Patent Publication Number: US-11041532-B2

Title: Synchronizer for transmission

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2019-0048280, filed on Apr. 25, 2019, which is incorporated herein by reference in its entirety. 
     FIELD 
     The present disclosure relates to a synchronizer apparatus for a transmission. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     A synchronizer apparatus for a transmission of a vehicle is typically a device for synchronously coupling a shifting gear (i.e., a speed gear) to an output shaft by an operation of a shift fork. 
       FIG. 1  is a cross-sectional view of a typical synchronizer apparatus for a transmission. 
     Referring to  FIG. 1 , in a typical synchronizer apparatus for a transmission, a hub  3  is spline coupled with an output shaft  1 , and a sleeve  5  is arranged on teeth on an exterior circumference of the hub  3 , movably in an axial direction. 
     A clutch gear  9  is arranged in an interior side of the shifting gear  7  to integrally rotate with the shifting gear  7 . 
     A synchronizer ring  11  is disposed between the clutch gear  9  and the hub  3 , to synchronously couple the clutch gear  9  and the hub  3  by a movement of the sleeve  5 . 
     Key balls  13  are disposed between the hub  3  and the sleeve  5  at three circumferential locations such that the sleeve  5  may have a principal location. The key balls  13  is elastically abutted by a spring  17  installed in a key ball housing  15  formed at the hub  3 , and thereby maintains contact with a groove G formed on an interior circumference of the sleeve  5 . 
     When the sleeve  5  moves in the axial direction during the shifting operation, the synchronizer ring  11  is coupled with a cone portion  9   a  of the clutch gear  9  by a conical clutch operation. 
     During a shifting operation, the shift fork  21  activated by an actuator  20  pushes the sleeve  5  in the axial direction. 
     Then, the sleeve  5  moves with the key ball  13  in the groove G, thereby also moving the key ball housing  15 , and therefore the synchronizer ring  11  is engaged with the cone portion  9   a  of the clutch gear  9 . 
     When the synchronizer ring  11  and the clutch gear  9  are engaged, the hub  3  and the shifting gear  7  are rotationally synchronized. In this state, the sleeve  5  further moves in the axial direction to be engaged with the clutch gear  9  as well as the synchronizer ring  11 . 
     Since the hub  3  and the shifting gear  7  are mechanically engaged, the shifting gear  7  is synchronously coupled with the output shaft  1  to rotate at a same rotation speed. 
     According to such a synchronizer apparatus, a neutral period, during which torque transmission is cut off, is involved during the shifting process, thereby deteriorating shift feel and increasing a shifting time. 
     According to such a conventional synchronizer apparatus, it is notable that only an engaged synchronizer ring  11  is synchronized with a speed gear, and a disengaged synchronizer ring  11  is not synchronized with a speed gear. 
     We have discovered that when the shift fork moves fast during the shifting operation, the sleeve  5  may collide with a stopper of the clutch gear  9  to produce an impact noise, thereby deteriorating shift feel. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY 
     The present disclosure provides a synchronizer apparatus for a transmission of a vehicle which is typically a device for synchronously coupling a shifting gear (i.e., a speed gear) to an output shaft by an operation of a shift fork. 
     According to an exemplary form of the present disclosure, the synchronizer apparatus for a transmission includes first and second shifting gears rotatably disposed on an output shaft, a hub spline-coupled with the output shaft and having a plurality of slope portions having slanted surfaces at both sides of the hub. In addition, the plurality of slope portions is symmetrically disposed along a circumferential direction. The synchronizer apparatus further includes a sleeve spline-coupled with an external circumference of the hub, a plurality of keys that are respectively disposed to the plurality of slope portions and correspond to an interior side of the sleeve, first and second clutch gears disposed between the hub and the first and second shifting gears, respectively, first and second outer rings and first and second inner rings disposed between the hub and the first and second clutch gears, respectively, first and second synchronizer cones between the first inner and outer rings and the second inner and outer rings and engaged with the first and second clutch gears, respectively, and a plurality of poppet ball units disposed at interior circumference of the sleeve at locations corresponding to the plurality of keys. Furthermore, the poppet ball units contact an exterior surface of the plurality of keys through a poppet ball. 
     The synchronizer apparatus for a transmission further includes a plurality of first and second push blocks disposed at both slanted surfaces of the plurality of slope portions, respectively, and a plurality of first and second push springs inserted in spring holes formed at outer surfaces of the plurality of first and second push blocks, respectively. In addition, the first and second pushing springs are abutted by interior surfaces of both bent ends of the plurality of keys such that cam surfaces of the push blocks may tightly contact the slanted surfaces of the plurality of slope portions of the hub, respectively. 
     According to a further aspect of the present disclosure, each of the plurality of keys includes a pressurizing surface formed as a central flat surface on an exterior surface to contact a poppet ball of the poppet ball unit, and a pressure release surface formed as a curved surface extending from both sides of and downward form the pressurizing surface. 
     According to a further aspect of the present disclosure, each of the plurality of keys is formed as a bent plate with a predetermined thickness. 
     According to a further aspect of the present disclosure, each of the plurality of keys includes bend ends, thereby forming interior surfaces to abut the first and second push springs. 
     According to a further aspect of the present disclosure, the poppet ball unit includes a ball housing inserted in an installation hole formed in interior circumference of the sleeve, and a poppet ball being inserted in the ball housing to form a rolling contact with the exterior surface of the key and pressurizing the key in a radially inward direction. 
     According to a further aspect of the present disclosure, each of the first and second push blocks includes an outer surface facing an interior surface of a corresponding outer ring, a coupling recess to be coupled with the key, and a cam surface formed as a slant surface to form a surface contact with a corresponding slanted surface of the slope portion the hub. 
     According to a further aspect of the present disclosure, each of the first and second push blocks further includes a protrusion end formed upward from a central upper surface forming the coupling recess, thereby forming a line contact with a bottom surface of the key. 
     According to a further aspect of the present disclosure, the protrusion end protrudes in a semicircular cross-section. 
     According to a further aspect of the present disclosure, the first and second push springs are inserted in spring holes formed at outer surfaces of the first and second push blocks, respectively, and the first and second push springs are formed as coil springs abutted by the interior surfaces of the key. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view of a typical synchronizer apparatus for a transmission; 
         FIG. 2  is a cross-sectional perspective view of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure; 
         FIG. 3  is a cross-sectional view of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure; 
         FIG. 4  is a perspective view of a hub applied to a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure; 
         FIG. 5  and  FIG. 6  are respectively a perspective view and a cross-sectional perspective view of an assembly of a push block and a push spring applied to a key of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure; 
         FIG. 7  is a perspective view of a push block applied to a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure; 
         FIG. 8  illustrates an operation of a push block applied to a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure; 
         FIG. 9  illustrates forces applied to first and second push blocks of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure depending on position of a poppet ball; and 
         FIG. 10A  to  FIG. 10E  illustrate a shifting process of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure. 
     
    
    
     The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
       FIG. 2  is a cross-sectional perspective view of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure.  FIG. 3  is a cross-sectional view of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure. 
     Referring to  FIG. 2  and  FIG. 3 , a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure is disposed between first and second shifting gears G 1  and G 2  mounted on an output shaft S of a transmission, and synchronously connect the first and second shifting gears G 1  and G 2  with the output shaft S, selectively. 
     Such a synchronizer apparatus for a transmission includes, on the output shaft S, a hub  31 , a sleeve  33 , three keys  35 , first and second clutch gears CG 1  and CG 2 , first and second outer rings OR 1  and OR 2 , first and second inner rings IR 1  and IR 2 , first and second synchronizer cones SC 1  and SC 2 , a poppet ball unit  40 , first and second push blocks PB 1  and PB 2 , and first and second push springs PS 1  and PS 2 . 
     The first and second shifting gears G 1  and G 2  are disposed on the output shaft S through a bearing B, to rotate relatively to the output shaft S. 
       FIG. 4  is a perspective view of a hub applied to a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure. 
     Referring to  FIG. 4 , the hub  31  is disposed between the first and second shifting gears G 1  and G 2  and spline coupled with the output shaft S. The hub  31  includes three slope portions  37  symmetrically disposed along a circumferential direction. Each of the slope portions  37  has a slanted surface CF 1  protruding in an axial direction. Although  FIG. 4  illustrates only a single side of the hub  31 , it may be understood that an opposite side of the hub  31  may be identically formed. 
     It may be understood that the number of the slope portions  37  may not be limited to three, and the number of the slope portions  37  may be decided in consideration of design factors. 
     Referring back to  FIGS. 2 and 3 , a gear sleeve  51  is mounted on the output shaft S and fits with the second shifting gear G 2 . The gear sleeve  51  supports the hub  31  and limits an axial movement of the hub  31 . 
     The sleeve  33  is spline coupled with an external circumference of the hub  31 , and is disposed movable in the axial direction. 
       FIG. 5  and  FIG. 6  are respectively a perspective view and a cross-sectional perspective view of an assembly of a push block and a push spring applied to a key of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure. 
     According to an exemplary form, as shown in  FIGS. 2 and 3 , three keys  35  are respectively disposed to the three slope portions  37 , and correspond to an interior side of the sleeve  33 . 
     It may be understood that the number of keys  35  is not limited to three, and may vary with the number of the slope portions  37 . 
     The key  35  is formed as a bent plate with a predetermined thickness, and includes a pressurizing surface T 1  and pressure release surfaces T 2  extending from the pressurizing surface T 1 . The pressurizing surface T 1  is formed as a central flat surface on an exterior surface to contact a poppet ball  45  of the poppet ball unit  40 . The pressure release surface T 2  is formed as a curved surface extending from both sides of and downward from the pressurizing surface T 1 . 
     Bent ends  35   a  are formed at ends of the pressure release surfaces T 2  of the key  35 , thereby forming receiving spaces to receive the first and second push blocks PB 1  and PB 2 , and first and second push springs PS 1  and PS 2  at both sides of the key  35 . 
     Referring back to  FIG. 2  and  FIG. 3 , the first and second clutch gears CG 1  and CG 2  are disposed between the hub  31  and the first and second shifting gears G 1  and G 2 , respectively. The first and second clutch gears CG 1  and CG 2  integrally rotate with the first and second shifting gears G 1  and G 2 , to mediate power delivery to the first and second shifting gears G 1  and G 2 . 
     The first and second clutch gears CG 1  and CG 2  are selectively engaged with the sleeve  33  through spline coupling, by a movement of the sleeve  33 . 
     The first and second outer rings OR 1  and OR 2  are disposed between the hub  31  and the first and second clutch gears CG 1  and CG 2 , respectively. The first and second outer rings OR 1  and OR 2  are selectively engaged with the sleeve  33  through spline coupling, by a movement of the sleeve  33 . 
     The first and second inner rings IR 1  and IR 2  are disposed between the hub  31  and the first and second clutch gears CG 1  and CG 2 , respectively, being radially interior to the first and second outer rings OR 1  and OR 2 . The first and second inner rings IR 1  and IR 2  may be engaged with the first and second clutch gears CG 1  and CG 2  respectively, by conical clutch operation. 
     The first and second synchronizer cones SC 1  and SC 2  are disposed between the first inner and outer rings IR 1  and OR 1  and the second inner and outer rings IR 2  and OR 2 , respectively. The first and second synchronizer cones SC 1  and SC 2  are engaged with the first and second clutch gears CG 1  and CG 2 . 
     The first and second synchronizer cones SC 1  and SC 2  may limit relative rotation of the first and second outer rings OR 1  and OR 2  and the first and second inner rings IR 1  and IR 2  with respect to the hub  31 . 
     The first and second outer rings OR 1  and OR 2  and first and second inner rings IR 1  and IR 2  may function as conventional synchronizer rings. When the sleeve  33  moves in an axial direction during the shifting process, the first and second outer rings OR 1  and OR 2  receive an axial directional force from the key  35 , and thereby engaged with the corresponding clutch gears CG 1  and CG 2  through the first and second inner rings IR 1  and IR 2  by the conical clutch operation. 
     Referring to  FIG. 6 , the poppet ball unit  40  is disposed at in interior circumference of the sleeve  33  at locations corresponding to the three keys  35 . The poppet ball unit  40  contacts an exterior surface of the key  35  through the poppet ball  45 . 
     According to the poppet ball unit  40 , a ball housing  41  is disposed in an installation hole H formed in an interior circumference of the sleeve  33 , and a poppet ball  45  is rotatably inserted in the ball housing  41 . 
     While the poppet ball  45  inserted in the ball housing  41  forms a rolling contact with the exterior surface of the key  35 , and the poppet ball  45  pressurizes the key  35  in a radially inward direction. 
       FIG. 7  is a perspective view of a push block applied to a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure. 
     Referring back to  FIG. 3 , the first and second push blocks PB 1  and PB 2  are disposed at both slanted surfaces CF 1  of the slope portion  37  of the hub  31 . 
     Referring to  FIG. 6  and  FIG. 7 , each of the first and second push blocks PB 1  and PB 2  includes an outer surface OF facing an interior surface of corresponding outer rings OR 1  and OR 2 , a coupling recess  53  to be coupled with the key  35 , and a cam surface CF 2  formed at an interior surface of the push blocks PB 1  and PB 2  and formed as a slant surface to form a surface contact with a corresponding slanted surface CF 1  of the slope portion  37  of the hub  31 . 
     As shown in  FIG. 7 , the first and second push blocks PB 1  and PB 2  further include a protrusion end  57  formed upward from a central upper surface  55  forming the coupling recess  53 . The first and second push blocks PB 1  and PB 2  may form a line contact with a bottom surface of the key  35  through the protrusion end  57 . 
     The protrusion end  57  may protrude in a semicircular cross-section, i.e., in a half-cylindrical shape. 
     Referring back to  FIG. 6 , the first and second push springs PS 1  and PS 2  are inserted in spring holes SH formed at outer surfaces of the first and second push blocks PB 1  and PB 2 , respectively. The first and second push springs PS 1  and PS 2  are abutted by interior surfaces of both bent ends  35   a  of the key  35  such that the cam surfaces CF 2  of the push blocks PB 1  and PB 2  may tightly contact the slanted surfaces CF 1  of the slope portion  37  of the hub  31 . 
     The first and second push springs PS 1  and PS 2  may be formed as coil springs, and each of the spring hole SH is formed at a central portion between the outer surfaces OF forming the coupling recess  53 . 
     In such a synchronizer apparatus for a transmission, the key  35  itself is elastic, and may rotated with the hub  31  while allowing a motion in the radial direction. 
     A radial directional position of the key  35  is determined by a width-wise location of the poppet ball  45  contacting the exterior surface of the key  35 . The force of the key  35  pressing the poppet ball  45  acts as a force on the synchronizer ring of the outer and inner rings OR 1  and IR 1  (or OR 2  and IR 2 ). 
       FIG. 8  illustrates an operation of a push block applied to a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure.  FIG. 9  illustrates forces applied to first and second push blocks of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure depending on position of a poppet ball. 
     Referring to  FIG. 8 , the first and second push blocks PB 1  and PB 2  are coupled with the key through the coupling recess  53 , thereby limiting the rotational movement of the key  35 . 
     In addition, the cam surfaces CF 2  of the first and second push blocks PB 1  and PB 2  tightly contacts the slanted surface CF 1  of the slope portion  37  of the hub  31 . 
     The first and second push springs PS 1  and PS 2  is inserted in the spring holes SH formed at the first and second push blocks PB 1  and PB 2 , and abutted by the both bent ends  35   a  of the key  35 , thereby forming an elastic force pushing the push blocks PB 1  and PB 2  toward the hub  31 . 
     By being in a tight contact with the slanted surfaces CF 1  of the hub  31  through the cam surfaces CF 2 , the first and second push blocks PB 1  and PB 2  are applied with a biasing force in a radial direction by the elastic force of the first and second push springs PS 1  and PS 2 . Such a radially biasing force on the push blocks PB 1  and PB 2  may inhibit or prevent a gap between the key  35  and the poppet ball  45  while the sleeve  33  moves. 
     When the first and second push blocks PB 1  and PB 2  move radially outward, a distance between the first and second push blocks PB 1  and PB 2  are decreased, i.e., the first and second push blocks PB 1  and PB 2  become closer. When the first and second push blocks PB 1  and PB 2  move radially inward, a distance between the first and second push blocks PB 1  and PB 2  are increased, i.e., the first and second push blocks PB 1  and PB 2  become farther. 
     As shown  FIG. 8 , in a state ST 1 , when the poppet ball  45  is located on the pressure release surface T 2  of the key  35  according to a movement of the sleeve  33 , the first and second push blocks PB 1  and PB 2  move radially outward along the slanted surface CF 1  of the hub  31 . Then, the first and second push blocks PB 1  and PB 2  become closer in the axial direction, and therefore become apart from the first and second outer rings OR 1  and OR 2 , such that the force of the poppet ball  45  is not transmitted to the outer rings OR 1  and OR 2 . 
     As shown  FIG. 8 , in a state ST 2 , when the poppet ball  45  is located on the pressurizing surface T 1  of the key  35  according to a movement of the sleeve  33 , the first and second push blocks PB 1  and PB 2  move radially inward along the slanted surface CF 1  of the hub  31 , together with the key  35 , by the force of the poppet ball  45  pressing the key  35 . Then, the first and second push blocks PB 1  and PB 2  become farther from each other in the axial direction, and therefore contacts the first and second outer rings OR 1  and OR 2 , such that the force of the poppet ball  45  is transmitted to the outer rings OR 1  and OR 2 . 
     Referring to  FIG. 9 , between the contact points P 1  and P 3  of the protrusion ends  57  with the key  35 , the force on the first and second push blocks PB 1  and PB 2  are inversely proportional to a distance from a contact point between the poppet ball  45  and the key  35  to a contact point between the key  35  and the protrusion ends  57  of the push blocks PB 1  and PB 2 , respectively. A sum of the forces on the first and second push blocks PB 1  and PB 2  equals to the force of the poppet ball  45  on the key  35 . 
       FIG. 9  illustrates forces applied to first and second push blocks of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure depending on position of a poppet ball  45 . 
       FIG. 10A  to  FIG. 10E  illustrate a shifting process of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure. 
     Referring to  FIG. 10A  to  FIG. 10E , shifting operations of a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure are described in detail. 
     In  FIG. 10A , the sleeve  33  is disposed to the right in the drawing, and the hub  31  is spline-coupled with the second outer ring OR 2  and the second clutch gear CG 2  through the sleeve  33 . Therefore, the sleeve  33  synchronously interconnects the second shifting gear G 2  with the output shaft S. 
     In this state, the poppet ball  45  is located on the right pressure release surface T 2  of the key  35 , and the first and second push blocks PB 1  and PB 2  are positioned radially outward by moving along the slanted surface CF 1  of the hub  31  together with the key  35 . The first and second push blocks PB 1  and PB 2  become closer in the axial direction, and therefore become apart from the first and second outer rings OR 1  and OR 2 , such that the force of the poppet ball  45  is not transmitted to the outer rings OR 1  and OR 2 . 
     As shown in  FIG. 10B , when the sleeve  33  moves to the left in the drawing in the axial direction such that the poppet ball  45  is located at a rightmost end of the pressurizing surface T 1  of the key  35 , the sleeve  33  is now spline-coupled with the hub  31  and the second outer ring OR 2 , releasing the coupling with the second clutch gear CG 2 . Therefore, the synchronous coupling between the second shifting gear G 2  and the output shaft S is released. 
     Then, the poppet ball  45  presses the key  35 , and accordingly, the first and second push blocks PB 1  and PB 2  moves radially inward along the slanted surface CF 1  of the hub  31  together with the key  35 . The first and second push blocks PB 1  and PB 2  become farther from each other in the axial direction, and therefore contacts the first and second outer rings OR 1  and OR 2 , such that the force of the poppet ball  45  is transmitted to the outer rings OR 1  and OR 2 . 
     In this state, the force of the poppet ball  45  is mostly transmitted to the second push block PB 2 , and accordingly the synchronizer ring of the second outer and inner rings OR 2  and IR 2  maintains the function. Therefore, the torque of the output shaft S is still transmitted to the second shifting gear G 2  which is to be released. 
     Then, as shown in  FIG. 10C , the sleeve  33  moves to a neutral position and the poppet ball  45  is located at a center of the pressurizing surface T 1  of the key  35 . In this case, the sleeve  33  is spline-coupled with the hub  31  only. 
     Then, the force of the poppet ball  45  on the key  35  equally acts on the first and second outer rings OR 1  and OR 2  through the outer surfaces of the first and second push blocks PB 1  and PB 2 . 
     In this state, the force of the poppet ball  45  is equally transmitted to the first and second outer rings OR 1  and OR 2  and the first and second inner rings IR 1  and IR 2  through the first and second push blocks PB 1  and PB 2 . Therefore, a torque phase process is realized, which is an interim phase where the torque transmission from the hub  31  is being changed from the second outer ring OR 2  to the first outer ring OR 1 . 
     Subsequently in  FIG. 10D , the sleeve  33  moves to the left in the drawing in the axial direction and the poppet ball  45  is located to the left portion of the pressurizing surface T 1  of the key  35 . In this case, the sleeve  33  is spline-coupled with the hub  31  and the first outer ring OR 1 , and the first shifting gear G 1  and the output shaft S are synchronously interconnected. 
     Then, by the force of the poppet ball  45  on the key  35 , outer surfaces of the first and second push blocks PB 1  and PB 2  respectively contact the first and second outer rings OR 1  and OR 2 . 
     In this state, the force of the poppet ball  45  is mostly transmitted to the first push block PB 1 , and accordingly the synchronizer ring of the first outer and inner rings OR 1  and IR 1  maintains the function. Therefore, the torque of the output shaft S is still transmitted to the first shifting gear G 1  which is to be engaged. 
     Subsequently in  FIG. 10E , the sleeve  33  is disposed to the left in the drawing, and the hub  31  is spline-coupled with the first outer ring OR 1  and the first clutch gear CG 1  through the sleeve  33 . Therefore, the sleeve  33  synchronously interconnects the first shifting gear G 1  with the output shaft S. 
     In this state, the poppet ball  45  is located on the left pressure release surface T 2  of the key  35 , and the first and second push blocks PB 1  and PB 2  are positioned radially outward by moving along the slanted surface CF 1  of the hub  31  together with the key  35 . 
     In addition, the first and second push blocks PB 1  and PB 2  become closer in the axial direction, and therefore become apart from the first and second outer rings OR 1  and OR 2 , such that the force of the poppet ball  45  is not transmitted to the outer rings OR 1  and OR 2 . 
     According to a synchronizer apparatus for a transmission according to an exemplary form of the present disclosure, push blocks PB 1  and PB 2  and push springs PS 1  and PS 2  are employed to both sides of the key  35  and the hub  31 . During a shifting operation from a to-be-released shifting gear to a to-be-engaged shifting gear, torque transmission to the to-be-released shifting gear is maintained until the to-be-engaged shifting gear is actually initiated to be engaged. Therefore, interruption of torque transmission during the shifting operation is reduced or minimized. 
     In addition, forces on synchronizer rings at opposite sides of the hub are inversely proportional to each other according to movement of the sleeve, and accordingly, a shift shock and/or an impact noise possibly caused by a stopper and a sleeve may be reduced. 
     While this disclosure has been described in connection with what is presently considered to be practical exemplary forms, it is to be understood that the disclosure is not limited to the disclosed forms. On the contrary, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 
     DESCRIPTION OF SYMBOLS 
     
         
         
           
             S: output shaft 
             G 1 , G 2 : first and second shifting gears 
             CG 1 , CG 2 : first and second clutch gears 
             OR 1 , OR 2 : first and second outer rings 
             IR 1 , IR 2 : first and second inner rings 
             SC 1 , SC 2 ; first and second synchronizer cones 
               31 : hub 
               33 : sleeve 
               35 : key 
               35   a : bent end 
               37 : slope portion 
             CF 1 : slanted surface 
             CF 2 : cam surface 
               40 ; poppet ball unit 
               41 : ball housing 
               45 : poppet ball 
               51 : gear sleeve 
               57 : protrusion end 
             PB 1 , PB 2 : push block 
             PS 1 , PS 2 : push spring 
             T 1 : pressurizing surface 
             T 2 : pressure release surface 
             B: bearing