The present disclosure claims the benefit of Japanese Patent Application No. 2020-201070 filed on Dec. 3, 2020 with the Japanese Patent Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

Field of the Disclosure

The present disclosure relates to the art of a shifting mechanism for shifting a gear stage or an operating mode in a power transmission unit in which the gear stage or the operating mode can be selected from a plurality of stages or modes.

Discussion of the Related Art

JP-A-2000-9217 describes a synchronizing device of a transmission comprising: a clutch hub that is splined onto an output shaft; a sleeve that is fitted onto the clutch hub in a slidable manner; and a shift arm (or shift fork) whose leading end is fitted into a groove formed on an outer circumferential surface of the sleeve. In the synchronizing device taught by JP-A-2000-9217, a tapered guide surface is formed on the leading end of the shift fork to deliver lubrication oil to a contact portion between the leading end of the shift fork and the groove of the sleeve.

JP-A-2016-070371 describes a shift fork of a manual transmission in which a contact portion between a sleeve and the shift fork can be lubricated effectively. According to the teachings of JP-A-2016-070371, the shift fork comprises a plurality of first pads attached to both ends of the shift fork, and a plurality of second pad attached to an intermediate portion of the shift fork between the first pads. Specifically, a width of each of the first pads in an axial direction is wider than a width of each of the second pads in the axial direction. In other words, a clearance between the second pad and the groove of the sleeve in the axial direction is wider than a clearance between the first pad and the groove of the sleeve in the axial direction. According to teachings of JP-A-2016-070371, therefore, a lubrication oil splashed by a gear of the transmission can be supplied smoothly into the clearance between the second pad and the groove of the sleeve, but the lubrication oil is not allowed to flow easily out of the clearance between the first pads and the groove of the sleeve.

JP-A-2020-118294 describes a shifting mechanism in which an endplay between a shift fork and a cam groove in an axial direction is reduced. The shifting mechanism taught by JP-A-2020-118294 comprises: a cam follower that is connected to the shift fork while being fitted into a cam groove, and a spring as a pushing member that pushes the cam follower onto one of guide walls of the cam groove. Since the cam follower is pushed onto one of the guide walls of the cam groove by the spring, the endplay between the shift fork and the cam groove can be reduced so that an axial length of the shifting mechanism taught by JP-A-2020-118294 can be reduced.

As described in JP-A-2000-9217 and JP-A-2016-070371, the leading end of the shift fork is fitted into the groove of the sleeve so that the shift sleeve is moved in the axial direction by shifting the shift fork in the axial direction. In this situation, the sleeve is rotated relatively to the shift fork by a rotation of the output shaft, and if the sleeve is moved in the axial direction together with the shift fork, the leading end of the shift fork slides on the guide wall of the groove. In order to prevent seizure and wear at the contact portion between the shift fork and the groove, according to the teachings of JP-A-2000-9217, the tapered guide surface is formed on the leading end of the shift fork to deliver lubrication oil to the above-mentioned contact portion. However, in order to allow the shift fork to slide smoothly on the groove, and to supply the lubricating oil to the contact portion, it is necessary to maintain a clearance (or endplay) between the leading end of the shift fork and the groove of the sleeve. Therefore, the sleeve may be tilted with respect to the shift fork. If the sleeve is tilted or inclined with respect to the shift fork, the sleeve may not slide smoothly along the groove in the axial direction. That is, a speed change operation of the transmission may not be carried out smoothly. In order to avoid such disadvantage, an inclination of the sleeve with respect to the shift fork may be reduced by increasing a width of the leading end of the shift fork in the axial direction. Nonetheless, if the clearance between the leading end of the shift fork and the groove of the sleeve is reduced as a result of increasing the width of the leading end of the shift fork, the lubrication oil may not be supplied smoothly to between the shift fork and the groove.

SUMMARY

Aspects of preferred embodiments of the present disclosure have been conceived noting the foregoing technical problems, and it is therefore an object of the present disclosure to provide a shifting mechanism in which an inclination of a sleeve with respect to a shift fork is reduced while maintaining a clearance between the shift fork and the sleeve.

An exemplary embodiment of the present disclosure relates to a shifting mechanism, comprising: a shift sleeve that is arranged around a rotary shaft to be rotated integrally with the rotary shaft; a shift fork that reciprocates in an axial direction of the rotary shaft; an engagement groove that is formed entirely on an outer circumference of the shift sleeve; and a pair of cutouts formed on a bottom of the engagement groove. In the shifting mechanism, the shift sleeve is reciprocated in the axial direction by the shift fork but allowed to rotate relatively to the shift fork. The shift fork comprises: an arcuate fork section that is fitted onto the shift sleeve; and at least a pair of engagement ridges formed on an inner circumference of both ends of the fork section, each of which is engaged with the engagement groove. In order to achieve the above-explained objective, according to the exemplary embodiment of the present disclosure, an angle toward the shift fork between: a line drawn between one end of the fork section and a rotational center of the shift sleeve; and a line drawn between the other end of the fork section and the rotational center of the shift sleeve, is a reflex angle. A first diameter of the shift sleeve between a predetermined site of the bottom of the engagement groove within the cutout and a symmetrical site across the rotational center is shorter than a second diameter of the shift sleeve between predetermined sites of the bottom of the engagement groove where the cutout is not formed. In addition, a straight-line distance between the engagement ridges formed on both ends of the fork section is longer than the first diameter but shorter than the second diameter.

In a non-limiting embodiment, the engagement ridges may be formed on the inner circumference of both ends of the fork section across the rotational center of the shift sleeve.

In a non-limiting embodiment, each of the engagement ridges may protrude radially inwardly to be fitted into the engagement groove, and a chord length of the engagement ridge between radially innermost portions in the circumferential direction may be longer than a chord length of the cutout between end portions in the circumferential direction.

In a non-limiting embodiment, the shift fork may further comprise another engagement ridge formed between the engagement ridges formed on the inner circumferences of both ends of the fork section.

Thus, in the shifting mechanism according to the exemplary embodiment of the present disclosure, the engagement ridges formed on both ends of the shift fork are engaged with the engagement groove formed on the outer circumference of the shift sleeve. As described, the first diameter of the shift sleeve between predetermined sites of the bottom of the engagement groove within the cutouts is shorter than the second diameter of the shift sleeve between predetermined sites of the bottom of the engagement groove where the cutout is not formed. In addition, an angle toward the shift fork between: a line drawn between one end of the fork section and a rotational center of the shift sleeve; and a line drawn between the other end of the fork section and the rotational center of the shift sleeve, is a reflex angle. Further, the straight-line distance between the engagement ridges is longer than the first diameter of the shift sleeve, but shorter than the second diameter of the shift sleeve. To this end, circumferential lengths of the shift fork and the engagement ridges formed on the end portions of the shift fork are elongated so that the engagement ridges formed on the end portions of the shift fork are situated further than the rotational center of the shift sleeve in the circumferential direction, between the outer circumference of the shift sleeve and the bottom of the engagement groove in the radial direction. According to the exemplary embodiment of the present disclosure, therefore, an inclination (tan θ) of the shift sleeve with respect to the shift fork can be reduced. For this reason, the shift sleeve may be reciprocated in the axial direction without jamming thereby executing a shifting operation smoothly.

In addition, according to the exemplary embodiment of the present disclosure, the straight-line distance between the engagement ridges formed on both ends of the shift fork is longer than the first diameter of the shift sleeve between the cutouts, but shorter than the second diameter of the shift sleeve between predetermined sites of the bottom of the engagement groove where the cutout is not formed. Therefore, although the engagement ridges formed on both ends of the shift fork are elongated in the circumferential direction, the shift fork may be fitted onto the shift sleeve by inserting the engagement ridges formed on end portions of the shift fork within the engagement groove of the shift sleeve through the cutouts.

Further, the chord length between the end portions of the cutout in the circumferential direction is shorter than the chord length between the radially innermost portions of the engagement ridge of the end portions of the shift fork in the circumferential direction. Therefore, even if the shift fork fitted onto the shift sleeve is displaced in the radial direction by some sort of external factor during rotation of the shift sleeve, the engagement ridge will not enter the cutout. That is, a leading end of the engagement ridge will not come into contact to the end portion of the cutout during rotation of the shift sleeve, and hence undesirable collision noises and vibrations will not be generated. In addition, the engagement ridge will not be worn undesirably.

