Transmission for tractor

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.