Drive shaft for ATVs

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 J1 and J2 on the inboard and outboard sides, rubber boots are used as constant velocity joint boots.

BACKGROUND OF THE INVENTION

1. Field of the Invention

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).

2. Brief Description of the Prior Art

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 inFIG. 4, is constructed such that, for example, the power from an engine21is outputted from the output shafts on the front and rear sides via a speed change mechanism in the interior and is inputted to differential gears24and25on the front and rear sides via power transmission means22and23, such as chains or propeller shafts. And, the engine power inputted to the differential gears24and25is reduced in speed by the mechanism of the differential gears24and25and is converted to a rotational power in a direction orthogonal thereto, whereupon it is transmitted to the front wheels28and rear wheels29through drive shafts26and27. In the example shown in the same figure, constant velocity joints are used for joints A between the drive shaft26on the front side and the differential gear24and for joints B in the front wheels28. In addition, there are cases where constant velocity joints are used for joints C between the drive shaft27on the rear side and the differential gear25and for joints D in the rear wheels29. Further, when propeller shafts are used as the power transmission means22and23, 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)21and joints G and H in the differential gears24and25.

FIG. 5shows the drive shaft26on the front side. In order to allow the drive shaft26to make angular displacement and axial displacement following the movement of the front wheel28during cornering, traversing uneven terrains or the like movement, a slide type constant velocity joint30and a fixed type constant velocity joint31are used in pair for joining the drive shaft26. 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 shaft26is joined to the differential gear24(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 shaft26is joined to the wheel28(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 numerals32and33denote boots for the constant velocity joints.

Heretofore, as the DOJ and BJ, those for passenger cars have been frequently converted to be used as such. Refer toFIGS. 6 and 7of Japanese Patent Application Laid Open under No. 2001-97063.

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.

In this connection, drive shaft for passenger cars employ Hytrel or other resin boots for constant velocity joint boots33on 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

A main object of this invention is to improve the steering feeling for ATVs.

In order to achieve the object, the invention provides 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.

The boots of the constant velocity joints on the outboard side may advantageously be of rubber in respect of improving the steering feeling.

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.

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.

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.

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.

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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A drive shaft for ATVs, as shown inFIG. 1, comprises a constant velocity joint J1on the outboard side, a constant velocity joint J2on the inboard side, and an intermediate shaft1joining the two joints J1and J2. The constant velocity joint J1on the outboard side is joined to a wheel, while the constant velocity joint J2on the inboard side is joined to a differential gear (seeFIG. 4).

The constant velocity joint J1on the outboard side is constituted by an undercut free type constant velocity joint (UJ).FIG. 2illustrates a state assumed when the working angle θ of the undercut free type constant velocity joint is 0°. This constant velocity joint J1comprises an outer joint member2(outer ring) with a spherical inner peripheral surface2aaxially formed with a plurality (six or eight) of bottom-curved track grooves3, an inner joint member4(inner ring) with a spherical outer peripheral surface4aformed with a plurality (six or eight) of bottom-curved axial track grooves5, a plurality (six or eight) torque transmitting balls6disposed in ball track formed by the opposed track grooves3and5of both joint members2and4, and a cage8interposed between both joint members2and4and receiving and holding the torque transmitting balls6in a plurality of window-shaped pockets7. And, the intermediate shaft1of the drive shaft (seeFIG. 1) is joined to the inner joint member4through serrations4c(or splines) formed in the inner periphery thereof, while a wheel-side member is joined to the stem2xof the outer joint member2.

As shown inFIG. 2, the ball track formed from the track groove3of the outer joint member2and the track groove5of the inner joint member4exhibits 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 groove3of the outer joint member2, and the outboard-side region of the track groove5of the inner joint member4are formed with straight sections2band4b, respectively, whose groove bottoms are linear as seen in a longitudinal section. The presence of the straight sections2band4bresults in the maximum working angle being set at 50°, which is greater than the conventional passenger car BJ maximum working angle (46.5°).

The center Od of the inner peripheral spherical surface8bof the cage8is 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 surface8b, the center Q of the torque transmitting ball6, and the joint center O, that is, the offset angle of the inner spherical surface8bof the cage8is 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 surface8aof the cage8is 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 surface8a, the center Q of the torque transmitting ball6, 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 surface2aof the outer joint member2and the diameter of the inner peripheral spherical surface8bof the cage8are smaller at the opposite ends than at the axial center, though not shown, while the diameter of the outer peripheral spherical surface8aof the cage8and the diameter of the spherical outer peripheral surface4aof the inner joint member4are larger at the opposite ends than at the axial center. Thereby, the inner peripheral surface2aof the outer joint member2and the outer spherical surface8aof the cage8contact only at the axial opposite ends, and the inner spherical surface8bof the cage8and the outer peripheral surface4aof the inner joint member4also contact only at the axial opposite ends.

On the other hand, the center Oa of the track grooves3of the outer joint member2is 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 grooves3of the outer joint member2, the center Q of the torque transmitting ball6, and the joint center O is found the offset angle of the track of the outer joint member3, which is φa-φc. This offset angle of the track grooves3of the outer joint member3is set at 4°-6° (in this embodiment, 5°). Further, the center Ob of the track grooves5of the inner joint member4is 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 member4found from a total offset angle consisting of ∠ObQO defined by the center Ob of the track grooves5of the inner joint member4, the center Q of the torque transmitting ball6, and the joint center O is also set at 4°-6° (in this embodiment, 5°), in the same manner as above.

The diameter Dx of the opening8xin the end of the cage8on the outboard side is set larger than the diameter Dy of the opening8yin the end on the inboard side, the arrangement being such that the inner joint member4can be removably inserted in the cage8through the opening8xon the outboard side. In this case, the diameter Dy of the opening8yon the inboard side is set small with such a value that the inner joint member4cannot be removably inserted in the cage8.

More specifically, whereas the outer peripheral surface8aof the cage8is spherical substantially over the entire region (the region excluding the chamfered portions of the axial opposite ends), the inner peripheral surface8bthereof is a spherical surface8b1in the axial central region (the region equal to or slightly larger than the axial width of the pocket7), and the surface continuous with this spherical surface8b1is a cylindrical surface8b2on the outboard side and is a spherical surface8b3on the inboard side. In this case, the cylindrical surface8b2on the outboard side continuously extends to the end edge with substantially the same diameter, while the side which is further inboard of the spherical surface8b3on the inboard side is continuously formed with a cylindrical surface8b4which is smaller in diameter and in axial width than the cylindrical surface8b2on the outboard side.

Therefore, the thickness of the cage8gradually 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 cage8, 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 surface8bof the cage8and the outer peripheral surface4aof the inner joint member4is 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 pocket7in the inner peripheral surface8bof the cage8and the outer peripheral surface4aof the inner joint member4is set such that it is very narrow on the outboard side but becomes wider than that on the inboard side.

Further, the inboard-side end of the cage8projects from the inboard-side end of the outer joint member2, whereby the axial width of the cage8is relatively long-sized. Further, the plurality of pockets7formed at equal intervals peripherally of the cage8are set the same in size (the same in axial width and peripheral length).

In this case, the value obtained by subtracting the diameter d1of the torque transmitting ball6from the axial width L of the pocket7of the cage8(before the torque transmitting balls6are fitted), that is, the axial pocket clearance δ between the pocket7of the cage8and the torque transmitting ball6is 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 balls6are fitted in the pockets7of the case8in such a manner as to be peripherally slightly movable.

The constant velocity joint J2on the inboard side is constituted by a double offset type constant velocity joint (DOJ). The DOJ, as shown inFIG. 3, comprises an outer ring (outer member)12whose cylindrical inner peripheral surface12ais axially formed with a plurality (for example, 6) of linear track grooves12b, an inner ring (inner member)13whose spherical outer peripheral surface13ais axially formed with a plurality (for example, 6) of linear track grooves13b, a plurality (for example, 6) of torque transmitting balls14disposed in a ball track defined by cooperation between the track grooves12bof the outer ring12and the track grooves13bof the inner ring13, and a cage15for receiving the torque transmitting balls14. The stem12cof the outer ring12is joined to a differential gear, and the intermediate shaft1is joined to the inner periphery of the inner ring13through serrations or the like.

The cage15is an annulus comprising an outer spherical surface15acontractually guided by the inner peripheral surface12aof the outer ring12, an inner spherical surface15bcontractually guided by the outer peripheral surface13aof the inner ring13, and a plurality (for example, 6) of pockets15cfor receiving the torque transmitting balls14. The spherical center Oco of the outer spherical surface15aand the spherical center OCIof the inner spherical surface15bare axially offset by equal spacings in mutually opposite directions with respect to the joint center O.

When this joint is transmitting torque while assuming working angle, the cage15rotates to the position of the torque transmitting balls14which move on the ball track according to the inclination of the inner ring13, the cage15holding the torque transmitting balls14in a plane which bisects the working angle. Thereby, the constant velocity nature of the joint is secured. Further, when the outer and inner rings12and13axially relatively move, slippage occurs between the outer spherical surface15aof the cage15and the inner peripheral surface12aof the outer ring12, enabling smooth axial movement (plunging).

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°.

In the DOJ, the cage offset angle φc (∠OcoQO or ∠OCIQO) defined by the spherical center Oco of the cage outer spherical surface15a, the ball center Q, and the joint center O is set such that 7°≦φc<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 cage15can be made large, thus making it possible to reliably prevent the balls14from jumping out of the cage pockets15cwhen the allowable maximum working angle is taken.

The drive shaft described so far can be used not only on the front side but also on the rear side of ATVs.

In addition, the outboard-side constant velocity joint J1and inboard-side constant velocity joint J2are generally provided with boots10and11, as shown inFIG. 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 boot10for the constant velocity joint J1on 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.