KR20220087259A - Power Transmission Apparatus for Electric Vehicle - Google Patents

Abstract

A power transmission includes a drive shaft, a driven shaft disposed parallel to the drive shaft, and a first layshaft disposed in a skew position with respect to the two shafts. A first worm gear installed in the reverse direction between the drive shaft and the first auxiliary shaft transmits power. The second worm gear installed in the forward direction between the first auxiliary shaft and the driven shaft transmits power.
A power transmission path having a structure similar to that of the power transmission path passing through the first auxiliary shaft but having a different rotation ratio may be additionally provided. It may further include a power transmission selection means for selecting one of the power transmission paths having two different rotation ratios. According to a further aspect, the power transmission device may detect the rotational position of the driven shaft side and control the rotational position of the driving shaft side accordingly to reduce shift shock.

Description

Power Transmission Apparatus for Electric Vehicle

Disclosed is a technology related to a device for transmitting rotational power using a high-density gear, an enveloping worm gear (Enveloping Worm and Enveloping Worm wheel).

Worm gear has a large reduction ratio and transmits power so that the input and output directions of power are at right angles. Since the worm has a rod shape and the worm wheel meshing with the worm's teeth has a disk shape, the contact area between the worm and the worm wheel is small, and the bite rate may decrease. In order to solve this problem, a long worm gear, for example, a structure such as a Hindley worm is known. The long-ball type worm reduces gear wear by increasing the number of teeth meshing through the structure in which the outer peripheral surface of the worm is curved in a long-ball shape and spiral teeth are formed in the curved area, and the contact line stands vertically when viewed from the tooth surface of the long-ball type worm wheel for lubricity. As this is good, the power transmission efficiency is high and large power can be transmitted.

Multi-speed transmissions typically mechanically alternate the power transmission path from a drive shaft to a driven shaft. The drive shaft and the driven shaft are generally arranged to be parallel. When the drive shaft and the driven shaft are parallel, if the distance between the drive shaft and the driven shaft increases, it is directly connected to the reduction ratio, so using a general parallel shaft power transmission gear such as a helical gear or a spur gear is inefficient due to low design freedom and complicated structure. It is advantageous to adopt However, a worm gear having a low reduction ratio of 5:1 or less is difficult to design and process, so power transmission using the worm gear is not common.

The proposed invention aims to propose a new structure in which the inter-axis distance and the design of the reduction ratio are independent in a power transmission that can be applied when the driving shaft and the driven shaft are parallel.

Furthermore, it is an object of the proposed invention to propose a new structure of a multi-speed gearbox to which design specifications of an inter-axis distance and a reduction ratio can be flexibly applied when a driving shaft and a driven shaft are parallel.

Further, the proposed invention aims to reduce shift shock in a multi-speed gearbox that can be applied when a drive shaft and a driven shaft are parallel.

According to one aspect, a power transmission includes a drive shaft, a driven shaft disposed parallel to the drive shaft, and a first layshaft disposed in a skew position with respect to the two axes. A first worm gear installed in the reverse direction between the drive shaft and the first auxiliary shaft transmits power. For reverse driving, the reduction ratio between the driving shaft and the first auxiliary shaft is designed to be relatively low, for example, 1:1 to 2:1. The second worm gear installed in the forward direction between the first auxiliary shaft and the driven shaft transmits power.

According to another aspect, a power transmission path having a structure similar to that of the power transmission path passing through the first auxiliary shaft but having a different rotation ratio may be additionally provided. It may further include a power transmission selection means for selecting one of the power transmission paths having two different rotation ratios.

According to a further aspect, the power transmission may detect the rotational position of the driven shaft side and control the rotational position of the driving shaft side accordingly to reduce shift shock.

According to the proposed invention, the distance between the parallel shafts can be freely designed by connecting the power transmission between the two parallel shafts by a straight shaft arranged in a skewed position. Furthermore, by driving one of the worm gears in the reverse direction with a low reduction ratio, the size of the overall power transmission can be reduced, and the rotation ratio can be relatively lowered while maintaining high efficiency despite passing through the worm gear twice. For example, in the power transmission according to the proposed invention, the overall reduction ratio can be freely designed from 1:1 to 100:1, and even an increase of 0.5:1 becomes possible. Furthermore, it is possible to reduce shift shock by controlling the drive motor.

1 is a plan view and a right side view illustrating a configuration of a power transmission device according to an embodiment.
2 is a plan view, a right side view, and a front view illustrating a configuration of a power transmission device according to another embodiment.
3 is a block diagram illustrating a shift control configuration of a power transmission device according to an exemplary embodiment.

The foregoing and additional aspects are embodied through the embodiments described with reference to the accompanying drawings. It is understood that various combinations of elements of each embodiment are possible within one embodiment or with elements of other embodiments, as long as there is no contradiction between them or other mentions. Based on the principle that the inventor can appropriately define the concept of a term to describe his invention in the best way, the terms used in the present specification and claims shall have meanings consistent with the description or the proposed technical idea. and should be interpreted as a concept. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a plan view and a right side view illustrating a configuration of a power transmission device according to an embodiment. As shown, the power transmission device according to an embodiment includes a driving shaft 110 and a driven shaft 150 disposed parallel to the driving shaft 110 . In the right side view, the driving shaft and the driven shaft are omitted for better understanding. A first power transmission unit via a first layshaft 130 is provided between the driving shaft 110 and the driven shaft 150 . The first auxiliary shaft 130 is disposed in a skewed position with respect to the driving shaft 110 and the driven shaft 150 . The first power transmission unit includes first worm gears 111 and 131 for transmitting power between the driving shaft 110 and the first sub-shaft 130 , and a second worm gear for transmitting power between the first sub-shaft 130 and the driven shaft 150 . (113,133). According to an aspect of the proposed invention, the first worm gears 111 and 131 are driven in the reverse direction, and the second worm gears 113 and 133 are driven in the forward direction. The worms 131 and 133 are fixed to the first auxiliary shaft 130 , and the worm wheels 111 and 151 are fixed to the driving shaft 110 and the driven shaft 150 . In the illustrated embodiment, the worms 131 and 133 of the first auxiliary shaft 130 are integrally cut with the first auxiliary shaft 130 , and the worm wheels 111 and 151 of the driving shaft 110 and the driven shaft 150 are processed separately. is fastened to the driving shaft 110 and the driven shaft 150 .

In general, the worm gear transmits power in the forward direction, that is, from the worm to the worm wheel. The larger the lead angle of the worm, the smaller the sliding friction loss and the better the transmission efficiency. In general, when the lead angle is 6 degrees (friction coefficient 0.1) or more, and if the reduction ratio is about 30:1 or less, reverse power transmission is also possible. The efficiency in forward driving becomes similar. According to an aspect of the proposed invention, when designing a tooth shape, a lead angle of 35 to 55 degrees and a reduction ratio of 1:1 to 0.5:1 can be designed to improve transmission efficiency even when driving in the reverse direction. The second worm gears 113 and 133 have the same center distance as the first worm gear, but the reduction ratio may be determined according to the reduction ratio of the entire reduction gear, which is a design target. Accordingly, the power transmission according to the proposed invention can support an overall free reduction ratio. According to an aspect of the proposed invention, by driving one of the worm gears in the reverse direction with a very low gear ratio, the size of the overall reducer can be minimized and the rotation ratio can be relatively lowered while maintaining high efficiency despite passing through the worm gear twice. According to an aspect of the present invention, by driving one of the worm gears in the reverse direction with a low reduction ratio, the size of the overall power transmission device can be reduced, and the rotation ratio can be relatively lowered while maintaining high efficiency despite passing through the worm gear twice.

In one embodiment, the first worm gear includes a first driving worm wheel 111 and a first worm-shaped worm 131 . The first driving worm wheel 111 is rotationally fixed to the driving shaft 110 . In this specification, the expression 'rotationally fixed' means that the rotation is operable so that the rotation of the axis is dependent on the rotation. Gears completely fixed to the shaft or integrally formed, sliding gears that are slidable in the axial direction but rotate together, or gears coupled to the shaft so that power transmission is interrupted by a clutch are further included. The first long worm 131 rotates in engagement with the first driving worm wheel 111 , and is fixed to one side of the first auxiliary shaft 130 . For example, the first long-shaped worm 131 is manufactured as a separate part from the first auxiliary shaft 130 and fitted, or integrally formed with the first auxiliary shaft 130, or through cutting, the first auxiliary shaft ( 130) and may be manufactured integrally. In the illustrated embodiment, the first long worm 131 is machined integrally with the first auxiliary shaft 130 . Power is transmitted in the reverse direction from the first driving worm wheel 111 of the driving shaft 110 to the first elongated worm 131 of the first auxiliary shaft 130 .

In one embodiment, the second worm gear includes a second long worm 133 and a first driven worm wheel 151 . The second long worm 133 is spaced apart from the first long worm 131 and fixed to the other side of the first auxiliary shaft 130 . The first driven worm wheel 151 is rotationally fixed to the driven shaft 150 , and rotates in engagement with the second long worm 133 . For example, the second long worm 133 is manufactured as a separate part from the first auxiliary shaft 130 and fitted, or integrally formed with the first auxiliary shaft 130, or through cutting, the first auxiliary shaft ( 130) and may be manufactured integrally. In the illustrated embodiment, the second long worm 133 is machined integrally with the first auxiliary shaft 130 . Power is transmitted in a forward direction from the second long worm 133 of the first auxiliary shaft 130 to the first driven worm wheel 151 of the driven shaft 150 . In the illustrated embodiment, the first driven worm wheel 151 is separately cut and fixed to the driven shaft 150 .

In the illustrated embodiment, the drive shaft 110 is rotatably fixed by a pair of supports 115 and 117 and bearings at both ends. In addition, the first auxiliary shaft 130 is rotatably fixed at both ends by a pair of supports 135 and 137 and bearings. In addition, the driven shaft 150 is rotatably fixed at both ends by a pair of supports 153 and 155 and bearings. However, the proposed invention is not limited thereto, and the rotation shafts may be fixed to a common frame or a divided frame, or may be fastened with a motor or other parts. For example, the driven shaft may be a differential gear, and in this case, the first driven worm wheel 151 may be fixed to the housing of the differential gear.

2 is a plan view, a right side view, and a front view illustrating a configuration of a power transmission device according to another embodiment. For ease of understanding, some corresponding components are omitted from the right side view and the front view. Configurations corresponding to the embodiment shown in FIG. 1 are referred to by the same reference numerals. According to another aspect of the proposed invention, another power transmission path different from that via the first auxiliary shaft 130 may be provided between the driving shaft 110 and the driven shaft 130 . The added power transmission path is via the second sub-shaft 170, and has a structure similar to the power transmission path via the first sub-shaft 130, but has a different rotation ratio. Here, the rotation ratio refers to the ratio of the rotation of the input shaft to the rotation of the output shaft. The power transmission device according to an aspect may further include a power transmission selection means, eg, a clutch, for selecting one of the power transmission paths having two different rotation ratios.

A power transmission device according to a further aspect will be described with reference to FIG. 2 . As shown, the second via a second layshaft 170 separately from the first power transmission path via the first layshaft 130 between the driving shaft 110 and the driven shaft 150 . A power transmission path is provided. According to a further aspect, the second power transmission path has a different rotation ratio than the first power transmission path. The second minor shaft 170 is parallel to the first minor shaft 130 and is disposed in a skew position with respect to the driving shaft 110 and the driven shaft 150 .

The second power transmission path includes the third worm gears 113 and 173 between the driving shaft 110 and the second sub shaft 170 , and the fourth worm gears 171 , 153 between the second sub shaft 170 and the driven shaft 150 . The third worm gears 113 and 173 are driven in the reverse direction, and the fourth worm gears 171 and 153 are driven in the forward direction, and have a structure similar to the first power transmission path. The worms 171 and 173 are fixed to the second auxiliary shaft 170 , and the worm wheels 111 , 113 , 151 and 153 are fixed to the driving shaft 110 and the driven shaft 150 . In the illustrated embodiment, the long-shaped worm screws 171 and 172 of the second auxiliary shaft 170 are cut integrally with the second auxiliary shaft 170, and the long-shaped worm wheels 111 and 113 on the driving shaft 110 are separately After being cut, it is assembled to the drive shaft 110 through a bearing. In addition, the long worm wheels 151 and 153 of the driven shaft 150 are separately cut and fixed.

According to a further aspect, the power transmission further includes a variable speed drive 121 . The variable speed driving unit 121 selectively transmits power to one of the first power transmission unit and the second power transmission unit. In the illustrated embodiment, the variable speed drive is embodied as a dog clutch 121 installed on the drive shaft. The dog clutch 121 is a mechanical element that controls power transmission as the two rotating bodies are driven in a state in which they are engaged with each other and in a state in which they are spaced apart. In the illustrated embodiment, the dog clutch 121 is shifted left and right on the driving shaft 110 to transmit the power to either the first driving worm wheel 111 or the second driving worm wheel 113 . However, the proposed invention is not limited thereto, and various known variable speed drive structures applicable to the structure in which the drive shaft 110 , the driven shaft 150 , the first auxiliary shaft 130 , and the second auxiliary shaft 170 are disposed may be applied. . For example, the dog clutch may be implemented as a dog clutch applied to the driven shaft 150 instead of the driving shaft 110 . As another example, in addition to the dog clutch, a structure employing a multiple plate clutch or a centrifugal clutch may be applied. According to the aspect of the proposed invention, gears having the same shape are arranged on one shaft, so that the application of the clutch structure is facilitated. For example, since worms are excluded from the drive shaft 110 and only worm wheels exist, various clutch structures can be easily applied.

Although the illustrated embodiment illustrates two power transmission paths, the proposed invention is not limited thereto and may be extended to a power transmission including three or more power transmission paths. That is, in the illustrated embodiment, a three-speed or more multi-stage power transmission may be provided by adding a power transmission path having a structure similar to that of the first power transmission unit or the second power transmission unit but having a different rotation ratio.

In one embodiment, the third worm gear includes a second driving worm wheel 113 and a third long worm 171 . The second driving worm wheel 113 is rotationally fixed to the driving shaft 110 . The third long worm 171 rotates in engagement with the second driving worm wheel 113 , and is fixed to one side of the second auxiliary shaft 170 . For example, the third long worm 171 is manufactured as a separate part from the second auxiliary shaft 170 and fitted, or integrally formed with the second auxiliary shaft 170, or through cutting, the second auxiliary shaft ( 170) and can be manufactured in one piece. In the illustrated embodiment, the second driving gear-type worm wheel 113 is separately cut and assembled to the driving shaft 110 as a bearing and is fastened in an idle state. The third long worm screw 171 is integrally cut and fixed to one side of the second auxiliary shaft 170 . Power is transmitted in the reverse direction from the first driving worm wheel 111 of the driving shaft 110 to the first elongated worm 131 of the first auxiliary shaft 130 .

In one embodiment, the second worm gear includes a second long worm 133 and a first driven worm wheel 151 . The second long worm 133 is spaced apart from the first long worm 131 and fixed to the other side of the first auxiliary shaft 130 . The first driven worm wheel 151 is rotationally fixed to the driven shaft 150 , and rotates in engagement with the second long worm 133 . For example, the second long worm 133 is manufactured as a separate part from the first auxiliary shaft 130 and fitted, or integrally formed with the first auxiliary shaft 130, or through cutting, the first auxiliary shaft ( 130) and may be manufactured integrally. Power is transmitted in a forward direction from the second long worm 133 of the first auxiliary shaft 130 to the first driven worm wheel 151 of the driven shaft 150 .

In the illustrated embodiment, the drive shaft 110 is rotatably fixed by a pair of supports 115 and 117 and bearings at both ends. In addition, the first auxiliary shaft 130 is rotatably fixed at both ends by a pair of supports 135 and 137 and bearings. In addition, the driven shaft 150 is rotatably fixed at both ends by a pair of supports 153 and 155 and bearings. However, the proposed invention is not limited thereto, and the rotation shafts may be fixed to a common frame or a divided frame, or may be fastened with a motor or other parts. For example, the driven shaft worm wheels 151 and 153 may be fixed to the housing of the differential gear.

According to an additional aspect, the variable speed driving unit may intermittently transmit the driving force from the driving shaft 110 to a selected one of the first sub shaft 130 and the second sub shaft 170 . For example, the variable speed driving unit may include a dog clutch for intermittent transmission of the rotational force of the drive shaft to any one selected from the first worm gears 111 and 131 and the third worm gears 113 and 173 . In one embodiment, the variable speed drive unit includes a dog clutch 121 that intermittently controls transmission of rotational force from the driving shaft 110 to the first driving worm wheel 111 , and a rotational force from the driving shaft 110 to the second driving worm wheel 113 . It may include a dog clutch 123 for intermittent transmission of the. One of the first power transmission path and the second power transmission path is selected according to which of the two dog clutches 121 and 123 is bitten by the corresponding worm wheel. Although the illustrated embodiment has a structure in which the two dog clutches 121 and 123 intermittently intermittently transmit the torque to the corresponding worm wheel, one dog clutch selectively transmits the torque to one of the worm wheels of both sides. It can also be implemented as a control structure.

According to an additional aspect, the variable speed driving unit may intermittently transmit the driving force from the selected one of the first sub-axis 130 or the second sub-axis 170 to the driven shaft. As an example, the variable speed driving unit may include a dog clutch for intermittent transmission of a rotational force selected from the second worm gears 113 and 133 and the fourth worm gears 171 and 153 . In one embodiment, the variable speed drive unit includes a dog clutch for intermittent transmission of rotational force from the first auxiliary shaft 130 to the first driven worm wheel 151 , and from the second auxiliary shaft 170 to the second driven worm wheel 153 . It may include a dog clutch for intermittent transmission of the rotational force. One of the first power transmission path and the second power transmission path is selected according to which of the two dog clutches is bitten by the corresponding worm wheel. Although this embodiment has a structure in which two dog clutches intermittently the transmission of rotational force to the corresponding worm wheel, one dog clutch can be implemented with a structure in which the transmission of rotational force to one of both worm wheels is selectively intercepted. may be

According to a further aspect, the power transmission may detect the rotational position of the driven shaft side and control the rotational position of the driving shaft side accordingly to reduce shift shock. 3 is a block diagram illustrating a shift control configuration of a power transmission device according to an exemplary embodiment. As shown, the power transmission device according to an embodiment may further include a first rotational position detection unit 11 , a second rotational position detection unit 13 , and a shift control unit 50 .

In one embodiment, the first rotational position detecting unit 11 detects the rotational position of the first driving worm wheel 111 that rotates depending on the driven shaft 150 . The first rotational position detecting unit 11 may be implemented as, for example, a magnetic encoder coupled to one side of the first driving worm wheel 111 to detect the rotational position. In the illustrated embodiment, the transmission of the driving force between the first driving worm wheel 111 and the driving shaft 110 is interrupted by the dog clutch 121, and in a state in which the dog clutch 121 is disconnected, the first driving worm wheel 111 ) is dependent on the driven shaft 150 .

The second rotational position detecting unit 13 detects the rotational position of the second driving worm wheel 113 which rotates depending on the driven shaft 150 . The second rotational position detecting unit 13 may be implemented as, for example, a magnetic encoder coupled to one side of the second driving worm wheel 113 to detect the rotational position. In the illustrated embodiment, the transmission of the driving force between the second driving worm wheel 113 and the driving shaft 110 is interrupted by the dog clutch 123, and the second driving worm wheel 113 in a state in which the dog clutch 123 is disconnected. ) is dependent on the driven shaft 150 .

The shift control unit 50 shifts to reduce shift shock by controlling the rotational phase of the drive shaft according to the output value of any one selected from the first rotational position detection unit 11 and the second rotational position detection unit 13 in response to the shift instruction do. To this end, the power transmission device may include an additional encoder for detecting the rotational position of the drive shaft 110 connected to the drive motor. For example, it may include a magnetic encoder coupled to one side of any one of the dog clutches 121 and 123 to detect the rotational position thereof.

For example, when a shift is made from the first power transmission unit to the second power transmission unit, the dog clutch 121 operates so that the rotational force of the driving shaft 110 is connected to the first driving worm wheel 111 , and the second driving worm wheel Reference numeral 113 is engaged with the fourth worm gear 171 to maintain a position and speed dependent on the rotation of the driven shaft 150 . If the shift control unit 50 immediately drives the dog clutches 121 and 123 to shift in response to the shift instruction, shift shock occurs while the second driving worm wheel 113 and the dog clutch 123 engage. According to an additional aspect of the proposed invention, in response to the shift instruction, the shift control unit 50 detects the rotational position and speed of the second driving worm wheel 113 from the output value of the second rotational position detecting unit 13, and the driving shaft 110 ), the drive motor 31 is controlled so that the rotational position of the dog clutch 123 fixed to the rotation matches the identified rotational position and speed. The shift control unit 50 may be implemented as a microprocessor or sequence control logic.

Although the present invention has been described above with reference to the accompanying drawings, the present invention is not limited thereto, and it should be construed to encompass various modifications that can be apparent from those skilled in the art. The claims are intended to cover such modifications.

11: first rotational position detection unit 13: second rotational position detection unit
31: drive motor 50: shift control unit
110: drive shaft 111: first drive worm wheel
113: second driving worm wheel
121: dog clutch
130: first auxiliary shaft 131: first long-shaped worm
133: second long worm
150: driven shaft 151: first driven worm wheel
153: second driven worm wheel
170: second support shaft 171: third long worm
173: 4th long worm-shaped worm

Claims (11)

a driving shaft;
a driven shaft disposed parallel to the drive shaft;
a first layshaft disposed in a skew position with respect to the drive shaft and the driven shaft; a first worm gear that transmits power between the drive shaft and the first auxiliary shaft and is driven in the opposite direction; a first power transmission unit that transmits power between the driven shafts and includes a second worm gear driven in a forward direction;
A power transmission comprising a. The power transmission device according to claim 1, wherein the first worm gear has a reduction ratio of 1:1 to 0.5:1. The method according to claim 1,
The first worm gear is:
a first driving worm wheel rotationally fixed to the driving shaft;
It rotates in engagement with the first driving worm wheel and includes a first long worm screw fixed to one side of the first layshaft,
The second worm gear is:
A second long-shaped worm fixed to be spaced apart from the first long-shaped worm on the other side of the first auxiliary shaft, and a first driven worm wheel that is rotationally fixed to the driven shaft and rotates in engagement with a second long-shaped worm power transmission. The method according to claim 1, wherein the power transmission comprises:
a second layshaft having a rotation ratio different from that of the first power transmission unit, parallel to the first auxiliary shaft and disposed in a skew position with respect to the drive shaft and the driven shaft, the drive shaft and the second minor shaft a second power transmission unit including a third worm gear that transmits power between the shafts and is driven in a reverse direction, and a fourth worm gear that transmits power between the second auxiliary shaft and a driven shaft and is driven in a forward direction;
a variable speed driving unit that selectively transmits power to one of the first power transmission unit and the second power transmission unit;
Power transmission further comprising a. The method according to claim 3, wherein the power transmission comprises:
a second layshaft having a rotation ratio different from that of the first power transmission unit, parallel to the first auxiliary shaft and disposed in a skew position with respect to the drive shaft and the driven shaft, the drive shaft and the second minor shaft a second power transmission unit including a third worm gear that transmits power between the shafts and is driven in a reverse direction, and a fourth worm gear that transmits power between the second auxiliary shaft and a driven shaft and is driven in a forward direction;
a variable speed driving unit that selectively transmits power to one of the first power transmission unit and the second power transmission unit;
Power transmission further comprising a. 5. The method according to claim 4,
The third worm gear is:
a second drive worm wheel rotationally fixed to the drive shaft;
It rotates in engagement with the second driving worm wheel, and includes a third long worm fixed to one side of the second auxiliary shaft,
The fourth worm gear is:
A power transmission comprising a fourth long gear-shaped worm fixed to the other side of the second sub-axis while being spaced apart from a third long gear-shaped worm, and a second driven worm wheel that is rotationally fixed to the driven shaft and rotates in engagement with the fourth long gear-shaped worm Device. 6. The method of claim 5,
The third worm gear is:
a second drive worm wheel rotationally fixed to the drive shaft;
It rotates in engagement with the second driving worm wheel, and includes a third long worm fixed to one side of the second auxiliary shaft,
The fourth worm gear is:
A power transmission comprising a fourth long gear-shaped worm fixed to the other side of the second sub-axis while being spaced apart from a third long gear-shaped worm, and a second driven worm wheel that is rotationally fixed to the driven shaft and rotates in engagement with the fourth long gear-shaped worm Device. The power transmission device according to claim 5, wherein the speed change driving unit includes at least one dog clutch for intermittent transmission of a driving force from a driving shaft to a selected one of a first sub shaft and a second minor shaft. The power transmission device according to claim 7, wherein the variable speed drive unit includes a dog clutch that controls transmission of the rotational force of the drive shaft to one of the first drive worm wheel and the second drive worm wheel. The power transmission device according to claim 5, wherein the speed change driving unit includes at least one dog clutch for intermittent transmission of a driving force from a selected one of a first auxiliary shaft and a second auxiliary shaft to a driven shaft. 10. The method of claim 9, wherein the power transmission comprises:
a first rotational position detection unit for detecting a rotational position of the first driving worm wheel;
a second driving phase detection unit detecting a rotational position of the second driving worm wheel;
a shift control unit configured to change the speed to reduce shift shock by controlling the rotation position of the drive shaft according to an output value of any one selected from the first rotation position detection unit and the second rotation position detection unit in response to a shift instruction;
Power transmission further comprising a.

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