JP2013155787A - Worm gear mechanism, and electric power steering device using worm gear mechanism - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance productivity of a worm, while enhancing durability of an oblique axis worm gear mechanism.SOLUTION: In a worm gear mechanism 44, an axis of a worm wheel 48 is positioned in a direction for crossing an axis 46 of a worm 47 at an angle being not a right angle. A tooth 47a of the worm is constructed of a metallic material. Teeth 48a of the worm wheel are constructed of a resin material. The plurality of teeth of the worm wheel can simultaneously mesh with a plurality of places of the tooth of the worm. In case torque acts on the plurality of teeth of the worm wheel from the plurality of places of the tooth of the worm, a range for contacting with the teeth of the worm wheel in both end places in the worm shaft direction among the plurality of places is named "a contact area A1". A tooth surface 47af of the tooth of the worm has an auxiliary tooth surface area A2 over the whole range of the contact area. The auxiliary tooth surface area is positioned in the vicinity of an addendum of the tooth of the worm, and is set in a pressure angle αlarger than a pressure angle αof the other part.

Description

  The present invention relates to a worm gear mechanism with improved durability and an electric power steering apparatus using the worm gear mechanism, and more particularly to a technique for improving the tooth profile of a worm.

  Some worm gear mechanisms are used in electric power steering devices. The worm gear mechanism used in the electric power steering apparatus is composed of a worm connected to an electric motor and a worm wheel connected to a load. Torque generated by the electric motor is transmitted from the worm to the load via the worm wheel.

  In recent years, there has been a growing demand for miniaturization and weight reduction of electric power steering devices, and there has been a demand for higher output of electric motors. In order to reduce the size of the electric power steering device, the worm wheel shaft is arranged in a direction intersecting at a non-perpendicular angle with respect to the worm shaft, that is, a worm gear mechanism having an oblique shaft with a shaft angle not 90 °. It is possible. Such oblique shaft worm gear mechanisms are known from Patent Documents 1-4.

  The electric power steering apparatus known from Patent Documents 1 to 4 employs a worm gear mechanism having an oblique axis. The worm gear mechanism includes a worm having metal teeth and a worm wheel having resin teeth. In the worm gear mechanism known from Patent Document 2, a plurality of teeth of the worm wheel can simultaneously mesh with a plurality of locations of the teeth of the worm.

  However, in the worm gear mechanism of the oblique axis as known in Patent Document 1, Patent Document 2 and Patent Document 4, when torque acts on a plurality of teeth of the worm wheel from a plurality of worm teeth. It is said that the stress tends to concentrate at locations at both ends in the worm axis direction among a plurality of locations. When the stress is concentrated, a region where the surface pressure is locally increased is generated between the teeth of the worm and the teeth of the worm wheel. There is room for improvement to increase the durability of the worm gear mechanism.

  On the other hand, in the worm tooth profile known from Patent Document 3, the tooth thickness at the end of the tooth gradually decreases from the reference pitch line to the tooth tip, whereas the tooth thickness at the tooth root gradually decreases from the root to the reference pitch line. The degree of gradual decrease is set large. However, since it forms in such a complicated shape over the whole tooth | gear of a worm | warm, it is disadvantageous when improving productivity.

JP 2007-239849 A JP 2009-156443 A JP 2010-270908 A JP 2011-033197 A

  An object of the present invention is to provide a technique capable of improving the productivity of a worm while enhancing the durability of the worm gear mechanism of the oblique shaft.

  According to the first aspect of the present invention, the worm wheel axis is located in a direction intersecting the worm axis at an angle other than a right angle, the worm teeth are made of a metal material, and the worm wheel teeth Is made of a resin material, and in a worm gear mechanism capable of simultaneously engaging a plurality of teeth of the worm wheel with a plurality of positions of the teeth of the worm, torque from a plurality of positions of the worm teeth to a plurality of teeth of the worm wheel When the worm wheel acts, a range where the teeth of the worm wheel are in contact with each other at the both ends of the worm shaft in the plurality of locations is defined as a contact region, and the tooth surface of the worm teeth is the entire range of the contact region. The auxiliary tooth surface region is located near the tooth tip of the tooth of the worm, and the pressure of other parts It is set to a larger pressure angle than, the worm gear mechanism is provided, characterized in that.

  As described in claim 2, preferably, the range of the auxiliary tooth surface region is set over a wider range than the contact region.

  As described in claim 3, preferably, the worm gear mechanism according to claim 1 or 2, the steering system from the steering wheel of the vehicle to the steering wheel, torque is generated and the torque is transmitted via the worm gear mechanism. There is provided an electric power steering device equipped with a worm gear mechanism characterized by comprising an electric motor for transmitting to the steering system.

  In the invention according to claim 1, in the worm gear mechanism of the oblique shaft, when torque acts on a plurality of teeth of the worm wheel from a plurality of positions of the worm teeth, at a position at both ends in the worm axis direction among the plurality of positions, The contact area is the range where the teeth of the worm wheel come into contact. The tooth surface of the worm tooth has an auxiliary tooth surface region over at least the entire range of the contact region. The auxiliary tooth surface region is located in the vicinity of the tooth tip of the worm tooth, and is set to a pressure angle larger than the pressure angle of the other part.

  That is, in the worm teeth, only the contact area where the surface pressure is locally increased between the teeth of the worm wheel and the teeth of the worm wheel is set to a pressure angle larger than the pressure angle of other parts. As a result, there is no portion between the worm teeth and the worm wheel teeth where the surface pressure increases due to local stress concentration. For this reason, durability of a worm gear mechanism can be improved. In addition, since only the worm teeth are corrected locally, productivity can be increased as compared with the conventional case where the worm teeth are formed in a complicated shape over the entire worm teeth.

  In the invention according to claim 2, since the range of the auxiliary tooth surface region is set over a range wider than the contact region, some dimensional tolerance of the worm gear mechanism can be absorbed. For this reason, dimensional management of the worm gear mechanism is easy.

  According to a third aspect of the present invention, there is provided an electric power steering apparatus employing the oblique shaft worm gear mechanism according to the first or second aspect. By using such a worm gear mechanism, a small electric power steering device with excellent durability can be provided.

It is a schematic diagram of an electric power steering device using a worm gear mechanism according to the present invention. It is a whole block diagram of the electric power steering apparatus shown by FIG. FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 2. It is the perspective view which looked at the relationship of mesh | engagement of the worm | warm and worm wheel which were shown by FIG. 3 from diagonally upward. FIG. 5 is a side view of the worm and worm wheel shown in FIG. 4. FIG. 6 is a cross-sectional view showing a basic tooth profile of the worm shown in FIG. 5. FIG. 7 is a cross-sectional view taken along line 7-7 in FIG. FIG. 8 is a cross-sectional view taken along line 8-8 in FIG. It is sectional drawing which shows the tooth profile of the auxiliary tooth surface area | region of the worm | worm shown by FIG.

  EMBODIMENT OF THE INVENTION The form for implementing this invention is demonstrated below based on an accompanying drawing.

  An electric power steering apparatus using a worm gear mechanism according to an embodiment will be described. FIG. 1 schematically shows an electric power steering apparatus 10. The electric power steering apparatus 10 includes a steering system 20 that extends from a steering wheel 21 of the vehicle to steering wheels (for example, front wheels) 29 and 29 of the vehicle, and an auxiliary torque mechanism 40 that applies auxiliary torque to the steering system 20.

  The steering system 20 includes a steering wheel 21, a pinion shaft 24 connected to the steering wheel 21 via a steering shaft 22 and universal joints 23, 23, and a pinion shaft 24 connected to the pinion shaft 24 via a rack and pinion mechanism 25. Rack shaft 26, and left and right steering wheels 29, 29 (for example, left and right front wheels 29, 29) connected to both ends of the rack shaft 26 via left and right tie rods 27, 27 and knuckles 28, 28, respectively. Become. The rack and pinion mechanism 25 includes a pinion 31 formed on the pinion shaft 24 and a rack 32 formed on the rack shaft 26.

  According to the steering system 20, when the driver steers the steering wheel 21, the left and right steering wheels 29, 29 are steered by the steering torque via the rack and pinion mechanism 25 and the left and right tie rods 27, 27. Can do.

  The auxiliary torque mechanism 40 detects the steering torque of the steering system 20 applied to the steering wheel 21 with the steering torque sensor 41, and generates a control signal with the control unit 42 based on the torque detection signal of the steering torque sensor 41. An auxiliary torque corresponding to the steering torque is generated by the electric motor 43 based on the signal, transmitted to the pinion shaft 24 via the worm gear mechanism 44, and further transmitted to the rack 32 via the pinion 31 from the pinion shaft 24. This is the configuration.

  The steering torque sensor 41 detects a torque applied to the pinion shaft 24 and outputs it as a torque detection signal, and is constituted by a magnetostrictive torque sensor, for example.

  According to such an electric power steering apparatus 10, the steering wheels 29 and 29 can be steered by the rack shaft 26 by a combined torque obtained by adding the auxiliary torque of the electric motor 43 to the steering torque of the driver.

  As shown in FIGS. 2 and 3, the pinion shaft 24, the rack and pinion mechanism 25, the rack shaft 26, the steering torque sensor 41, and the worm gear mechanism 44 are housed in a housing 51. The housing 51 extends in the vehicle width direction (left-right direction in FIG. 2). The upper opening of the housing 51 is closed by the upper cover portion 54. The steering torque sensor 41 is attached to the upper cover portion 54. The rack shaft 26 is supported by a rack guide 60 so as to be slidable in the axial direction.

  As shown in FIG. 3, the pinion shaft 24 is a substantially vertical shaft extending in the vertical direction of the vehicle body, and the positions of the upper end portion, the central portion, and the lower end portion are the first bearing 55 and the second bearing, respectively. 56 and a third bearing 57 are rotatably supported by the housing 51. The second bearing 56 is positioned on the housing 51 by a lock nut 58.

  2 and 3, the rack shaft 26 is a horizontal shaft extending in the vehicle width direction and is slidable in the longitudinal direction of the shaft. Tie rods 27 and 27 are connected to both ends in the longitudinal direction of the rack shaft 26 via ball joints 52 and 52, respectively. Further, both ends of the rack shaft 26 are covered with dust seal boots 53 and 53.

  As shown in FIG. 3, the worm gear mechanism 44 is an auxiliary torque transmission mechanism that transmits the auxiliary torque generated by the electric motor 43 to the pinion shaft 24, that is, a booster mechanism. The worm gear mechanism 44 includes a drive-side worm 47 formed integrally with the worm shaft 46 and a driven-side worm wheel 48 that meshes with the worm 47. One end of the worm shaft 46 is connected to a motor shaft 43a (see FIG. 1) by a coupling (not shown). As the steering wheel 21 (see FIG. 1) is steered left and right, the electric motor 43 drives the worm 47 in the normal direction and the reverse direction so that the worm wheel 48 can be rotated in the normal direction and the reverse direction. .

  Both ends of the worm shaft 46 are rotatably supported by the housing 51. A worm wheel 48 (hereinafter simply referred to as “wheel 48”) is provided on the pinion shaft 24. The wheel 48 is restricted from relative movement in the axial direction with respect to the pinion shaft 24, and relative rotation is restricted.

  FIG. 4 shows the meshing relationship between the worm 47 and the wheel 48 shown in FIG. FIG. 5 shows the meshing relationship between the worm 47 and the wheel 48 shown in FIG. 4 as viewed from the side.

  The worm 47 has, for example, two threads 47a (that is, teeth 47a) and a constant pitch of the threads 47a. The advance angle of the thread 47a of the worm 47 is set slightly larger than the friction angle of the thread surface. For this reason, the worm 47 can be rotated from the wheel 48 side (reverse transmission property).

  As shown in FIGS. 3 and 4, the tooth 48 a of the wheel 48 is formed in a so-called “indented tooth shape” in which a central portion of the tooth width is recessed in an arc shape from the tooth tip surface toward the radially inner side. ing.

  As shown in FIGS. 3 to 5, the worm shaft 46 (the shaft 46 of the worm 47) is angled with respect to the pinion shaft 24 (the shaft 24 of the wheel 48), that is, with respect to the center line CL of the pinion shaft 24. It is tilted by “90 ° ± γ °”. More specifically, the worm gear mechanism 44 is a so-called oblique crossing in which the shaft 46 of the worm 47 is arranged at a non-perpendicular angle with respect to the shaft 24 of the wheel 48 and the wheel 48 is engaged with the worm 47. This is a shaft worm gear mechanism.

  Thus, the angle (axis angle, crossing angle) between the center line WL of the shaft 46 of the worm 47 and the center line CL of the shaft 24 of the wheel 48 is not 90 °, but “90 ° ± γ °”. It is. For this reason, the center line WL of the shaft 46 of the worm 47 has an inclination of “90 ° −γ” with respect to the axial plane of the wheel 48 (the center line Lw of the wheel 48 in the tooth width direction). Here, the angle γ is referred to as “an oblique angle γ”. The value of the oblique angle γ is such that when the twist angle λ of the tooth 48a of the wheel 48 is 0 °, the meshing state between the tooth 47a of the worm 47 (that is, the thread 47a) and the tooth 48a of the wheel 48 is good. Thus, it is set in the range of “0 ° <γ ° <90 °”.

  As is apparent from the above description, the worm gear mechanism 44 is constituted by a wheel 48 provided on the pinion shaft 24 and a worm 47 that meshes to drive the wheel 48. The shaft 46 of the worm 47 is arranged in a direction intersecting with the shaft 24 of the wheel 48 at an angle other than a right angle, that is, an oblique axis whose axis angle is not 90 °. The twist angle of the teeth 48a of the wheel 48 was set to 0 °.

  In the worm 47, at least the teeth 47a are made of a metal material, preferably the whole is made of a metal material. As this metal material, for example, steel such as carbon steel for machine structure (JIS-G-4051) is adopted.

  The wheel 48 has at least teeth 48a made of a resin material such as nylon resin. For example, the wheel 48 includes a metal boss portion 71 attached to the pinion shaft 24 by fitting, and a resin wheel main body 72 formed integrally with the boss portion 71. A plurality of teeth 48 a are formed on the outer peripheral surface of the wheel body 72 over the entire circumference. Alternatively, the wheel 48 is entirely formed of a molded product made of resin.

  Since the tooth surface of the resin tooth 48a of the resin wheel 48 is meshed with the tooth surface of the metal tooth 47a of the worm 47, the meshing can be made relatively smooth and noise can be reduced. It can be further reduced.

  Furthermore, since the teeth 47a of the worm 47 are made of metal, they have high rigidity and are difficult to elastically deform. On the other hand, since the teeth 48a of the wheel 48 are made of resin, they have relatively low rigidity and are more easily elastically deformed than the worm 47. When the wheel 48 is rotated by the worm 47, the teeth 48a of the wheel 48 can be elastically deformed according to the magnitude of the auxiliary torque. As a result, the plurality of teeth 48 a of the wheel 48 can simultaneously mesh with the plurality of teeth 47 a of the worm 47.

When the wheel 48 is rotated by the worm 47, a series of changes in the meshing of the teeth 47a of the worm 47 with respect to one of the teeth 48a of the wheel 48 are as follows.
(1) First, the root part of the tooth surface of the tooth 47a of the worm 47 contacts and pushes the tooth tip of the tooth 48a of the wheel 48 (first contact step).
(2) Subsequently, the root portion of the tooth surface of the tooth 47a of the worm 47 contacts with the end portion of the tooth surface of the tooth 48a of the wheel 48, thereby further pressing (second contact step). ).
(3) Subsequently, the portion of the pitch circle in the tooth surface of the tooth 47a of the worm 47 contacts the portion of the pitch circle in the tooth surface of the tooth 48a of the wheel 48, thereby continuing to push further (third Contact step).
(4) Subsequently, the end portion of the tooth surface of the tooth 47a of the worm 47 contacts with the root portion of the tooth surface of the wheel 48, thereby further pressing (fourth contact step).

  As described above, when a plurality of teeth 48a of the wheel 48 mesh with the teeth 47a of the worm 47 at the same time, the elastic deformation amount (deflection amount) of these teeth 48a is substantially the same. However, the contact points at which the tooth surfaces of the teeth 48a contact the teeth 47a of the worm 47 are different from each other. That is, the load that the tooth surface receives from the teeth 47a of the worm 47 varies depending on each contact point so that the plurality of teeth 48a bend by the same amount of deflection. Therefore, the contact pressure that each contact point receives is different. In particular, in the case of the first contact step and the fourth contact step, the contact pressure received by the contact point is larger than in the case of other contact steps.

  Moreover, as the worm 47 rotates, the contact point of the tooth surface of the tooth 48a of the wheel 48 with respect to the tooth 47a of the worm 47 changes in the direction of the "tooth line" (tooth width direction) of the tooth surface.

Hereinafter, the teeth 47a of the worm 47 and the teeth 48a of the wheel 48 will be described in detail. The teeth 48a of the wheel 48 are “helical teeth”, and the tooth profile of the teeth 48a is, for example, “involute”. As shown in FIG. 6, the tooth profile of the tooth 47 a of the worm 47 is basically “involute” or “substantially trapezoidal”, for example. Pressure angle of the teeth 47a of the worm 47 is basically the alpha _1. With respect to the pressure angle alpha ₁ of the teeth 47a of the worm 47, the pressure angle of the teeth 48a of the wheel 48 is the same.

  As described above, the plurality of teeth 48 a of the wheel 48 can simultaneously mesh with the plurality of teeth 47 a of the worm 47. As shown in FIGS. 5, 7, and 8, when torque is applied to the plurality of teeth 48 a of the wheel 48 from the plurality of positions of the teeth 47 a of the worm 47, A range A1 in which the teeth 48a of the wheel 48 are in contact with each other at locations Pe and Pe (see FIG. 5) is referred to as a “contact region A1”. The tooth surface 47af of the tooth 47a of the worm 47 has an auxiliary tooth surface region A2 over the entire range of the contact region A1.

The auxiliary tooth surface area A2 is positioned in the vicinity of the tip 47ae tooth 47a of the worm 47 is set to pressure angle alpha ₂ shown in FIG. Pressure angle alpha ₂ of the auxiliary tooth surface area A2, the pressure angle alpha ₁ of the other _part, that is, is set to a value greater than the basic pressure angle alpha ₁ (pressure angle α _{1 <α 2).}

Thus, among the teeth 47a of the worm 47, only the contact area A1 locally surface pressure is increased between the teeth 47a and the teeth 48a of the wheel 48 is greater than the pressure angle alpha ₁ of the other part It is set to the pressure angle α _2. As a result, there is no portion between the teeth 47a of the worm 47 and the teeth 48a of the wheel 48 where the surface pressure increases due to local stress concentration. For this reason, the durability of the worm gear mechanism 44 can be enhanced. In addition, since only the teeth 47a of the worm 47 are corrected locally, the productivity can be improved as compared with a case where the worm teeth are formed in a complicated shape as in the prior art.

  Furthermore, the range of the auxiliary tooth surface region A2 is preferably set over a wider range than the contact region A1. Therefore, some dimensional tolerance of the worm gear mechanism 44 can be absorbed. For this reason, dimensional management of the worm gear mechanism 44 is easy.

  The size of the auxiliary tooth surface region A2 is preferably set to the following range. As shown in FIG. 9, the height Th of the addendum is obtained by the difference between ½ of the diameter do of the tip circle and ½ of the diameter dp of the pitch circle. That is, the relationship is “Th = ((do / 2) − (dp / 2))”. The height Ah in the tooth height direction of the auxiliary tooth surface region A2, that is, the size Ah in the tooth root direction with respect to the tip circle is set to be larger than ½ of the tooth tip height Th. Is preferable (Ah> (Th / 2)). If it is this height Ah, it will exceed the height of contact area A1.

  More preferably, the height Ah in the tooth height direction of the auxiliary tooth surface region A2 is set in a range up to 1.2 times 1/2 of the height Th of the addendum. That is, the relationship is “(1.2 × Th / 2) ≧ Ah> (Th / 2”. With this height Ah, the dimensional tolerance of the worm gear mechanism 44 can be absorbed and an unnecessary range is set. There is nothing.

  Further, as shown in FIG. 5, the size (width) of the auxiliary tooth surface region A2 in the winding direction (spiral direction) of the teeth 47a of the worm 47 is larger than the size of the contact region A1 in the same direction. Is set. In this way, the contact area A1 enters the auxiliary tooth surface area A2.

  Furthermore, by using such an oblique axis worm gear mechanism 44, it is possible to provide a small electric power steering apparatus 10 (see FIG. 1) having excellent durability.

  In the present invention, the electric power steering apparatus 10 transmits the torque generated by the electric motor 43 based on the steering input of the steering wheel 21 to the steering wheels 29, 29 via the worm gear mechanism 44, thereby turning the vehicle. What is necessary is just the structure which steers. For example, the worm gear mechanism 44 and the manufacturing method thereof according to the present invention can also be applied to a so-called steer-by-wire (abbreviated as “SBW”) electric power steering apparatus. The steer-by-wire type electric power steering apparatus mechanically separates the pinion shaft 24 from the steering wheel 21, and transmits the steering torque generated by the electric motor 43 based on the steering input via the worm gear mechanism 44. In this configuration, the steering wheels 29 and 29 are steered by transmitting to the pinion shaft 24.

  In the worm gear mechanism 44 of the present invention, the steering torque generated by the steering wheel 21 is detected by the steering torque sensor 41, and the electric motor 43 generates auxiliary torque in accordance with the detection signal of the steering torque sensor 41, and this auxiliary torque is generated. This is suitable for the electric power steering apparatus 10 for a vehicle that transmits to the steering system 20 via the worm gear mechanism 44.

DESCRIPTION OF SYMBOLS 10 ... Electric power steering apparatus, 20 ... Steering system, 21 ... Steering wheel, 24 ... Worm wheel shaft, 29 ... Steering wheel, 40 ... Auxiliary torque mechanism, 43 ... Electric motor, 43a ... Motor shaft, 44 ... Worm gear mechanism, 46 ... Worm shaft, 47 ... Worm, 47a ... Worm tooth, 47ae ... Tooth tip, 47af ... Tooth surface, 48 ... Worm wheel, 48a ... Worm wheel tooth, A1 ... Contact area, A2 ... Auxiliary tooth surface area, Pe: Locations at both ends in the worm axis direction among a plurality of locations, α ₁ ... Pressure angle at other locations, α ₂ … Pressure angle.

Claims (3)

The worm wheel axis is located in a direction intersecting the worm axis at a non-perpendicular angle, the worm teeth are made of a metal material, the worm wheel teeth are made of a resin material, and the worm wheel In a worm gear mechanism capable of simultaneously meshing a plurality of teeth of the worm wheel with a plurality of teeth,
When a torque acts on a plurality of teeth of the worm wheel from a plurality of teeth of the worm, a range in which the teeth of the worm wheel are in contact with each other at positions at both ends in the worm axial direction among the plurality of positions age,
The tooth surface of the worm tooth has an auxiliary tooth surface region over the entire range of the contact region;
The auxiliary tooth surface region is located in the vicinity of the tooth tip of the worm tooth, and is set to a pressure angle larger than the pressure angle of other portions.   The worm gear mechanism according to claim 1, wherein a range of the auxiliary tooth surface region is set over a range wider than the contact region.   A worm gear mechanism according to claim 1 or 2, a steering system from a steering wheel of a vehicle to a steering wheel, and an electric motor that generates torque and transmits the torque to the steering system via the worm gear mechanism. An electric power steering apparatus equipped with a worm gear mechanism characterized by comprising.

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