BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to a drive shaft for ATVs (All Terrain Vehicle: a mounted type vehicle designed to traverse uneven terrains, also called a four-wheeled buggy).  
         [0003]     2. Brief Description of the Prior Art  
         [0004]     An ATV, which is a mounted type of four-wheeled or three-wheeled vehicle designed to traverse uneven terrains, is equipped with balloon tires to freely traverse such uneven terrains as wasteland and sandy place. The power transmission device for ATVs, as conceptually shown in  FIG. 4 , is constructed such that, for example, the power from an engine  21  is outputted from the output shafts on the front and rear sides via a speed change mechanism in the interior and is inputted to differential gears  24  and  25  on the front and rear sides via power transmission means  22  and  23 , such as chains or propeller shafts. And, the engine power inputted to the differential gears  24  and  25  is reduced in speed by the mechanism of the differential gears  24  and  25  and is converted to a rotational power in a direction orthogonal thereto, whereupon it is transmitted to the front wheels  28  and rear wheels  29  through drive shafts  26  and  27 . In the example shown in the same figure, constant velocity joints are used for joints A between the drive shaft  26  on the front side and the differential gear  24  and for joints B in the front wheels  28 . In addition, there are cases where constant velocity joints are used for joints C between the drive shaft  27  on the rear side and the differential gear  25  and for joints D in the rear wheels  29 . Further, when propeller shafts are used as the power transmission means  22  and  23 , there are cases where constant velocity joints are used for joints E and F between the propeller shafts and the output shafts of the engine (speed change mechanism)  21  and joints G and H in the differential gears  24  and  25 .  
         [0005]      FIG. 5  shows the drive shaft  26  on the front side. In order to allow the drive shaft  26  to make angular displacement and axial displacement following the movement of the front wheel  28  during cornering, traversing uneven terrains or the like movement, a slide type constant velocity joint  30  and a fixed type constant velocity joint  31  are used in pair for joining the drive shaft  26 . Here, the fixed type constant velocity joint means a constant velocity joint that allows only an angular displacement between two shafts, while the slide type constant velocity joint means a constant velocity joint that allows not only an angular displacement between two shafts but also an axial displacement (plunging). In the example shown in the same figure, the inboard side of the drive shaft  26  is joined to the differential gear  24  (at the joining section A) through the slide type constant velocity joint (double offset type constant velocity joint, hereinafter referred to as “DOJ”)  30 , while the outboard side of the drive shaft  26  is joined to the wheel  28  (at the joining section B) through a fixed type constant velocity joint (Rzeppa type constant velocity joint: ball fixed joint, hereinafter referred to as “BJ”)  31 . The numerals  32  and  33  denote boots for the constant velocity joints.  
         [0006]     Heretofore, as the DOJ and BJ, those for passenger cars have been frequently converted to be used as such. Refer to  FIGS. 6 and 7  of Japanese Patent Application Laid Open under No. 2001-97063.  
         [0007]     Since vehicle weight restrictions are severe particularly for ATVs, further weight reduction or size compaction has been required of the drive shafts thereof. Further, since ATVs are small in size, narrow in width and high in vehicle height, the normal working angle of the constant velocity joints installed in the drive shaft is nearly twice that of those for passenger cars. For this reason, for passenger car specifications, the working stability of the constant velocity joints would be impaired depending on usage conditions or the like. Further, about half of the durability (life) of constant velocity joints for passenger cars or the like is sufficient for constant velocity joints for ATV as considered from balance between market performance and the term of guarantee; therefore, considered on the basis of passenger car specifications as they are, there is a feeling of excessive quality consciousness. As to the frequency of use, about half for passenger car specifications is sufficient as considered from balance with vehicle speed, and the same may be said. On the other hand, in the aspect of strength, such as twist strength, the same degree as for passenger car specifications is required.  
         [0008]     In this connection, drive shaft for passenger cars employ Hytrel or other resin boots for constant velocity joint boots  33  on the outboard side where they are frequently subjected to flying sands or disturbances. However, the resin boot, which is high in rigidity, performs the function of increasing the resistance to the bending of the constant velocity joint. Particularly, concerning the drive shaft on the front side for driving the front wheels, i.e., the steering wheel which takes steer angles through steering links, the resistance to the bending of the constant velocity joint due to the hardness of the resin boots gives a large influence to the steering feeling. The influence on the steering feeling due to the hardness of such resin boots could be ignored in the case of passenger cars in which power steering is employed. However, in the case of steering mechanism for ATVs, since the handle bar is mechanically joined to the steering links, the bending resistance of the drive shaft on the front side directly influences the steering feeling.  
       SUMMARY OF THE INVENTION  
       [0009]     A main object of this invention is to improve the steering feeling for ATVs.  
         [0010]     In order to achieve the object, the invention provide a drive shaft which is installed in a mounted type vehicle designed to traverse uneven terrains and which transmits drive power to the wheels through constant velocity joints on the inboard and outboard sides, wherein the boot for at least one of the constant velocity joints is made of a boot material whose hardness, in terms of JISK6253 Durometer Hardness A Type, is 65 or less at normal temperature (25° C.) and is 77 or less at low temperature (−20° C.). Specifically, the boot material corresponds to such boot material as CR (chloroprene rubber) or neoprene rubber (NR). Employing such boot material leads to reduced bending resistance of the constant velocity joint, contributing much to improvement in the steering feeling.  
         [0011]     The boots of the constant velocity joints on the outboard side may advantageously be of rubber in respect of improving the steering feeling.  
         [0012]     The constant velocity joint on the outboard side may comprise an outer ring having a plurality of axially extending track grooves disposed circumferentially of an inner spherical surface, an inner ring having a plurality of axially extending track grooves disposed circumferentially of an outer spherical surface, torque transmitting balls which engage the track grooves of the outer and inner rings, and a cage interposed between the inner spherical surface of the outer ring and the outer spherical surface of the inner ring and having pockets for receiving the torque transmitting balls, wherein the groove center of the track grooves of the outer ring and the groove center of the track grooves of the inner ring are axially offset by equal spacings in mutually opposite directions on opposite sides of the joint center, as viewed in a longitudinal joint section.  
         [0013]     The opening-side end of the track grooves of the outer ring may be provided with a straight section, and the opposite opening-side end of the track grooves of the inner ring is provided with a straight section.  
         [0014]     The relation between the axial dimension L of the pockets of the cage and the diameter d of the torque transmitting balls may be −30 μm≦(L−d)≦0. Whereas an excessive interference has conventionally been imparted, the interference is reduced, thereby allowing the torque transmitting balls to smoothly roll when the constant velocity joints are bent. Therefore, the resistance to the bending of the constant velocity joints is reduced, improving the steering feeling.  
         [0015]     The center of curvature of the outer spherical surface of the cage and the center of curvature of the inner spherical surface of the cage may be axially offset. Here, there are two types of offset: an offset of center of curvature of the ball track of the inner and outer rings, i.e., a track offset, and an offset of the centers of curvature of the inner and outer spherical surfaces of the cage, i.e., a cage offset. Let the sum total of the two be called a total offset. If the total offset is set to be excessively small, this aggravates the workability of the joint. Application of the cage offset results in local material thickening in the axial direction, which leads to aggravation of window punching and directionality, so that assemblage is also aggravated. However, if the track offset alone is set to be large, the groove depth on the innermost side of the outer ring track becomes shallow, thus aggravating the ride-on durability. Accordingly, minimizing the track offset and making up for the shortage by cage offset makes it possible to secure workability and to maintain ride-on durability.  
         [0016]     According to the invention, as compared with the steering feeling for a conventional drive shaft using a resin boot, the steering feeling for ATVs is remarkably improved.  
         [0017]     Embodiments of the invention will now be described with reference to the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a longitudinal sectional view of drive shaft for ATVs, showing an embodiment of the invention;  
         [0019]      FIG. 2  is a longitudinal sectional view of an UJ in the drive shaft of  FIG. 1 ;  
         [0020]      FIG. 3  is a longitudinal sectional view of a DOJ in the drive shaft of  FIG. 1 ;  
         [0021]      FIG. 4  is a conceptual view of a power train for ATVs; and  
         [0022]      FIG. 5  is a longitudinal sectional view of conventional drive shaft for ATVs. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     A drive shaft for ATVs, as shown in  FIG. 1 , comprises a constant velocity joint J 1  on the outboard side, a constant velocity joint J 2  on the inboard side, and an intermediate shaft  1  joining the two joints J 1  and J 2 . The constant velocity joint J 1  on the outboard side is joined to a wheel, while the constant velocity joint J 2  on the inboard side is joined to a differential gear (see  FIG. 4 ).  
         [0024]     The constant velocity joint J 1  on the outboard side is constituted by an undercut free type constant velocity joint (UJ).  FIG. 2  illustrates a state assumed when the working angle θ of the undercut free type constant velocity joint is 0°. This constant velocity joint J 1  comprises an outer joint member  2  (outer ring) with a spherical inner peripheral surface  2   a  axially formed with a plurality (six or eight) of bottom-curved track grooves  3 , an inner joint member  4  (inner ring) with a spherical outer peripheral surface  4   a  formed with a plurality (six or eight) of bottom-curved axial track grooves  5 , a plurality (six or eight) torque transmitting balls  6  disposed in ball track formed by the opposed track grooves  3  and  5  of both joint members  2  and  4 , and a cage  8  interposed between both joint members  2  and  4  and receiving and holding the torque transmitting balls  6  in a plurality of window-shaped pockets  7 . And, the intermediate shaft  1  of the drive shaft (see  FIG. 1 ) is joined to the inner joint member  4  through serrations  4   c  (or splines) formed in the inner periphery thereof, while a wheel-side member is joined to the stem  2   x  of the outer joint member  2 .  
         [0025]     As shown in  FIG. 2 , the ball track formed from the track groove  3  of the outer joint member  2  and the track groove  5  of the inner joint member  4  exhibits a shape (a wedge shape) which is wide in the inboard side (the right side in the same figure), gradually diminishing toward the outboard side (the left side in the same figure). In this case, the inboard-side region of the track groove  3  of the outer joint member  2 , and the outboard-side region of the track groove  5  of the inner joint member  4  are formed with straight sections  2   b  and  4   b , respectively, whose groove bottoms are linear as seen in a longitudinal section. The presence of the straight sections  2   b  and  4   b  results in the maximum working angle being set at 50°, which is greater than the conventional passenger car BJ maximum working angle (46.5°).  
         [0026]     The center Od of the inner peripheral spherical surface  8   b  of the cage  8  is offset by a distance Lc taken axially from the joint center O to the outboard side. A cage offset angle φ c consisting of ∠OdQO defined by the center Od of the inner peripheral spherical surface  8   b , the center Q of the torque transmitting ball  6 , and the joint center O, that is, the offset angle of the inner spherical surface  8   b  of the cage  8  is set greater than 0° but less than 1° (preferably, 0.5°-0.8°, and in this embodiment, 0.7°). Further, the center Oc of the outer peripheral spherical surface  8   a  of the cage  8  is offset by the same distance Lc as above taken axially from the joint center O to the inboard side. A cage offset angle consisting of ∠OcQO defined by the center Oc of the outer peripheral spherical surface  8   a , the center Q of the torque transmitting ball  6 , and the joint center O is set greater than 0° but less than 1° (preferably, 0.5°-0.8°, and in this embodiment, 0.7°), in the same manner as above. In addition, the diameter of the spherical inner peripheral surface  2   a  of the outer joint member  2  and the diameter of the inner peripheral spherical surface  8   b  of the cage  8  are smaller at the opposite ends than at the axial center, though not shown, while the diameter of the outer peripheral spherical surface  8   a  of the cage  8  and the diameter of the spherical outer peripheral surface  4   a  of the inner joint member  4  are larger at the opposite ends than at the axial center. Thereby, the inner peripheral surface  2   a  of the outer joint member  2  and the outer spherical surface  8   a  of the cage  8  contact only at the axial opposite ends, and the inner spherical surface  8   b  of the cage  8  and the outer peripheral surface  4   a  of the inner joint member  4  also contact only at the axial opposite ends.  
         [0027]     On the other hand, the center Oa of the track grooves  3  of the outer joint member  2  is offset by a distance La taken axially from the joint center O to the inboard side. From a total offset angle φa consisting of ∠OaQO defined by the center Oa of the track grooves  3  of the outer joint member  2 , the center Q of the torque transmitting ball  6 , and the joint center O is found the offset angle of the track of the outer joint member  3 , which is φa-φc. This offset angle of the track grooves  3  of the outer joint member  3  is set at 4°-6° (in this embodiment, 5°). Further, the center Ob of the track grooves  5  of the inner joint member  4  is offset by the same distance La as above taken axially from the joint center O to the outboard side, and the track offset angle of the inner joint member  4  found from a total offset angle consisting of ∠ObQO defined by the center Ob of the track grooves  5  of the inner joint member  4 , the center Q of the torque transmitting ball  6 , and the joint center O is also set at 4°-6° (in this embodiment, 5°), in the same manner as above.  
         [0028]     The diameter Dx of the opening  8   x  in the end of the cage  8  on the outboard side is set larger than the diameter Dy of the opening  8   y  in the end on the inboard side, the arrangement being such that the inner joint member  4  can be removably inserted in the cage  8  through the opening  8   x  on the outboard side. In this case, the diameter Dy of the opening  8   y  on the inboard side is set small with such a value that the inner joint member  4  cannot be removably inserted in the cage  8 .  
         [0029]     More specifically, whereas the outer peripheral surface  8   a  of the cage  8  is spherical substantially over the entire region (the region excluding the chamfered portions of the axial opposite ends), the inner peripheral surface  8   b  thereof is a spherical surface  8   b   1  in the axial central region (the region equal to or slightly larger than the axial width of the pocket  7 ), and the surface continuous with this spherical surface  8   b   1  is a cylindrical surface  8   b   2  on the outboard side and is a spherical surface  8   b   3  on the inboard side. In this case, the cylindrical surface  8   b   2  on the outboard side continuously extends to the end edge with substantially the same diameter, while the side which is further inboard of the spherical surface  8   b   3  on the inboard side is continuously formed with a cylindrical surface  8   b   4  which is smaller in diameter and in axial width than the cylindrical surface  8   b   2  on the outboard side.  
         [0030]     Therefore, the thickness of the cage  8  gradually decreases as the cage extends from the axial central region to the outboard side, but gradually increases due to the cage offset as it extends for a predetermined distance from the axial central region to the inboard side. In other words, the average thickness of the inboard-side region, rather than the axial central region of the cage  8 , is set such that it is greater than the average thickness of the outboard-side region. Further, the area of contact between the inner peripheral surface  8   b  of the cage  8  and the outer peripheral surface  4   a  of the inner joint member  4  is set such that it is narrower on the outboard side than on the inboard side. Along with this, the area of contact between the axial opposite sides of the pocket  7  in the inner peripheral surface  8   b  of the cage  8  and the outer peripheral surface  4   a  of the inner joint member  4  is set such that it is very narrow on the outboard side but becomes wider than that on the inboard side.  
         [0031]     Further, the inboard-side end of the cage  8  projects from the inboard-side end of the outer joint member  2 , whereby the axial width of the cage  8  is relatively long-sized. Further, the plurality of pockets  7  formed at equal intervals peripherally of the cage  8  are set the same in size (the same in axial width and peripheral length).  
         [0032]     In this case, the value obtained by subtracting the diameter d 1  of the torque transmitting ball  6  from the axial width t 1  of the pocket  7  of the cage  8  (before the torque transmitting balls  6  are fitted), that is, the axial pocket clearance δ between the pocket  7  of the cage  8  and the torque transmitting ball  6  is set such that −30 μm≦δ≦0 μm. More preferably, this axial pocket clearance δ is set such that −20 μm≦δ≦0 μm. In addition, the torque transmitting balls  6  are fitted in the pockets  7  of the cage  8  in such a manner as to be peripherally slightly movable.  
         [0033]     The constant velocity joint J 2  on the inboard side is constituted by a double offset type constant velocity joint (DOJ). The DOJ, as shown in  FIG. 3 , comprises an outer ring (outer member)  12  whose cylindrical inner peripheral surface  12   a  is axially formed with a plurality (for example, 6) of linear track grooves  12   b , an inner ring (inner member)  13  whose spherical outer peripheral surface  13   a  is axially formed with a plurality (for example, 6) of linear track grooves  13   b , a plurality (for example, 6) of torque transmitting balls  14  disposed in a ball track defined by cooperation between the track grooves  12   b  of the outer ring  12  and the track grooves  13   b  of the inner ring  13 , and a cage  15  for receiving the torque transmitting balls  14 . The stem  12   c  of the outer ring  12  is joined to a differential gear, and the intermediate shaft  1  is joined to the inner periphery of the inner ring  13  through serrations or the like.  
         [0034]     The cage  15  is an annulus comprising an outer spherical surface  15   a  contractually guided by the inner peripheral surface  12   a  of the outer ring  12 , an inner spherical surface  15   b  contractually guided by the outer peripheral surface  13   a  of the inner ring  13 , and a plurality (for example, 6) of pockets  15   c  for receiving the torque transmitting balls  14 . The spherical center Oco of the outer spherical surface  15   a  and the spherical center O CI  of the inner spherical surface  15   b  are axially offset by equal spacings in mutually opposite directions with respect to the joint center O.  
         [0035]     When this joint is transmitting torque while assuming working angle, the cage  15  rotates to the position of the torque transmitting balls  14  which move on the ball track according to the inclination of the inner ring  13 , the cage  15  holding the torque transmitting balls  14  in a plane which bisects the working angle. Thereby, the constant velocity nature of the joint is secured. Further, when the outer and inner rings  12  and  13  axially relatively move, slippage occurs between the outer spherical surface  15   a  of the cage  15  and the inner peripheral surface  12   a  of the outer ring  12 , enabling smooth axial movement (plunging).  
         [0036]     The allowable maximum working angle of the DOJ disposed on the inboard side is smaller than that of the constant velocity joint (UJ) on the outboard side, being set, for example, at 30.5°.  
         [0037]     In the DOJ, the cage offset angle φc (∠OcoQO or ∠O CI QO) defined by the spherical center Oco of the cage outer spherical surface  15   a , the ball center Q, and the joint center O is set such that 7°≦φc&lt;9°. In conventional passenger car specifications DOJs, this cage offset angle φc has been set at 9° or above. However, herein it is set at smaller values than before for light weight and size compaction. Even if the cage offset angle φc is reduced in this manner, the track groove depths of the inner and outer rings can be made shallow, provided that the durability is set at about 70% of that for passenger car specifications. Thereby, the thickness of the cage  15  can be made large, thus making it possible to reliably prevent the balls  14  from jumping out of the cage pockets  15   c  when the allowable maximum working angle is taken.  
         [0038]     The drive shaft described so far can be used not only on the front side but also on the rear side of ATVs.  
         [0039]     In addition, the outboard-side constant velocity joint J 1  and inboard-side constant velocity joint J 2  are generally provided with boots  10  and  11 , as shown in  FIG. 1 , in order to prevent leakage of the internally filled grease and entry of foreign matter from outside. However, concerning the front-side drive shaft, in relation to steering performance, particularly as the boot  10  for the constant velocity joint J 1  on the outboard side, use is made of a boot made of a boot material whose hardness, in terms of JISK6253 Durometer Hardness A Type, is 65 or less at normal temperature (25° C.) and is 77 or less at low temperature (−20° C.). Specifically, the boot material corresponds to such boot material as CR (chloroprene rubber) or neoprene rubber (NR). Employing such relatively soft boot material leads to reduced bending resistance of the constant velocity joint, improving the steering feeling.
In a drive shaft installed in a mounted type vehicle to traverse uneven terrains and adapted to transmit drive power to wheels through constant velocity joints J 1 and J 2 on the inboard and outboard sides, rubber boots are used as constant velocity joint boots.