JP5172754B2 - Electric power steering device - Google Patents

Description

  The present invention relates to an electric power steering apparatus including a speed reduction mechanism constituted by a worm gear.

  When the tooth face of the screw thread (tooth), which is a staggered gear element, is formed in a concave arc shape and is forwardly displaced, it is called a Neiman tooth profile or a Neiman worm as described in Patent Document 1, and a contact line stands. At the same time, it is said that the relative curvature radius of the tooth surface is increased to increase efficiency and load capacity.

Japanese Utility Model Publication No. 4-56250

  However, what is called a Neiman tooth profile or Neiman worm, as described above, has a concave arc shape on the thread tooth surface, so that the surface of the worm thread tip and the worm wheel tooth face. The pressure tends to increase. In particular, when the worm wheel is formed of a resin material, the worm wheel is bent and deformed in response to input from the worm, and the surface pressure between the tooth tip on the worm side and the tooth surface on the worm wheel side is large. As a result, there has been a problem that the meshing of the worm gear becomes unstable.

  The present invention has been devised in view of such technical problems, and provides an electric power steering apparatus including a worm gear that can reduce the surface pressure between the tooth tip on the worm side and the tooth surface on the worm wheel side. It is to provide.

  The invention according to claim 1 of the present invention is, in particular, a metal material having a tooth-shaped thread formed in a concave arc shape having a first radius whose first tooth surface is a single radius. And a tooth portion made of a resin material and having a second radius corresponding to the tooth profile on the worm shaft side, the second tooth surface being a single radius. In an electric power steering system including a worm wheel that meshes with the worm shaft with a tooth shape that is convexly formed by a cutting tool, the first radius is set larger than the second radius, and the first radius The first pressure angle that is the pressure angle of the tooth surface is set to be larger than the second pressure angle that is the pressure angle on the pitch line of the gear cutting tool.

  The invention according to claim 4 of the present invention has, in particular, a worm shaft made of a metal material having a tooth-shaped thread in which the first tooth surface, which is the tooth surface, is formed in a concave shape, and a tooth portion made of a resin material. And a worm wheel that meshes with the worm shaft with a tooth shape having a convex second tooth surface corresponding to the tooth shape on the worm shaft side. The meshing of the first tooth surface and the second tooth surface is performed in an unloaded state where the rotational force of the electric motor is not transmitted from the first tooth surface side to the second tooth surface side. A non-contact portion where the first tooth surface and the second tooth surface are not in contact with each other is provided on the tooth end side of the screw thread, and the first tooth surface and the Contact with the second tooth surface And the first tooth surface and the second tooth in a load state where the rotational force of the electric motor is transmitted from the first tooth surface side to the second tooth surface side. It is characterized in that the surface pressure of the first tooth surface generated by meshing with the surface is set to be greater on the tooth root side than on the tooth end side.

  The invention according to claim 5 of the present invention includes, in particular, a tooth-shaped thread having a tooth tip correction portion having a convex profile at the tooth tip and the first tooth surface which is the tooth surface formed in a concave shape. A worm shaft made of a metal material and a tooth portion made of a resin material, and having a tooth profile in which the second tooth surface, which is the tooth surface corresponding to the tooth profile on the worm shaft side, is formed in a convex shape. In the electric power steering apparatus including a worm wheel that meshes with a shaft, the meshing between the first tooth surface and the second tooth surface is performed from the first tooth surface side to the second tooth surface side. In a no-load state where the rotational force of the electric motor is not transmitted, the first tooth surface and the second tooth surface are not in contact with the end of the worm shaft, except for the tooth tip correction portion of the thread. Contact part is provided In addition, in a load state in which the rotational force of the electric motor is transmitted from the first tooth surface side to the second tooth surface side, it is set so as to contact the entire tooth height except the tooth tip correction portion. It is characterized by that.

  According to the present invention, compared with the case where the first radius and the second radius and the first pressure angle and the second pressure angle are the same, the tooth tip on the worm shaft side is the tooth surface on the worm wheel side. It will change in the direction away from. Thereby, the contact of the tooth tip on the worm shaft side with the tooth surface on the worm wheel side is relieved, and the meshing as the worm gear can be made smooth.

It is a longitudinal cross-sectional view explaining the structure of the electric power steering apparatus which concerns on this invention. It is a fragmentary sectional view which follows the AA line of FIG. It is a perspective view which shows the meshing state of the worm shaft and worm wheel which comprise the deceleration mechanism of FIG. FIG. 4 is an enlarged cross-sectional view taken along line BB in FIG. 3 showing the first embodiment of the present invention and showing the tooth profile of the thread on the worm shaft side. It is an expanded sectional view showing the section of an axis perpendicular direction of a tooth part of a gear cutting tool which performs gear cutting by the side of a worm wheel. FIG. 4 is an enlarged cross-sectional view corresponding to a cross section taken along line BB of FIG. 3 when only the radius of curvature is increased for the tooth profile of the thread on the worm shaft side. FIG. 5 is an enlarged cross-sectional view corresponding to a cross section taken along line BB in FIG. 3 when only the pressure angle on the pitch line is increased for the tooth profile of the thread on the worm shaft side. FIG. 5 is an enlarged cross-sectional view corresponding to a cross section taken along line B-B in FIG. 3 showing a second embodiment of the present invention and showing a tooth profile of a thread on the worm shaft side.

  Hereinafter, embodiments of an electric power steering apparatus according to the present invention will be described in detail with reference to the drawings. In each embodiment, the electric power steering device is applied to, for example, a rack and pinion type steering device for an automobile.

  1 to 7 show a first embodiment of an electric power steering apparatus according to the present invention. As shown in FIG. 1, one end of the electric power steering apparatus is linked to a steering wheel (not shown) so as to be integrally rotatable. The other end of the output shaft 2 is connected to the other end of the input shaft 1 via a well-known rack and pinion mechanism 4 and connected to a steered wheel (not shown) via a torsion bar 3 so as to be relatively rotatable. These constitute the steering mechanism of the present invention. The rack and pinion mechanism 4 includes a pinion tooth 2 a created at one end of the output shaft 2 and a rack shaft 5 having rack teeth 5 a meshing with the pinion tooth 2 a in a predetermined axial range. Both ends of are connected to the steered wheels via tie rods not shown.

  With this configuration, when the steering wheel is rotated, the torsion bar 3 is twisted by rotating the input shaft 1 in synchronization therewith, and the output shaft 2 follows the input shaft 1 by the elastic force of the torsion bar 3. It will rotate. Then, the rotational movement of the output shaft 2 is converted into the linear (axial direction) movement of the rack shaft 5, and the knuckle (not shown) is pulled left and right through the tie rod as the rack shaft 5 moves in the axial direction. The steered wheels will be steered.

  Further, as shown in FIG. 2 in addition to FIG. 1, the electric power steering device is housed in a housing 6 provided on the outer peripheral side of the connection portion between the input shaft 1 and the output shaft 2, A torque sensor 7 that detects a steering torque based on the relative rotational displacement amount of the input shaft 1 and the output shaft 2, and is driven and controlled by a control unit (not shown) based on a detection result of the torque sensor 7 and a vehicle speed signal. An electric motor 8 capable of rotating in the forward and reverse directions that generates an auxiliary torque corresponding to the torque, and a speed reduction mechanism 10 that transmits the rotational force of the electric motor 8 to the output shaft 2 are provided. Thus, when the driver performs steering, the rotational drive direction of the electric motor 8 is switched according to the steering direction, and the auxiliary torque according to the driver's steering torque is output from the output shaft 2 with the output of the speed reduction mechanism 10. To be granted.

  The speed reduction mechanism 10 is a worm gear that is a kind of staggered gear as shown in FIG. 3, and is coaxially connected to the tip of the drive shaft 8a of the electric motor 8 so that both ends thereof are A worm shaft 11 rotatably supported by the first and second bearings 18a and 18b, and a worm wheel fixed to the outer periphery of the output shaft 2 and meshing with the worm shaft 11 in a twisted relationship (axial angle is 90 °). 12.

  Furthermore, a backlash adjusting mechanism 30 for adjusting the backlash between the worm shaft 11 and the worm wheel 12 is provided at one end of the worm shaft 11 that is supported by the first bearing 18a. The backlash adjusting mechanism 30 is fitted to the first bearing 18 a from above in FIG. 2 and is fitted to the outer peripheral side of the bearing holding member 31. The bearing holding member 31 fits and holds the second bearing 18 b. And an O-ring 32. The bearing holding member 31 is provided on a side opposite to the fixed portion 31a for holding the first bearing 18a and the worm wheel 12, and is movable so as to be relatively deformable with respect to the fixed portion 31a in the radial direction of the worm wheel 12. And the first bearing 18a and one end of the worm shaft 11 are urged toward the worm wheel 12 side by the tight force of the O-ring 32 so as to adjust the backlash of the both 11 and 12. It has become.

  Here, the worm shaft 11 and the worm wheel 12 constituting the worm gear are configured so that different materials are engaged with each other at least at the engagement portion between them. That is, the worm shaft 11 is formed of a metal material, while the worm wheel 12 is formed of a resin material that does not include reinforcing fibers such as glass fibers, for example, a surface layer portion (a so-called rim portion) 12a including a tooth profile. At the same time, a body portion (including a portion called a boss and a portion called an arm) 12b, which is the other portion, is formed in a gear shape from a metal material, and both 12a and 12b are formed by so-called insert molding. It is integrally molded.

  The worm shaft 11 is formed as a multi-threaded worm shaft having three threads 13 as tooth portions, and the tooth profile of the thread 13 is indicated by a solid line in FIG. The tooth surfaces (the first tooth surface of the present invention) 14 on both sides are formed in a concave arc shape having a single radius of curvature (the first radius of the present invention) R1, and the so-called Neiman tooth profile described above or It is configured as a so-called Neiman Warm. On the other hand, the tooth portion 15 of the worm wheel 12 has teeth 15 having tooth surfaces 22 on both sides of the tooth profile formed in a concave arc shape with a single curvature radius (second radius of the present invention) R2. The tooth is cut (cut) by the gear cutting tool 20 having the portion 21, and the tooth surface (second tooth surface of the present invention) 17 corresponding to the tooth shape on the worm shaft 11 side is formed in a convex arc shape. It meshes with the worm shaft 11. Here, the tooth tip of the screw thread 13 of the worm shaft 11 is provided with a tooth tip correcting portion 15 having a convex profile formed by chamfering in a taper or arcuate shape, so that the mesh with the worm wheel 12 is more engaged. It has been adjusted to be smooth.

  In addition, as described above, what is called a Neiman tooth profile or Neiman worm has a tendency that the surface pressure on the end of the tooth (particularly in the vicinity of the tooth tip) tends to increase when meshing with the worm wheel. In the worm shaft 11 according to the embodiment, the tooth surface 14 on the side of the addendum 14a is shown as a solid line in FIG. 4 with respect to the conventional tooth profile X as a reference as shown by a broken line in FIG. The following unique adjustments have been made. Specifically, in the worm shaft 11 according to the present embodiment, the curvature radius R1 of the tooth surface 14 of the thread 13 is larger than the curvature radius R2 of the tooth surface 22 of the tooth portion 21 of the gear cutting tool 20. As described above, the pressure angle (first pressure angle of the present invention) α1 on the pitch line M of the tooth surface 14 is equal to the pressure angle (pitch of the present invention on the pitch line N of the tooth surface 22 of the gear cutting tool 20). (Second pressure angle) is set to be larger than α2, that is, (R1> R2) ∩ (α1> α2).

  That is, when the worm shaft 11 and the worm wheel 12 are engaged with each other, the worm shaft 11 is in a state in which the rotational force of the electric motor 8 is not transmitted from the tooth surface 14 of the worm shaft 11 to the tooth surface 17 of the worm wheel 12 (no load state). A non-contact portion where the tooth surfaces 14 and 17 are not in contact with each other is provided on the end of the tooth tip 14a which is the tooth tip side of the pitch line M of the screw thread 13 and the pitch line of the screw thread 13 of the worm shaft 11 A contact portion is provided in which the tooth surfaces 14 and 17 on the tooth base 14b side, which is the tooth root side of M, contact each other, and the tooth surface 14 on the worm wheel 12 side from the tooth surface 14 on the worm shaft 11 side. The worm shaft generated by the meshing of the tooth surfaces 14 and 17 in a state where the rotational force of the electric motor 8 is transmitted to the motor (load state) Is configured to be larger at the tooth root side than the surface pressure of the tooth surface 14 of one side addendum side. The load state means a state in which the vehicle speed is 0 km / h and the steering is stationary.

  In other words, when the worm shaft 11 and the worm wheel 12 are engaged with each other, the worm shaft 11 does not transmit the rotational force of the electric motor 8 from the tooth surface 14 of the worm shaft 11 to the tooth surface 17 of the worm wheel 12 (no load state). A non-contact portion where the tooth surfaces 14, 17 are not in contact with each other is provided on the end of the tooth tip 14a, which is the tooth tip side of the pitch line M excluding the tooth tip correction portion 15 of the thread 13 of the shaft 11, and In a state where the rotational force of the electric motor 8 is transmitted from the tooth surface 14 on the worm shaft 11 side to the tooth surface 17 on the worm wheel 12 side (load state), the tooth surface 14 excluding the tooth tip correction portion 15 on the worm shaft 11 side. Is configured to contact the tooth surface 17 on the worm wheel 12 side in the entire region in the tooth height direction.

  In this manner, the tooth surface 14 of the thread 13 of the worm shaft 11 is configured as described above, so that the tooth surface is more than the pitch line M as compared with the shape (profile) of the tooth profile X which is a conventional reference. The tooth end 14a side, which is the tip 14c side, is set to have a tapered profile, and the surface pressure on the tooth end 14a side (particularly in the vicinity of the tooth tip 14c) at the time of meshing with the worm wheel 12 is configured to be low. .

  Here, as shown in FIG. 6, the tooth end 14 a side can be tapered by changing (enlarging) only the radius of curvature R 1 as shown in FIG. Since the base 14b side also becomes thin, it cannot be said that it is effective as a means for adjusting the profile of the tooth end 14a when the profile of the tooth base 14b is appropriate, and as shown in FIG. In addition, the tooth end 14a side can be tapered by changing (enlarging) only the pressure angle α1, but in this case, the tooth base 14b side becomes thicker, so the tooth base 14b side This is not an effective means for adjusting the profile on the end of the tooth end 14a when the profile is appropriate. In FIGS. 6 and 7, the broken line indicates the conventional tooth profile as the reference, and the solid line indicates a modification of the conventional tooth profile.

  Therefore, as shown in FIG. 4, by changing both the radius of curvature R1 and the pressure angle α1 as in the configuration of the present embodiment, the effect of changing only the radius of curvature R1 and the change of only the pressure angle α1. As a result, the profile on the end 14a side is tapered, while the profile on the base 14b side is hardly changed. That is, in other words, it is possible to change only the profile on the tooth end 14a side in a tapered manner without changing the profile on the tooth base 14b side.

  Further, with respect to the worm shaft 11, the difference between the pressure angle α 1 of the tooth surface 14 and the pressure angle α 2 of the tooth surface 22 of the gear cutting tool 20 is determined from the pitch circle of the worm shaft 11 and the worm wheel 12. The shaft 11 is set so as to become smaller toward the tooth root 14b and to be substantially equal in the vicinity of the tooth bottom 14d. In other words, in the worm shaft 11 according to the present embodiment, only the profile on the tooth tip 14c side of the tooth profile is changed and the profile near the tooth bottom 14d is changed due to the unique configuration (adjustment). There is no end to it. That is, according to this setting, the profile on the tooth root 14b side (near the tooth bottom 14d) can be maintained, which is effective when it is desired to change only the profile on the tooth end 14a side.

  The worm gear constituting the speed reduction mechanism 10 according to the present embodiment as described above is manufactured by the following procedure.

  That is, in the worm shaft 11, first, both end portions of the material cut to a predetermined axial length are cut to a predetermined outer diameter, and a gear cutting tool such as a hob is used for the intermediate portion thereof. To form the thread 13. Subsequently, after heat treatment such as carburizing and quenching, finishing is performed by grinding. In particular, the tooth surface 14 of the screw thread 13 is ground with a circular grindstone having the same maximum radius on the entire circumference, and the entire tooth surface 14 is ground. If grinding is performed with the circular grindstone as described above, the circular grindstone wears in the same manner on the entire circumference thereof, so that the occurrence of uneven wear of the circular grindstone is suppressed. As a result, even if the grindstone is worn, the finished shape of the tooth surface 14 on the worm shaft 11 side can be made uniform.

  Here, in the worm gear using the so-called Neiman worm, the surface pressure between the tooth tip 14 c of the thread 13 and the tooth surface 17 of the worm wheel 12 tends to increase due to the tooth profile of the thread 13 of the worm shaft 11. Therefore, the meshing of the worm shaft 11 and the worm wheel 12 is made smooth by grinding the end of the thread 13 that increases the surface pressure, that is, the end 14a side of the pitch line M. Is possible. Further, when grinding the tooth root 14b side, that is, the tooth bottom 14d side from the pitch line M, by grinding the range from the pitch line M to the tooth bottom 14d, the boundary portion due to the presence or absence of grinding. This eliminates the possibility of stress concentration in the worm shaft, thereby making it possible to improve the bending stress of the worm shaft. From this, it can be said that it is desirable to grind the entire tooth surface 14 of the thread 13.

  On the other hand, for the worm wheel 12, the body portion 12b is formed by cutting or sintering, and the resin material constituting the surface layer portion 12a is integrated on the outer periphery of the body portion 12b by insert molding or the like, It completes by forming the tooth | gear part 16 with the said cutting tool 20, such as a hob, in the outer peripheral part of the resin material used as the surface layer part 12a.

  As described above, according to the electric power steering apparatus according to this embodiment, by configuring (adjusting) the tooth surface 14 of the screw thread 13 of the worm shaft 11 as described above, the radius of curvature of the tooth surface 14 is obtained. R1, the radius of curvature R2 of the tooth surface 22 of the gear cutting tool 20, the pressure angle α1 at the pitch line M of the tooth surface 14 and the pressure angle α2 at the pitch line N of the tooth surface 22 of the gear cutting tool 20 are the same. Compared to the conventional technique, the tooth tip 14c on the worm shaft 11 side changes in a direction away from the tooth surface 17 on the worm wheel 12 side. Thereby, the contact of the tooth tip 14c on the worm shaft 11 side with the tooth surface 17 on the worm wheel 12 side is alleviated, and the meshing as the worm gear can be made smooth.

  Moreover, according to such a configuration, the tooth profile of the thread 13 of the worm shaft 11 is maintained while maintaining the profile of the conventional tooth profile X as a reference on the tooth root 14b side (particularly in the vicinity of the tooth bottom 14d). Only the 14a side (particularly in the vicinity of the tooth tip 14c) can be formed into a tapered profile. That is, in other words, it is possible to suppress only an increase in the surface pressure on the side of the addendum 14a (particularly in the vicinity of the addendum 14c) at the time of meshing with the worm wheel 12, which is a problem of the Neiman worm. As a result, it is possible to obtain a smooth mesh with the tooth surface 17 on the worm wheel 12 side over the entire tooth surface 14 on the worm shaft 11 side.

  Further, in the electric power steering apparatus according to the present embodiment, since the backlash adjusting mechanism 30 is provided, the meshing angle between the worm shaft 11 and the worm wheel 12 by the backlash adjusting action by the backlash adjusting mechanism 30. However, by making the tooth profile on the side of the worm shaft 11 unique as described above, the shift of the meshing angle between the both 11 and 12 is reduced, and the gap between the both 11 and 12 is good. It is possible to ensure proper meshing.

  Further, in the worm wheel 12, since the surface layer portion 12a constituting the tooth portion 16 is formed of a resin material, it is possible to reduce the rattling noise when meshing with the worm shaft 11. Of course, the effect of reducing backlash by elastically deforming and thermally expanding the tooth portion 16 can also be expected. Moreover, since the resin material does not contain reinforcing fibers, the work efficiency when cutting the teeth 16 of the worm wheel 12 is improved, and the thread 13 of the worm shaft 11 is not damaged. . In other words, by forming the tooth surface 17 of the tooth portion 16 on the worm wheel 12 side into a convex arc shape, the tooth thickness is increased and the strength of the tooth portion 16 is increased, so the surface layer portion 12a is formed. Sufficient strength can be secured without mixing reinforcing fibers in the resin material.

  And since the tooth part 16 of the worm wheel 12 is formed of a resin material that does not contain reinforcing fibers, the tooth part 16 is easily elastically deformed when meshed with the worm shaft 11. Thereby, the input from the road surface in the load state and the like can be absorbed by the tooth portion 16.

  Further, in the Neiman worm, the pressure angle is large on the basis of its tooth profile, and this tends to increase the shaft bending rigidity. However, the worm shaft 11 has a unique configuration as described above, so that the pressure angle is increased. The load on the side of the tooth 14b having a high height increases, and the bending force of the shaft increases accordingly. However, in the speed reduction mechanism 10 used in the electric power steering apparatus according to this embodiment, the worm Since the thread 13 of the shaft 11 is formed as a three-row multi-worm worm shaft, the rigidity of the worm shaft 11 in the cross section perpendicular to the axis is also increased.

  FIG. 8 shows a second embodiment of the present invention, in which the configuration of the tooth surface 14 of the thread 13 of the worm shaft 11 in the first embodiment is changed.

  That is, the tooth profile of the thread 13 of the worm shaft 11 according to the present embodiment is such that the root 14b is formed in a concave arc shape with the same radius of curvature R1 as in the first embodiment, and the addendum 14a is The pitch line M is formed in a straight line extending by a tangent to the radius of curvature R1.

  Even with this configuration, the tooth tip 14c on the worm shaft 11 side changes in a direction away from the tooth surface 17 on the worm wheel 12 side when the worm shaft 11 and the worm wheel 12 are engaged, as in the first embodiment. Thus, the contact of the tooth tip 14c on the worm shaft 11 side with the tooth surface 17 on the worm wheel 12 side is alleviated, and as a result, the meshing as the worm gear can be smoothed.

  In other words, even in the configuration according to this embodiment, the tooth profile of the screw thread 13 of the worm shaft 11 is the same as that of the first embodiment, which is a standard on the end of the tooth end 14a (particularly in the vicinity of the tooth tip 14c). The profile of the conventional tooth profile X serving as a reference can be maintained on the tooth base 14b side (particularly in the vicinity of the tooth bottom 14c) while forming a profile that is tapered more than the tooth profile X of FIG. That is, it is possible to suppress only an increase in the surface pressure on the end of the tooth end 14a (particularly in the vicinity of the tooth tip 14c) at the time of meshing with the worm wheel 12, which is a problem of the Neiman worm, and for the tooth surface 17 on the worm wheel 12 side, Smooth meshing can be obtained over the entire tooth surface 14 on the worm shaft 11 side.

  The present invention is not limited to the configuration of each of the above-described embodiments. For example, the number of threads 13 and the number of teeth of the worm shaft 11, the number of teeth of the worm wheel 12, and the like are the specifications of the electric power steering device. It can be changed freely according to.

The technical ideas other than those described in the above claims ascertained from the embodiments will be described below.
(1) The electric power steering apparatus according to claim 1, wherein the worm shaft is ground by a circular grindstone having a radius of a maximum diameter portion which is the same in the entire circumference.

According to this configuration, the circular grindstone is similarly worn on the entire circumference, and the occurrence of uneven wear is suppressed. Thereby, even if the said grindstone wears, the finished shape of the tooth surface by the side of a worm shaft can be made uniform.
(2) The electric power steering device according to claim 1, wherein the first tooth surface is ground from the pitch circle of the worm shaft and the worm wheel to the tooth tip.

In a worm gear using a Neiman worm, the surface pressure between the tooth tip of the worm thread and the tooth surface of the worm wheel tends to increase depending on the tooth profile of the worm. Accordingly, by grinding the tooth tip side of the worm, which is a factor for increasing the surface pressure, it becomes possible to make the meshing of both smooth.
(3) The electric power steering apparatus according to claim 1, wherein the first tooth surface is ground from the pitch circle of the worm shaft and the worm wheel to the tooth bottom.

When grinding is not performed to the root at the root, a boundary portion is formed between the portion subjected to the grinding on the pitch circle side and the portion not subjected to the grinding on the bottom side. , Stress concentration occurs at the boundary. Thus, by grinding to the bottom of the tooth as described above, there is no risk of stress concentration occurring in part, and as a result, the bending stress of the worm shaft can be improved.
(4) The apparatus according to claim 1, further comprising a backlash adjusting mechanism that reduces backlash between the two by biasing the worm shaft toward the worm wheel based on an elastic force of an elastic member. Electric power steering device.

When the backlash adjustment mechanism is provided, the meshing angle between the worm shaft and the worm wheel may be shifted. The shift of the engagement angle between the two can be reduced, and a good engagement can be secured between the two.
(5) The electric power steering device according to claim 1, wherein a tooth portion of the worm wheel formed of the resin material is formed of a resin material not including a reinforcing fiber.

By setting it as this structure, the efficiency at the time of gear-cutting the tooth surface by the side of a worm wheel can be achieved.
(6) The electric power steering apparatus according to claim 1, wherein the worm shaft is constituted by three teeth.

The Neiman worm has a tendency that the pressure angle is large and the shaft bending rigidity tends to be large based on its tooth profile, and the load at the tooth root having a high pressure angle is increased by configuring the worm shaft as in claim 1. As a result, the bending component of the shaft also increases. Therefore, by setting the worm shaft to three teeth, it is possible to improve the rigidity in the cross section perpendicular to the axis of the worm shaft.
(7) The electric power steering apparatus according to claim 4, wherein the load state is a state where the vehicle speed is 0 km / h and the steering wheel is fixed.

In the state where the steering is turned off at least when the vehicle speed is zero, the tooth part on the worm wheel side formed of a resin material is elastically deformed so that the input from the road surface or the like is received by the tooth part on the worm wheel side As a result, the steering feeling can be improved.
(8) The electric power steering apparatus according to claim 4, wherein the first tooth surface is ground from a pitch circle of the worm shaft and the worm wheel to a tooth tip.

In a worm gear using a Neiman worm, the surface pressure between the tooth tip of the worm thread and the tooth surface of the worm wheel tends to increase depending on the tooth profile of the worm. Accordingly, by grinding the tooth tip side of the worm, which is a factor for increasing the surface pressure, it becomes possible to make the meshing of both smooth.
(9) The electric power steering device according to claim 4, wherein the first tooth surface is ground from the pitch circle of the worm shaft and the worm wheel to the tooth bottom.

When grinding is not performed to the root at the root, a boundary portion is formed between the portion subjected to the grinding on the pitch circle side and the portion not subjected to the grinding on the bottom side. , Stress concentration occurs at the boundary. Thus, by grinding to the bottom of the tooth as described above, there is no risk of stress concentration occurring in part, and as a result, the bending stress of the worm shaft can be improved.
(10) The backlash adjusting mechanism further reduces a backlash between the two by biasing the worm shaft toward the worm wheel based on an elastic force of an elastic member. Electric power steering device.

When the backlash adjustment mechanism is provided, the meshing angle between the worm shaft and the worm wheel may be shifted, but by making the profile of the tooth surface of the worm shaft and the worm wheel unique as in claim 4, The shift of the engagement angle between the two can be reduced, and a good engagement can be secured between the two.
(11) The electric power steering device according to claim 4, wherein a tooth portion of the worm wheel formed of the resin material is formed of a resin material not including reinforcing fibers.

By setting it as this structure, the efficiency at the time of gear-cutting the tooth surface by the side of a worm wheel can be achieved.
(12) The electric power steering apparatus according to claim 5, wherein the load state is a state where the vehicle speed is 0 km / h and the steering wheel is stationary.

In the state where the steering is turned off at least when the vehicle speed is zero, the tooth part on the worm wheel side formed of a resin material is elastically deformed so that the input from the road surface or the like is received by the tooth part on the worm wheel side As a result, the steering feeling can be improved.
(13) The electric power steering apparatus according to claim 5, wherein the first tooth surface is ground from a pitch circle of the worm shaft and the worm wheel to a tooth tip.

In a worm gear using a Neiman worm, the surface pressure between the tooth tip of the worm thread and the tooth surface of the worm wheel tends to increase depending on the tooth profile of the worm. Accordingly, by grinding the tooth tip side of the worm, which is a factor for increasing the surface pressure, it becomes possible to make the meshing of both smooth.
(14) The electric power steering device according to claim 5, wherein the first tooth surface is ground from the pitch circle of the worm shaft and the worm wheel to the tooth bottom.

When grinding is not performed to the root at the root, a boundary portion is formed between the portion subjected to the grinding on the pitch circle side and the portion not subjected to the grinding on the bottom side. , Stress concentration occurs at the boundary. Thus, by grinding to the bottom of the tooth as described above, there is no risk of stress concentration occurring in part, and as a result, the bending stress of the worm shaft can be improved.
(15) The apparatus according to claim 5, further comprising a backlash adjusting mechanism that reduces backlash between the two by biasing the worm shaft toward the worm wheel based on an elastic force of an elastic member. Electric power steering device.

  When the backlash adjusting mechanism is provided, the meshing angle between the worm shaft and the worm wheel may be shifted, but by making the profile of the tooth surface of the worm shaft and the worm wheel unique as in claim 5, The shift of the engagement angle between the two can be reduced, and a good engagement can be secured between the two.

DESCRIPTION OF SYMBOLS 1 ... Input shaft 2 ... Output shaft 8 ... Electric motor 10 ... Deceleration mechanism 11 ... Worm shaft 12 ... Worm wheel 13 ... Screw thread 14 ... Tooth surface 14a ... Addendum 14b ... Tooth base 14c ... Tooth tip 14d ... Tooth bottom 15 ... Tooth tip correction part 16 ... tooth part 17 ... tooth surface 20 ... gear cutting tool 21 ... tooth part 22 ... tooth surface R1 ... radius of curvature (first radius)
R2 ... radius of curvature (second radius)
α1 ... Pressure angle (first pressure angle)
α2 ... Pressure angle (second pressure angle)

Claims (3)

An input shaft linked to the steering wheel;
An output shaft provided so as to be rotatable relative to the input shaft and linked to the steered wheels;
An electric power steering comprising: an electric motor that applies a rotational force to the output shaft via a predetermined speed reduction mechanism;
The speed reduction mechanism includes a worm shaft made of a metal material having a tooth-shaped thread formed in a concave arc shape having a first radius of which the first tooth surface is a single radius, and a resin. A tooth portion made of a material is formed, and a second tooth surface corresponding to the tooth profile on the worm shaft side is convexly formed by a gear cutting tool having a second radius that is a single radius. A worm wheel meshing with the worm shaft with a formed tooth profile,
The first radius is set to be larger than the second radius, and the first pressure angle that is the pressure angle of the first tooth surface is the pressure angle on the pitch line of the gear cutting tool. An electric power steering device characterized by being set to be larger than the pressure angle.   In the worm shaft or the worm wheel, the difference between the first pressure angle and the second pressure angle becomes smaller from the pitch circle of the worm shaft and the worm wheel toward the tooth root side of the worm shaft. The electric power steering apparatus according to claim 1, wherein the electric power steering apparatus is set as follows.   In the worm shaft or the worm wheel, a difference between the first pressure angle and the second pressure angle is set to be substantially equal in the vicinity of a tooth root of the worm shaft. Item 3. The electric power steering device according to Item 2.

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