on the right end side. Therefore, the balls existing in the load action phase positions φp and φp ′ are an upper row and a lower row surrounded by a rectangular frame as shown in FIG. 10A, and the number of balls supporting the load. Therefore, the load bearing force decrease region LA with a small amount exists at the load action phase positions φp and φp ′.

  On the other hand, in the present embodiment, at the stroke ends at both ends, as shown in FIGS. 11A and 11B, a load bearing force reduction region with a small number of balls in the load zone is, for example, about 90 in the circumferential direction. The phase position is shifted by °. In this case, the load bearing force decrease region LA having a small number of balls supporting the load is deviated from the load action phase positions φp and φp ′. For the radial load Fr, as shown in FIG. In addition, a radial load can be received by the three rows of balls surrounded by the upper rectangular frame, and the contact surface pressure in the ball screw mechanism 11 can be reduced. Similarly, for the moment loads F1 and F2, as shown in FIG. 11B, the left end side receives the moment load F1 in two rows surrounded by the lower square frame, and the right end side receives the upper square frame. The moment load F2 can be received in the two enclosed rows, and the contact surface pressure generated in the ball screw mechanism 11 can be reduced.

  Therefore, also in the second embodiment, when the tube method is adopted, the load supporting force reduction region LA where the number of balls supporting the load is reduced at both stroke ends is removed from the load action phase positions φp and φp ′, Since the phase of the ball screw groove 9 is adjusted so that the other areas are the load action phase positions φp and φp ′, the load is applied to both the radial load and the moment load without adopting a special structure. The number of balls received can be increased, and the contact surface pressure generated in the ball screw mechanism 11 can be reduced, and the life of the ball screw mechanism 11 can be extended.

  In the second embodiment, the case where the ball circulation path BR4 is shifted by about 90 ° from the load action phase position φa has been described. However, the present invention is not limited to this, and a load support corresponding to the ball circulation path 31 is provided. It is only necessary to adjust the phase so that the force reduction region LA is out of the load action phase positions φp and φp ′.

Next, a third embodiment of the present invention will be described with reference to FIGS.
In the third embodiment, an end deflector system is adopted as a ball screw nut circulation system.
That is, in the third embodiment, as shown in FIG. 12, an end deflector type circulation system is employed in which a ball end nut is provided at both ends of the ball screw nut 15 to provide a function for rescuing and returning the ball. .

  In this end deflector system, as shown in FIGS. 12 (a) and 12 (b), the ball circulation path 32 of the ball screw nut 15 is positioned upward at the stroke ends at both ends, and for example, the ball scooping return position is at the end deflector. For example, when the ball return position is about 90 ° at about 135 °, the load action phase position φp is as shown in FIG. 12 (b) when a radial load Fr directed from below to above is applied to the rack shaft 7. As shown in the figure, when the ball rescue position, the ball circulation path 32, and the ball return position are located at the upper 90 ° position and the load action phase position φp, the ball scooping connected to the ball circulation path is removed. And the ball of the ball return portion do not exist at the load action phase position φp, and the balls receiving the radial load Fr are only two rows surrounded by the upper rectangular frame. It becomes, so that the load acting phase position load supporting force reduction area LA is smaller number of balls .phi.p is present in the load acting phase position .phi.p.

  Similarly, as shown in FIGS. 13A and 13B, when the moment loads F1 and F2 are received, the load action phase position φp is upward on the right end side as in the second embodiment described above. A load acting phase position φp ′ is formed below the left end side. For this reason, the balls 14 existing in the load action phase positions φp and φp ′ are an example of an upper part and an example of a lower part surrounded by a rectangular frame as shown in FIG. The load bearing force reduction region LA having a small number of balls is present at the load action phase positions φp and φp ′.

  On the other hand, in the present embodiment, at the stroke ends at both ends, as shown in FIGS. 14A and 14B, the load supporting force reduction region LA with a small number of balls supporting the load is, for example, in the circumferential direction. The phase position is shifted by about 90 °. In this case, the load bearing force decrease region LA having a small number of balls supporting the load is deviated from the load action phase positions φp and φp ′. For the radial load Fr, as shown in FIG. Moreover, the radial load Fr can be received by the upper three rows of balls, and the contact surface pressure generated in the ball screw mechanism 11 can be reduced. Similarly, with respect to the moment loads F1 and F2, as shown in FIG. 14B, the right end receives the moment load F2 in two rows surrounded by the upper rectangular frame, and the lower two rows on the left end side. It is possible to receive the moment load F1 and to reduce the contact surface pressure generated in the ball screw mechanism 11.

  Therefore, also in the third embodiment, when the end deflector method is adopted, the load supporting force reduction region LA where the number of balls supporting the load is reduced at both stroke ends is removed from the load action phase positions φp and φp ′. Since the phase of the ball screw groove 9 is adjusted so that the other region is the load action phase position φp, the load is applied to both the radial load and the moment load without adopting a special structure. The number of balls can be increased, the contact surface pressure generated in the ball screw mechanism 11 can be reduced, and the life of the ball screw mechanism 11 can be extended.

  In the third embodiment, the case where the ball circulation path 32 is shifted by about 90 ° from the load acting phase position φp has been described. However, the present invention is not limited to this and corresponds to each of the ball circulation paths BR1 to BR4. It is only necessary to adjust the phase so that the load bearing force reduction region LA to be moved deviates from the load action phase positions φp and φp ′.

  In the first to third embodiments, the case where the circulation method is any of the top method, the tube method, and the end deflector method has been described. However, the ball is attached to the end caps provided at both ends of the ball screw nut 15. In the end cap system having a function of scooping up and returning, the load supporting force reduction area LA having a small number of balls supporting the load is removed from the load action phase positions φp and φp ′ and the other areas are removed from the load action phase position φp and By arranging at φp ′, it is possible to obtain the same operational effects as in the above embodiment.

  In the first to third embodiments described above, the case where the load supporting force reduction region of the ball screw nut is removed from the load zone at both the rack portion side stroke end and the ball screw portion side stroke end will be described. However, the present invention is not limited to this, and the load supporting force reduction region of the ball screw nut may be removed from the load zone only at any one of the stroke ends. In this case, it is preferable that the load supporting force decreasing region of the ball screw nut is removed from the load zone at the ball screw portion side stroke end shown in FIG.

In the first to third embodiments, the case where the ball screw portion 9 is formed integrally with the rack shaft 7 has been described. However, the present invention is not limited to this, and the rack shaft 7 and the ball screw portion 9 are not limited thereto. Can be applied to an electric power steering apparatus in which the two are arranged coaxially as separate members and are connected by a connecting member.
In the above embodiment, the case where the speed reduction mechanism 13 is configured by the small-diameter pulley 19, the large-diameter pulley 20, and the timing belt 21 has been described. However, a normal belt can be applied instead of the timing belt 21, and A reduction gear can be applied, and any reduction mechanism can be employed.

  DESCRIPTION OF SYMBOLS 1 ... Steering gear mechanism, 2 ... Pinion mechanism, 3 ... Rack mechanism, 5 ... Pinion shaft, 7 ... Rack shaft, 8 ... Rack part, 9 ... Ball screw groove, 10 ... Electric power steering apparatus, 11 ... Ball screw mechanism, DESCRIPTION OF SYMBOLS 12 ... Electric motor for steering assistance, 13 ... Reduction mechanism, 14 ... Ball, 15 ... Ball screw nut, 17 ... Rolling bearing, 19 ... Small diameter pulley, 20 ... Large diameter pulley, 21 ... Timing belt, 30 ... Tie rod, 31 ... Tube, 32 ... Ball circuit

Claims (4)

An electric motor;
A rack shaft connected between the steering mechanism and the tie rod;
A power transmission mechanism for transmitting power from the electric motor to the rack shaft,
The power transmission mechanism is connected to or integrated with the rack shaft and has a male screw groove;
A ball screw mechanism having a nut disposed around the screw shaft and having a female screw groove, and a plurality of rolling elements capable of rolling in a rolling path formed between the male screw groove and the female screw groove; An electric power steering device
In at least one of the stroke ends of the rack shaft, the load bearing force reduction region of the nut is a region other than the phase in the rack shaft circumferential direction defined by the plane formed by the rack shaft and the tie rod connected to the rack shaft. Electric power steering device characterized in that it is set to   The electric power steering apparatus according to claim 1, wherein the load bearing force reduction region is a region in which the number of balls in a load zone that receives a load is minimized.   2. The electric motor according to claim 1, wherein the load bearing force reduction region is a region where the number of balls in a load zone receiving the radial load is minimum with respect to the radial load input to the ball screw. Power steering device.   2. The electric motor according to claim 1, wherein the load bearing force reduction region is a region where the number of balls in a load zone receiving the moment load is minimum with respect to the moment load input to the ball screw. Power steering device.

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