WO2020116087A1

WO2020116087A1 - Power transmitting device and electric power steering device with same

Info

Publication number: WO2020116087A1
Application number: PCT/JP2019/043880
Authority: WO
Inventors: 修平千葉
Original assignee: Ｋｙｂ株式会社
Priority date: 2018-12-07
Filing date: 2019-11-08
Publication date: 2020-06-11
Also published as: JP2020091018A

Abstract

A power transmitting device (60) is provided with: an electric motor (50) having a rotating shaft (51); an worm gear (43) provided to the rotating shaft (51); a worm wheel (44) meshing with the worm gear (43); and a housing (45) for accommodating the worm gear (43) and the worm wheel (44). A first axis (C1) which is the center of rotation of the worm gear (43) is tilted relative to a second axis (C2) which is the center of rotation of the worm wheel (44) so that the worm wheel (44) can be mounted after the worm gear (43) is mounted to the housing (45).

Description

Power transmission device and electric power steering device including the same

The present invention relates to a power transmission device and an electric power steering device including the power transmission device.

JP2003-113909A discloses a power transmission device that transmits the power of an electric motor via a worm mechanism. A worm gear is formed on the rotary shaft of the electric motor of the power transmission device.

In the power transmission device described in JP2003-113909A, since the worm gear supported by the housing is formed on the rotary shaft of the electric motor, the performance of the electric motor is confirmed in the state of being assembled in the housing. However, since the worm wheel is also attached to the housing, the performance of the electric motor is confirmed when the worm gear and the worm wheel are in mesh with each other. When various performances such as cogging torque and torque ripple of the electric motor are measured in such a state, it is clear whether the measurement result is caused by the electric motor or whether the worm gear and the worm wheel are affected by the meshing condition. Not. Therefore, it is difficult to accurately determine whether or not the electric motor satisfies a predetermined performance.

　In order to accurately measure the performance of the electric motor when assembled in the housing, it is possible to assemble the worm wheel in the housing after confirming the performance of the electric motor. However, since the worm gear formed on the rotary shaft of the electric motor is a spiral gear, it is difficult to assemble the worm wheel into the housing after the worm gear is assembled.

The present invention aims to improve the performance measurement accuracy of an electric motor used in a power transmission device.

According to an aspect of the present invention, a power transmission device includes an electric motor having an output shaft, a worm gear provided on the output shaft, a worm wheel that meshes with the worm gear, and a housing that houses the worm gear and the worm wheel. The first axis that is the rotation center of the worm gear can be attached to the second axis that is the rotation center of the worm wheel after the worm gear is attached to the housing. Is inclined to become.

FIG. 1 is a configuration diagram of an electric power steering device in which a power transmission device according to an embodiment of the present invention is used. FIG. 2 is a cross-sectional view of the power transmission device according to the embodiment of the present invention. FIG. 3 is a cross-sectional view of the power transmission device taken along the line III-III in FIG. FIG. 4 is a schematic diagram showing the relationship between the first axis and the second axis projected on the projection surface. FIG. 5 is a view for explaining the assembling procedure of the power transmission device, and is a cross-sectional view showing the same cross section as FIG. 2. FIG. 6 is a view for explaining the assembling procedure of the power transmission device, and is a cross-sectional view showing the same cross section as FIG. 3.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

With reference to FIG. 1, a power transmission device 60 according to an embodiment of the present invention and an electric power steering device 100 in which the power transmission device 60 is used will be described. FIG. 1 is a configuration diagram of an electric power steering device 100 in which a power transmission device 60 is used.

The electric power steering device 100 is a device that is installed in a vehicle and assists a steering force with respect to a steering device that steers the wheels 1 by converting a steering torque applied to a steering wheel 10 as a steering wheel by a driver. ..

As shown in FIG. 1, the electric power steering apparatus 100 includes a steering shaft 20 that rotates by a steering torque input from a steering wheel 10, a rack shaft 30 that steers the wheels 1 as the steering shaft 20 rotates, The power transmission device 60 applies a rotational torque to the steering shaft 20 to assist the steering torque.

The steering shaft 20 includes an input shaft 21 that rotates in response to a steering operation performed by a driver who operates the steering wheel 10, an output shaft 22 that displaces the rack shaft 30, and a torsion bar that connects the input shaft 21 and the output shaft 22. 23 and.

The rack shaft 30 is connected to the wheel 1 via a tie rod 31 and a knuckle arm 32, and the wheel 1 is steered by the displacement of the rack shaft 30.

The output shaft 22 and the rack shaft 30 are connected to each other via a transmission unit 41. The transmission unit 41 is a rack and pinion mechanism including a pinion gear 41a provided on the end of the output shaft 22 and a rack gear 41b provided on the rack shaft 30. The pinion gear 41a and the rack gear 41b mesh with each other, and the torque of the output shaft 22 is converted into an axial load of the rack shaft 30 and transmitted to the rack shaft 30 via the pinion gear 41a and the rack gear 41b. As a result, the rack shaft 30 is displaced in the axial direction by the transmitted torque and steers the wheel 1.

The power transmission device 60 includes an electric motor 50 that is a brushless motor, and a speed reduction unit 42 that decelerates the rotation of the electric motor 50 and transmits the decelerated rotation to the steering shaft 20. The specific configuration of the power transmission device 60 will be described later.

The reduction unit 42 is a worm gear mechanism including a worm gear 43 as a drive gear provided on a rotating shaft 51, which will be described later, as an output shaft of the electric motor 50, and a worm wheel 44 as a driven gear provided on the output shaft 22. .. The worm gear 43 and the worm wheel 44 mesh with each other, and the torque of the electric motor 50 is transmitted to the output shaft 22 via the worm gear 43 and the worm wheel 44. The torque transmitted from the electric motor 50 to the output shaft 22 is further transmitted to the rack shaft 30 via the pinion gear 41a and the rack gear 41b.

The electric power steering apparatus 100 further includes a torque sensor 72 that detects a torque acting on the torsion bar 23, and a controller 70 that controls driving of the electric motor 50 according to a detection value of the torque sensor 72.

The controller 70 includes a CPU (Central Processing Unit) that performs arithmetic processing, a ROM (Read-Only Memory) that stores a control program executed by the CPU, and a RAM (random access memory) that stores the arithmetic result of the CPU. And a microcomputer including. The controller 70 may be composed of a single microcomputer or may be composed of a plurality of microcomputers.

The torque sensor 72 detects the steering torque applied to the input shaft 21 according to the steering operation by the driver, and outputs a voltage signal corresponding to the detected steering torque to the controller 70. The controller 70 calculates the torque output by the electric motor 50 based on the voltage signal from the torque sensor 72, and controls the drive of the electric motor 50 so that the torque is generated.

As described above, the electric power steering apparatus 100 detects the steering torque applied to the input shaft 21 by the torque sensor 72, and controls the drive of the electric motor 50 by the controller 70 based on the detection result, thereby controlling the driver. Assist steering operation.

Next, a specific configuration of the power transmission device 60 will be described with reference to FIGS. 2 and 3. 2 is a cross-sectional view of the power transmission device 60, and FIG. 3 is a cross-sectional view of the power transmission device 60 taken along the line III-III of FIG.

As described above, the power transmission device 60 includes the electric motor 50 and the speed reducer 42. As shown in FIG. 2, the electric motor 50 has a rotating shaft 51 as an output shaft, a rotor 52 fixed to the rotating shaft 51, and a stator 53 arranged so as to face the rotor 52, and is decelerated. The portion 42 includes a worm gear 43 provided on the rotating shaft 51 of the electric motor 50 and a worm wheel 44 provided on the output shaft 22. Further, a controller 70 having a control board 71 for controlling the drive of the electric motor 50 is arranged adjacent to the outside of the rotating shaft 51 of the electric motor 50 in the axial direction. The controller 70 may be arranged at a position away from the electric motor 50.

The shaft provided with the worm gear 43 is not connected to the rotary shaft 51 of the electric motor 50 via a coupling such as a coupling, but is formed integrally with the shaft to which the rotor 52 is fixed, that is, the rotary shaft 51. There is.

The worm gear 43 rotates about a first axis C1 that is the axis of the rotating shaft 51, and the worm wheel 44 rotates about a second axis C2 that is the axis of the output shaft 22.

The power transmission device 60 further includes a housing 45 that accommodates the worm gear 43 and the worm wheel 44. As shown in FIGS. 2 and 3, the housing 45 has a first accommodation hole 46 for accommodating the worm gear 43 and a second accommodation hole 47 as a accommodation hole for accommodating the worm wheel 44. It is formed so as to intersect the one accommodation hole 46. Further, an insertion hole 48 through which the output shaft 22 is inserted is formed in the housing 45 coaxially with the second accommodation hole 47.

The first accommodation hole 46 and the second accommodation hole 47 partially overlap and communicate with each other. As described above, the worm gear 43 and the worm wheel 44 mesh with each other in the portion where the first accommodation hole 46 and the second accommodation hole 47 communicate with each other.

A first bearing 54 and a second bearing 55 that rotatably support the rotating shaft 51 are held in the first housing hole 46 that houses the worm gear 43. The first bearing 54 and the second bearing 55 are so-called rolling bearings configured by an inner ring, an outer ring, and a plurality of rolling elements arranged between the inner ring and the outer ring. The first bearing 54 is arranged between the rotor 52 and the worm gear 43, and the second bearing 55 is arranged on the opposite side of the first bearing 54 with the worm gear 43 interposed therebetween.

Here, when the worm gear 43 supported by the housing 45 via the

bearings

54 and 55 is formed on the rotating shaft 51 of the electric motor 50 as described above, the performance of the electric motor 50 can be confirmed by checking the electric motor 50. Generally, it is performed in a state where it is assembled to the housing 45, that is, in a state where the worm gear 43 and the worm wheel 44 mesh with each other.

However, when various performances such as the cogging torque and the torque ripple of the electric motor 50 are measured in such a state, the influence of the meshing condition between the worm gear 43 and the worm wheel 44 may be attributed to whether the measurement result is due to the electric motor 50. I'm not sure if I have received it. Therefore, it is difficult to accurately determine whether or not the electric motor 50 satisfies the predetermined performance.

Therefore, in the present embodiment, in order to enable the performance of the electric motor 50 to be measured in a state where the worm wheel 44 is not assembled to the housing 45, the first axis C1 that is the rotation center of the worm gear 43 is set. , The first axis C1 and the second axis C2 so that the worm wheel 44 can be assembled after the worm gear 43 is assembled to the housing 45 with respect to the second axis C2 that is the rotation center of the worm wheel 44. Are inclined from the state where they are orthogonal to each other.

Next, a specific relationship between the first axis C1 and the second axis C2 will be described with reference to FIG. FIG. 4 shows the relationship between the first axis C1 and the second axis C2 projected on the projection plane P1 which is a plane including the second axis C2 and parallel to the first axis C1 from the direction indicated by the arrow A in FIG. FIG. In FIG. 4, only the worm gear 43 and the worm wheel 44 are shown for the sake of explanation.

The worm gear 43 has a tooth top line 43a formed in a spiral shape with a predetermined advance angle γ. In FIG. 4, the broken line of the worm gear 43 indicates the tip line 43b on the side facing the worm wheel 44.

The worm wheel 44 is a spur gear and has a plurality of tooth tip lines 44a parallel to the second axis C2.

As shown in FIG. 4, a predetermined first angle α1 is formed between the first projection axis PC1 obtained by projecting the first axis C1 onto the projection plane P1 and the second axis C2, and the worm wheel 44 is provided with the predetermined first angle α1. A predetermined second angle α2 is formed between the projected tooth top line P43b obtained by projecting the tooth top line 43b of the worm gear 43 which is opposed to the projection plane P1 and the second axis C2. The magnitude of the second angle α2 changes the magnitude of the first angle α1, that is, the positional relationship between the first axis C1 that is the rotation center of the worm gear 43 and the second axis C2 that is the rotation center of the worm wheel 44. It changes with that.

Here, when the magnitude of the first angle α1 is set so that the magnitude of the second angle α2 is equal to or larger than the advance angle γ of the worm gear 43, the worm wheel 44 should be attached to the housing 45 in which the worm gear 43 is attached. Even so, the teeth of the worm gear 43 prevent the worm wheel 44 from entering the housing 45. Therefore, it is impossible to assemble the worm wheel 44 to the housing 45 after the worm gear 43 is assembled to the housing 45.

On the other hand, when the magnitude of the first angle α1 is set so that the magnitude of the second angle α2 is smaller than the advance angle γ of the worm gear 43, the tooth tip line 43b of the worm gear 43 facing the worm wheel 44 is Since the state is almost parallel to the second axis C2, the teeth of the worm gear 43 are less likely to prevent the worm wheel 44 from entering the housing 45. Therefore, even after the worm gear 43 is assembled to the housing 45, the worm wheel 44 can be assembled to the housing 45.

If the magnitude of the first angle α1 is set so that the magnitude of the second angle α2 is smaller than the advance angle γ of the worm gear 43, a spur gear is used as the worm wheel 44 instead of a helical gear. It becomes possible to do. As described above, by using the worm wheel 44 as a spur gear that is relatively easy to process, the manufacturing cost of the power transmission device 60 can be reduced. Moreover, since the tooth surface of the worm wheel 44 becomes flat by using the spur gear as the worm wheel 44, when the worm wheel 44 is assembled to the housing 45 in which the worm gear 43 is assembled, the tooth surface of the worm gear 43 causes the worm wheel 43 to move. It is easy for 44 to be guided. Therefore, the worm wheel 44 can be easily assembled to the housing 45 in which the worm gear 43 is assembled.

Further, when the magnitude of the first angle α1 is set such that the magnitude of the second angle α2 becomes zero, that is, the projection tip line P43b is parallel to the second axis C2, the worm wheel 44 The tooth tip line 43b of the worm gear 43 and the tooth tip line 44a of the worm wheel 44 facing each other are parallel to each other. Therefore, the teeth of the worm gear 43 do not prevent the worm wheel 44 from entering the housing 45. Therefore, the worm wheel 44 can be easily assembled to the housing 45 in which the worm gear 43 is assembled.

In this way, by setting the magnitude of the first angle α1, that is, the inclination of the worm gear 43 with respect to the worm wheel 44, so that the magnitude of the second angle α2 becomes smaller than the advance angle γ of the worm gear 43, the worm gear 43 The worm wheel 44 can be assembled to the housing 45 even after the assembly is assembled to the housing 45.

Next, with reference to FIGS. 5 and 6, an assembly procedure of the power transmission device 60 having the above configuration will be described. 5 shows a cross section of the power transmission device 60 of the same portion as FIG. 2, and FIG. 6 shows a cross section of the power transmission device 60 of the same portion as FIG.

First, as shown in FIG. 5, the housing 45 is in a state where the electric motor 50 including the worm gear 43 and the controller 70 are assembled, that is, the worm wheel 44 is not assembled.

Then, in this state, the performance of the electric motor 50 is measured. By measuring the performance of the electric motor 50 in the state where the worm wheel 44 is not assembled, the performance of the electric motor 50 alone can be accurately measured. As a result, it becomes possible to easily determine whether or not the electric motor 50 satisfies the predetermined performance.

Next, the worm wheel 44 is assembled to the housing 45 whose performance of the electric motor 50 has been measured.

Specifically, as shown in FIG. 6, the worm wheel 44 integrated with the output shaft 22 is inserted into the second accommodation hole 47 along the direction of the arrow.

Here, as described above, the relationship between the first axis C1 and the second axis C2 is preferably such that the magnitude of the second angle α is smaller than the advance angle γ of the worm gear 43, preferably the second angle α. Is set to be zero. Therefore, the worm wheel 44 is inserted into the second accommodating hole 47 while meshing with the worm gear 43, and finally, as shown in FIG. 3, is accommodated in the second accommodating hole 47 while meshing with the worm gear 43. Will be done.

As described above, in the present embodiment, the performance of the electric motor 50 is improved by intersecting the first axis C1 that is the rotation center of the worm gear 43 and the second axis C2 that is the rotation center of the worm wheel 44 at a predetermined angle. The worm wheel 44 can be assembled to the housing 45 after the measurement is completed.

According to the above embodiment, the following effects are achieved.

In the power transmission device 60, the first axis C1 that is the rotation center of the worm gear 43 is attached to the second axis C2 that is the rotation center of the worm wheel 44 after the worm gear 43 is attached to the housing 45. In order to be able to do so, the first axis C1 and the second axis C2 are inclined from a state where they are orthogonal to each other.

Specifically, a plane that includes the second axis C2 that is the rotation center of the worm wheel 44 and that is parallel to the first axis C1 that is the rotation center of the worm gear 43 is defined as the projection plane P1, and the first axis C1 is projected onto the projection plane P1. The angle formed between the first projection axis PC1 and the second projection axis C2 is set to a first angle α1, and the tooth tip line 43b of the worm gear 43 facing the worm wheel 44 is projected onto the projection plane P1. , The second axis C2 is set to a second angle α2, the magnitude of the first angle α1 is set so that the second angle α2 becomes smaller than the advance angle γ of the worm gear 43. .

By thus reducing the second angle α2, it is possible to assemble the worm wheel 44 to the housing 45 even after the rotary shaft 51 on which the worm gear 43 is formed is assembled to the housing 45. Therefore, it is possible to measure the performance of the electric motor 50 in a state where the worm wheel 44 is not attached to the housing 45. As a result, the performance measurement accuracy of the electric motor 50 used in the power transmission device 60 can be improved.

Next, a modification of the above embodiment will be described. The following modifications are also within the scope of the present invention, and the configurations shown in the variations and the configurations described in the above embodiments may be combined, or the configurations described in the following different variations may be combined. Is also possible.

In the above-described embodiment, the rack-and-pinion mechanism having the pinion gear 41a and the rack gear 41b is used as the transmission unit 41 that transmits the rotation of the output shaft 22 to the wheels 1, but the transmission unit is coupled to the output shaft 22. A link mechanism having a pitman arm may be used.

In addition, in the above-described embodiment, the power transmission device 60 applies the output of the electric motor 50 to the output shaft 22 that is connected to the input shaft 21 that rotates in accordance with the steering operation. The power transmission device 60 is not limited to this as a shaft that gives the output of the electric motor 50. For example, when the power transmission device 60 has an output shaft that is not directly connected to the input shaft, such as a dual pinion type or a steer-by-wire type, The output of the electric motor 50 may be applied to the output shaft by the power transmission device 60.

Further, in the above embodiment, the steering wheel is the steering wheel 10. Instead of this, the steering handle may be of other forms such as a bar handle or a control stick.

The following will collectively describe the configuration, operation, and effect of the embodiment of the present invention.

The power transmission device 60 includes an electric motor 50 having a rotary shaft 51 as an output shaft, a worm gear 43 provided on the rotary shaft 51, a worm wheel 44 that meshes with the worm gear 43, and a housing 45 that houses the worm gear 43 and the worm wheel 44. The first axis C1 which is the center of rotation of the worm gear 43, and the second axis C2 which is the center of rotation of the worm wheel 44 are mounted on the housing 45 after the worm gear 43 is mounted on the housing 45. Is inclined so that it is possible.

In this configuration, the first axis C1 that is the rotation center of the worm gear 43 may be attached to the second axis C2 that is the rotation center of the worm wheel 44 after the worm gear 43 is attached to the housing 45. Sloped to be possible. The first axis C1 is changed to the second axis C2 so that the worm wheel 44 can be assembled to the housing 45 even after the rotary shaft 51 having the worm gear 43 formed therein is assembled to the housing 45. By tilting with respect to the housing 45, it becomes possible to measure the performance of the electric motor 50 in a state where the worm wheel 44 is not assembled to the housing 45. As a result, the performance measurement accuracy of the electric motor 50 used in the power transmission device 60 can be improved.

A plane that includes the second axis C2 and is parallel to the first axis C1 is the projection plane P1, and the angle formed by the first projection axis PC1 that projects the first axis C1 on the projection plane P1 and the second axis C2 is When the first angle α1 is set, and the angle formed by the projection tip line P43b obtained by projecting the tooth tip line 43b of the worm gear 43 facing the worm wheel 44 onto the projection plane P1 and the second axis C2 is set as the second angle α2. The magnitude of the first angle α1 is set so that the second angle α1 is smaller than the advance angle γ of the worm gear 43.

In this configuration, the size of the second angle α1 that is the angle formed by the projected tip line P43b obtained by projecting the tooth tip line 43b of the worm gear 43 facing the worm wheel 44 on the projection plane P1 and the second axis C2 has a size of the worm gear 43. The magnitude of the first angle α1, which is the angle formed by the first projection axis PC1 and the second projection axis C2 obtained by projecting the first axis C1 on the projection surface P1, is set so as to be smaller than the advance angle γ of. By reducing the second angle α2 in this way, the worm wheel 44 can be assembled in the housing 45 even after the rotary shaft 51 having the worm gear 43 formed therein is assembled in the housing 45. Therefore, the performance of the electric motor 50 can be measured in a state where the worm wheel 44 is not attached to the housing 45, and as a result, the performance measurement accuracy of the electric motor 50 used in the power transmission device 60 can be improved. it can.

Also, the magnitude of the first angle α1 is set so that the second angle α1 becomes zero.

With this configuration, the second angle α1 that is the angle formed by the projection tip line P43b obtained by projecting the tip line 43b of the worm gear 43 facing the worm wheel 44 onto the projection plane P1 and the second axis C2 becomes zero. , The magnitude of the first angle α1, which is the angle formed by the first projection axis PC1 that projects the first axis C1 onto the projection plane P1 and the second axis C2, is set. By setting the second angle α2 to zero in this manner, the worm wheel 44 can be easily assembled to the housing 45 even after the rotary shaft 51 having the worm gear 43 is assembled to the housing 45. is there. Therefore, the performance of the electric motor 50 can be measured in a state where the worm wheel 44 is not attached to the housing 45, and as a result, the performance measurement accuracy of the electric motor 50 used in the power transmission device 60 can be improved. it can.

Also, the worm wheel 44 is a spur gear.

In this configuration, the worm wheel 44 is a spur gear. If the size of the first angle α1 is set so that the size of the second angle α2 is smaller than the advance angle γ of the worm gear 43, a spur gear instead of a helical gear should be adopted as the worm wheel 44. Is possible. As described above, by using the worm wheel 44 as a spur gear that is relatively easy to process, the manufacturing cost of the power transmission device 60 can be reduced. Moreover, since the tooth surface of the worm wheel 44 becomes flat by using the spur gear as the worm wheel 44, when the worm wheel 44 is assembled to the housing 45 in which the worm gear 43 is assembled, the tooth surface of the worm gear 43 causes the worm wheel 43 to move. It is easy for 44 to be guided. Therefore, the worm wheel 44 can be easily assembled to the housing 45 in which the worm gear 43 is assembled. Therefore, the performance of the electric motor 50 can be measured in a state where the worm wheel 44 is not attached to the housing 45, and as a result, the performance measurement accuracy of the electric motor 50 used in the power transmission device 60 can be improved. it can.

Further, in the electric power steering device 100 including the power transmission device 60 having the above-described configuration, the worm wheel 44 is provided on the steering shaft 20 that rotates by receiving the steering torque from the steering wheel 10, and the electric motor 50 is connected to the worm gear 43. The steering shaft 20 is provided with a rotational torque that assists the steering torque by rotationally driving the worm wheel 44.

In this configuration, the power transmission device 60 having the above configuration is used in the electric power steering device 100. As described above, by adopting the power transmission device 60 including the electric motor 50 that is confirmed to satisfy the predetermined performance, the rotational torque that assists the steering torque is stably applied to the steering shaft 20 by the power transmission device 60. Will be granted. Therefore, the operation of the electric power steering device 100 can be stabilized.

Although the embodiment of the present invention has been described above, the above embodiment merely shows a part of the application example of the present invention, and the technical scope of the present invention is limited to the specific configuration of the above embodiment. Absent.

The present application claims priority based on Japanese Patent Application No. 2018-229871 filed with the Japan Patent Office on December 7, 2018, the entire contents of which are incorporated herein by reference.

Claims

A power transmission device,
An electric motor having an output shaft,
A worm gear provided on the output shaft,
A worm wheel that meshes with the worm gear,
A housing that houses the worm gear and the worm wheel,
The first axis that is the center of rotation of the worm gear is inclined with respect to the second axis that is the center of rotation of the worm wheel so that the worm wheel can be assembled after the worm gear is assembled to the housing. Power transmission device.
The power transmission device according to claim 1, wherein
A plane including the second axis and parallel to the first axis is a projection plane,
An angle formed by a first projection axis line obtained by projecting the first axis line on the projection surface and the second axis line is defined as a first angle,
When the angle formed by the projection tip line obtained by projecting the tip line of the worm gear facing the worm wheel onto the projection surface and the second axis is the second angle,
The magnitude of the first angle is set such that the second angle is smaller than the lead angle of the worm gear.
The power transmission device according to claim 2, wherein
The magnitude of the said 1st angle is a power transmission device set so that the said 2nd angle may become zero.
The power transmission device according to any one of claims 1 to 3,
The power transmission device in which the worm wheel is a spur gear.
An electric power steering apparatus comprising the power transmission device according to claim 1.
The worm wheel is provided on a steering shaft that rotates when steering torque is input from a steering wheel,
The electric power steering apparatus, wherein the electric motor applies a rotational torque for assisting the steering torque to the steering shaft by rotationally driving the worm wheel via the worm gear.