KR101522497B1

KR101522497B1 - Transmission of Vehicle

Info

Publication number: KR101522497B1
Application number: KR1020140029629A
Authority: KR
Inventors: 김택성; 김상현
Original assignee: 엘에스엠트론 주식회사
Priority date: 2014-03-13
Filing date: 2014-03-13
Publication date: 2015-05-21

Abstract

A transmission for a tractor according to the present invention comprises: a driving input shaft connected to an output unit of an engine; and a carrier shaft and a linear gear shaft receive a rotational power from the driving input shaft to rotate. The transmission for a tractor further comprises: a first auxiliary transmission driving gear and a third auxiliary transmission driving gear mounted on the carrier shaft; a second auxiliary transmission driving gear and a forth auxiliary transmission driving gear disposed to operate with the linear gear shaft; a first and a third auxiliary transmission synchro-transmission instrument provided with a first and a third auxiliary transmission driven gear disposed on a first and a third auxiliary transmission driven shaft having a first and a third auxiliary transmission clutch driving gear to receive a driving power from the first and the second auxiliary transmission driving gear; a second and a forth auxiliary transmission synchro-transmission instrument provided with a second and a forth auxiliary transmission driven shaft rotatably mounted on the first and the third auxiliary transmission driven shaft, and a second and a forth auxiliary transmission clutch driving gear disposed on the second and the forth auxiliary transmission driven shaft; an output clutch shaft rotatably mounted on the driving input shaft; a front and a rear driving gear disposed to operate with the a clutch output shaft; and an auxiliary transmission clutch provided with a second and a fourth clutch driven gear mounted on the clutch output shaft to receive a driving power from a second and a forth clutch driving gear and a first and a third clutch driven gear to receive a driving power of a first and a third clutch driving gear.

Description

[0001] Transmission of Vehicle [0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission suitable for a working vehicle such as a tractor, and more particularly to a transmission suitable for a working vehicle such as a tractor and the like. It is.

Hydraulic mechanical transmission (HMT) is used as a power transmission system of a working vehicle such as a tractor by combining a hydraulic type continuously variable transmission (HST) and a mechanical differential.

The hydraulic mechanical transmission is a transmission designed to maximize the benefits of both HST with excellent operability and differential gear with excellent transmission efficiency.

In addition, a conventional working vehicle generally uses a multi-channel transmission having two or more power transmission paths using one or more planetary gears and a continuously variable transmission. This is because, by using a continuously variable transmission in one path, It is an unauthorized transmission.

However, most of the conventional continuously variable-speed tractors employ a wet multi-plate clutch and usually employ a wet multi-plate clutch corresponding to a single number. Therefore, in consideration of the number of stages of a general tractor, at least four to eight sets are adopted.

The operation of the wet type multi-plate clutch is performed by driving the hydraulic pump. Since the driving of the hydraulic pump uses the power of the engine, the more the number of the wet type multi-plate clutch is employed, the more the engine power is used. Therefore, there is a structural problem in that the power of the engine is lost in view of the transmission and the working machine.

In addition, since the wet multi-plate clutch is structurally large and occupies a large space for installation, it is also required to install a cooling device for lubrication for cooling. As a result, the size of the transmission increases, There was a difficult problem.

Korean Patent Laid-Open Publication No. 10-2012-0086822

SUMMARY OF THE INVENTION It is an object of the present invention to provide a tractor transmission having a simple structure and excellent power transmission efficiency while minimizing a wet type multi-plate clutch which is difficult to install and manage.

In order to achieve the above object, according to the present invention, there is provided a transmission comprising a drive input shaft connected to an output portion of the engine, and a carrier shaft and a sun gear shaft which are rotated by receiving a rotational force of the drive input shaft, A third reduction gear and a third reduction gear; A second shift stage and a fourth shift stage, which are provided in association with the sun shaft; And a third shift speed clutch drive gear that transmits the power of the above-mentioned first shift stage drive gear and the third shift stage drive gear. A first and a third speed change synchronous transmission mechanism coupled with a driven gear; Second and fourth driven gears that are rotatably engaged with the first and third driven gears and second and fourth driven gears, respectively, Transmission synchro transmission; A clutch output shaft rotatably coupled to the drive input shaft; A forward driving gear and a reverse driving gear provided to interlock with the clutch output shaft; A second-speed clutch driven gear coupled to the clutch output shaft and receiving power from the second-stage clutch drive gear, and a third-speed clutch driven gear that receives power from the first- A clutch for a tractor, comprising: a clutch;

In addition, in the above-described first-stage to second-speed shifting, the 1 < st > -to-3 speed change-speed synchronizer transmission gear is connected to the above- The transmission clutch disengages the power connected to the 1st and 3rd driven shafts, and is connected to the 2nd and 4th driven shafts. When the 2nd and 3rd shafts are engaged, In the state where the transmission is connected from the 1st speed drive of the 1st speed reduction gear to the 2nd speed 3rd speed drive gear, and the 2nd speed gearshift transmission gear is connected to 2nd speed, the auxiliary speed change clutch disengages the power of the 2nd speed / There is provided a transmission for a tractor to which power transmission is connected to an auxiliary transmission 1 or 3 driven shaft.

The sun gear shaft is freely rotatably connected to the drive input shaft, the carrier shaft is freely rotatably connected to the sun gear shaft, and the clutch output shaft is rotatably connected to the drive input shaft on the drive input shaft. Is provided.

A first sun gear fixed to the drive input shaft; A second sun gear formed on the sun shaft; A ring gear rotatable about the driving input shaft and installed to surround the second sun gear; Further comprising a plurality of planetary gears having first pinion gears connected to the first sun gear and second pinion gears connected to the second sun gear and ring output gears of the ring gear at the same time, Wherein the carrier is coupled to the plurality of planetary gears at the same time.

The driving input shaft is connected to the HST input shaft of the HST unit and the input unit of the ring gear is connected to the HST output shaft of the HST unit A carrier shaft coupled with the power of the drive input shaft and the power of the HST unit to interlock with the carrier, and a tractor transmission transmitted to the sun shaft.

Further, there is provided a tractor transmission in which a driving device driving shaft is connected to the driving input shaft.

The forward and reverse output shafts through which the power of the forward driving gear and the reverse driving gear are transmitted are equipped with forward and backward synchronized transmission shafts having forward driven gears and reverse driven gears respectively corresponding to the forward driving gear and the reverse driving gear, And a reverse idle gear for transmitting the power of the reverse drive gear to the reverse driven gear is engaged between the reverse drive gear and the reverse drive gear.

1 is a diagram showing a configuration of a tractor transmission according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Although the present invention has been described with reference to the embodiments shown in the drawings, it is to be understood that the technical idea of the present invention and its essential structure and operation are not limited thereby.

1 is a diagram showing a configuration of a transmission 1 according to an embodiment of the present invention.

1, the transmission 1 according to the present embodiment is constituted around a drive input shaft 5 connected to an output portion of the engine 3 and rotated by the power of the engine 1. As shown in Fig.

A first sun gear 7 is fixed to the most upstream side of the drive input shaft 5 and an HST input drive gear 9 is fixed to the middle of the downstream side.

The working device driving shaft 11 for driving the working device of the working vehicle is connected to the most downstream side of the driving input shaft 5 via the clutch 13 so that the working device connected to the working device driving shaft 11 is moved up and down And so on.

As used herein, the term " upstream "means a side to which power is input by one member, and the term" downstream " means a side to which power input from the member is transmitted.

The HST input drive gear 9 interlocked with the drive input shaft 5 is connected to the HST input driven gear 19 fixed on the HST input shaft 17 of the hydraulic type continuously variable transmission (HST) unit 15.

The HST unit 15 includes a hydraulic pump 21 and a hydraulic motor 23. The rotational force of the drive input shaft 5 driven by the engine 3 is transmitted to the HST input drive gear 9 and the HST input driven gear 19. The hydraulic pump 21 is driven by the driving force of the HST input shaft 17 .

The hydraulic pump 21 is configured to adjust the capacity of the internal flow rate by adjusting the inclination angle of the swash plate provided therein. The hydraulic pump 21 drives the hydraulic motor 23. The HST unit 15 can adjust the driving ratio of the hydraulic motor 23 to the hydraulic pump 21 by adjusting the capacity of the hydraulic pump 21, have. The hydraulic motor 23 rotates the HST output shaft 25 of the HST unit 15.

Since the structure of the hydraulic type continuously variable transmission that receives and decelerates the driving force of the engine 3 like the HST unit 15 is already known, a detailed description of the structure and power transmission characteristics of the HST unit 15 will be omitted .

On the other hand, on the downstream side of the first sun gear 7 of the drive input shaft 5, there are provided a carrier shaft 27 freely rotatable on the drive input shaft 5, A carrier 29 is provided and the second sun gear 31 is provided at the upstream end of the carrier shaft 27.

A plurality of planetary gears 33 are connected to the first sun gear 7 and the second sun gear 31. The plurality of planetary gears 33 are connected to the first sun gear 7 via a first pinion gear 35 And a second pinion gear 37 connected to the second sun gear 31.

As shown in the figures, the carrier 29 is coupled to the plurality of planetary gears 33 at the same time. According to this embodiment, in order to combine the driving force of the HST unit 15 and the rotational driving force of the driving input shaft 5, and a ring gear 39 is installed to surround the second sun gear 31.

The ring output gear 41 of the ring gear 39 is connected to the second pinion gear 37 of the planetary gear 33 and the ring input gear 40 is connected to the HST output shaft 25 of the HST unit 15 And connected to the formed HST output drive gear 43. The ring gear 39 is freely rotatable with respect to the drive input shaft 5 by rotational drive of the HST output shaft 25. [

Therefore, when the engine 3 is driven, the driving input shaft 5 is rotated, and the rotational driving force of the driving input shaft 5 is transmitted to the HST input shaft 12 through the HST input driving gear 9 and the HST input driven gear 19. [ (17).

The rotational driving force of the HST input shaft 17 is input to the HST unit 15 and the HST unit 15 rotates the HST output shaft 25 in accordance with the capacity of the predetermined HST unit 15. When the HST output shaft 25 rotates, the ring gear 39 connected to the HST output drive gear 43 rotates about the drive input shaft 5.

On the other hand, when the drive input shaft 5 is rotated by the driving force of the engine 3, the first sun gear 7 fixed to the drive input shaft 5 also rotates. A plurality of planetary gears 33 connected to the first sun gear 7 and the ring gear 39 are connected to the first sun gear 7 and the ring gear 39, respectively, as the first sun gear 7 and the ring gear 39 rotate, (43) around the drive input shaft (5).

At this time, since the revolution speeds of the respective planetary gears 33 are all the same, the carrier 29 which is simultaneously engaged with the planetary gears 33 rotates about the drive input shaft 5.

The carrier 29 and the carrier shaft 27 connected to the second pinion gear 37 of the planetary gear 33 are rotated by the planetary gears 33, And rotates about the drive input shaft 5 by the rotational motion and the idle motion of the drive shaft 33.

That is, according to the present embodiment, the rotational driving force of the driving input shaft 5 directly driven by the engine 3 and the rotational driving force of the HST output shaft 25 driven by the HST unit 15 are transmitted to the planetary gear 33 And the combined driving force is combined with the power of the first sun gear 7 to drive the carrier shaft 27 and the sun shaft 53 to rotate.

More specifically, the auxiliary transmission 1 drive gear 45 and the auxiliary transmission three-stage drive gear 47 are coupled to each other so as to interlock with the carrier shaft 27. The auxiliary transmission 1 drive gear 45 is connected to the sub- Stage driven gear 47 to transmit the power to the below-described third speed driven gear 51, which will be described later.

A second speed change stage second drive gear 55 and a fourth speed change drive gear 57 are coupled to the sun gear shaft 53 to be interlocked with the second sun gear 31, Stage drive gear 57 transmits the power to the downshift four-stage driven gear 61, which will be described later, .

First, the auxiliary speed change driven one-side driven gear 49 and the auxiliary speed change three-step driven gear 51 are rotatably coupled to the auxiliary speed change one-side driven gears 70, 49 and the auxiliary transmission three-stage driven gear 51. The auxiliary transmission 1-side driven gear 49 or the auxiliary transmission 3 is selectively connected to the auxiliary transmission 1, And a three-step speed change synchromesh mechanism 63 for transmitting the power of the three-stage driven gear 51 to the reduction gears 1 and 3 or the driven shaft 70. [

In addition, the first and third stages of drive shafts 70 are coupled to the first and second stage clutch drive gears 65 in an interlocking manner. Therefore, when the power of the negative speed first driven gear 49 or the negative third speed driven gear 51 selected through the selective shifting operation of the first-speed shift synchronous transmission mechanism 63 is shifted to the negative speed ratio 1, Driven shaft 70, so that the first-stage clutch drive gear 65 rotates.

In addition, the auxiliary speed-change first-speed and third-speed driven shafts 70 are provided with second-speed, second-speed, fourth-speed And a driven shaft 71 is rotatably coupled. The driven side second driven gear 59 and the driven side fourth driven gear 61 receive power from the second reduction gear second drive gear 55 and the third reduction gear fourth drive gear 57, respectively.

Further, the second speed-change driven gear 59 or the fourth speed-change driven gear 61 is selected through the selective shifting operation of the second-speed shift synchronous transmission mechanism 69, and the selected second speed- The power of the gear 59 or the reduction gears of the four-stage driven gear 61 is transmitted to the reduction gears 4,

Meanwhile, a clutch output shaft 73 coupled to the drive input shaft 5 such that the forward drive gear 81 and the reverse drive gear 83 are coupled to each other is rotatably coupled.

The clutch output shaft 73 is provided with a four-stage clutch driven gear 75 that receives power from the second-stage clutch drive gear 67 and a second-stage clutch driven gear 75 that receives power from the first- And a third speed change clutch 79 having a three-speed clutch driven gear 77 are mounted and coupled.

Thus, the second-stage clutch driven gear 75 and the first-stage and third-stage clutch driven gears 77 are driven by the corresponding two-stage clutch drive gear 67 and the first- So that the second-stage clutch driven gear 75 or the first-stage clutch driven gear 77 is selected through the selective shifting operation of the auxiliary shift clutch 79. [

The power of the second-stage clutch driven gear 75 or the first-stage clutch driven gear 77 selected here is transmitted to the clutch output shaft 73.

The forward drive gear 81 and the reverse drive gear 83 are coupled to the clutch output shaft 73 so that the forward drive gear 81 and the reverse drive gear 83 interlock with the clutch output shaft 73 do. A forward-reverse output shaft 85, through which power is transmitted from the clutch output shaft 73, is disposed at the downstream end of the drive input side 5.

The forward-rearward output shaft 85 is provided with a forward-backward synchronizing transmission mechanism 91 having a forward driven gear 87 and a reverse driven gear 89 which are respectively opposed to the forward drive gear 81 and the reverse drive gear 83, A reverse idle gear 90 is engaged between the reverse drive gear 83 and the reverse driven gear 89 so that the driving force of the reverse drive gear 83 is transmitted to the reverse driven gear 90 through the reverse idle gear 90, And is transmitted to the gear 89. The reverse driven gear 89 rotates in a direction opposite to the rotation direction of the forward driven gear 87. [

As described above, the forward driven gear 87 and the reverse driven gear 89 receive power from the forward drive gear 81 and the reverse drive gear 83, respectively, The forward driven gear 87 and the reverse driven gear 89 are selected through the shift operation and the power of the selected forward driven gear 87 or the reverse driven gear 89 is transmitted to the forward and reverse output shaft 85 .

The power transmitted to the forward / rearward output shaft 85 is connected to the forward / reverse output shaft 85 and is connected to the main clutch 93 for final output.

Hereinafter, the operation of the transmission 1 according to the present embodiment will be described.

[Power transmission path]

As described above, when the engine 3 is rotated to rotate the drive input shaft 5, the rotational drive force of the drive input shaft 5 and the rotational drive force of the HST output shaft 25 driven by the HST unit 15 become HST And the combined driving force is combined with the power of the first sun gear 7 to rotate the carrier shaft 27 and the second sun gear shaft 31 . At this time, the HST unit 15 can convert the output rotational speed and use it to adjust the speed of the carrier shaft 27 and the sun gear shaft 53 of the compound planetary gear 33.

The power transmitted to the carrier shaft 27 is transmitted to the carrier shaft 27 through the auxiliary speed change one stage drive gear 45, the auxiliary speed third stage drive gear 47, the auxiliary speed change one stage driven gear 49, And the selected power is transmitted to the negative change-speed clutch 79 via the selective shifting operation of the transmission mechanism 63 of the first-speed and third-speed change-speed synchronizer.

The power transmitted to the sun shaft 53 is transmitted to the sun gear shaft 53 through the auxiliary speed change two-stage drive gear 55, the auxiliary speed change fourth speed drive gear 57, the second speed change speed second speed driven gear 59, 61, and the power selected through the selective shifting operation of the transmission mechanism 69 is transmitted to the negative change-speed clutch 79.

Each of the power transmitted to the auxiliary speed change clutch 79 is transmitted to the transmission mechanism 91 through the forward / backward synchromesh via the operation of the auxiliary speed change clutch 79, and is transmitted through the forward / backward shift transmission mechanism 91 After the forward or reverse operation is selected, power is transmitted to the main clutch 93 and transmitted to the wheel.

[Sub shift operation]

First, the components of the auxiliary speed change section will be described. The auxiliary speed change clutch 79 is connected together with two synchronous transmission gears, that is, the first-stage speed change synchronous transmission mechanism 63 and the second- .

The first and third speed change synchronous transmission mechanism 63 is connected to the above-described second speed change stage first drive gear 45. The second and third speed change speed mechanism mechanisms 69, The auxiliary shift clutch 79 is disconnected from the power connected to the auxiliary shifting and first and third driven shafts 70 and connected to the auxiliary shifting clutches 4 and 5 in the state of being connected to the second driving gear 55, The transmitted power is transmitted downstream.

In the second-stage to third-speed shifting, the first-speed-change-speed synchronizer transmission mechanism 63 is connected from the first-speed shift stage drive gear 45 to the third shift stage drive gear 47, In the state where the synchromesh transmission mechanism 69 is connected to the second stage, the auxiliary shift clutch 79 is disengaged from the power of the auxiliary transmission 4 and the auxiliary transmission shaft 71 and connected to the auxiliary transmission 1, To transmit the received power to the downstream.

[Before and after operation]

First, components of the forward and backward portions are constituted by the forward-backward synchronizing transmission mechanism 91 and the main clutch 93. And selects the forward driven gear 87 or the reverse driven gear 89 for forward or backward movement of the transmission mechanism 91 via the forward and backward transmission gears 91 when the forward or backward selection is selected and transmits the selected power to the forward and rearward output shaft 85 .

At this time, the subtransmission clutch 79 immediately before shifting to the forward / backward synchro is released, whereby the power transmitted from the auxiliary speed change portion is temporarily shut off to synchronize the gear ratio. In addition, the main clutch 93 disengages the forward and reverse output shafts 85 to increase the stability of the transmission mechanism and the overall system by synchronizing the forward and backward shifts during shifting as a part of the safety device.

In the transmission 1 according to the present embodiment as described above, in place of the wet type multi-plate clutch, there are provided two synchronous transmission gears, that is, a three-step transmission synchronous transmission mechanism 63, The use of the wet type multi-plate clutch is minimized, so that it can be effectively applied to a vehicle such as a tractor having a power of 100 horsepower or less, or the like, which is limited in the space for mounting the component.

Further, since each of the synchro transmission mechanisms 63 and 69 described above operates with an electric actuator, there is no need for a heat generation and a hydraulic pressure supply, so that a related cooling device is not required, so that the structure of the transmission 1 is simplified, The size can be made compact.

1: Transmission 3: Engine
5: drive input shaft 7: first sun gear
9: HST input drive gear 11: working device drive shaft
13: Clutch 15: Hydraulic stepless speed change unit
17: HST input shaft 19: HST input driven gear
21: Hydraulic pump 23: Hydraulic motor
25: HST output shaft 27: carrier shaft
29: carrier 31: second sun gear
33: planetary gear 35: first pinion gear
37: second pinion gear 39: ring gear
40: ring input gear 41: ring output gear
43: HST output drive gear 45: Negative speed first drive gear
47: Third speed reduction gear drive gear 49: 1st speed reduction gear drive gear
51: Third speed lowering gear 53:
55: Second speed shift second gear drive gear 57: Second shift gear fourth gear drive gear
59: Second speed shifting second driven gear 61: Third shifting fourth speed gear
63: 1, 3-speed shift synchronous transmission gear 65: 1, 3-speed clutch drive gear
67: 2,4-speed clutch drive gear 69: 4, 2-speed shift synchromesh transmission gear
71: Second speed shift 2nd / 4th speed driven shaft 73: Clutch output shaft
75: 2nd and 4th stage clutch driven gear 77: 1st and 3rd stage clutch driven gear
79: auxiliary shift clutch 81: forward gear
83: reverse gear 85: forward / reverse output shaft
87: forward driven gear 89: reverse driven gear
90: Reverse idle gear 91: Forward / reverse synchro transmission
93: Main clutch

Claims

A transmission comprising a drive input shaft (5) connected to an output portion of an engine (3), a carrier shaft (27) and a sun gear shaft (53) rotated by receiving a rotational force of the drive input shaft (5)
A first shift stage and a third shift stage drive gear (45, 47) coupled to the carrier shaft (27);
A second shift stage and a fourth shift stage drive gear (55, 57) provided in association with the sun gear shaft (53);
Speed driven gears (first and second speed-change driven gears) 45 and 45, which are provided with a third speed change clutch drive gear 65 to which the power of the above- 70, and a third-speed change-speed synchronous transmission mechanism 63 coupled to the first-speed shift stage and the third-stage shift stage three-stage driven gears 49, 51;
A second shift stage driven by the second shift stage 71 and a second shift stage driven by the second shift stage, A 2 < st > speed change speed synchronous transmission mechanism 69 equipped with a gear 67;
A clutch output shaft (73) rotatably coupled to the drive input shaft (5);
A forward drive gear 81 and a reverse drive gear 83 provided to interlock with the clutch output shaft 73;
Stage clutch-driven gear 67 that is coupled to the clutch output shaft 73 and receives power from the second-stage clutch drive gear 67 and a second- A subtransmission clutch (79) provided with a first and a third stage clutch driven gear;
And a control unit for controlling the drive of the tractor.

The method according to claim 1,
The first and third speed change synchronous transmission mechanism 63 is connected to the above-described auxiliary speed change one-stage drive gear 45 and the four-speed synchronous transmission mechanism 69 is connected to the above- The auxiliary shift clutch 79 disengages the power connected to the auxiliary shifting and first and third driven shafts 70 while the power is transmitted to the auxiliary shifting gears 55 and 55. Thus, Connected,
In the second-stage to third-speed shifting, the first-speed-change-speed synchronizer transmission mechanism 63 is connected from the first-speed shift stage drive gear 45 to the third shift stage drive gear 47, In the state in which the synchromesh transmission mechanism 69 is connected to the second stage, the auxiliary shift clutch 79 disengages the power of the auxiliary transmission / second / fourth driven shaft 71 and engages the auxiliary transmission / Is connected to the transmission.

The method according to claim 1,
The sun gear shaft 53 is freely rotatably connected to the drive input shaft 5 and the carrier shaft 27 is freely rotatably connected to the sun gear shaft 53. The clutch output shaft 73, Is rotatably connected on the drive input shaft (5).

The method of claim 3,
A first sun gear 7 fixed to the drive input shaft 5;
A second sun gear 31 formed on the sun shaft 53;
A ring gear 39 freely rotatable about the driving input shaft 5 and installed to surround the second sun gear 31;
A first pinion gear 35 connected to the first sun gear 7 and a second pinion gear 37 connected simultaneously to the ring output gear 41 of the second sun gear 31 and the ring gear 39, , And a plurality of planetary gears (33)
Wherein the carrier (29) interlocked with the carrier shaft (27) is simultaneously engaged with the plurality of planetary gears (33).

5. The method of claim 4,
And an HST unit (15) for adjusting the magnitude of the output power by adjusting the internal hydraulic pressure,
The driving input shaft 5 is connected to the HST input shaft 17 of the HST unit 15,
The input portion of the ring gear 39 is connected to the HST output shaft 25 of the HST unit 15 so that the power of the drive input shaft 5 and the power of the HST unit 15 are combined, Is transmitted to the carrier shaft (27) and the sun shaft (53) which are linked together.

The method according to claim 1,
And a working device drive shaft (11) is connected to the drive input shaft (5).

The method according to claim 1,
The forward and reverse output shafts 85 to which the power of the forward drive gear 81 and the reverse drive gear 83 are transmitted are connected to the forward driven gears 87 and 87 which are respectively opposed to the forward drive gear 81 and the reverse drive gear 83 A forward / backward synchronizing transmission mechanism 91 having a reverse driven gear 89 is mounted,
And a reverse idle gear (90) for transmitting the power of the reverse drive gear (83) to the reverse driven gear (89) is engaged between the reverse drive gear (83) and the reverse driven gear (89) .