Transmission for tractor

1. A transmission for a tractor comprising:

a forward/reverse transmission unit for selectively changing a rotational direction of power of a first shaft connected with an engine and transmitting the power to a second shaft connected with an axle shaft;

a main transmission unit installed between the second shaft and the axle shaft and having a plurality of gears to speed-change the power transmitted from the second shaft in four-speed; and

a sub-transmission unit installed between the main transmission unit and the axle shaft and having a plurality of gears to speed-change again the power transmitted from the main transmission unit in four-speed,

wherein the main transmission unit includes:

a third shaft connected with the second shaft;

a fourth shaft selectively connected with the third shaft;

a fifth shaft arranged parallel to the third and fourth shafts;

a sixth gear installed on the third shaft;

a seventh gear, an eighth gear and a ninth gear installed in parallel on the fourth shaft;

a tenth gear installed on the fifth shaft and engaged with the sixth gear;

an eleventh gear installed on the fifth shaft and engaged with the seventh gear;

a twelfth gear installed on the fifth shaft and engaged with the eighth gear;

a thirteenth gear installed on the fifth shaft and engaged with the ninth gear;

a first coupling gear installed on one side of the sixth gear;

a second coupling gear installed on one side of the seventh gear opposite to the first coupling gear;

a first hub linearly movably installed on the fourth shaft to be selectively connected with the first or second coupling gear;

a third coupling gear installed on one side of the eighth gear;

a fourth coupling gear installed on one side of the ninth gear opposite to the third coupling gear; and

a second hub linearly movably installed on the fourth shaft to be selectively connected with the third or fourth coupling gear.

2. The transmission as claimed in claim 1 , wherein the forward/reverse transmission unit includes:

a forward clutch section and a reverse clutch section both installed in parallel on the first shaft;

a first gear coupled to one side of the forward clutch section to be selectively connected with the first shaft by an operation of the forward clutch section;

a second gear installed on the second shaft and engaged with the first gear;

a third gear coupled to one side of (the reverse clutch section to be selectively connected with the first shaft by an operation of the reverse clutch section;

a fourth gear engaged with the third gear; and

a fifth gear installed on the second shaft and engaged with the fourth gear.

3. The transmission as claimed in claim 2 , wherein the forward/reverse clutch sections are installed on the second shaft.

4. The transmission as claimed in claim 1 , wherein the sub-transmission unit includes:

a sixth shaft arranged parallel to the fourth shaft and receiving power from the fourth shaft;

a seventh shaft arranged coaxially with the fourth shaft;

an eighth shaft selectively connected with the sixth shaft;

a ninth shaft having one end selectively connected with the seventh shaft and the other end connected with the axle shaft;

a fourteenth gear installed on a distal end of the fourth shaft;

an eighteenth gear installed on one end of the sixth shaft and engaged with the fourteenth gear;

a nineteenth gear installed on the other end of the sixth shaft;

a fifteenth gear installed on one end of the seventh shaft and engaged with the nineteenth gear;

a sixteenth gear installed on the other end of the seventh shaft;

a twentieth gear installed on one end of the eighth shaft and engaged with the sixteenth gear;

a twenty-first gear installed on the other end of the eighth shaft; and

a seventeenth gear installed on the ninth shaft and engaged with the twenty-first gear.

5. The transmission as claimed in claim 4 , wherein the sub-transmission unit further includes:

a fifth coupling gear installed an one side of the fourteenth gear;

a sixth coupling gear installed on one side of the fifteenth gear opposite to the fifth coupling gear;

a third hub linearly movably installed on the ninth shaft to be selectively connected with the fifth or sixth coupling gear;

a seventh coupling gear installed on one side of the nineteenth gear;

an eighth coupling gear installed on one side of the twentieth gear opposite to the seventh coupling gear; and

a fourth hub linearly movably installed on the eighth shaft to be selectively connected with the seventh or sixth coupling gear.

6. A transmission for a tractor comprising:

a forward/reverse transmission unit for selectively changing a rotational direction of power of a first shaft connected with an engine and transmitting the power to a second shaft connected with an axle shaft;

a main transmission unit installed between the second shaft and the axle shaft and having a plurality of gears to speed-change the power transmitted from the second shaft in four-speed; and

a sub-transmission unit installed between the main transmission unit and the axle shaft and having a plurality of gears to speed-change again the power transmitted from the main transmission unit in four-speed,

wherein the main transmission unit includes:

a third shaft connected with the second shaft;

a fourth shaft selectively connected with the third shaft;

a fifth shaft arranged parallel to the third and fourth shafts;

a sixth gear installed on the third shaft;

a seventh gear, an eighth gear, and a ninth gear installed in parallel on the fourth shaft;

a tenth gear installed on the fifth shaft and engaged with the sixth gear;

an eleventh gear installed on the fifth shaft and engaged with the seventh gear;

a twelfth gear installed on the fifth shaft and engaged with the eight gear; and

a thirteenth gear installed on the fifth shaft and engaged with the ninth gear; and

wherein the sub-transmission unit includes:

a sixth shaft arranged parallel to the fourth shaft and receiving power from the fourth shaft;

a seventh shaft ranged coaxially with the fourth shaft;

an eighth shaft selectively connected with the sixth shaft;

a ninth shaft having one end selectively connected with the seventh shaft and the other end connected with the axle shaft;

a fourteenth gear installed on a distal end of the fourth shaft;

an eighteenth gear installed on one end of the sixth shaft and engaged with the fourteenth gear;

a nineteenth gear installed on the other end of the sixth shaft;

a fifteenth gear installed on one end of the seventh shaft and engaged with the nineteenth gear;

a sixteenth gear installed on the other end of the seventh shaft;

a twentieth gear installed on one end of the eighth shaft and engaged with the sixteenth gear;

a twenty-first gear installed on the other end of the eighth shaft; and

a seventeenth gear installed on the ninth shaft and engaged with the twenty-first gear.

7. The transmission as claimed in claim 6 , wherein the forward/reverse transmission unit includes;

a forward clutch section and a reverse clutch section both installed in parallel on the first shaft;

a first gear coupled to one side of the forward clutch section to be selectively connected with the first shaft by an operation of the forward clutch section;

a second gear installed on the second shaft and engaged with the first gear;

a third gear coupled to one side of the reverse clutch section to be selectively connected with the first shaft by an operation of the reverse clutch section,

a fourth gear engaged with the third gear; and

a fifth gear installed on the second shaft and engaged with the fourth gear.

8. The transmission as claimed in claim 6 , wherein the sub-transmission unit further includes:

a fifth coupling gear installed on one side of the fourteenth gear;

a sixth coupling gear installed on one side of the fifteenth gear opposite to the fifth coupling gear;

a third hub linearly movably installed on the ninth shaft to be selectively connected with the fifth or sixth coupling gear;

a seventh coupling gear installed on one side of the nineteenth gear;

an eighth coupling gear installed on one side of the twentieth gear opposite to the seventh coupling gear; and

a fourth hub linearly movably installed on the eighth shaft to be selectively connected with the seventh or sixth coupling gear.

9. The transmission as claimed in claim 7 , wherein the forward/reverse clutch sections are installed on the second shaft.