BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a seal assembly and a crawler-track connection structure.  
         [0003]     2. Description of the Related Art  
         [0004]     Generally, as shown in  FIG. 6 , a crawler vehicle such as a construction machine, e.g., a bulldozer tractor or a hydraulic shovel tractor, has an endless link chain  81  and a plurality of ground-contacting shoe plates (not shown) mounted on the link chain  81 . The link chain  81  comprises a plurality of links  82   a,  . . . and  82   b,  . . . that are disposed parallel to each other, and a crawler-track connection Structure  84  for pivotably connecting the links  82   a  and  82   b  opposing each other. Specifically, the links  82   a  and  82   b  comprise an intermediate portion  85  where the shoe plates (not shown) are mounted, and connecting portions  86  and  87  protruding from the intermediate portion  85 . A pin insertion opening  88  is provided in the connecting portion  86 , and a bushing insertion opening  89  is provided in the connecting portion  87 . The links  82   a  and  82   a  and the links  82   b  and  82   b  are individually connected together via the crawler-track connection structure  84  such that the connecting portion  86  and the connecting portion  87  are overlapped with each other.  
         [0005]     The crawler-track connection structure  84  comprises a pin  90  and a bushing  91  externally fitted around the pin  90 . End portions of the pin  90  extend outward from the bushing  91  In the axial direction, and the pin  90  is press-fitted into the pin insertion opening  88  of the link  82 . End portions of the bushing  91  are press-fitted into the bushing insertion openings  89 . An opening portion of the pin insertion opening  88  on the side of the bushing is formed as a large-diameter portion  92 . Space portions  94  are formed by the large-diameter portion  92 , an outer end surface of the bushing  91 , and an outer peripheral surface  93  of the pin  90 . A seal assembly  95  is fitted into the space portion  94 . In this case, the bushing  91  is externally fitted to be rotatable around the pin  90 , the pin  90  is unitized with the connecting portion  86 , and the bushing  91  is unitized with the connecting portion  87 . Thereby, end portions of the links  82  and  82  to be connected, i.e., the connecting portions  86  and  87 , are pivotably connected together. In addition, an oil injection opening  96  is provided in the pin  90 . Oil in the oil injection opening  96  flows to the side of the outer peripheral surface  93  of the pin  90  through a path (not shown) and serves as a lubricant between the pin  90  and the bushing  91 .  
         [0006]     As shown in  FIG. 7 , the seal assembly  95  comprises a seal ring  98  having a lip portion  97 , a support ring  99  for supporting the seal ring  98 , and a load ring  100  for receiving a pressure from the lip portion  97  of the seal ring  98 . Thereby, the seal assembly  95  prevents overflow of the aforementioned lubricant to the outside.  
         [0007]     In the above-described conventional crawler-track connection structure, to cause the lip portion  97  to press-engage with a corresponding wall (an end surface of the bushing  91  in  FIG. 6 ), the load ring  100  needs to receive a pressure from an outer peripheral side thereof. Therefore, In the conventional structure, housings (space portions  94 ) need to be formed to insert the seal assembly  95 , thereby complicating the overall structure that makes the manufacturing processing to be difficult. In addition, since the seal assembly  95  must be inserted in the space portions  94 , the overall assembly requires complicated steps, thereby reducing the productivity.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is made to solve the above-described problems with the conventional case. Accordingly, an object of the invention is to provide a seal assembly that has a simple assembly structure and that provides secure sealing effects. Another object of the invention is to provide a crawler-track connection structure that allows the seal assembly to be easily mounted, and in addition, that securely prevents overflow of a lubricant and the like to the outside.  
         [0009]     To these ends, according to first aspect of the invention, a seal assembly comprises a pair of seal rings  1  and  1  individually comprising lip portions  23  and  23  disposed such that each of the lip portions  23  and  23  protrudes in a direction opposing an axial direction; and a load seal ring  2  compressed and inserted between the seal rings  1  and  1 , the load seal ring  2  exerting reaction forces on the lip portions  23  and  23  outwardly in the axial direction.  
         [0010]     In the seal assembly according to the first aspect of the invention, with an axial-direction compressive force being exerted on the load seal ring  2 , the load seal ring  2  presses the respective lip portions  23  and  23  of the seal rings  1  and  1  outwardly in the axial direction. Therefore, when the seal assembly is mounted between walls that form a gap of which the length is less than the axial-direction length of the seal assembly in a free state, the individual lip portions  23  and  23  are press-engaged with the corresponding walls, thereby allowing an inner-diameter side and an outer-diameter side of the lip portion  23  and  23  to be hermetically enclosed. That is, with the seal assembly, assembly thereof into a crawler-track connection structure and the like can easily be performed to thereby improving the efficiency in the assembly work.  
         [0011]     The seal assembly according to the first aspect of the invention may further comprise an outer-diameter controller body  32  for controlling the displacement of the load seal ring  2  in a periphery outer direction.  
         [0012]     In this case, the displacement of the load seal ring  2  in the periphery outer direction can be controlled by the outer-diameter controller body  32 , and reaction forces of the load seal ring  2  in the axially-outer direction can be securely exerted on the lip portions  23  and  23 . Thereby, conventionally required outer-peripheral-side controller walls (space portions  94  shown in  FIG. 6 ) can be avoided. That is, processing for spaces used for mounting a seal assembly S is not required. This facilitates manufacturing processing, and concurrently, allows high-precision sealing effects to be provided.  
         [0013]     The seal assembly according to the first aspect of the invention may further comprise an inner-diameter controller body  32  for controlling the displacement of the load seal ring  2  in a periphery inner direction.  
         [0014]     In this case, the displacement of the load seal ring  2  in the periphery outer direction can be controlled by the Inner-diameter controller body  32 , and reaction forces of the load seal ring  2  in the axially-outer direction can be securely exerted on the lip portions  23  and  23 . In addition, the inner-diameter controller body  33  functions as a spacer disposed in an inner-diameter side of the seal assembly to thereby allow a mounting portion of the seal assembly to easily be secured.  
         [0015]     In the seal assembly according to the first aspect of the invention, one of the seal rings  1  and  1  may comprise the outer-diameter controller body  32  for controlling the displacement of the load seal ring  2  in the periphery outer direction, and the other one of the seal rings  1  and  1  comprise the inner-diameter controller body  32  for controlling the displacement of the load seal ring  2  in the periphery inner direction.  
         [0016]     In this case, the displacement of the load seal ring  2  in the periphery outer direction can be controlled by the outer-diameter controller body  32 , and the displacement of the load seal ring  2  in the in the periphery inner direction can be controlled by the inner-diameter controller body  33 . Reaction forces in the axial direction according to the load seal ring  2  can be securely exerted on the lip portions  23  and  23 . Therefore, a higher-precision function can be produced.  
         [0017]     In the seal assembly according to the first aspect of the invention, the load seal ring may comprise a circumferential groove  27  that tolerates axial-direction compression.  
         [0018]     In this case, according to the provision of the circumferential groove  27 , when the compressive force is exerted on the load seal ring  2  in the axial direction, the load seal ring  2  is compressed in the axial direction, and reaction forces in the axially-outer direction according to the load seal ring  2  can be securely exerted on the lip portions  23  and  23 . Thereby, stable sealing effects can be produced.  
         [0019]     In the above seal assembly according to the first aspect of the invention, a cross section of the seal assembly may be symmetric with respect to a radial-direction line passing the center thereof.  
         [0020]     In this case, since the cross section of the seal assembly is symmetric with respect to the radial-direction line passing the center thereof, the obverse side and the reverse side of the seal assembly are the same. This provides the advantage of facilitating mounting work of the seal assembly. Furthermore, only one type of pair of components, such as the seal rings  1  and  1 , may be formed, the manufacturing cost can thereby be reduced.  
         [0021]     According to a second aspect of the present invention, a crawler-track connection structure comprises a pin  8  to be inserted through superposed end portions of links  5  and  5 ; and a seal assembly S externally fitted on the pin  8  for preventing overflow of a lubricant to the outside, the lubricant being supplied to an outer peripheral side of the pin  8 . One of the links  5  and  5  is immobilized on the pin  8 , and the other one of the links  5  and  5  is supported on the pin  8  to be rotatable thereon. The seal assembly comprises a load seal ring  2  disposed between radial-direction walls W and W opposing each other along an axial direction, a first seal ring  1  comprising a lip portion  23  press-engaged with one of the radial-direction walls W and W according to a pressure exerted from the load seal ring  2 , and a second seal ring  1  comprising a lip portion  23  press-engaged with the other one of the radial-direction walls W and W according to a pressure exerted from the load seal ring  2 .  
         [0022]     In the crawler-track connection structure according to the second aspect of the invention, with the seal assembly S being mounted between the radial-direction walls W and W opposing each other along the axial direction, the lip portions  23  and  23  are press-engaged with the corresponding walls W and W to thereby allow an inner-diameter side and an outer-diameter side of the lip portions  23  and  23  to be hermetically enclosed. That is, in the seal assembly, the conventional space portions  94  shown in  FIG. 6  are not required. Therefore, manufacturing processing and mounting work of the seal assembly S can be easily performed, and high-precision sealing effects can be produced.  
         [0023]     The crawler-track connection structure according to the second aspect of the invention may further comprise a bushing  12  immobilized in the other one of the links  5  and  5  to be rotatable on the pin  8 , and an end surface of the bushing  12  functions as the one of the radial-direction walls W and W.  
         [0024]     In this case, one of the radial-direction walls W and W for receiving the seal assembly S can be formed using an end surface of the bushing  12 . Thereby, the crawler-track connection structure can be simplified overall.  
         [0025]     In addition, the crawler-track connection structure according to the second aspect of the invention may further comprise a bushing  12  immobilized in the other one of the links  5  and  5  to be rotatable on the pin  8 , and a bushing  13  on the side of a sprocket  18 , wherein the seal assembly S is inserted between the bushing  12  and the bushing  13 .  
         [0026]     In this case, the radial-direction walls W for receiving the seal assembly S can be formed using the bushing  12 . Thereby, the crawler-track connection structure can be simplified overall, and the assembly work is facilitated, improving the productivity thereof.  
         [0027]     The crawler-track connection structure according to the second aspect of the invention may further comprise a ring body  31  disposed in an inner-diameter side of the load seal ring  2  for controlling the displacement of the load seal ring  2  in a periphery inner direction.  
         [0028]     In this case, the displacement of the load seal ring  2  in the periphery inner direction can be controlled by the inner-diameter controller body  31 , and reaction forces of the load seal ring  2  in the axially-outer direction can be securely exerted on the lip portions  23  and  23 . In addition, the inner-diameter controller body  31  functions as a spacer disposed in an inner-diameter side of the seal assembly to thereby allow a mounting portion of the seal assembly to easily be secured.  
         [0029]     The crawler-track connection structure according to the second aspect of the invention may further comprise a dust seal ring  37  disposed in an outer peripheral side of the seal assembly S.  
         [0030]     In this case, the dust seal ring  37  prevents the entrance of dust, mud, muddy water, and the like to the seal assembly S from the outer peripheral side. Thereby, the seal assembly S provides stable sealing effects, the quality of the crawler track can be improved, and the durability of the crawler track can be improved.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1A  is an essential-portion cross-sectional view of a seal assembly (premounted) according to an embodiment of the present invention;  
         [0032]      FIG. 1B  is an essential-portion cross-sectional view of the seal assembly (postmounted);  
         [0033]      FIG. 2  is a cross-directional view of a crawler-track connection structure according to an embodiment of the present invention;  
         [0034]      FIG. 3  is an essential-portion cross-sectional view showing a disposed state of a dust seal ring according to the present invention;  
         [0035]      FIG. 4A  is an essential-portion cross-sectional view of a seal assembly (premounted) according to another embodiment of the present invention;  
         [0036]      FIG. 4B  is an essential-portion cross-sectional view of the seal assembly (postmounted);  
         [0037]      FIG. 5A  is an essential-portion cross-sectional view of a seal assembly (premounted) according to still another embodiment of the present invention;  
         [0038]      FIG. 5B  is an essential-portion cross-sectional view of the seal assembly (postmounted);  
         [0039]      FIG. 6  is a cross-sectional view of a conventional crawler-track connection structure; and  
         [0040]      FIG. 7  is a cross-sectional view of a conventional seal assembly. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]     Hereinbelow, referring to the accompanying drawings, practical embodiments of the present invention will be described in detail.  FIGS. 1A and 1B  are essential-portion cross-sectional views of an embodiment of a seal assembly according to the present invention. A seal assembly S is used for, for example, a crawler-track connection structure. The seal assembly S has a pair of seal rings  1  and  1  and load seal ring  2  disposed between the seal rings  1  and  1 , and each of the seal rings  1  and  1  is supported by a support ring  3  that has an L-shaped cross section. In the cross-sectional view, the seal assembly S is formed symmetric with respect to a radial line passing the center of the assembly S. The crawler-track connection structure is intended for use in a traveling crawler of a crawler vehicle, such as a construction machine, e.g., a bulldozer tractor or a hydraulic shovel tractor.  
         [0042]     As shown in  FIG. 2 , the above-described connection structure connects links  5  and  5  in a state where a connecting portion  6  at an end portion of one of the links  5  and  5  is superposed on a connecting portion  7  at an end portion of the other link  5 . Specifically, the individual link  5  has the connecting portion  6  on one end side, and the connecting portion  7  on the other end side, in which the connecting portion  6  and the connecting portion  7  of the adjacent links  5  and  5  are connected together via the connection structure. As shown in  FIG. 6 , in the individual links  5 , an intermediate portion (not shown) Is formed between the connecting portion  6  and the connecting portion  7 , and shoe plates are disposed in the intermediate portion. In addition, although not shown in  FIG. 2 , links  5  and  5  are similarly connected together at the other end portion of a pin  8 .  
         [0043]     The connection structure comprises a fallout prevention pin  9  externally fitted around the pin  8 . Specifically, a pin insertion opening  10  is provided in the connecting portion  6  of the link, and a bushing insertion opening  11  is provided in the connecting portion  7  of the link  5 , in which an and portion of the pin  8  is press-fitted into the pin insertion opening  10 , and a bushing  12  is press-fitted into the bushing insertion opening  11  of the connecting portion  7  to be rotatable around the pin  8 . In addition, another bushing  13  externally fitted around the pin  8 , that is, a bushing on the side of a sprocket  18  described below, is provided between the connecting portions  6  of the links  5  and  5  opposing each other at a predetermined space (in the drawing, opposing links  5  and  5  on the other side are omitted). The seal assembly S of the present invention is provided between the bushings  12  and  13 . An end surface of the bushing  12  on the one side and an end surface of the bushing  13  on the other side function as radial-direction walls W and W, on which lip portions  23  and  23  are provided, respectively. A conventional seal assembly Sa is provided between the bushing  12  and the connecting portion  6  of the link  5  on the one side.  
         [0044]     A circumferential U-shaped groove  14  is provided at an end portion of the pin  8 , and a tapered face  15  is provided around a peripheral portion of the pin insertion opening  10  to reduce the diameter of the opening inwardly in the axial direction. The tapered face  15  and the circumferential U-shaped groove  14  together form a circular space  16 . The fallout prevention pin  9  is elastically flexible in diameter, and  1 t is fitted in the circular space  16  to thereby fix the link  5  and the pin  8  together. The links  5  are thus connected to form a link chain. The link chain is engaged with a sprocket  18  (refer to  FIG. 2 ) of a crawler vehicle, such as a construction machine. An oil injection opening (not shown) is provided in the pin  8 . Oil in the oil injection opening flows to the side of an outer peripheral face  19  and serves as a lubricant between the pin  8  and the bushing  12 ,  
         [0045]     As shown in  FIGS. 1A and 1B , the seal ring  1  in the seal assembly S is formed of, for example, a urethane resin having the hardness of about Hs  95 . The seal ring  1  comprises a first portion  20 , a second portion  21 , and a pendulous peripheral wall  22 . The first portion  20  is provided on an outer peripheral side, and extends in the axial direction. The second portion  21  inwardly extends in a radial direction from an axial-direction outer portion of the first portion  20 . The pendulous peripheral wall  22  inwardly extends in the radial direction to an axial-direction inner portion of the first portion  20 . The lip portion  23  has a triangular cross section outwardly protruding in the axial direction, and it is provided in the second portion  21 .  
         [0046]     The support ring  3  is formed of metal, and it is disposed on a reverse-face side of the seal rings  1 . The support ring  3  comprises a first portion  24  and a second portion  25 . The first portion  24  engages with the first portion  20  of the seal ring  1 , the second portion  25  engages with the second portion  21  of the seal ring  1 , and an inner surface of the pendulous peripheral wall  22  of the seal ring  1  engages with an end surface of the support ring  3 . Thus, the aforementioned portions are unitized with the seal ring  1 .  
         [0047]     The load seal ring  2  is formed of, for example, NBR having the hardness of about Hs  90 . The load seal ring  2  is formed of a ring body that has a substantially trapezoidal cross section in which a circumferential groove  27  is provided on an outer peripheral surface  26 . Specifically, cutout portions  29  and  29  are provided on two end surfaces  28  and  28 , respectively, in which the axial-direction length of an inner peripheral surface  30  is less than the axial-direction length of the outer peripheral surface  26 . Therefore, when an axial direction compressive force is exerted on the load seal ring  2 , the axial-direction length of the load seal ring  2  is reduced. On the other hand, the outer peripheral surface  26  of the load seal ring  2  engages with an inner peripheral surface of the first portion  24  of the support ring  3 , and the end surface  28  of the load seal ring  2  engages with an inner surface of the second portion  25  of the support ring  3 .  
         [0048]     In a free state shown in  FIG. 1A , the axial-direction length of the seal assembly S is larger than the dimension between the bushings  12  and  13 , and a ring body  31  is provided between the bushings  12  and  13 . In this case, the outer diameter of the ring body  31  is arranged to be substantially the same as the inner diameter of the aforementioned load seal ring  2 . Thereby, the ring body  31  controls the displacement of the load seal ring  2  in the periphery inner direction, and controls the reduction in the dimension between the bushings  12  and  13 .  
         [0049]     In the free state, the seal assembly S configured as described above is greater than the dimension between the bushings  12  and  13 . Therefore, as shown in  FIG. 1B , when seal assembly S is disposed between the bushings  12  and  13 , it receives an axial-direction compressive force. When the seal assembly S receives the axial-direction compressive force, the axial-direction length of the load seal ring  2  is reduced. In this case, the ring body  31  functions as an inner-diameter controller body  33  for controlling the displacement of the load seal ring  2  in the periphery inner direction. In addition, the first portion  24  of the load seal ring  2  functions as an outer-diameter controller body  32  for controlling the displacement of load seal ring  2  in the periphery outer direction. This ensures that reaction forces are exerted on the lip portions  23  and  23  outwardly in the axial direction from the load seal ring  2 .  
         [0050]     As described above, when the load seal ring  2  is compressed in the axial direction, the reaction forces can be obtained. According to the reaction force, the lip portion  23  of the first seal ring  1  (which hereinbelow will refer to the seal ring  1  on the side of the bushing  12 ) is press-engaged with the end surface (that is, the radial-direction wall W) of the bushing  12 . Concurrently, the lip portion  23  of the second seal ring  1  (which hereinbelow will refer to the seal ring  1  on the side of the bushing  13 ) is press-engaged with the end surface (that is, the radial-direction wall W) of the bushing  32 . Thereby, an inner-diameter side and an outer-diameter side of the lip portion  23 , 23  can be hermetically enclosed to allow a seal function to be implemented. Therefore, use of the seal assembly S avoids the necessity of the provision of walls for receiving the outer peripheral side and the space portion  94 . In addition, the use of the seal assembly S avoids the necessity of the conventionally required space portions  94  (shown in  FIG. 6 ) provided to insert the seal assembly S. Thereby, the crawler-track connection structure can be simplified and furthermore, the seal assembly S need not be inserted Into the housings  94 . This allows the efficiency In assembly to be improved. In addition, the seal assembly S is formed to have the cross section symmetric with respect to the radial-direction line passing the center of the assembly. Therefore, the obverse side and the reverse side of the seal assembly S are the same. This provides an advantage in that the seal assembly S can be easily inserted. Furthermore, since only one type of pair of components, such as the seal rings  1  and  1  or the support rings  3  and  3 , may be formed, the manufacturing cost can thereby be reduced. In the above, there is still another advantage In that since the load seal ring  2  is also formed symmetric with respect to the aforementioned radial-direction line, the manufacture thereof is facilitated.  
         [0051]     As shown in  FIG. 2 , in the above-described crawler-track connection structure, the conventional seal assembly Sa is inserted between the connecting portions  6  and  7  of the links  5  and  5  in the manner described above. The seal assembly Sa comprises the seal ring  98  having the lip portion  97 , the support ring  99  for supporting the seal ring  98 , and the load ring  100  for receiving a pressure from the lip portion  97  of the seal ring  98 . More specifically, with the pin insertion opening  10 , the opening portion on the busing side is used as a large-diameter portion, a space portion (housing)  34  is formed using the large-diameter portion  35 , the end surface of the bushing  12 , and the outer peripheral face  19  of the pin  8 . Therefor, the seal assembly Sa prevents overflow of the lubricant from the side of the outer peripheral face  19  of the pin  8  to the outside between the links  5  and  5 , which are connected together. In addition, a spacer  36  to be fitted around the pin  8  is disposed on the inner-diameter side of the load ring  100 .  
         [0052]     In the crawler-track connection structure shown in  FIG. 2 , since the outer peripheral side of the seal assembly S is in an open state, mud, muddy water, dust, and the like can enter the seal assembly S through the outer peripheral side. To prevent the entrance, as shown in  FIG. 3 , it is preferable that a dust seal ring  37  be disposed. The dust seal ring  37  is formed of a ring body having a cross section substantially shaped as an irregular rectangle. In addition, the dust seal ring  37  has lip portions  38  and  38  on two end surfaces, and is provided between a connecting portion  7  of a link  5  and a bushing  13 . This configuration securely prevents the entrance of dust and the like from the outside. Furthermore, the above configuration prevents overflow of the lubricant and the like from the interior of the seal assembly S to the outside.  
         [0053]      FIGS. 4A and 4B  show another embodiment of a seal assembly. This embodiment is different from the above-described embodiment in that a pair of seal ring  1  and  1  are shaped different from each other. Specifically, one of the seal rings  1  and  1  (which hereinbelow will be referred to as a first seal ring  1 ) is formed of a ring body having a cross section substantially shaped as a trapezoid. A first support ring  3  for supporting the first seal ring  1  comprises a first portion  40  and a second portion  41 . The first portion  40  is disposed in an inner-diameter side and extends in the axial direction, and the second portion  41  extends outwardly in the radial direction from an axial-direction outer end portion of the first portion  40 . The second portion  41  is buried in the first seal ring  1 . On the other hand, the other-seal ring  1  (which hereinbelow will be referred to as a second seal ring  1 ) comprises a first portion  42  and a second portion  43 . The first portion  42  is disposed in an outer-diameter side, and extends in the axial direction. The second portion  43  extends outwardly in the radial direction from an axial-direction outer end portion of the first portion  42 , and a lip portion  23  is provided in the second portion  43 . A second support ring  3  for receiving the second seal ring  1  comprises a first portion  44  and a second portion  45 . The first portion  44  is disposed in an outer-diameter side, and extends in the axial direction. The second portion  45  extends inwardly in the radial direction from an axial-direction outer end portion of the first portion  44 . The second portion  45  is buried in the second portion  43  of the second seal ring  1 .  
         [0054]     In a load seal ring  2 , a cutout portion  46  is formed on the side of the first seal ring  1  of an outer peripheral surface  26 , and a cutout portion  47  is formed on the side of the second seal ring  1  of an inner peripheral surface  30 . In addition, when the load seal ring  2  is provided, the outer peripheral surface  26  engages with an inner peripheral surface of the first portion  44  of the second support ring  3 , an outer end surface  48  (a surface corresponding to the second seal ring  1 ) thereof engages with an inner surface of the second portion  43  of the second seal ring  1 , the inner peripheral surface  30  thereof engages with an outer peripheral surface of the first portion  40  of the first support ring  3 , and an outer end surface  49  (a surface corresponding to the first seal ring  1 ) thereof engages with an inner surface of the first seal ring  1 .  
         [0055]     Also in this case, the axial-direction length in a free state is larger than the length between the bushings  12  and  13 , and as shown in  FIG. 4B , the seal assembly S is disposed between the bushings  12  and  13 . In thin case, the aforementioned cutout portions  46  and  47  are used as circumferential grooves  27  and  27  each tolerating axial-direction compression of the load seal ring  2 , and the axial-direction length of the seal assembly S is thereby reduced. Concurrently, the first portion  40  of the first support ring  3  receives the load seal ring  2  from the Inner-diameter side, and the first portion  44  of the second support ring  3  receives the load seal ring  2  from the outer-diameter side. Thereby, the outer end surface  49  functions as an inner-diameter controller body  33  for controlling the displacement of the load seal ring  2  in the periphery inner direction. In addition, the first portion  44  of the load seal ring  2  functions as an outer-diameter controller body  32  for controlling the displacement of load seal ring  2  in the periphery outer direction. Accordingly, the displacement of the load seal ring  2  in the radial direction is controlled, reaction forces are exerted on the lip portions  23  and  23  outwardly in the axial direction, and the lip portions  23  and  23  closely contact the bushings  12  and  13 , respectively. Thereby, a high-precision seal function can be implemented.  
         [0056]      FIGS. 5A and 5B  show a still another embodiment of a seal assembly. In this case, each of seal rings  1  and  1  is formed of a ring body having a cross section substantially shaped as a trapezoid, and a corner portion on an outer side thereof in the axial direction is used as a lip portion  23 . A support ring  3 , 3  is formed of a first portion  51  and a second portion  52 . In an inner-diameter side, the first portion  51  extends in the axial direction, and the second portion  52  extends in the radial direction from an outer end portion of the first portion  51 . In this case, the second portion  52  expands outwardly in the axial direction; and it is formed of an inner diameter portion  52   a,  an intermediate portion  52   b,  and an outer diameter portion  52   c,  and is buried in the seal ring  1 . Specifically, although the intermediate portion  52   b  and the outer diameter portion  52   c  are completely buried therein, an inner surface of the inner diameter portion  52   a  is exposed to the outside. When θ 1  represents the expansion angle of the inner diameter portion  52   a, θ   2  represents the expansion angle of the inner diameter portion  52   b,  and θ 3  represents the expansion angle of the inner diameter portion  52   c,  the relationship thereof is θ 2 &lt;θ 1 &lt;θ 3 . However, the relationship is not restricted thereto.  
         [0057]     In the present embodiment, a load seal ring  2  is formed of a ring body having a cross section substantially shaped as a letter V turned upside down. A circumferential groove  27  is formed on an inner peripheral surface  30 . Two end surfaces are each formed of an inner-diameter-side slanting surface  53  and an outer-diameter-side slanting surface  54 . The diameter of the inner-diameter-side slanting surface  53  increases along the direction of an outer-diameter side, whereas the diameter of the outer-diameter-side slanting surface  54  decreases along the direction of an outer-diameter side. An inner peripheral surface  30  of the load seal ring  2  engages with an outer peripheral surface of a first portion  51  of a support ring  3 , and the inner-diameter-side slanting surface  53  engages with the inner diameter portion  52   a  of the support ring  3  or an inner surface of the seal ring  1 .  
         [0058]     A dust seal ring  37  is disposed on an outer-diameter side of the load seal ring  2 . In this case, a core member  55  is buried in the dust seal ring  37 . Specifically, the dust seal ring  37  is used to form an outer-diameter controller body  32  for controlling the displacement of load seal ring  2  in the direction of the outer-diameter side.  
         [0059]     Also in this case, the axial-direction length in a free state is larger than the length between the bushings  12  and  13 , and as shown in  FIG. 5B , the seal assembly S is disposed between the bushings  12  and  13 . In this case, since the circumferential groove  27  is provided, the load seal ring  2  tolerates compression in the axial direction of the load seal ring  2 , and the axial-direction length of the seal assembly S is thereby reduced. Concurrently, the first portion  51  of the first support ring  3  functions as an inner-diameter controller body  33  for controlling the displacement of the load seal ring  2  in the direction of the inner-diameter side. In addition, the dust seal ring  37  functions as an outer-diameter controller body  32  for controlling the displacement of the load seal ring  2  in the direction of the outer-diameter direction. Accordingly, the displacement of the load seal ring  2  in the radial direction is controlled, reaction forces are exerted on the lip portions  23  and  23  outwardly in the axial direction, and the lip portions  23  and  23  closely contact the bushings  12  and  13 , respectively. Thereby, a high-precision seal function can be implemented. Furthermore, the above-described structure securely prevents the entrance of dust and the like from the outside.  
         [0060]     As above, while the present invention has been described with reference to the practical embodiments of the seal assembly, the invention is not limited thereto. On the contrary, the invention may be implemented with various modifications within the spirit and scope of the invention. For example, in the embodiment of the seal assembly shown in  FIGS. 1A and 1B , the cross section of the circumferential groove  27  is not limited to be semicircular, but may be modified to have various other shapes, for example, a semielliptical or semi-lengthen-circular shape, a V shape, and a rectangular shape. This may also be applied to the shape of the circumferential groove  27  of the seal assembly shown in  FIGS. 5A and 5B . In addition, in the seal assembly shown in  FIGS. 4A and 4B , the diameter of the seal ring  1  on the side of the bushing  13  is relatively large, and the seal ring  1  on the side of the bushing  12  is relatively small. However, the relationship of the diameters may be reverse. That is, the diameter of the seal ring  1  on the side of the bushing  13  may be relatively small, and the seal ring  1  on the side of the bushing  12  may be relatively large. Furthermore, in the crawler-track connection structure shown in  FIG. 2 , the seal assembly S may be used instead of the seal assembly Sa disposed in the housing  34 .
A seal assembly has simple assembly structures, and provides secure sealing effects. A crawler-track connection structure allows the seal assembly to be easily mounted, and securely prevents overflow of a lubricant and the like to the outside. A pair of seal rings ( 1) and ( 1) individually having lip portions ( 23) and ( 23) are disposed such that each of the lip portions ( 23) and ( 23) protrudes in a direction opposing an axial direction, and a load seal ring ( 2) is compressed and inserted between the pair of seal rings ( 1) and ( 1). The load seal ring ( 2) exerts reaction forces on the pair of lip portions ( 23) and ( 23) outwardly in the axial direction.