JP2012163179A - Speed reduction mechanism and electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a speed reduction mechanism which is small, inexpensive and excellent in durability.SOLUTION: A reduction gear 31 is used for meshing with a worm and formed by forging. The tooth surface 34 of each tooth part 32 of the reduction gear 31 has a worm wheel tooth shape part 35 and a concave part 36 which is recessed from the worm wheel tooth shape part 35 to avoid engagement with the worm 20. The concave part 36 [Fig.4(a)] is provided in an area E1 where an angle B, formed by the contact line CL of the worm wheel tooth shape and worm 20, and the sliding direction S1 of the worm 20, is less than or equal to a threshold value B1 when no concave part 36 is formed [Fig.4(b)].

Description

  The present invention relates to a speed reduction mechanism and an electric power steering device.

  There has been proposed a reduction mechanism for an electric power steering device in which a worm wheel is composed of a cored bar formed by a cold forging process and a synthetic resin gear part fixed to the outer periphery of the cored bar (for example, Patent Documents). 1).

JP 2007-255524 A

With the recent increase in output of electric power steering devices, the speed reduction mechanism has become larger and the layout on the vehicle has become very strict. When a resin is used as the gear portion of the worm wheel as in Patent Document 1, the worm wheel tends to be large in order to satisfy the strength. In addition, a resin that can withstand high strength is expensive, and the manufacturing cost increases.
Therefore, it is conceivable to use a metal gear. However, for example, when a worm and a helical gear as a reduction gear are engaged with each other, contact is almost made at a point. For this reason, wear in the initial stage of use becomes large. As a result, the backlash increases and the rattling sound (rattle sound) increases.

  On the other hand, when a cylindrical worm (for example, JIS3 type whose cross-sectional tooth profile is trapezoidal) and a drum-shaped worm wheel (a cross-sectional tooth shape is involute) as a reduction gear are meshed, In a region where the angle (crossing angle) formed with the sliding direction of the worm is small, the lubricating oil film is easily cut. Therefore, wear and heat generation occur, and durability when used for a long time is deteriorated.

In order to improve this, it is conceivable to use a drum-shaped worm whose contact line can be in the toothing direction. However, the drum-shaped worm is complicated in processing and has a high manufacturing cost.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a reduction mechanism and an electric power steering device that are small, inexpensive, and excellent in durability.

  In order to achieve the object, the invention of claim 1 includes a worm (20), a metal reduction gear (31; 131) meshing with the worm, and a lubricant interposed between the worm and the reduction gear. The speed reduction gear includes a tooth surface (34; 41, 51), and the tooth surface is a worm wheel tooth profile (35; 43, 43) formed at least at the center in the tooth width direction (W1). 53) and a recess (36; 44, 54) that is recessed from the worm wheel tooth profile and avoids meshing with the worm.

According to a second aspect of the present invention, the concave portion includes an angle (B) formed between a contact line (CL) when the worm wheel tooth profile of the worm wheel tooth profile portion contacts the worm and the worm slip direction (S1). ) May be arranged in the region (E1) that is equal to or less than the threshold value (B1).
As in claim 3, the recess may be formed by forging.

According to a fourth aspect of the present invention, there is provided an electric power steering device (1) for decelerating the rotation of the steering assisting electric motor (18) using the speed reduction mechanism.
In the above description, numbers in parentheses represent reference numerals of corresponding components in the embodiments described later, but the scope of the claims is not limited by these reference numerals.

According to the invention of claim 1, since the metal reduction gear is used, it is small and has high strength. Further, since the tooth contact between the worm wheel tooth profile of the reduction gear and the worm is in line contact, a good contact state can be secured from the beginning, and the occurrence of initial wear can be prevented.
Generally, when the worm wheel tooth profile of the reduction gear is meshed with, for example, a cylindrical worm, the central portion in the tooth width direction of the tooth surface of the reduction gear is a region where the contact line between the worm wheel tooth profile and the worm matches the sliding direction of the worm. including. In the present invention, since the concave portion is provided in the central portion to avoid meshing with the worm, it is possible to suppress wear at a portion where there is a possibility that it becomes the most severe for durability. Can be improved.

Further, by adopting the above-described configuration of the reduction gear, it is possible to use an inexpensive cylindrical worm, and the manufacturing cost can be reduced. As a result, a reduction mechanism that is small, inexpensive, and excellent in durability can be realized.
According to the second aspect of the present invention, the worm wheel tooth profile of the worm wheel tooth profile portion and the region where the angle formed between the contact line of the worm and the sliding direction of the worm is less than the threshold value are provided with the recess, Is avoided. As a result, in the meshing area excluding the concave portion, the angle formed by the contact line and the sliding direction of the worm exceeds the threshold value, so that the oil film does not easily break on the entire tooth surface, and smooth lubrication can be ensured. it can. Therefore, wear can be suppressed over a long period of time, and the durability can be remarkably improved.

  Moreover, in the area | region except a recessed part among worm wheel tooth profile shape parts, the angle which the said contact line and the sliding direction of a worm make becomes small and it is easy to produce oil film cutting | disconnection, so that it approaches a recessed part regarding a tooth width direction. In other words, there is a recess where the lubricant accumulates closest to the region where the oil film is most likely to be cut (corresponding to the edge of the recess in the tooth width direction and the vicinity thereof). Therefore, since effective lubrication can be performed on the region where the oil film is most likely to be cut, the overall lubrication effect is high. Moreover, since the lubricant tends to be sent to the recess from the worm wheel tooth profile portion by the so-called wedge effect (a phenomenon in which the lubricant is sucked into a narrow gap as it rotates), the lubricant in the recess Can be kept abundantly. Also from this point, the lubricating effect can be enhanced.

Moreover, when the said recessed part is formed by forging like Claim 3, a tooth surface can be formed easily and manufacturing cost can be made cheap.
According to the invention of claim 4, it is possible to realize an electric power steering device which is small, inexpensive and excellent in durability.

1 is a schematic diagram showing a schematic configuration of an electric power steering apparatus including a reduction gear according to an embodiment of the present invention. It is a schematic perspective view of a reduction gear. It is a schematic perspective view of the principal part of a reduction gear. (A) is typical sectional drawing which shows the worm wheel tooth profile of a reduction gear, and the meshing state of a worm. (B) is typical sectional drawing as a reference figure which shows the worm wheel tooth profile of the reduction gear when not providing a recessed part, and the meshing state of a worm. It is a schematic perspective view of the principal part of the reduction gear of another embodiment of this invention.

Embodiments of the present invention will be specifically described below with reference to the drawings.
FIG. 1 is a schematic diagram of an electric power steering apparatus including a reduction gear according to an embodiment of the present invention. Referring to FIG. 1, an electric power steering device 1 assists steering by a steering wheel 2 as a steering member, a steering mechanism 4 that steers the steered wheels 3 in conjunction with rotation of the steering wheel 2, and a driver. A steering assist mechanism 5 is provided. The steering wheel 2 and the steering mechanism 4 are mechanically connected via a steering shaft 6 and an intermediate shaft 7.

In the present embodiment, a description will be given according to an example in which the steering assist mechanism 5 applies assist force (steering assist force) to the steering shaft 6. However, the present invention can also be applied to a structure in which the steering assist mechanism 5 applies assist force to the pinion shaft described later.
The steering shaft 6 includes an input shaft 8 connected to the steering wheel 2 and an output shaft 9 connected to the intermediate shaft 7. The input shaft 8 and the output shaft 9 are connected via a torsion bar 10 so as to be relatively rotatable on the same axis.

  A torque sensor 11 disposed around the steering shaft 6 detects the steering torque input to the steering wheel 2 based on the relative rotational displacement amounts of the input shaft 8 and the output shaft 9. The torque detection result of the torque sensor 11 is input to an ECU (Electronic Control Unit) 12 as a motor control device for assisting steering. Further, the vehicle speed detection result from the vehicle speed sensor 90 is input to the ECU 12. The intermediate shaft 7 connects the steering shaft 6 and the steering mechanism 4.

The steering mechanism 4 includes a rack and pinion mechanism including a pinion shaft 13 and a rack shaft 14 as a steered shaft. A steered wheel 3 is connected to each end of the rack shaft 14 via a tie rod 15 and a knuckle arm (not shown).
The pinion shaft 13 is connected to the intermediate shaft 7. The pinion shaft 13 rotates in conjunction with the steering of the steering wheel 2. A pinion 16 is provided at the tip (lower end in FIG. 1) of the pinion shaft 13.

  The rack shaft 14 extends linearly along the left-right direction of the automobile. A rack 17 that meshes with the pinion 16 is formed in the middle of the rack shaft 14 in the axial direction. By the pinion 16 and the rack 17, the rotation of the pinion shaft 13 is converted into the axial movement of the rack shaft 14. The steered wheel 3 can be steered by moving the rack shaft 14 in the axial direction.

When the steering wheel 2 is steered (rotated), this rotation is transmitted to the pinion shaft 13 via the steering shaft 6 and the intermediate shaft 7. The rotation of the pinion shaft 13 is converted into an axial movement of the rack shaft 14 by the pinion 16 and the rack 17. Thereby, the steered wheel 3 is steered.
The steering assist mechanism 5 includes a steering assist electric motor 18 and a speed reduction mechanism 19 as a transmission mechanism for transmitting the output torque of the electric motor 18 to the steering mechanism 4. The reduction mechanism 19 includes a worm 20 as a drive gear and a reduction gear 31 that meshes with the worm 20. The speed reduction mechanism 19 is accommodated in the gear housing 21. In the gear housing 21, at least a meshing region between the worm 20 and the reduction gear 31 is filled with a lubricant such as grease, and the lubricant is interposed between the tooth surfaces of the worm 20 and the reduction gear 31. Yes.

The worm 20 is connected to a rotating shaft (not shown) of the electric motor 18 via a joint (not shown). The worm 20 is rotationally driven by the electric motor 18. The reduction gear 31 is connected to the steering shaft 6 so as to be integrally rotatable.
When the electric motor 18 rotationally drives the worm 20, the reduction gear 31 is rotationally driven by the worm 20, and the reduction gear 31 and the steering shaft 6 rotate integrally. The rotation of the steering shaft 6 is transmitted to the pinion shaft 13 via the intermediate shaft 7. The rotation of the pinion shaft 13 is converted into the axial movement of the rack shaft 14. Thereby, the steered wheel 3 is steered. That is, the steered wheels 3 are steered by rotating the worm 20 by the electric motor 18.

  The electric motor 18 is a three-phase brushless motor, and is controlled by the ECU 12 as a motor control device. The ECU 12 controls the electric motor 18 based on the torque detection result from the torque sensor 11, the vehicle speed detection result from the vehicle speed sensor 90, and the like. Specifically, the ECU 12 determines a target assist amount using a map in which the relationship between the torque and the target assist amount is stored for each vehicle speed, and controls the assist force generated by the electric motor 18 to approach the target assist amount. To do.

FIG. 2 is a schematic perspective view of the reduction gear 31 according to an embodiment of the present invention. The reduction gear 31 is made of metal and is formed by forging. The reduction gear 31 has a center hole 31a. The reduction gear 31 has a large number of teeth 32 having an advance angle A1 on its outer periphery 31b. A tooth gap 33 is formed between adjacent tooth portions 32, 32.
As shown in FIG. 3, each tooth portion 32 of the reduction gear 31 has a tooth surface 34. The tooth surface 34 includes a worm wheel tooth profile portion 35 as a reference tooth profile shape and a recess 36 that is recessed from the worm wheel tooth profile shape portion 35. The position of the concave portion 36 is arranged at the center portion in the tooth width direction W1, and the concave portion 36 is recessed from the worm wheel tooth profile portion 35 so that the concave portion 36 and the worm 20 are not engaged with each other. The recess 36 is preferably formed by forging in order to reduce the manufacturing cost.

  Specifically, the recess 36 is formed at a position that satisfies the following conditions. FIG. 4B is a schematic cross-sectional view as a reference diagram showing the meshing state of the reduction gear 31 ′ and the worm 20 when the entire tooth surface 34 is the worm wheel tooth profile portion 35. As shown in FIG. 4B, the contact line CL when the worm wheel tooth profile of the worm wheel tooth profile portion 35 contacts the worm 20 is indicated by a solid line. Further, the sliding direction S1 of the worm 20 is indicated by a one-dot chain line as a concentric circle with the central axis of the worm 20 as the center.

  In FIG. 4B, an angle B formed by the contact line CL and the sliding direction S1 is equal to or less than a threshold value B1 (B ≦ B1. The threshold value B1 is set to a value within a range of 5 to 10 °, for example, 10 °. In the region E <b> 1, it is difficult to form an oil film with a lubricant. In the region E1, as shown in FIG. 4A, a recess 36 is formed so that the engagement between the recess 36 and the worm 20 is avoided.

Although the threshold value B1 is not shown, when the cylinder corresponding to the worm is brought into line contact with the tooth shape of the worm wheel and the cylinder is slid at an angle corresponding to the threshold value B1, the oil film in the tooth shape of the worm wheel is formed. It is determined to be a preferable value by experimentally determining the formation state.
According to the present embodiment, since the metal reduction gear 31 is used, it is small and has high strength. Further, since the tooth contact between the tooth surface 34 (worm wheel tooth profile portion) of the reduction gear 31 and the worm 20 is in line contact, a good contact state can be secured from the beginning, and the occurrence of initial wear can be prevented. .

In general, when the worm wheel tooth profile of the reduction gear is meshed with, for example, a cylindrical worm 20, as shown in FIG. 4B, the central portion of the tooth surface of the reduction gear 31 'in the tooth width direction W1 is the worm wheel tooth profile. And a contact line CL of the worm 20 includes a region D1 where the worm 20 has a sliding direction S1.
In the present embodiment, as shown in FIG. 4 (a), a recess 36 is provided in the central portion in the tooth width direction W1 to avoid meshing with the worm 20, so that there is a possibility that it will be the most severe for durability. Wear can be reliably suppressed at the portion, and as a result, the durability of the speed reduction mechanism 19 as a whole can be improved. In addition, with the reduction gear 31 having the above-described configuration, it is possible to use an inexpensive cylindrical worm 20 and to reduce the manufacturing cost.

  In particular, in the recess 36 (see FIG. 4A) for avoiding meshing with the worm 20, the angle B formed between the worm wheel tooth profile and the contact line CL of the worm 20 and the sliding direction S1 of the worm 20 is equal to or less than the threshold value B1. It is provided in the region E1 (see FIG. 4B). As a result, in the meshing region excluding the recess 36, the angle B formed by the contact line CL and the sliding direction S1 of the worm 20 becomes a value exceeding the threshold value B1 (B ≧ B1). It is difficult to occur and smooth lubrication can be ensured. Therefore, wear can be suppressed over a long period of time, and the durability can be remarkably improved.

  Further, in the region of the worm wheel tooth profile portion 35 excluding the recess 36, the closer to the recess 36 in the tooth width direction W1, the smaller the angle B formed between the contact line CL and the sliding direction S1 of the worm 20, and the oil film breakage occurs. It tends to occur easily. In other words, the lubricant accumulates closest to the region where oil film breakage is most likely to occur (corresponding to the edges 36a and 36b of the recess 36 and the vicinity thereof in the tooth width direction W1 in FIG. 4A). There is a recess 36. Therefore, since effective lubrication can be performed on the region where the oil film is most likely to be cut, the overall lubrication effect is high. Moreover, since the lubricant tends to be sent from the worm wheel tooth profile portion 35 to the recess 36 due to a so-called wedge effect (a phenomenon in which the lubricant is sucked into a narrow gap as it rotates), the recess 36 is lubricated. The agent can be kept amply. Also from this point, the lubricating effect can be enhanced.

The lubricant may be filled in the recess 36 from the beginning of use, but if not, it is collected in the recess due to the wedge effect. However, if the recess 36 is filled from the beginning of use, it is preferable in that the lubricant can be retained in the recess 36 from the initial use for a long time.
Moreover, when the recessed part 36 is formed by forging, the tooth surface 34 can be formed easily and manufacturing cost can be made cheap. Therefore, the reduction mechanism 19 that is small, inexpensive, and excellent in durability can be realized, and eventually, the electric power steering device 1 that is small, inexpensive, and excellent in durability can be realized.

  In the above-described embodiment, the entire tooth surface 34 of the reduction gear 31 forms a worm wheel tooth profile. However, the present invention is not limited to this, and as shown in FIG. One tooth surface 41 of the surfaces 41, 51 is formed as a reference tooth profile shape on a main body portion 42 having a helical tooth profile shape and a part of the tooth surface 41 (corresponding to a tooth contact area with the worm 20). There may be provided a worm wheel tooth profile portion 43 made of a recessed portion and a recess 44 recessed from the worm wheel tooth profile portion 43.

Similarly, the other tooth surface 51 is a concave portion formed in a main portion 52 having a helical tooth shape as a reference tooth shape and a part of the tooth surface 51 (corresponding to a tooth contact area with the worm 20). The worm wheel tooth profile portion 53 may be provided with a recess 54 that is recessed from the worm wheel tooth profile portion 53.
The worm wheel tooth profile part 43 of one tooth surface 41 and the worm wheel tooth profile part 53 of the other tooth surface 51 are arranged at substantially the same position in the tooth width direction. Accordingly, the positions of the worm wheel tooth profile portions 43 and 53 correspond to the actual tooth contact area when the reduction gear 131 is engaged with the worm 20. The positions of the recesses 44 and 54 in the worm wheel tooth profile portions 43 and 53 are arranged at the center in the tooth width direction W1 as shown in FIG.

Although not shown, each of the recesses 44 and 54 is the angle formed between the worm wheel tooth profile and the contact line CL of the worm 20 and the sliding direction S1 of the worm 20 in the same manner as shown in FIGS. 4 (a) and 4 (b). It is provided in a region where B is equal to or less than the threshold B1.
Also in this embodiment, the same effect as the embodiment of FIG. 3 can be obtained. In addition, since the reference tooth profile of the tooth surfaces 41 and 51 is a helical tooth profile, the worm wheel tooth profile and the recess can be formed by forging from the axial direction, which makes it easier to manufacture.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims.

  DESCRIPTION OF SYMBOLS 1 ... Electric power steering device, 4 ... Steering mechanism, 5 ... Steering assist mechanism, 6 ... Steering shaft, 18 ... Electric motor, 19 ... Reduction mechanism, 20 ... Worm, 31; 131 ... Reduction gear, 32; 132 ... Tooth part 33 ... tooth gap, 34 ... tooth surface, 35 ... worm wheel tooth profile, 36 ... recess, 36a, 36b ... edge, 41, 51 ... tooth surface, 42, 52 ... main part, 43, 53 ... worm wheel Tooth profile portion, 44, 54 ... concave, B ... angle, B1 ... threshold, CL ... contact line, S1 ... sliding direction, W1 ... tooth width direction

Claims (4)

A worm, a metal reduction gear meshing with the worm, and a lubricant interposed between the worm and the reduction gear,
The reduction gear includes a tooth surface,
The tooth surface includes a worm wheel tooth profile portion formed at least in the center in the tooth width direction, and a recess that is recessed from the worm wheel tooth profile portion to avoid meshing with the worm.   2. The deceleration according to claim 1, wherein the concave portion is disposed in a region where an angle formed between a contact line when the worm wheel tooth profile of the worm wheel tooth profile portion contacts the worm and a sliding direction of the worm is equal to or less than a threshold value. mechanism.   The reduction mechanism according to claim 1 or 2, wherein the recess is formed by forging.   An electric power steering apparatus that decelerates rotation of an electric motor for assisting steering using the speed reduction mechanism according to claim 1.

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