referring to fig1 there is illustrated an automotive vehicle power transmission including coupling sleeves made in accordance with the present invention . the power transmission comprises a main drive gear 2 rotatable with rotation of the engine ( not shown ), a third gear 4 , a second gear 6 , and a first gear 8 , all of which are mounted on a main shaft 10 for free rotation . the main drive gear 2 and the third gear 4 are provided on their facing surfaces with clutch gears 12 and 14 , respectively , and the second and first gears 6 and 8 are provided on their facing surfaces with clutch gears 16 and 18 , respectively . the power transmission also comprises a gear 20 in mesh with the main drive gear 2 , a third - speed gear 22 in mesh with the third gear 4 , a second - speed gear 24 in mesh with the second gear 6 , and a first - speed gear 26 in mesh with the first gear 8 . all of the gears 20 to 26 are fixedly secured on a counter gear shaft 28 . fixedly secured on the main shaft 10 between the main drive gear 2 and the third gear 4 is a first synchromesh mechanism 30 which comprises a synchrohub 32 rigidly fixed on the main shaft 10 , a coupling sleeve 34 splined to the outer periphery of the synchrohub 32 and formed in its peripheral surface with a fork groove 36 , and two fork rings 38 . the coupling sleeve 34 comes into mesh with the clutch gear 12 of the main drive gear 2 for transmitting rotation of the main drive gear 2 to the main shaft 10 when axially moved to the left . if the coupling sleeve 34 is axially moved to the right , it comes into mesh with the clutch gear 14 of the third gear 4 for transmitting rotation of the third gear 4 to the main shaft 10 . a second synchromesh mechanism 40 , which is similar in structure to the first synchromesh mechanism 30 , is also fixedly secured on the main shaft 10 . the coupling sleeve 44 of the second synchromesh mechanism 40 comes into mesh with the clutch gear 16 of the second gear 6 for transmitting rotation of the second gear 6 to the main shaft 10 when axially moved to the left and comes into mesh with the clutch gear 18 of the first gear 8 for transmitting rotation of the first gear 8 to the main shaft 10 when axially moved to the right . referring to fig2 there is illustrated an operating mechanism for operating the first and second synchromesh mechanisms 30 and 40 . the operating mechanism 50 comprises fork rods 52 and 54 extending in parallel with the main shaft 10 , shift forks 56 and 58 having their base portions secured to the respective fork rods 52 and 54 and their semi - circular forked portion engaged with the fork grooves 36 and 46 of the respective coupling sleeves 34 and 44 so that the shift forks 56 and 58 can be moved together with the respective coupling sleeves 34 and 44 with movement of the respective fork rods 52 and 54 . another fork rod 60 is provided which has a fork 62 secured thereto for sliding a reverse gear 64 on the main shaft 10 through a bush 66 . the fork rods 52 , 54 and 60 are selectively slid on the main shaft 10 by a striking rod 68 . referring to fig3 there is illustrated in greater detail the coupling sleeve 34 which is substantially similar in structure to the coupling sleeve 44 . a wear - resistive member 70 having high self - lubricating and antiwear properties and u - shaped cross section is rigidly fitted in the fork groove 36 of the coupling sleeve 34 . the wear - resistive member 70 may preferably comprise synthetic resin high in antiwear and self - lubricating properties . such materials include , but are in no way limited to , polyamide resin available from dupont under the tradename nylon , ethylene tetrafluoride available from dupont under the tradename teflon , and acetal resin available from dupont under the tradename delrin . furthermore , the wear - resistive member 70 may comprise metal having high antiwear and self - lubricating properties such as sintered oleo - metal . if the wear - resistive member 70 is formed of synthetic resin , it may be fixed in place by coating , sticking , or fitting projections formed on the wear - resistive member 70 in recesses formed in the coupling sleeve 34 . if the wear - resistive member 70 is formed of metal , it may be fixed in place by fusing or sticking . the coupling sleeve structure of the present invention is effective to protect the coupling sleeve from wear and assures smooth and certain speed ratio changing operation for a long time . furthermore , since the wear - resistive member 70 is formed of synthetic resin or soft metal , the coupling sleeve structure of the present invention can reduce the wear of the associated shift fork particularly where the shift fork is formed of material having relatively poor wear resistance for purpose of weight reduction . fig4 illustrates a second embodiment of the present invention in which wear - resistive members 70 &# 39 ; are secured to the side wall surface of the fork groove 36 in the same manner as described in connection with fig3 . the coupling sleeve structure of this embodiment provides the same effect as described in conjunction with the first described embodiment since most of the force of the shift fork exerted on the coupling sleeve 34 is centered on the side wall surfaces of the fork groove 36 and almost no force is exerted on the bottom surface of the fork groove 36 . fig5 shows another embodiment in which the fork groove 36 is formed on opposite sides of its bottom surface with small grooves 82 each receiving the wear - resistive member 70 &# 39 ; and formed in each of its side wall surfaces with a recess 74 receiving the turned portions of the wear - resistive member 70 &# 39 ;. fig6 illustrates a third embodiment of the present invention in which the coupling sleeve 34 is formed on its peripheral surface with an annular projection 80 fitted in groove 82 formed in the shift fork 56 . wear - resistive members 70 &# 34 ; are secured to the opposite side surfaces of the annular projection 80 . the wear - resistive members 70 &# 34 ; may be secured to the projection 80 in the same manner as described in connection with the first embodiment . it is to be noted , of course , that the annular projection 80 of the coupling sleeve 34 may be covered with an annular wear - resistive member of u - shaped cross section . the coupling sleeve structure of this embodiment provides the same effect as described in connection with the first embodiment . fig7 shows another attachment in which the peripheral surface of the coupling sleeve 34 is formed on opposite sides of the projection 80 with slits 84 each receiving a wear - resistive member 70 &# 34 ; and the projection 80 is formed on each of its opposite side surface with a recess 86 receiving the turned end portions of the wear - resistive member 70 &# 34 ;. it is therefore apparent from the foregoing that there has been provided , in accordance with the present invention , a coupling sleeve structure capable of increasing the life and reducing the wear of the coupling sleeve and the associated shift fork , thereby permitting smooth and certain speed ratio changing operations for a long time that fully satisfies the objects , aims and advantages set forth above . while the present invention has been described in conjunction with specific embodiments thereof , it is evident that many alternatives , modifications and variations will be apparent to those skilled in the art . accordingly , it is intended to embrace all alternatives , modifications and variations that fall within the spirit and broad scope of the appended claims .