As is well known, the traveling apparatus of the crawler vehicle is required two functions of obtaining a great traction force and reducing a vibration and an impact of the vehicle, at a time of traveling the ground surface having the great rolling plain or the irregularity such as rocks or the like.
In particular, in the crawler vehicle such as a tractor, a bulldozer and the like working on the basis of the traction force, a traction force at a time when a crawler belt of the traveling apparatus slips due to a dozing load or a traction load (hereinafter, refer to as a shoe slip) is a critical traction force of the vehicle. Accordingly, the traveling apparatus of the vehicle is required to be structured such that the shoe slip can be inhibited from being generated as much as possible under a predetermined vehicle mass, whereby a high traction force can be obtained.
In order to solve the problem mentioned above, there has been conventionally considered various kinds of traveling apparatus of the crawler vehicle. As a first example, in Japanese patent document 1: Japanese Unexamined Patent Publication No. 57-107964, there is described a traveling apparatus of a crawler vehicle in which an improved road-hugging property of the traveling apparatus is secured at a time of traveling on the road surface having the great rolling or the irregularity such as the rocks or the like, and both of securing the traction force and cushioning the vibration and the impact of the vehicle are intended.
Further, as a second example, in Japanese patent document 2: Japanese Unexamined Patent Publication No. 2001-225770, there is described a traveling apparatus of a crawler vehicle in which a backward and forward stability is secured by bracing of front and rear end portions of the traveling apparatus against the dozing load or the traction load, thereby inhibiting the shoe slip from being generated and intending to secure a high traction force.
First, a description will be given of the first example of the traveling apparatus of the crawler vehicle in accordance with the conventional art by exemplifying the structure described in the publication 1, with reference to FIG. 9. FIG. 9 is a side elevational view of a bulldozer described in the publication 1.
In FIG. 9, a bulldozer 2 is provided with a pair of right and left traveling apparatuses 70 and 70. In each of the traveling apparatuses 70, a track frame 71 is swingably attached to a vehicle body via a pivot shaft 72. First arms 73 and 83 are respectively attached to both front and rear end portions of the track frame 71 by pins 73a and 83a so as to freely swing. Idlers 74 and 84 are rotatably attached to outer end portions of the first arms 73 and 83, respectively.
Approximately center portions of second arms 75 and 85 are swingably attached to inner lower end portions of the first arms 73 and 83, respectively. Track rollers 76 and 76 and track rollers 86 and 86 are rotatably attached to both end portions of the second arms 75 and 85, respectively.
On the other hand, base end portions of the first arms 77 and 87 are swingably attached to two positions in a center lower portion of the track frame 71 by pins 77a and 87a so as to be apart from each other in a back and forth direction, respectively. Approximately center portions of the second arms 78 and 88 are swingably attached to leading end portions of the first arms 77 and 87. Track rollers 79 and 79 and track rollers 89 and 89 are rotatably attached to both end portions of the second arms 78 and 88, respectively.
Further, a sprocket 81 is rotatably attached to the vehicle body above a rear end portion of the track frame 71, and a crawler belt 82 is wound around the sprocket 81, the idlers 74 and 84 and the track rollers 76, 79, 86 and 89.
In accordance with the structure shown in FIG. 9, the respective track frames 71 in the left and right traveling apparatuses 70 and 70 swing around the pivot shaft 72, whereby it is possible to secure a road-hugging property of the left and right traveling apparatuses 70 and 70 even at a time of traveling on the ground surface having a great rolling. Further, the idlers 74 and 84 and the track rollers 76, 79, 86 and 89 respectively swing, whereby it is possible to secure an improved road-hugging property of the traveling apparatus 70 even at a time of traveling on the ground surface having the irregularity such as the rocks or the like. As a result, it is possible to intend to secure the traction force and cushion the vibration and impact of the vehicle.
However, in the traveling apparatus 70 mentioned above, there is a problem that the shoe slip tends to be generated with respect to the great dozing load or the traction load. This will be in detail described with reference to FIG. 10. FIG. 10 is a view describing an example of the traction load of the bulldozer. In this case, the same reference numerals are attached to the same constituting elements as those shown in FIG. 9, and a description thereof will be omitted below.
In FIG. 10, in the case that a ripper apparatus 5 is attached to a rear portion of the bulldozer 2, and the rock is excavated by the ripper apparatus 5, it is hard to compress and destroy the rock as is well known. Therefore, the excavation is performed on the basis of a tension destruction by applying a force to the rock in a direction of an arrow D in the drawing toward a free space on the ground surface. As a result, an excavation reaction force R1 is going to lift up a front side of the bulldozer 2. However, the bulldozer 2 stands against the lift-up by a mass W and a supporting point reaction force R2. In this case, when the rear idler 84 of the traveling apparatus 70 swings upward at this time, the front portion of the traveling apparatus 70 tends to lift up, whereby the shoe slip tends to be generated. As a result, it is hard to increase a value of the traction force F to the maximum.
As a means for solving the problem mentioned above, the patent publication 2 mentioned above describes a traveling apparatus of a crawler vehicle in which a track frame, an idler and a sprocket are arranged approximately linearly. A description will be given of the second example in accordance with the conventional art by exemplifying the described structure. FIG. 11 is a side elevational view of the bulldozer described in the document 2, and FIG. 12 is a side elevational view of a traveling apparatus thereof. In this case, the same reference numerals are attached to the same constituting elements as those in FIG. 10, and a description thereof will be omitted below.
In FIGS. 11 and 12, traveling apparatuses 60 and 60 are arranged in left and right sides of the bulldozer 3. In each of the traveling apparatuses 60, a track frame 61, an idler 12 and a sprocket 63 are arranged approximately linearly. A plurality of track rollers 23, 24 and 25 are rotatably mounted to a lower portion of the track frame 61, and a crawler belt 14 is wound around the idler 12, the sprocket 63 and the track rollers 23, 24 and 25.
Further, the idler 12 is attached to the track frame 61 so as to be prevented from moving in a vertical direction, and the sprocket 63 is rotatably attached to a vehicle body frame (not shown). The track roller 25 positioned in a front side is rotatably attached to a leading end portion of an arm 26 which is swingably attached to the track frame 61 by a pin 26a. The track rollers 23 and 24 positioned sequentially positioned in a rear side of the track roller 25 are rotatably attached to both end portions of a second arm 22 which is swingably attached in an approximately center portion with respect to a leading end portion of a first arm 21 swingably attached to the track frame 61 by a pin 21a. 
In accordance with the structure mentioned above, since the idler 12 and the sprocket 63 do not move in the vertical direction, the front portion or the rear portion of the traveling apparatus 60 is hard to lift up even in the case that a great dozing load or traction load is applied. Therefore, it is possible to inhibit the shoe slip from being generated and it is possible to obtain a high traction force.
However, in the traveling apparatus 60 disclosed in the publication 2, the following problems are generated. In other words, since the idler 12 and the sprocket 63 do not move in the vertical direction, the impact applied to an operator cab 4 is reduced by half by the swing motion (not shown) of the track frame 61 of the traveling apparatus 60 in the run-on side at a time of forward moving, for example, in the case that any one side of the right and left traveling apparatuses 60 runs on the rock during the traveling on the ground surface having the irregularity such as the rock or the like, however, the impact caused by the run-on of the sprocket 63 is directly transmitted to the operator cab 4 at a time of backward moving. As a result, since the impact applied to the operator cab 4 is great at a time of backward moving on the ground surface having the irregularity such as the rock or the like, there is a problem that a fatigue of an operator is increased and an operability within the operator cab 4 is lowered.