Crawler travel unit

1. A crawler type travel unit comprising:

a rear axle case of a travel machine body;

a track frame swingably supported by said rear axle case through a rocking shaft;

a front driven wheel disposed on a front end portion of said track frame;

a rear driven wheel disposed on a rear end portion of said track frame;

three idling wheels aligned between said front driven wheel and said rear driven wheel at intervals in a longitudinal direction;

a rear axle supported by said rear axle case;

a drive wheel which is disposed upward of a longitudinal intermediate portion of said track frame and is configured to receive power transmitted from said rear axle; and

a crawler belt wrapped around said drive wheel, said front driven wheel, said rear driven wheel and said idling wheels,

wherein a horizontal distance during contact with the ground between a center of the rocking shaft and a center of a spindle of said front driven wheel is set longer than a horizontal distance between the center of the rocking shaft and a center of a spindle of said rear driven wheel, and said rocking shaft is disposed downward of a center of said drive wheel, so as to give an upward force to a front portion of said travel unit when tension is applied to said crawler belt;

wherein said rocking shaft is disposed at a position immediately below a center of said drive wheel and displaced frontward from the position;

wherein each of two idling wheels on a rear side of said three idling wheels is pivoted on a first rocking link, and a longitudinal intermediate portion of said first rocking link is pivoted on said track frame through a first pivot shaft;

wherein a horizontal distance between said center of said rocking shaft and said center of the first pivot shaft is set shorter than a horizontal distance between said center of said rocking shaft and a wheel shaft of the idling wheel on a front side, and a wheel shaft of a forward one of the two rear idling wheels pivoted on the first rocking link is arranged forwardly of the rocking shaft, so as to support a load acting on the track frame by the two rear idling wheels;

wherein, as seen in vertical section from behind, the track frame has an upper section with a larger right/left width and a lower section with a smaller right/left width; and

wherein the idler wheels extend across a right/left width of the track frame so that the idler wheels are located on lateral sides of the smaller-width lower section and directly below lateral sides of the larger-width upper section.

2. The crawler type travel unit according to claim 1 ,

wherein each of the front driven wheel and the rear driven wheel has a diameter larger than a diameter of each of the idling wheels, and

wherein said first pivot shaft of said first rocking link has a height which is substantially the same as a height of a wheel shaft of each of said idling wheels on the rear side, and is arranged in the middle between said wheel shafts.

3. The crawler type travel unit according to claim 1 , further comprising:

a tension-adjusting mechanism provided on a front portion of said track frame and configured to support said front driven wheel and to bias said front driven wheel in a direction that adjusts tension,

said tension-adjusting mechanism including: an axle support configured to support a spindle of said front driven wheel from both sides of said spindle; and a pair of right and left tension springs configured to frontward bias the axle support.

4. A crawler type travel unit comprising:

a rear axle case of a travel machine body;

a track frame swingably supported by said rear axle case through a rocking shaft;

a front driven wheel disposed on a front end portion of said track frame;

a rear driven wheel disposed on a rear end portion of said track frame;

four idling wheels aligned between said front driven wheel and said rear driven wheel at intervals in a longitudinal direction;

a rear axle supported by said rear axle case;

a drive wheel which is disposed upward of a longitudinal intermediate portion of said track frame and is configured to receive power transmitted from said rear axle; and

a crawler belt wrapped around said drive wheel, said front driven wheel, said rear driven wheel and said idling wheels,

wherein said four idling wheels are composed of the two idling wheels on the rear side and two idling wheels on a front side relative to the two idling wheels on the rear side;

wherein each of two idling wheels on a rear side of said four wheels is pivoted on a first rocking link, and a longitudinal intermediate portion of said first rocking link is pivoted on said track frame through a first pivot shaft;

wherein each of the two idling wheels on the front side is pivoted on a second rocking link, and a longitudinal intermediate portion of said second rocking link is pivoted on said track frame through a second pivot shaft;

wherein a horizontal distance during contact with the ground between said center of said rocking shaft and said center of said spindle of said front driven wheel is set longer than a horizontal distance between said center of said rocking shaft and said center of said spindle of said rear driven wheel, and said rocking shaft is disposed downward and forward of said center of said drive wheel, so as to give an upward force to a front portion of said travel unit when tension is applied to said crawler belt;

wherein a horizontal distance between said center of said rocking shaft and said center of said first pivot shaft is set shorter than a horizontal distance between said center of said rocking shaft and said center of said second pivot shaft;

wherein, as seen in vertical section from behind, the track frame has an upper section with a larger right/left width and a lower section with a smaller right/left width; and

wherein the idler wheels extend across a right/left width of the track frame so that the idler wheels are located on lateral sides of the smaller-width lower section and directly below lateral sides of the larger-width upper section.

5. The crawler type travel unit according to claim 4 ,

wherein each of the front driven wheel and the rear driven wheel has a diameter larger than a diameter of each of the idling wheels, and

wherein said first pivot shaft of said first rocking link has a height which is substantially the same as a height of a wheel shaft of each of said idling wheels on the rear side, and is arranged in the middle between said wheel shafts.

6. The crawler type travel unit according to claim 5 ,

wherein said second pivot shaft of said second rocking link has a height which is substantially the same as a height of said wheel shaft of each of said idling wheels on the front side, and is arranged in the middle between said wheel shafts.

7. The crawler type travel unit according to claim 4 , further comprising:

a tension-adjusting mechanism provided on a front portion of said track frame and configured to support said front driven wheel and to bias said front driven wheel in a direction that adjusts tension,

said tension-adjusting mechanism including: an axle support configured to support a spindle of said front driven wheel from both sides of said spindle; and a pair of right and left tension springs configured to frontward bias the axle support.