Patent Description:
In general, a drive shaft of an automobile includes an outboard-side constant velocity universal joint mounted to a wheel, an inboard-side constant velocity universal joint mounted to a differential gear, and an intermediate shaft coupling both the constant velocity universal joints to each other. Typically, as the outboard-side constant velocity universal joint, there is used a fixed type constant velocity universal joint that can form a large operating angle but is not displaced in an axial direction thereof. Meanwhile, as the inboard-side constant velocity universal joint, there is used a plunging type constant velocity universal joint that has a relatively small maximum operating angle and can be displaced in the axial direction while forming the operating angle.

A demand for reduction in weight of an automobile is as high as ever, and a power transmission mechanism including drive shafts is also required to achieve further reduction in weight and size. Accordingly, a plunging type constant velocity universal joint, which is incorporated into an inboard-side end portion of the drive shaft, is also required to achieve further reduction in weight and size.

As a representative of the plunging type constant velocity universal joint, a double offset constant velocity universal joint is known. In the double offset constant velocity universal joint, a curvature center of a spherical portion formed on an outer peripheral surface of a cage and a curvature center of a spherical portion formed on an inner peripheral surface of the cage are offset to opposite sides in an axial direction of the joint with respect to a joint center by an equal distance. With this configuration, balls are always retained within a plane obtained by bisection of an operating angle, thereby ensuring a constant velocity characteristic between an outer joint member and an inner joint member. The double offset constant velocity universal joint typically includes six torque transmission balls. In Patent Literature <NUM> below, a double offset constant velocity universal joint including eight torque transmission balls is disclosed. When the number of the balls is thus set to eight, reduction in weight and size can be achieved while ensuring strength, load capacity, and durability equivalent to or higher than those of the double offset constant velocity universal joint including the six balls.

The double offset constant velocity universal joint including the eight balls as disclosed in Patent Literature <NUM> above is put to practical use as a mass-produced product. The present invention has been made through study on further reduction in weight and size of the plunging type constant velocity universal joint of this type.

For example, in Patent Literature <NUM> above, a rear-wheel drive shaft is disclosed. In the rear-wheel drive shaft, a diameter of a spline formed in each end portion of an intermediate shaft (hollow shaft) is increased so that the hollow shaft has a sufficient margin of strength. Thus, reduction in thickness is available, and hence reduction in weight of the hollow shaft is also achieved. However, the invention proposed in Patent Literature <NUM> is made to achieve the reduction in weight and increase in strength of a hollow shaft to be used for a rear-wheel drive shaft. In Patent Literature <NUM>, no description is made of an object to achieve the reduction in weight and size of the plunging type constant velocity universal joint.

Therefore, an object to be achieved by the present invention is to further reduce a weight and a size of a plunging type constant velocity universal joint through study on internal specifications of the plunging type constant velocity universal joint to be used for a rear-wheel drive shaft, in particular, a double offset constant velocity universal joint comprising eight balls.

A fixed type constant velocity universal joint provided on an outboard side of a drive shaft is directly mounted to a wheel, and hence a maximum operating angle of the fixed type constant velocity universal joint significantly differs between a case in which the joint is mounted to a front wheel being a steered wheel and a case in which the joint is mounted to a rear wheel that is not steered. Meanwhile, a plunging type constant velocity universal joint provided on an inboard side of the drive shaft is not directly mounted to the wheel, and hence is hardly affected by a steering angle of the wheel. Accordingly, in view of, for example, mass production cost, the plunging type constant velocity universal joints having the same specifications are hitherto used for the front-wheel drive shaft and the rear-wheel drive shaft.

However, the inventors of the present invention have focused on the fact that, when the plunging type constant velocity universal joint is used exclusively for the rear-wheel drive shaft, the maximum operating angle can be reduced. That is, a large number of components are arranged at a vicinity of a front wheel of a vehicle, and hence a space is limited. Thus, for example, as illustrated in <FIG>, in some cases, it is inevitable that an axial center of a front wheel FW and an axial center of a differential gear G be arranged in an offset manner in a front-and-rear direction of the vehicle. In this case, in constant velocity universal joints J11 and J12 provided on a front-wheel drive shaft DS1, a normal operating angle (operating angle when an automobile runs straight at a constant speed) α in the front-and-rear direction of the vehicle is not <NUM>°, but the constant velocity universal joints J11 and J12 always rotate under a state of forming the operating angle in the front-and-rear direction of the vehicle. Therefore, the plunging type constant velocity universal joint J12 is affected in a complex manner by the above-mentioned normal operating angle α in the front-and-rear direction of the vehicle and an operating angle in an up-and-down direction accompanied with up-and-down movement of the wheel with respect to a vehicle body. Thus, the plunging type constant velocity universal joint J12 is required to have a relatively large operating angle.

In contrast, at a vicinity of a rear wheel of the vehicle, there is a relatively sufficient margin of arrangement space for components. Thus, typically, as illustrated in <FIG>, an axial center of a rear wheel RW and the axial center of the differential gear G are arranged under a state of being hardly offset to each other in a front-and-rear direction of the vehicle body. In this case, the constant velocity universal joints J21 and J22 for a rear-wheel drive shaft DS2 form a normal operating angle of about <NUM>° in the front-and-rear direction of the vehicle, and hence the plunging type constant velocity universal joint J22 to be used for the rear-wheel drive shaft DS2 may have an operating angle smaller than that of the plunging type constant velocity universal joint J21 to be used for the front-wheel drive shaft DS1. Therefore, when the plunging type constant velocity universal joint is used exclusively for the rear-wheel drive shaft, the maximum operating angle can be reduced.

Based on the knowledge described above, according to the present invention, there is provided a plunging type constant velocity universal joint according to appended claim <NUM>.

In the plunging type constant velocity universal joint, loads are applied evenly to the respective balls under a state in which an operating angle is <NUM>°. However, when the operating angle is formed, uneven loads are applied to the respective balls, and a difference in loads applied to the respective balls becomes larger as the operating angle becomes larger. Therefore, in a case of the large operating angle, maximum loads applied to the respective balls are large, and hence members (the outer joint member, the inner joint member, and the cage) held in contact with the balls are required to have thicknesses large enough to bear the maximum loads applied from the balls. Accordingly, when the plunging type constant velocity universal joint is used exclusively for the rear-wheel drive shaft to reduce the maximum operating angle as described above, the maximum loads applied to the balls are reduced, and each component held in contact with the balls has a sufficient margin of strength. Thus, without causing reduction in load capacity and durability, a thickness of each component, for example, a radial thickness of the inner joint member (specifically, a radial distance between a groove bottom of each of the track grooves of the inner joint member and a pitch circle of the spline hole) can be reduced. In this manner, the track grooves formed in the outer peripheral surface of the inner joint member can be closer to a radially inner side, and hence a pitch circle diameter of the track grooves, that is, the pitch circle diameter of the balls arranged in the track grooves can be reduced as compared to that of a conventional product (double offset constant velocity universal joint having a large operating angle and including eight balls, which is applicable to both the front-wheel drive shaft and the rear-wheel drive shaft). Thus, a size of the plunging type constant velocity universal joint in the radial direction can be reduced, and hence reduction in weight can be achieved.

When the plunging type constant velocity universal joint rotates under a state of forming the operating angle, the balls rotate while undergoing displacement in a circumferential direction with respect to the cage. Accordingly, in order to allow the balls to move in the circumferential direction, the pockets of the cage each have a thin shape elongated in the circumferential direction, and a circumferential dimension of each of the pockets is determined depending on the maximum operating angle of the constant velocity universal joint. The conventional product has a large maximum operating angle, and hence a circumferential length of each of the pockets is increased. Thus, it has been required to increase a diameter of the cage in order to ensure the circumferential length of each of the pockets. Therefore, a diameter of the outer peripheral surface of the inner joint member to be held in slide contact with the inner peripheral surface of the cage is increased. Consequently, the inner joint member has an excessively large thickness that is more than necessary in view of strength.

In contrast, when the plunging type constant velocity universal joint is used exclusively for the rear-wheel drive shaft to reduce the maximum operating angle as described above, the circumferential dimension of each of the pockets of the cage can be reduced. Accordingly, a diameter of the cage can be reduced, and the diameter of the outer peripheral surface of the inner joint member to be held in slide contact with the inner peripheral surface of the cage can be reduced. Thus, a radial thickness of the inner joint member is reduced as compared to that of the conventional product so that the radial thickness can be set to an appropriate value (minimum value necessary in view of strength). Accordingly, the pitch circle diameter of the balls is reduced as described above, thereby being capable of reducing a size of the plunging type constant velocity universal joint in the radial direction.

Incidentally, the constant velocity universal joints are mass-produced products. Thus, typically, stepwise sizes are set for the constant velocity universal joints in accordance with torque load capacity, and internal specifications (for example, dimensions and shapes of components) are set for each size (a series of the constant velocity universal joints is launched). In order to achieve reduction in weight and size of the constant velocity universal joint of respective sizes, when the ball diameter is reduced, contact pressure at contact portions between the balls and the track grooves is increased, which directly causes reduction in torque load capacity. Accordingly, when study is made on design change of the constant velocity universal joint, in order to maintain torque load capacity, the ball diameter is not changed in most cases unless the number of the balls is increased. Therefore, when a dimension of each component is represented by a ratio to the ball diameter, the internal specifications of the constant velocity universal joint in accordance with torque load capacity (that is, size of the constant velocity universal joint) can be shown. As descried above, the plunging type constant velocity universal joint is used exclusively for the rear-wheel drive shaft to reduce the maximum operating angle, and the dimension of each component with respect to the ball diameter {specifically, a ratio (PCDBALL/DBALL) of the pitch circle diameter of the balls to the ball diameter and a ratio (TI/DBALL) of the radial thickness of the inner joint member to the ball diameter} is reduced as compared to that of the conventional product. In this manner, a new series of plunging type constant velocity universal joints each having a small weight and a small size can be launched.

Further, the radial thickness of the inner joint member is reduced as described above, and hence a diameter of the spline hole formed along the axial center of the inner joint member can be increased. In this manner, a diameter of a shaft to be inserted into the spline hole is increased, thereby improving torsional strength of the shaft. According to the invention a ratio PCDSPL/DBALL of the pitch circle diameter PCDSPL of the spline hole of the inner joint member to the diameter DBALL of each of the balls is set from <NUM> to <NUM> (preferably from <NUM> to <NUM>).

When the maximum operating angle of the plunging type constant velocity universal joint is reduced, the pitch circle diameter of the balls can be reduced as described above, and hence a diameter of the outer joint member can be reduced. Further, when the maximum operating angle of the plunging type constant velocity universal joint is reduced, the thickness of the inner joint member can be reduced as described above, and hence a diameter of the spline hole of the inner joint member can be increased. From the above description, a ratio DO/PCDSPL of an outer diameter DO of the outer joint member to the pitch circle diameter PCDSPL of the spline hole of the inner joint member can be reduced, specifically, the ratio DO/PCDSPL can be set from <NUM> to <NUM>. In this manner, reduction in weight and size of the plunging type constant velocity universal joint, and improvement in strength of the intermediate shaft can be achieved at the same time.

The above-mentioned plunging type constant velocity universal joint has a maximum operating angle of <NUM>° or less.

As described above, according to the present invention, in the plunging type constant velocity universal joint to be used for the rear-wheel drive shaft, through setting of the internal specifications (the pitch circle diameter of the balls and the radial thickness of the inner joint member determined with respect to the ball diameter) based on a design concept different from that of the related art, further reduction in weight and size can be achieved while maintaining torque load capacity.

Now, an embodiment of the present invention is described with reference to the drawings.

<FIG> is an illustration of a power transmission mechanism for a rear-wheel drive vehicle (such as an FR vehicle) of an independent suspension type. In this power transmission mechanism, a rotational drive force output from an engine E is transmitted to a differential gear G through a transmission M and a propeller shaft PS, and then is transmitted to right and left rear wheels (wheel W) through right and left rear-wheel drive shafts <NUM>.

As illustrated in <FIG>, the rear-wheel drive shaft <NUM> comprises a plunging type constant velocity universal joint <NUM>, a fixed type constant velocity universal joint <NUM>, and an intermediate shaft <NUM>. The plunging type constant velocity universal joint <NUM> is provided on an inboard side (right side in <FIG>) and is configured to allow both axial displacement and angular displacement. The fixed type constant velocity universal joint <NUM> is provided on an outboard side (left side in <FIG>) and is configured to allow only angular displacement. The rear-wheel drive shaft <NUM> has the structure in which both the constant velocity universal joints <NUM> and <NUM> are coupled by the intermediate shaft <NUM>. The plunging type constant velocity universal joint <NUM> on the inboard side is coupled to the differential gear G, and the fixed type constant velocity universal joint <NUM> on the outboard side is coupled to the wheel W (see <FIG>).

As illustrated in <FIG>, the plunging type constant velocity universal joint <NUM> comprises an outer joint member <NUM>, an inner joint member <NUM>, eight balls <NUM>, and a cage <NUM>. The outer joint member <NUM> is mounted to the differential gear G (see <FIG>). The inner joint member <NUM> is mounted to an inboard-side end portion of the intermediate shaft <NUM> (see <FIG>). The eight balls <NUM> are configured to transmit torque between the outer joint member <NUM> and the inner joint member <NUM>. The cage <NUM> is configured to retain the eight balls <NUM>.

The outer joint member <NUM> integrally comprises a mouth section 21a and a stem section 21b. The mouth section 21a has a cup shape that is open toward one side in an axial direction of the joint (outboard side or left side in <FIG>). The stem section 21b extends from a bottom portion of the mouth section 21a to another side in the axial direction (inboard side or right side in <FIG>). Eight linear track grooves 21d extending in the axial direction are formed in a cylindrical inner peripheral surface 21c of the mouth section 21a. A spline 21e to be inserted into a spline hole of the differential gear G is formed in an outer peripheral surface of an inboard-side end portion of the stem section 21b. The mouth section 21a and the stem section 21b may be integrally made of the same material, or may be joined to each other by, for example, welding after the mouth section 21a and the stem section 21b are formed into separate sections.

A spline hole 22c into which the intermediate shaft <NUM> is to be inserted is formed along an axial center of the inner joint member <NUM>. Eight linear track grooves 22e extending in the axial direction are formed in a spherical outer peripheral surface 22d of the inner joint member <NUM>. That is, the inner joint member <NUM> integrally comprises a cylindrical portion 22a and a plurality of protruding portions 22b. The cylindrical portion 22a has the spline hole 22c. The plurality of protruding portions 22b protrude from the cylindrical portion 22a radially outward. The track grooves 22e are formed in circumferential regions between the plurality of protruding portions 22b. Radially outer surfaces of the plurality of protruding portions 22b form the spherical outer peripheral surface 22d of the inner joint member <NUM>.

The track grooves 21d of the outer joint member <NUM> and the track grooves 22e of the inner joint member <NUM> are opposed to each other in a radial direction to form eight ball tracks, and the balls <NUM> are arranged one by one in the ball tracks, respectively. A transverse sectional shape of each of the track grooves 21d and 22e is an elliptic shape or a Gothic arch shape. With this configuration, the track grooves 21d and 22e and the balls <NUM> are held in contact with each other at a contact angle of from about <NUM>° to about <NUM>°, in other words, held in so-called angular contact with each other. A transverse sectional shape of each of the track grooves 21d and 22e may be an arc shape, and the track grooves 21d and 22e and the balls <NUM> may be held in so-called circular contact with each other.

The cage <NUM> has eight pockets 24a configured to retain the balls <NUM>. All the eight pockets 24a have the same shape, and are arranged at equal intervals in a circumferential direction of the cage <NUM>. An outer peripheral surface of the cage <NUM> comprises a spherical portion 24b and conical portions 24c. The spherical portion 24b is held in slide contact with the cylindrical inner peripheral surface 21c of the outer joint member <NUM>. The conical portions 24c extend in tangential directions from both end portions of the spherical portion 24b in the axial direction. As illustrated in <FIG>, when the plunging type constant velocity universal joint <NUM> forms a maximum operating angle θ, each of the conical portions 24c functions as a stopper configured to restrain further increase in operating angle through linear contact with the inner peripheral surface 21c of the outer joint member <NUM>. An inclination angle of the conical portions 24c with respect to the axial center of the cage <NUM> is set to a half of a value of the maximum operating angle θ of the plunging type constant velocity universal joint <NUM>. A spherical portion 24d is formed on the inner peripheral surface of the cage <NUM>, and is held in slide contact with the spherical outer peripheral surface 22d of the inner joint member <NUM>. Through axial sliding of the spherical portion 24b of the outer peripheral surface of the cage <NUM> and the cylindrical inner peripheral surface 21c of the outer joint member <NUM>, axial displacement is allowed between the outer joint member <NUM> and the inner joint member <NUM>.

As illustrated in <FIG>, a curvature center O24b of the spherical portion 24b of the outer peripheral surface of the cage <NUM>, and a curvature center O24d of the spherical portion 24d of the inner peripheral surface of the cage <NUM> (that is, curvature center of the spherical outer peripheral surface 22d of the inner joint member <NUM>) are offset to opposite sides in the axial direction with respect to joint center O (s) by an equal distance. In the illustrated example, the curvature center O24b of the spherical portion 24b of the outer peripheral surface of the cage <NUM> is offset to the inboard side (joint deep side) with respect to the joint center <NUM>(s), and the curvature center O24d of the spherical portion 24d of the inner peripheral surface of the cage <NUM> is offset to the outboard side (joint opening side) with respect to the joint center O(s). With this configuration, at a freely-selected operating angle, the balls <NUM> retained by the cage <NUM> are always arranged within a plane obtained by bisection of the operating angle, thereby ensuring a constant velocity characteristic between the outer joint member <NUM> and the inner joint member <NUM>.

As illustrated in <FIG>, the fixed type type constant velocity universal joint <NUM> comprises an outer joint member <NUM>, an inner joint member <NUM>, eight balls <NUM>, and a cage <NUM>. The outer joint member <NUM> is mounted to the wheel W (see <FIG>). The inner joint member <NUM> is mounted to an outboard-side end portion of the intermediate shaft <NUM> (see <FIG>). The eight balls <NUM> are configured to transmit torque between the outer joint member <NUM> and the inner joint member <NUM>. The cage <NUM> is configured to retain the eight balls <NUM>.

The outer joint member <NUM> integrally comprises a mouth section 31a and a stem section 31b. The mouth section 31a has a cup shape that is open toward one side in an axial direction of the joint (inboard side or right side in <FIG>). The stem section 31b extends from a bottom portion of the mouth section 31a to another side in the axial direction (outboard side or left side in <FIG>). Eight arc-shaped track grooves 31d extending in the axial direction are formed in a spherical inner peripheral surface 31c of the mouth section 31a. A spline 31e to be inserted into a spline hole on the wheel W side is formed in an outer peripheral surface of the stem section 31b. The mouth section 31a and the stem section 31b may be integrally made of the same material, or may be joined to each other by, for example, welding after the mouth section 31a and the stem section 31b are formed into separate sections. Further, a through hole extending in the axial direction may be formed along the axial centers of the mouth section 31a and the stem section 31b.

A spline hole 32c into which the intermediate shaft <NUM> (see <FIG>) is to be inserted is formed along an axial center of the inner joint member <NUM>. Eight arc-shaped track grooves 32e extending in the axial direction are formed in a spherical outer peripheral surface 32d of the inner joint member <NUM>. That is, the inner joint member <NUM> integrally comprises a cylindrical portion 32a and a plurality of protruding portions 32b. The cylindrical portion 32a has the spline hole 32c. The plurality of protruding portions 32b protrude from the cylindrical portion 32a radially outward. The track grooves 32e are formed in circumferential regions between the plurality of protruding portions 32b. Radially outer surfaces of the plurality of protruding portions 32b form the spherical outer peripheral surface 32d of the inner joint member <NUM>.

The track grooves 31d of the outer joint member <NUM> and the track grooves 32e of the inner joint member <NUM> are opposed to each other in a radial direction to form eight ball tracks, and the balls <NUM> are arranged one by one in the ball tracks, respectively. A transverse sectional shape of each of the track grooves 31d and 32e is an elliptic shape or a Gothic arch shape. With this configuration, the track grooves 31d and 32e and the balls <NUM> are held in contact with each other at a contact angle of from about <NUM>° to about <NUM>°, in other words, held in so-called angular contact with each other. A transverse sectional shape of each of the track grooves 31d and 32e may be an arc shape, and the track grooves 31d and 32e and the balls <NUM> may be held in so-called circular contact with each other.

A curvature center O31d of the track grooves 31d of the outer joint member <NUM>, and a curvature center O32e of the track grooves 32e of the inner joint member <NUM> are offset to opposite sides in the axial direction with respect to a joint center O(f) by an equal distance. In the illustrated example, the curvature center O31d of the track grooves 31d of the outer joint member <NUM> is offset to the inboard side (joint opening side) with respect to the joint center O(f), and the curvature center O32e of the track grooves 32e of the inner joint member <NUM> is offset to the outboard side (joint deep side) with respect to the joint center O(f). With this configuration, at a freely-selected operating angle, the balls <NUM> retained by the cage <NUM> are always arranged within a plane obtained by bisection of the operating angle, thereby ensuring a constant velocity characteristic between the outer joint member <NUM> and the inner joint member <NUM>.

The cage <NUM> has eight pockets 34a configured to retain the balls <NUM>. All the eight pockets 34a have the same shape, and are arranged at equal intervals in a circumferential direction of the cage <NUM>. A spherical outer peripheral surface 34b of the cage <NUM> is held in slide contact with the spherical inner peripheral surface 31c of the outer joint member <NUM>. A spherical inner peripheral surface 34c of the cage <NUM> is held in slide contact with the spherical outer peripheral surface 32d of the inner joint member <NUM>. A curvature center of the outer peripheral surface 34b of the cage <NUM> (that is, curvature center of the spherical inner peripheral surface 31c of the outer joint member <NUM>), and a curvature center of the inner peripheral surface 34c (that is, curvature center of the spherical outer peripheral surface 32d of the inner joint member <NUM>) match with the joint center O(f).

As illustrated in <FIG>, as the intermediate shaft <NUM>, a hollow shaft having a through hole <NUM> extending in the axial direction can be used. The intermediate shaft <NUM> comprises a large-diameter portion <NUM>, small-diameter portions <NUM>, and tapered portions <NUM>. The large-diameter portion <NUM> is formed at a center of the intermediate shaft <NUM> in the axial direction. The small-diameter portions <NUM> are formed at both ends of the intermediate shaft <NUM> in the axial direction, respectively. Each of the tapered portions <NUM> connects the large-diameter portion <NUM> and the small-diameter portion <NUM>. An annular groove <NUM> for mounting a boot and a spline <NUM> are formed in the small-diameter portion <NUM> of the intermediate shaft <NUM>. The small-diameter portion <NUM> has a constant outer diameter except for the annular groove <NUM> and the spline <NUM>. The intermediate shaft <NUM> is not limited to the hollow shaft, and a solid shaft may also be used.

The spline <NUM> at an inboard-side end portion of the intermediate shaft <NUM> is press-fitted into the spline hole 22c of the inner joint member <NUM> of the plunging type constant velocity universal joint <NUM>. Thus, the intermediate shaft <NUM> and the inner joint member <NUM> are coupled to each other in a torque transmittable manner through spline fitting. An annular recessed groove is formed in the inboard-side end portion of the intermediate shaft <NUM>, and a snap ring <NUM> is fitted into the recessed groove. Through engagement of the snap ring <NUM> from the inboard side (shaft end side) of the inner joint member <NUM>, the intermediate shaft <NUM> and the inner joint member <NUM> are prevented from coming off.

The spline <NUM> at an outboard-side end portion of the intermediate shaft <NUM> is press-fitted into the spline hole 32c of the inner joint member <NUM> of the fixed type constant velocity universal joint <NUM>. Thus, the intermediate shaft <NUM> and the inner joint member <NUM> are coupled to each other in a torque transmittable manner through spline fitting. An annular recessed groove is formed in the outboard-side end portion of the intermediate shaft <NUM>, and a snap ring <NUM> is fitted into the recessed groove. Through engagement of the snap ring <NUM> from the outboard side (shaft end side) of the inner joint member <NUM>, the intermediate shaft <NUM> and the inner joint member <NUM> are prevented from coming off.

The plunging type constant velocity universal joint <NUM> and the fixed type constant velocity universal joint <NUM> described above are used exclusively for the rear-wheel drive shaft, and hence the maximum operating angle can be set smaller than that of a conventional product that is also usable for a front-wheel drive shaft. In this embodiment, both of the maximum operating angle of the plunging type constant velocity universal joint <NUM> and the maximum operating angle of the fixed type constant velocity universal joint <NUM> are set to <NUM>° or less. In this manner, reduction in weight and size of the plunging type constant velocity universal joint <NUM> and the fixed type constant velocity universal joint <NUM> can be achieved while maintaining load capacity. In the following, internal specifications of the plunging type constant velocity universal joint <NUM> are described in detail.

In Table <NUM> below and <FIG>, the internal specifications of the plunging type constant velocity universal joint <NUM> being the product of the present invention are shown and illustrated in comparison to a comparative product (double offset constant velocity universal joint having a maximum operating angle of <NUM>° and eight balls) having the same ball diameter as that of the product of the present invention. An upper half of each of <FIG> is a sectional view of the plunging type constant velocity universal joint <NUM> being the product of the present invention, and a lower half thereof is a sectional view of a plunging type constant velocity universal joint <NUM>' being the comparative product. Each component of the comparative product is denoted by the reference symbol obtained by adding a prime (') to the reference symbol of each component of the product of the present invention.

Parameters described above are defined as follows.

In the following, detailed description is made of a design concept leading to the above-mentioned internal specifications.

In the plunging type constant velocity universal joint <NUM>, as the operating angle is increased, a maximum load applied to each of the balls <NUM> is increased. Accordingly, when the maximum operating angle is reduced as described above, the maximum load applied to each of the balls <NUM> is reduced. Thus, the inner joint member <NUM> to be held in contact with the balls <NUM> has a sufficient margin of strength. As a result, the radial thickness of the inner joint member <NUM> can be reduced. Therefore, without causing reduction in load capacity and durability, the pitch circle diameter of the track grooves 22e of the inner joint member <NUM>, that is, the pitch circle diameter of the balls <NUM> arranged in the track grooves 22e can be reduced as compared to that of the comparative product {PCDBALL<PCDBALL', see the row (<NUM>) in Table <NUM> above. In this manner, a size of the plunging type constant velocity universal joint <NUM> in the radial direction is reduced, and thus reduction in weight can be achieved.

The comparative product has a large maximum operating angle, and hence a circumferential length of each of pockets 24a' of a cage <NUM>' is increased. Thus, it has been required to increase a diameter of the cage <NUM>' in order to ensure the circumferential length of each of the pockets 24a'. Therefore, a diameter of an outer peripheral surface of an inner joint member <NUM>' to be held in slide contact with an inner peripheral surface of the cage <NUM>' is increased. Consequently, the inner joint member <NUM>' has an excessively large thickness that is more than necessary in view of strength. In contrast, in the product of the present invention, when the maximum operating angle is reduced as described above, a movement amount of each of the balls <NUM> in the circumferential direction with respect to the cage <NUM> is reduced, thereby being capable of reducing the circumferential dimension of each of the pockets 24a of the cage <NUM> (Lp<Lp'). Accordingly, while maintaining the circumferential dimension of a pillar portion 24e between the pockets 24a (Lc≈Lc'), the diameter of the cage <NUM> can be reduced, and the diameter of the outer peripheral surface 22d of the inner joint member <NUM> to be held in slide contact with the spherical portion 24d of the inner peripheral surface of the cage <NUM> can be reduced. As a result, a thickness of the inner joint member <NUM> can be reduced so as to be set to a minimum thickness necessary in view of strength {TI<TI', see the row (<NUM>) in Table <NUM> above }. Further, the pitch circle diameter of the balls <NUM> is reduced as described above, thereby being capable of reducing a size of the plunging type constant velocity universal joint <NUM> in the radial direction.

Through reduction of the maximum operating angle of the plunging type constant velocity universal joint <NUM>, the maximum load applied to each of the balls <NUM> is reduced as described above, with the result that the cage <NUM> held in contact with the balls <NUM> has a sufficient margin of strength. Accordingly, an axial thickness of an annular portion formed at each end of the cage <NUM> in the axial direction can be reduced while maintaining durability equivalent to that of the comparative product. Thus, an axial width of the entire cage <NUM> can be reduced, and hence reduction in weight can be achieved {WC<WC', see the row (<NUM>) in Table <NUM> above}.

Through reduction of the maximum operating angle of the plunging type constant velocity universal joint <NUM>, an angle of the conical portions 24c of the outer peripheral surface of the cage <NUM> with respect to the axial center can be reduced, and can be set to <NUM>° or less in this embodiment. With this configuration, a thickness (for example, a thickness TC at a joint-opening-side end portion) of a thin portion of the cage <NUM> can be increased, and hence strength of the cage <NUM> can be increased.

When the maximum operating angle of the plunging type constant velocity universal joint <NUM> is reduced, the radial thickness TI of the inner joint member <NUM> can be reduced as described above, with the result that a diameter of the spline hole 22c of the inner joint member <NUM> can be increased {PCDSPL>PCDSPL', see the row (<NUM>) in Table <NUM> above}. In this manner, the intermediate shaft <NUM> to be inserted into the spline hole 22c is increased in diameter, and thus torsional strength can be enhanced. Further, when the maximum operating angle of the plunging type constant velocity universal joint <NUM> is reduced, the pitch circle diameter of the balls <NUM> can be reduced as described above, with the result that a diameter of the outer joint member <NUM> can be reduced. From the above description, in the product of the present invention, a ratio DO/PCDSPL of the outer diameter DO of the outer joint member <NUM> to the pitch circle diameter PCDSPL of the spline hole 22c of the inner joint member <NUM> can be set smaller than that of the comparative product {DO/PCDSPL<DO'/PCDSPL', see the row (<NUM>) in Table <NUM> above}. In this manner, reduction in weight and size of the plunging type constant velocity universal joint <NUM>, and improvement in strength of the intermediate shaft <NUM> can be achieved at the same time.

When the maximum operating angle of the plunging type constant velocity universal joint <NUM> is reduced, a movement amount of each of the balls <NUM> in the axial direction with respect to the inner joint member <NUM> is reduced. Specifically, as illustrated in <FIG>, an axial length (track effective length) of a locus of a contact point between the track groove 22e of the inner joint member <NUM> and the ball <NUM> is smaller in the product of the present invention having a small maximum operating angle than in the comparative product having a large maximum operating angle (ZI<ZI'). With this configuration, in the product of the present invention, the axial length of each of the track grooves 22e of the inner joint member <NUM> and the axial width of the entire inner joint member <NUM> can be reduced as compared to those of the comparative product {WI<WI', see the row (<NUM>) in Table <NUM> above}.

However, when the axial width of the inner joint member <NUM> is excessively small, the spline hole 22c formed along the axial center of the inner joint member <NUM> has an insufficient axial length, which may lead to insufficient strength of a spline fitting portion between the inner joint member <NUM> and the intermediate shaft <NUM> (see <FIG>). In the plunging type constant velocity universal joint <NUM> being the product of the present invention, through reduction of the maximum operating angle, the radial thickness of the inner j oint member <NUM> can be reduced as described above, and hence a diameter of the spline hole 22c of the inner joint member <NUM> can be increased. Accordingly, while maintaining the contact pressure for each spline tooth (that is, while maintaining strength of the spline fitting portion), the axial length of the spline hole 22c of the inner joint member <NUM> can be reduced. In the above-mentioned manner, through reduction of the axial length of each of the track grooves 22e of the inner joint member <NUM> and the axial length of the spline hole 22c, the axial width of the entire inner joint member <NUM> can be reduced as described above, and hence reduction in weight can be achieved.

As described above, according to the present invention, in consideration of various conditions obtained by reducing the maximum operating angle of the plunging type constant velocity universal joint, study is made on the internal specifications of the plunging type constant velocity universal joint, thereby reducing the weight and size of the plunging type constant velocity universal joint while maintaining torque load capacity equivalent to that of the comparative product. Thus, there can be launched a new series of plunging type constant velocity universal joints each having a small weight and a small size and being usable exclusively for the rear-wheel drive shaft.

Incidentally, when the plunging type constant velocity universal joint rotates under a state of forming a high operating angle, there is a risk in that noise is generated. In order to prevent generation of noise during such high operating angle, in the related-art plunging type constant velocity universal joint, a track PCD gap (difference between a pitch circle diameter of the track grooves of the outer joint member and a pitch circle diameter of the track grooves of the inner joint member), a gap between the outer joint member and the cage (difference between a diameter of the cylindrical inner peripheral surface of the outer joint member and a diameter of the spherical portion of the outer peripheral surface of the cage), and a spherical gap between the cage and the inner joint member (difference between the diameter of the spherical portion of the inner peripheral surface of the cage and a diameter of the spherical outer peripheral surface of the inner joint member) are each required to be set to a significantly small value. In contrast, in the above-mentioned plunging type constant velocity universal joint <NUM>, the maximum operating angle is small, and the risk of noise can be reduced. Accordingly, the above-mentioned gaps can be set to values larger than those of the conventional product, and hence the product of the present invention is advantageous in view of manufacture.

Further, the related-art plunging type constant velocity universal joint is used also for the front-wheel drive shaft. Thus, as countermeasures against idling vibration, the inner peripheral surface of the cage is processed into a special shape so that a relatively large axial gap is formed between the inner peripheral surface of the cage and the outer peripheral surface of the inner joint member (for example, see <FIG> and <FIG> in Patent Literature <NUM> above). In contrast, the above-mentioned plunging type constant velocity universal joint <NUM> is used exclusively for the rear-wheel drive shaft. Thus, the countermeasures against idling vibration are not required, and the inner peripheral surface (portion to be held in slide contact with the inner joint member) of the cage can be formed into a simple spherical shape. Accordingly, the product of the present invention is more advantageous than the conventional product in view of manufacture.

Preferable ranges of the gaps, which are set in consideration of the above-mentioned circumstances, are shown in Table <NUM> below (unit: mm). Axial gaps between the pocket surfaces of the cage and the balls are equal to those of the conventional product. When minute axial gaps are formed between the pocket surfaces of the cage and the balls, rolling performance of the balls is improved, and torque transmitting efficiency is improved.

Claim 1:
A plunging type constant velocity universal joint (<NUM>) for a rear-wheel drive shaft, comprising:
an outer joint member (<NUM>) having a cylindrical inner peripheral surface in which eight track grooves (21d) extending in an axial direction of the plunging type constant velocity universal joint (<NUM>) are formed;
an inner joint member (<NUM>) having a spherical outer peripheral surface in which eight track grooves (22e) extending in the axial direction are formed, and having a spline hole (22c) formed along an axial center of the inner joint member (<NUM>);
eight balls (<NUM>) arranged in ball tracks formed by the track grooves (21d) of the outer joint member (<NUM>) and the track grooves (22e) of the inner joint member (<NUM>); and
a cage (<NUM>), which has eight pockets (24a) configured to receive the balls (<NUM>), and is held in slide contact with the inner peripheral surface of the outer joint member (<NUM>) and the outer peripheral surface of the inner joint member (<NUM>),
wherein a curvature center of a spherical portion formed on an outer peripheral surface of the cage (<NUM>) and a curvature center of a spherical portion formed on an inner peripheral surface of the cage (<NUM>) are offset to opposite sides in the axial direction with respect to a joint center (O(s)) by an equal distance,
characterized in that
the plunging type constant velocity universal joint (<NUM>) has a maximum operating angle of <NUM>° or less,
a ratio PCDBALL/DBALL of a pitch circle diameter PCDBALL of the balls (<NUM>) to a diameter DBALL of each of the balls (<NUM>) is set from <NUM> to <NUM>,
a ratio TI/DBALL of a radial thickness TI of the inner joint member (<NUM>) to the diameter DBALL of each of the balls (<NUM>) is set from <NUM> to <NUM>,
a ratio PCDSPL/DBALL of a pitch circle diameter PCDSPL of the spline hole (22c) of the inner joint member (<NUM>) to the diameter DBALL of each of the balls (<NUM>) is set from <NUM> to <NUM>, and
a ratio WI/DBALL of an axial width WI of the inner joint member (<NUM>) to the diameter DBALL of each of the balls (<NUM>) is set from <NUM> to <NUM>.