JP2005016646A - Gearing mechanism - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gearing mechanism capable of securing both the strength of the gear and the contact ratio. <P>SOLUTION: At the tooth bottom of a rack 9 in the large pressure angle region, a thinned part 20 is formed so as not to intrude into the effective tooth height H as the range where a pinion 8 and the rack 9 are contacting actually. The thinned part 20 is continued smoothly to the rack meshing tooth face 15 through the first circular arc surface 22, which allows securing the contact ratio between the pinion 8 and the rack 9. A second circular arc surface 24 is formed in such a way as smoothly continued to the first circular arc surface 22. Each tooth bottom of the rack 9 is formed from two adjoining second circular arc surfaces 24. Accordingly the stress concentration generated on the rack is relieved by meshing with the pinion 8. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gear mechanism such as a rack and pinion mechanism.
[0002]
[Prior art]
An example of the gear mechanism is a rack and pinion mechanism. This rack-and-pinion mechanism meshes a rack and pinion (small gear) whose radius of the pitch circle of the circular gear is infinite, for example, to convert the rotational movement of the pinion into the reciprocating movement of the rack, or conversely the rack It is a mechanism that converts the reciprocating motion of the motor into the rotational motion of the pinion. In many such rack and pinion mechanisms, in order to ensure the strength of the rack, for example, an arc surface having a predetermined radius of curvature is formed on the bottom of the rack.
[0003]
[Patent Document 1]
JP-A-7-277207 [0004]
[Problems to be solved by the invention]
However, the conventional rack and pinion mechanism has the following problems. That is, depending on the reference pressure angle of the rack teeth (the angle of inclination of the rack teeth), the rack bottom may not be sufficiently secured. This is remarkable when, for example, the reference pressure angle is 30 degrees or more. In this case, for example, as shown in FIG. 5A, an arc surface 31 is formed on the tooth bottom of the rack 30 as much as possible. However, this makes it difficult to sufficiently secure the radius of curvature of the arc surface 31, and as a result, it is difficult to sufficiently relieve stress concentration due to engagement with the pinion 32.
[0005]
In order to solve such a problem, a rack and pinion mechanism as shown in FIG. 5B has been proposed. That is, a thinned portion 36 is formed on the bottom 35 of the rack 34 constituting the rack and pinion mechanism 33, and the meshed portion 36 and the meshing tooth surface 37 of the rack 34 have a predetermined radius of curvature. It continues smoothly through the circular arc surface 38 (see, for example, Patent Document 1). However, in the rack and pinion mechanism 33, the meat thin portion 36 is formed without considering the influence on the meshing between the rack 34 and the pinion 39.
[0006]
That is, the meat thinning portion 36 is formed so as to erode the meshing tooth surface 37 from the middle of the meshing tooth surface 37 involved in the meshing of the pinion 39 and the rack 34. In other words, the meat thinning portion 36 is formed so as to enter the effective toothpaste H that is a range where the pinion 39 and the rack 34 actually contact each other. For this reason, the meshing area (meshing length) between the teeth of the pinion 39 and the teeth of the rack 34 is reduced by the amount that the meat thinning portion 36 enters the effective toothpaste H, and consequently the meshing rate with the pinion 39. There was a problem that decreased.
[0007]
In order to solve such a problem, for example, a configuration as shown in FIG. That is, the tooth bottom is secured by displacing the tooth bottom height of the rack 40 in the tooth tip direction. As a result, a sufficient radius of curvature of the arc surface 41 formed on the tooth bottom of the rack 40 can be secured. However, in this case, it is necessary to reduce the total tooth depth 43 of the pinion 42 that meshes with the rack teeth by an amount corresponding to the height of the tooth bottom of the rack 40. Therefore, although the rack teeth can sufficiently secure the strength, it is difficult to secure the meshing rate with the pinion 42.
[0008]
As described above, in a rack having a reference pressure angle (for example, an angle of 30 degrees or more) that does not sufficiently secure the tooth bottom, one of the engagement ratio with the pinion and the rack tooth strength is sacrificed. Therefore, it was difficult to ensure a good balance between the two.
[0009]
The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a gear mechanism that can ensure both the strength and the engagement rate of the gear.
[0010]
[Means for Solving the Problems]
In order to solve the above problem, the invention according to claim 1 includes first and second gears meshing with each other, and at least a part of the tooth bottoms in either one of the first and second gears. In the gear mechanism in which the thinned portion is provided, the thinned portion is provided so as not to enter the effective toothpaste from the tooth tip surface of the first or second gear, and the first or second The gist is to include a circular arc surface continuous to the meshing tooth surface of the gear.
[0011]
According to a second aspect of the present invention, in the gear mechanism according to the first aspect, the meshing tooth surface of the first or second gear provided with the thinned portion is extended in a direction opposite to the tooth tip. The gist is that an inclined surface is formed by the above-described method, and the circular arc surface is smoothly continued to the inclined surface.
[0012]
According to a third aspect of the present invention, in the gear mechanism according to the first or second aspect, one of the first and second gears that forms the thinned portion is a rack, and the other gear. Is a pinion that meshes with the rack.
[0013]
The invention according to claim 4 is the gear mechanism according to claim 3, wherein the rack is a variable rack whose reference pressure angle increases toward the both ends or toward the center. To do.
[0014]
According to a fifth aspect of the present invention, in the gear mechanism according to the fourth aspect, a rack reference tooth having a reference pressure angle that is greater than or equal to the threshold reference pressure angle is set in advance so that a threshold reference pressure angle at which it is difficult to secure a tooth bottom It is summarized that the meat thinning portion is provided only on the bottom of the tooth.
[0015]
A sixth aspect of the present invention is the gear mechanism according to any one of the first to fifth aspects, wherein the thinning portion is different from the circular arc surface smoothly connected to the circular arc surface. The gist is to include an arc surface, and the other arc surface constitutes at least a part of the tooth bottom.
[0016]
(Function)
According to the first aspect of the present invention, the thinned portion is provided on at least a part of the tooth bottom of one of the first gear and the second gear. The meat thinning portion is provided so as not to enter the effective toothpaste from the tooth tip surface of the first or second gear. For this reason, the meshing area (meshing length) of the first gear and the second gear is ensured. Further, the thinned portion is continuous with the meshing tooth surface of the first or second gear via an arc surface. For this reason, the stress concentration generated in the tooth bottom of the first gear or the second gear is mitigated by the meshing of the first gear and the second gear. In addition, the said meat thinning part means the space provided intentionally unlike the crest which can be naturally formed between the tooth tip of a 1st gearwheel, and the tooth bottom of a 2nd gearwheel.
[0017]
According to a second aspect of the present invention, in addition to the action of the first aspect of the invention, the meshing tooth surface of the first or second gear provided with the thinned portion is extended in the direction opposite to the tooth tip. By doing so, an inclined surface is formed. The circular arc surface continues smoothly on this inclined surface. For this reason, the assembly error of the first and second gears, the machining error of the first and second gears, and the like are absorbed. That is, even if there is some assembling error or machining error, the meshing area (meshing length) of the first and second gears is ensured.
[0018]
In the invention according to claim 3, in addition to the action of the invention according to claim 1 or 2, the meshing area (meshing length) between the rack and the pinion is secured. Further, the stress concentration generated in the tooth bottom of the rack is alleviated by the engagement between the rack and the pinion.
[0019]
According to the invention described in claim 4, in addition to the action of the invention described in claim 3, the meshing area (meshing length) between the variable rack and the pinion is ensured. In addition, the stress concentration generated in the bottom of the variable bullrack is mitigated by the engagement between the variable bullrack and the pinion.
[0020]
In the invention described in claim 5, in addition to the action of the invention described in claim 4, a threshold reference pressure angle at which it is difficult to secure the root is preset. Here, securing the tooth bottom refers to securing the tooth bottom to such an extent that an arc surface having a predetermined radius of curvature capable of securing the strength of the rack can be secured while securing the height of the tooth bottom. And the said thinning part is provided only in the base of the rack tooth which has a reference pressure angle more than this threshold reference pressure angle. That is, the meat thinning portion is provided only in a necessary portion. By providing a thinned portion at the bottom of the rack tooth having a reference pressure angle larger than the threshold reference pressure angle, stress concentration generated at the bottom of the tooth is alleviated without reducing the meshing rate.
[0021]
According to a sixth aspect of the present invention, in addition to the action of the first aspect of the present invention, an arc surface different from the arc surface smoothly continuing to the arc surface is provided. The another circular arc surface is provided and constitutes at least a part of the tooth bottom. For this reason, the stress concentration generated in the tooth bottom is further alleviated.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment in which the present invention is embodied in, for example, a rack and pinion mechanism of an electric power steering apparatus will be described with reference to FIGS.
[0023]
As shown in FIG. 1, the electric power steering apparatus 1 includes a steering wheel 2, and a steering shaft 3 is connected to the steering wheel 2. A reduction gear 4 is fixed to the steering shaft 3, and a gear 6 attached to a rotation shaft (not shown) of a motor 5 is engaged with the reduction gear 4. A pinion 8 is fixed to the reduction gear 4 via a pinion shaft 7, and the pinion 8 meshes with the rack 9. Tie rods 10 are fixed to both ends of the rack 9, and knuckle 11 is rotatably connected to the tip portions of both tie rods 10, 10, respectively. Tire) 12 is fixed. A cross member 13 is rotatably connected between the knuckles 11 and 11. The pinion 8 and the rack 9 constitute a rack and pinion mechanism 14.
[0024]
Now, when the steering wheel 2 is turned by the driver, the steering shaft 3 rotates. This rotation is transmitted to the rack 9 via the pinion shaft 7 and the pinion 8 and is converted into the axial movement of the rack 9. Thereby, both front wheels 12 and 12 are steered via the tie rod 10. At this time, the motor 5 is driven so as to generate a predetermined steering assist torque (assist torque) based on the steering torque detected by the torque sensor (not shown) and the vehicle speed detected by the vehicle speed sensor (not shown). To do. The rotational speed of the motor 5 is reduced by the reduction gear 4 and transmitted to the pinion shaft 7. The rotation of the pinion shaft 7 is transmitted to the rack 9 via the pinion 8 and converted into the axial movement of the rack 9. In this manner, assist torque is applied to the steering of the front wheels 12 by the turning operation of the steering wheel 2.
[0025]
In the present embodiment, the pinion 8 corresponds to the first gear or the second gear of the present invention, and the rack 9 corresponds to the second gear or the first gear. The rack and pinion mechanism 14 corresponds to the gear mechanism of the present invention.
[0026]
[rack]
Next, the rack 9 constituting the rack and pinion mechanism 14 will be described in detail.
[0027]
As shown in FIG. 2, the rack 9 has a steering gear ratio (that is, the turning angle of the front wheel 12 with respect to the steering angle of the steering wheel 2) according to the meshing position with the pinion 8, that is, according to the operation of the steering wheel 2. Is a variable rack that changes. In the present embodiment, the rack 9 is formed so that the steering gear ratio at the small steering angle is small and the steering gear ratio at the large steering angle is large. For this reason, it reacts gently at a small steering angle, and reacts quickly at a large steering angle. Therefore, for example, the stability of the steering operation is ensured during high-speed traveling on an expressway operated in a small steering angle region and during low-speed traveling such as garage entry where a large steering angle is operated.
[0028]
As shown in FIGS. 2 and 3, the rack 9 is formed such that the tooth-to-tooth pitch P decreases toward the center, and conversely the tooth-to-tooth pitch P increases toward the outside. ing. That is, the rack 9 has a larger number of teeth toward the center, and conversely, the number of teeth decreases toward the outer side.
[0029]
The rack 9 is formed such that the reference pressure angle θ decreases toward the center, and conversely increases toward the outside. In the present embodiment, the rack 9 is formed so that the reference pressure angle θ changes in the range of 20 degrees to 40 degrees. Incidentally, the reference pressure angle θ refers to an angle between the virtual plane s orthogonal to the pitch line ps and the tooth surface 15 of the rack 9.
[0030]
[Small pressure angle region and large pressure angle region]
As shown in FIG. 2, the rack 9 is divided into a small pressure angle region A and a large pressure angle region B with a preset reference pressure angle θ (hereinafter referred to as “threshold reference pressure angle”) as a boundary. . The threshold reference pressure angle is a value serving as a reference for determining whether it is difficult to secure the bottom of the rack 9. If the reference pressure angle θ is less than the threshold reference pressure angle, a sufficient tooth bottom can be secured, and if it is equal to or greater than the threshold reference pressure angle, it is difficult to secure a sufficient tooth bottom.
[0031]
The threshold reference pressure angle is obtained in advance by an experiment using a mechanism model (rack model), a known theoretical calculation, or the like. In the present embodiment, the threshold reference pressure angle is 30 degrees. That is, a portion where the reference pressure angle θ is 20 degrees or more and less than 30 degrees is a small pressure angle region A, and a portion where the reference pressure angle θ is 30 degrees or more and 40 degrees or less is a large pressure angle region B.
[0032]
The small pressure angle region A is disposed on the center side of the rack 9, and the large pressure angle region B is disposed on both ends of the rack 9 so as to be continuous with the small pressure angle region A. Further, the heights of the tooth bottoms in the small pressure angle region A and the large pressure angle region B are the same. In the small pressure angle region A, the tooth bottom is secured to the extent that an arc surface having a predetermined radius of curvature can be formed in a state where the height of the tooth bottom is secured (that is, without changing the height of the tooth bottom). Has been. In the present embodiment, a reference pressure angle θ of 20 degrees or more and less than 30 degrees is set as θ1, and a reference pressure angle θ of 30 degrees or more and 40 degrees or less is set as θ2, so that they are distinguished from each other.
[0033]
[Gear bottom shape in small pressure angle region]
As shown in FIG. 3, a parabolic surface 17 having a predetermined radius of curvature is convex downward (on the side opposite to the tooth tip of the rack 9) on the tooth bottom 16 of the rack 9 in the small pressure angle region A. Is formed. Specifically, between a pair of adjacent rack teeth, an inclined surface 18a is provided on the rack bottom side of the meshing tooth surface 15 (on the side opposite to the tooth tip of the meshing tooth surface 15, that is, the lower side in FIG. 3). An arc surface 18b is formed so as to extend smoothly and to be smoothly continuous with the inclined surface 18a. The inclined surface 18a starts to extend from the intersection of the tooth tip line 19 of the pinion 8 meshing with the rack teeth and the meshing tooth surface 15 of the rack 9, and the arc surface 18b is the effective tooth depth from the tooth tip surface of the rack 9. It is formed at a position deeper than H (rack bottom side).
[0034]
That is, the paraboloid 17 is composed of a pair of inclined surfaces 18 a and 18 a and a circular arc surface 18 b, and the paraboloid 17 is provided at a portion not involved in the engagement between the pinion 8 and the rack 9. For this reason, in the small pressure angle region A, the engagement ratio between the pinion 8 and the rack 9 is ensured. Further, since the parabolic surface 17 is formed on the tooth bottom 16 of the rack 9, interference with the teeth of the pinion 8 is avoided, and stress concentration generated in the rack teeth due to the engagement with the pinion 8 is reduced. The
[0035]
[Dental shape in large pressure angle region]
As shown in FIG. 4, in the tooth bottom 23 of the rack 9 in the large pressure angle region B, a thinned portion 20 is formed at a portion not involved in the engagement between the pinion 8 and the rack 9. Specifically, between the pair of adjacent rack teeth, an inclined surface 21 is formed to extend on the rack bottom side (the side opposite to the tooth tip of the meshing tooth surface 15, that is, the lower side in FIG. 4). A first arc surface 22 having a predetermined radius of curvature is formed so as to be smoothly continuous with the inclined surface 21. The first arc surface 22 is located on the rack bottom side with respect to the tooth tip line 19 of the pinion 8 meshing with the rack teeth. That is, the first arc surface 22 is formed at a position deeper than the effective tooth depth H from the tooth tip surface of the rack tooth. In other words, the first circular arc surface 22 is formed at a portion not involved in the engagement between the pinion 8 and the rack 9.
[0036]
A second arc surface 24 having a predetermined radius of curvature is formed so as to be smoothly continuous with the first arc surface 22. A crank-shaped curved surface is constituted by the first arc surface 22 and the second arc surface 24. A pair of second arc surfaces 24 adjacent to each other are smoothly continuous to form one curved surface (arc surface-shaped tooth bottom 23).
[0037]
A portion where the thinned portion 20 composed of the inclined surface 21, the pair of first arc surfaces 22, 22 and the pair of second arc surfaces 24, 24 is not involved in the engagement between the pinion 8 and the rack 9, that is, the pinion 8 and the rack 9 are formed in a portion that is out of the effective tooth depth H, which is a range where the rack 9 actually contacts. For this reason, the meshing rate of the pinion 8 and the rack 9 in the large pressure angle region B is ensured. Interference with the teeth of the pinion 8 is also avoided. In addition, the first arc surface 22 is smoothly continued to the inclined surface 21, and the first arc surface 22 is further continued to the second arc surface 24, thereby engaging with the pinion 8. Thus, the stress concentration generated in the rack teeth is alleviated.
[0038]
Incidentally, it is not necessary to provide the thinned portion 20 on the bottom 16 of the rack teeth in the small pressure angle region A. This is because the rack tooth having the reference pressure angle θ1 can secure the root as compared with the rack tooth having the reference pressure angle θ2, and when the paraboloid 17 is formed on the tooth bottom 16, the paraboloid 17 (strictly This is because a large radius of curvature of the arc surface 18b) can be secured. Generally, when the height of the tooth bottom is the same, it becomes more difficult to secure the tooth bottom as the reference pressure angle θ is increased. Therefore, even if an attempt is made to form a paraboloid 17 (strictly speaking, an arcuate surface 18b) similar to the rack teeth of the small pressure angle region A on the bottom 23 of the rack teeth of the large pressure angle region B, this paraboloid The radius of curvature of the (arc surface) is smaller than the parabolic surface 17 (arc surface 18b) of the rack teeth in the small pressure angle region A. Therefore, there is a possibility that the stress concentration generated due to the engagement with the pinion 8 cannot be sufficiently relaxed. In the present embodiment, by providing the thinned portion 20 on the bottom of the rack tooth in the large pressure angle region B where the bottom of the tooth cannot be sufficiently secured, the height of the bottom is not displaced, and the pinion The strength of the rack teeth is ensured without lowering the meshing rate with 8.
[0039]
Therefore, according to the rack and pinion mechanism 14 in the present embodiment, both the meshing rate and the strength with the pinion 8 are ensured in a balanced manner in all the rack teeth in the small pressure angle region A and the large pressure angle region B. . Generally, the greater the meshing rate, the greater the number of teeth meshed at a time, and the vibration and noise associated with power transmission are suppressed. Therefore, quiet and smooth power transmission is possible in the rack and pinion mechanism 14, and the reliability and steering feeling of the electric power steering device are also improved.
[0040]
In the present embodiment, the first arc surface 22 corresponds to an arc surface of the present invention, and the second arc surface 24 corresponds to another arc surface.
[Effect of the embodiment]
Therefore, according to the present embodiment, the following effects can be obtained.
[0041]
(1) The tooth bottom 23 of the rack 9 in the large pressure angle region B does not participate in meshing with the pinion 8, that is, within a range where the pinion 8 and the rack 9 actually contact and participate in meshing. The meat thinning portion 20 is formed so as to be separated from a certain effective toothpaste H (that is, not to enter). For this reason, the meshing rate of the pinion 8 and the rack 9 is ensured. Further, by forming the first arc surface 22 so as to be continuous with the meshing tooth surface 15 of the rack 9, the tooth bottom 23 of the rack 9 (the inner bottom surface of the meat thinning portion 20) can be sufficiently secured, A sufficient radius of curvature of the second arcuate surface 24 can be ensured. For this reason, the stress concentration generated in the rack 9 (strictly speaking, the bottom of the rack 9) due to the engagement with the pinion 8 is alleviated. Therefore, both the strength of the rack 9 and the engagement ratio with the pinion 8 can be ensured.
[0042]
(2) The electric power steering apparatus 1 is provided with a rack and pinion mechanism 14. According to the rack and pinion mechanism 14, both the meshing rate and the strength with the pinion 8 are ensured in a balanced manner in all rack teeth in the small pressure angle region A and the large pressure angle region B. For this reason, the rack and pinion mechanism 14 can perform quiet and smooth power transmission. By providing such a rack and pinion mechanism 14, the reliability and steering feeling of the electric power steering apparatus 1 can be improved.
[0043]
(3) In the large pressure angle region B, an inclined surface 21 is formed by extending the meshing tooth surface 15 in a direction opposite to the tooth tip, and the first arc surface 22 is smoothly connected to the inclined surface 21. I tried to make it. The inclined surface 21 is obtained by extending the meshing tooth surface 15 as it is with the same inclination angle. For this reason, an assembly error between the pinion 8 and the rack 9 and a processing error of the pinion 8 and the rack 9 are absorbed. That is, even if there is a slight assembly error or processing error, the meshing area (meshing length) between the pinion 8 and the rack 9 is ensured. Therefore, the engagement ratio between the pinion 8 and the rack 9 can be ensured more reliably.
[0044]
[Another example]
In addition, you may implement this embodiment, changing into another example as follows.
In the present embodiment, the present invention is embodied in the rack and pinion mechanism 14. However, for example, the present invention may be embodied in a spur gear mechanism including a pair of spur gears (not shown) that mesh with each other. Even if it does in this way, the effect similar to the effect as described in (1)-(3) of this embodiment can be acquired.
[0045]
In the present embodiment, the rack 9 is a variable rack in which the pitch P and the reference pressure angle θ are displaced, but may be a constant track in which the pitch P and the reference pressure angle θ are fixed. This is particularly effective when the reference pressure angle θ2 is constant. Even if it does in this way, the effect similar to the effect as described in (1)-(3) of this embodiment can be acquired.
[0046]
In the present embodiment, the inclined surface 21 is formed by extending the meshing tooth surface 15 with the same inclination angle as it is, and the first arc surface 22 is smoothly continued to the inclined surface 21. It may be as follows. That is, the inclined surface 21 may be omitted, and the first arc surface 22 may be directly connected to the meshing tooth surface 15 of the rack 9. Even if it does in this way, the effect similar to the effect as described in (1)-(3) of this embodiment can be acquired.
[0047]
In the present embodiment, the second arc surface 24 is made continuous with the first arc surface 22, but a flat surface (flat surface) is formed between the first arc surface 22 and the second arc surface 24. Part) may be provided.
[0048]
In the present embodiment, in the large pressure angle region B, a pair of second arcuate surfaces 24 adjacent to each other is smoothly continued to form one curved surface, that is, an arcuate tooth bottom 23. A flat surface (a plane portion) may be provided between the second arcuate surfaces 24, 24.
[0049]
【The invention's effect】
As described above in detail, according to the present invention, both the strength and the meshing rate of the gear can be ensured.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an electric power steering apparatus according to an embodiment.
FIG. 2 is a cross-sectional view of a rack and pinion mechanism in the present embodiment.
FIG. 3 is an enlarged cross-sectional view of a main part of a rack and pinion mechanism in the present embodiment.
4A is an enlarged cross-sectional view of a main part of a rack and pinion mechanism in the present embodiment, and FIG. 4B is an enlarged cross-sectional view of a rack in the present embodiment.
FIGS. 5A to 5C are enlarged sectional views of main parts of a conventional rack and pinion mechanism.
[Explanation of symbols]
θ, θ1, θ2 ... reference pressure angle, 8 ... pinion (first gear or second gear),
9 ... Rack (second gear or first gear), 14 ... Rack and pinion mechanism,
20 ... Meat thinning portion, 21 ... Inclined surface, 22 ... First arc surface (arc surface),
23 ... tooth bottom, 24 ... second arc surface (another arc surface).

Claims (6)

A gear mechanism comprising first and second gears meshing with each other, wherein a thinned portion is provided on at least some of the tooth bottoms of either one of the first and second gears;
The meat thinning portion is provided so as not to enter the effective toothpaste from the tooth tip surface of the first or second gear, and includes an arc surface continuous to the meshing tooth surface of the first or second gear. Gear mechanism. An inclined surface is formed by extending the meshing tooth surface of the first or second gear provided with the thinning portion in a direction opposite to the tooth tip, and the arc surface is smoothly continued to the inclined surface. The gear mechanism according to claim 1, which is configured as described above. 3. The gear mechanism according to claim 1, wherein one of the first gear and the second gear that forms the thinned portion is a rack, and the other gear is a pinion that meshes with the rack. The gear mechanism according to claim 3, wherein the rack is a variable rack whose reference pressure angle increases toward the both end portions or toward the center portion. 5. The threshold reference pressure angle at which it is difficult to ensure the bottom of the tooth is set in advance, and the thinning portion is provided only on the bottom of the rack tooth having a reference pressure angle equal to or greater than the threshold reference pressure. Gear mechanism. The said thinning portion includes an arc surface different from the arc surface that is smoothly continuous with the arc surface, and the another arc surface constitutes at least a part of a tooth bottom. The gear mechanism of any one of Claims.

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