1. Field of the Invention
The present invention relates to a constant velocity joint, particularly to a constant velocity joint disposed on the outboard side of a drive axle for transmitting drive power to a wheel.
2. Description of the Prior Art
As is known in the art, in automobiles and other various vehicles similar thereto, constant velocity joints capable of transmitting rotational power at constant velocity even when there is an angular or axial displacement between two shafts have been installed in a power transmission path for transmission of drive power from the engine to a wheel. As an example of a vehicle having such power transmission path, a so-called ATV (All Terrain Vehicle, also called a four-wheeled buggy car) which is an uneven terrain traversing, four-wheeled or three-wheeled mounted type vehicle, equipped with balloon tires and designed to freely traverse wastelands, sandy beaches and the like has come to be widely known.
The power transmission device for these various vehicles will be explained with said ATV used as a representative example. As conceptually shown in FIG. 6, 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 orthogonal thereto, whereupon it is transmitted to the wheels 28 and 29 through left and right drive axles 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 wheels 28. In addition, there are cases where constant velocity joints are used for joints C between the drive axle 27 on the rear side and the differential gear 25 and for joints D in the wheels 29.
FIG. 7 shows the drive axle 26 on the front side. In order to allow the drive axle 26 to make angular displacement and axial displacement following the movement of the wheel 28 during cornering, traversing uneven terrains or the like movement, a slide type constant velocity joint (a constant velocity joint allowing angular displacement and axial displacement between two shafts) 30 and a fixed type constant velocity joint (a constant velocity joint allowing angular displacement between two shafts) 31 are used in pair for joining the drive axle 26. In the example shown in the same figure, one end (on the inboard side) of the drive axle 26 is joined to the differential gear 24 (at the joint A) through the slide type constant velocity joint (a double offset type constant velocity joint, hereinafter referred to as “DOJ”) 30, while the other end (the outboard side) of the drive axle 26 is joined to the wheel 28 (at the joint B) through a fixed type constant velocity joint (Rzeppa type constant velocity joint: ball fixed joint, hereinafter referred to as “BJ”) 31.
Heretofore, as said DOJ and BJ for vehicles such as ATVs, those for passenger cars have been frequently converted to be used as such. In vehicles small in size and narrow in width, such as ATVs, however, it is suitable to use a drive axle which is light-weight and compact and which has satisfactory operability. To meet such demand, Japanese Patent Laid-Open 2001-97063, for example, discloses the use of a double offset type constant velocity joint (DOJ) on the inboard side and an undercut free type constant velocity joint (hereinafter referred to as “UJ”) on the outboard side, in a drive axle for transmitting drive power to wheel through constant velocity joints on the inboard and outboard sides.
The UJ disposed on the outboard side of this drive axle, basically, as shown in FIG. 8, comprises an outer joint member 12 with a spherical inner peripheral surface 12a formed with a plurality of track grooves 13, an inner joint member 14 with a spherical outer peripheral surface 14a formed with a plurality of track grooves 15, a plurality of torque transmitting balls 16 disposed in ball tracks formed by the opposed track grooves 13 and 15 of both joint members 12 and 14, and a cage 18 interposed between both joint members 12 and 14 and formed with pockets 17 holding the plurality of torque transmitting balls 16. In consideration of the inner joint member 14 being incorporated in the cage 18, the cage 18 is internally formed at one end thereof with an opening 8x having a relatively large diameter for removably inserting the inner joint member 14 and at the other end thereof with an opening 18y having a relatively small diameter with such a value that the inner joint member 4 cannot be removably inserted in the cage 18.
In this case, the directivity of incorporation of the cage 18 is such that because of the demand for suppressing wear on and damage to the contact surfaces and for suppressing generation of heat and the like by increasing the area of contact between the inner joint member 14 on the outboard side (left side in the same figure) and the cage 18, which contact occurs during torque loading, so as to reduce the contact surface pressure, as shown in the same figure, it has been set that the opening 18x having a large diameter is disposed at the end of the inboard side (right side in the same figure). Specifically, with consideration given to the incorporatability of the outer joint member 12, cage 18 and inner joint member 14, and to decreases in durability and the like due to generation of heat, it is advantageous to position the large diameter opening 18x at the end of the cage 18 on the inboard side. Therefore, it has been common practice that the area of contact of the cage 18 with the inner and outer joint members 14 and 12 (particularly the area of contact of the cage 18 with the inner joint member 14) is large on the outboard side. With such matter taken into consideration, the direction of incorporation of the cage 18 has been set as described above.
On the other hand, the inner periphery of said inner joint member 14 is formed with serrations 14c (or splines) used for joining to an intermediate shaft 9 serving as the shaft member (see FIG. 7). Therefore, in this type of constant velocity joint (UJ), the dimensions or dimensional ratios of various constituent elements from the outer peripheral surface of the outer joint member 12 to the intermediate shaft 9 (serrations 14c) are very important, and these design values determine whether the strength and performance of the UJ are good or bad.
However, such vehicle as an ATV does not make a long-distance travel as in the case of passenger cars, and about half of the durability (life) of constant velocity joints for passenger cars is sufficient for constant velocity joints (UJs) as considered from balance between market performance and the term of guarantee. Despite this, considered on the basis of passenger car specifications as they are, there is a feeling of excessive quality. Further, 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. Therefore, even if the area of contact of the cage 18 particularly with the inner joint member 14 is not made as large as that for passenger cars, on the outboard side, sufficient durability is obtained. Therefore, there is also a feeling of excessive quality in establishing said state of contact of the cage 18.
In contrast therewith, the inboard side of the cage 18 is constantly subjected to a force from the torque transmitting balls 16. In other words, the force with which the torque transmitting balls 16 tend to jump out to the inboard side is supported by the inboard-side end periphery of the cage 18; therefore, it follows that a large pressing force from the torque transmitting balls 16 acts on the inboard-side end periphery of the cage 18. Despite this, if the large diameter opening 18x in the cage 18 is positioned at the inboard-side end as described above, this will lead to a shortage of strength due to less material in the inboard-side end periphery of the cage 18, forming a cause of damage or breakage to the cage 18.
Furthermore, of the constituent elements of UJ, that which influences most strongly is the cage 18. That is, despite the fact that the cage 18 cannot secure a larger (excessive) axial width, it is necessary to form six or eight pockets 17 for receiving and holding torque transmitting balls 16 having a sufficient diameter. Furthermore, during torque loading, a large pressing force from the torque transmitting balls 16 acts on the material in the periphery of each of the pockets 17 of the cage 18. Therefore, even in the case of short-term use of the UJ, cracks or damage occurs in the material between the pockets 17 of the cage 18 and on the material on axial opposite sides of each pocket 17, resulting in a decrease in the strength of the UJ.
It is only natural that increasing the material thickness of the cage 18 is effective in avoiding breakage to such cage 18. However, simply increasing the material thickness of the cage 18 makes it inevitable, for example, to shallow the track grooves 13 of the outer joint member 12 or the track grooves 15 of the inner joint member 14, thus increasing the percentage of risk of the torque transmitting balls 16 running up onto the shoulder. Specifically, in the conventional UJ shown in FIG. 9, the material thickness T1a of the cage 18 is thin, so that the track groove 13 of the outer joint member 12 and the track groove 15 of the inner joint member 14 are made sufficiently deep to make it possible to increase the contact angles αa and βa of the torque transmitting balls 16 with respect to these track grooves 13 and 15. However, in the case where the material thickness of the cage 18 is increased from such state, if the contact angles αa and βa are the same, the percentage of risk of the torque transmitting balls 16 running up onto the shoulder increases by the amount by which the track grooves 13 and 15 are shallowed.
And, since the outer and inner joint members 12 and 14 and the torque transmitting balls 16 have a very low probability of being damaged or broken, as compared with the cage 18, there is a feeling of excessive quality concerning the ratio of dimensions of their material thickness, diameter, etc. For this reason, it can hardly be said that the size and weight reductions of the UJ have been fully attained in relation to the intermediate shaft 9 serving as the shaft member. With the above taken into consideration, the strength of the cage 18 is improved and then excessive quality of the outer and inner joint members 12 and 14 and torque transmitting balls 16 is avoided to attain relative size and weight reductions of the UJ in connection with the shaft member. To attain this, the problem is to how to set the ratio of the dimensions of constituent elements so as to obtain the best result.