Abstract:
The transmission includes a number of male dowels disposed on gear shifters for engaging female dowels on gears. The male dowels are arranged in sets of high and low male dowels, the high and low male dowels being alternately arranged on a side surface of a gear shifter. The male dowels have curved engaging surfaces for engaging respective female dowels. The contact point between the male dowels and the female dowels is radially outward from a radial center of the male dowels, so that a thicker portion of the male dowels absorbs the impact between male and female dowel. The low male dowels need not be machined, which reduces the cost of producing the transmission.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a dog clutch type transmission suitable for use in a motorcycle or the like. 
     2. Background Art 
     A conventional dog clutch in a transmission includes a plurality of angular or round male dowels having the same height, and a plurality of female dowels adapted to engage the male dowels, thereby transmitting torque. This type of dog clutch is shown in Japanese Utility Model Publication Nos. Sho 59-4867 and Hei 4-22107. 
     Another conventional dowel structure has high male dowels and low male dowels of differing height, with the number of the female dowels being half the number of male dowels. Both the surface of each high male dowel and the surface of each low male dowel are machined. In this arrangement, a female dowel is a dowel hole or a recess adapted to engage a male dowel. 
     To increase the chance of engagement of the male dowels with the female dowels in the conventional dowel structure, the distance between neighboring ones of the male dowels must be increased, and each male dowel must therefore be reduced in size. However, the size reduction of each male dowel is limited because its strength must be sufficient. The ability to increase the chance of engagement is therefore limited in conventional devices. 
     In addition, in conventional devices, the play of each male dowel engaged with the corresponding female dowel must be adjusted in magnitude according to characteristics of individual types of transmissions, and the design of the male dowels is limited by the type of transmission in which it is utilitized. It is also necessary to set gears having different dowel specifications for manual and automatic transmissions. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the shortcomings associated with conventional devices and achieves other advantages not realized by conventional devices. 
     In one embodiment of the invention, a transmission shifts gears by engaging or disengaging a dog clutch, the dog clutch comprising a plurality of angular male dowels and a plurality of female dowels adapted to engage the male dowels. Each of the male dowels has a curved engaging surface, and each of the female dowels has a flat engaging surface for contacting the curved engaging surfaces. 
     A contact position between the curved engaging surface of each male dowel and the flat engaging surface of each female dowel is offset radially outward from a radial center of each male dowel. The male dowels comprise high male dowels and low male dowels that differ in height, and the high male dowels and the low male dowels are alternately arranged. 
     The number of female dowels is half the number of the male dowels. The tolerance of the height of each low male dowel is a maximum working tolerance by forging, and the surface of each low male dowel is an unmachined surface left after forging. 
     Each male dowel is angular, with its engaging surface formed as a curved surface. The engaging surface of each female dowel is formed as a flat surface. Accordingly, a pitch circle of engagement of the male and female dowels can be set, and a contact position between these engaging surfaces can be specified to a most effective position by adjusting the pitch circle. 
     The contact position between the engaging surfaces of the male and female dowels is offset radially outward by setting the pitch circle to thereby enlarge the radius of rotation of a contact portion between the male and female dowels. Accordingly, the contact portion is a thick-walled portion, and the strength of the male dowels can be ensured. Further, in a usual male dowel, stress concentration tends to occur at the root of a radially inner end of the male dowel. Accordingly, by radially outwardly offsetting the contact point, a stress portion can be spaced apart from the root of the radially inner end. As a result, the strength can be improved as compared with that of a conventional male dowel having a similar size. 
     The chance of dynamic engagement of the male and female dowels is determined by the start of engagement of these dowels. Accordingly, by alternately arranging the high male dowels and the low male dowels of differing height, the start of engagement is effected by the engagement of the high male dowels into the female dowels, thus ensuring the increased chance of engagement. 
     After completing the engagement of the high male dowels into the female dowels, the low male dowels are also engaged into the female dowels, so that the play determined after the engagement can be set to a small amount. Thus, the chance of engagement can be increased with the play reduced, and the degree of freedom of design can be increased. As a result, both a foot-operated gear change and a motor-operated gear change can be carried out by the same gear. 
     The maximum tolerance of the height of each low male dowel is set as a maximum working tolerance by forging. Accordingly, each low male dowel can be used as a forged part. Because the low male dowel is not machined, the number of production steps can be reduced, reducing costs. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is an enlarged elevational view illustrating the engagement of dowels according to one embodiment of the present invention; 
     FIG. 2 is a sectional view of a transmission having dowels according to an embodiment of the present invention; 
     FIG. 3 is an elevational view showing male dowels formed on one surface of a dog clutch; 
     FIG. 4 is an elevational view of female dowels to be engaged with the male dowels shown in FIG. 3; and 
     FIG. 5 is a cross section taken along the line  5 — 5  in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is an enlarged elevational view illustrating the engagement of dowels according to an embodiment of the present invention. FIG. 2 is a sectional view of a transmission having dowels according to the present invention. In the present specification, the term “male dowel” is used to indicate any surface projecting from another surface, and the term “female dowel” is used to indicate any aperture or recess suitable for receiving a male dowel. 
     As shown in FIG. 2, the transmission is of a constant-mesh type having a main shaft  1  and a counter shaft  2  parallel to the main shaft  1 . Gears provided on the shafts  1  and  2  are in mesh, and any one of first, second, and third shifters  3 ,  4 , and  5  is axially moved to select a desired pair of the meshing gears. The main shaft  1  is integrally formed with a first gear  11  and a second gear  12 . A third gear  13  is formed on the outer circumference of the first shifter  3  axially movably splined with the main shaft  1 . A fourth gear  14  and a fifth gear  15  are supported on the main shaft  1  so as to be idly rotatable. 
     A plurality of male dowels  31  are formed on one side surface of the first shifter  3  opposed to the fourth gear  14  so as to be allowed to engage a plurality of female dowels  31   a  formed on one side surface of the fourth gear  14  opposed to the one side surface of the first shifter  3 . Similarly, a plurality of male dowels  32  are formed on the other side surface of the first shifter  3  so as to be allowed to engage a plurality of female dowels  32   a  formed on one side surface of the fifth gear  15  opposed to the other side surface of the first shifter  3 . 
     A first gear  21 , a second gear  22 , a third gear  23 , and a reverse gear  20  are supported on the counter shaft  2  so as to be idly rotatable. A fourth gear  24  is formed on the outer circumference of the third shifter  5  splined with the counter shaft  2 , and a fifth gear  25  is fixedly mounted on the counter shaft  2 . The third shifter  5  is axially movably splined with a portion of the counter shaft  2  between the first gear  21  and the third gear  23 , and the second shifter  4  is axially movably splined with a portion of the counter shaft  2  between the second gear  22  and the reverse gear  20 . An output gear  26  is fixed to an axial end of the counter shaft  2  so as to be rotatable integrally therewith and to mesh with a driven output gear  61  mounted on an output shaft  6 . 
     A plurality of male dowels  51  are formed on one side surface of the third shifter  5  opposed to the first gear  21  so as to be allowed to engage a plurality of female dowels  51   a  formed on one side surface of the first gear  21  opposed to the one side surface of the third shifter  5 . Similarly, a plurality of male dowels  52  are formed on the other side surface of the third shifter  5  so as to be allowed to engage a plurality of female dowels  52   a  formed on one side surface of the third gear  23  opposed to the other side surface of the third shifter  5 . 
     A plurality of male dowels  41  are formed on one side surface of the second shifter  4  so as to be allowed to engage a plurality of female dowels  41   a  formed on one side surface of the second gear  22  opposed to the one side surface of the second shifter  4 . Similarly, a plurality of male dowels  42  are formed on the other side surface of the second shifter  4  so as to be allowed to engage a plurality of female dowels  42   a  formed on one side surface of the reverse gear  20  opposed to the other side surface of the second shifter  4 . 
     When the third shifter  5  is moved leftward as viewed in FIG. 2, the male dowels  51  come to engagement with the female dowels  51   a , thereby selecting the mesh of the first gears  11  and  21 . Similarly, when the second shifter  4  is moved leftward as viewed in FIG. 2, the male dowels  41  come into engagement with the female dowels  41   a , thereby selecting the mesh of the second gears  12  and  22 . Similarly, when the third shifter  5  is moved rightward as viewed in FIG. 2, the male dowels  52  come into engagement with the female dowels  52   a , thereby selecting the mesh of the third gears  13  and  23 . 
     When the first shifter  3  is moved leftward as viewed in FIG. 2, the male dowels  31  come into engagement with the female dowels  31   a , thereby selecting the mesh of the fourth gears  14  and  24 . When the first shifter  3  is moved rightward as viewed in FIG. 2, the male dowels  32  come into engagement with the female dowels  32   a , thereby selecting the mesh of the fifth gears  15  and  25 . 
     The reverse gear  20  is in mesh with an idle reverse gear (not shown) mounted on another shaft (not shown) and meshing with the first gear  11 . Accordingly, the reverse gear  20  is rotated in a direction reverse to the direction of rotation of the other shift gears on the counter shaft  2 . When the second shifter  4  is moved rightward as viewed in FIG. 2, the male dowels  42  come into engagement with the female dowels  42   a , thereby reverse rotating the counter shaft  2  through the mesh of the first gear  11 , the idle reverse gear, the reverse gear  20 , and the second shifter  4 , thus obtaining a reverse output. 
     The dowel structure on the fourth gear  24 , for example, will now be described (the shifters and the other gears have a similar dowel structure). FIG. 3 is an elevational view of the fourth gear  24  having the male dowels  51 , as viewed from the left side in FIG. 2, and FIG. 4 is an elevational view of the first gear  21  having the female dowels  51   a , as viewed from the right side in FIG.  2 . 
     As shown in FIG. 3, the eight male dowels  51  are equally spaced in the circumferential direction of the fourth gear  24 , and project in the axial direction of the fourth gear  24  integrally therewith. Each male dowel  51  is angular and has the shape of substantially quadrangular prism. The male dowels  51  may include four high male dowels  51 (A) and four low male dowels  51 (B). The four high male dowels  51 (A) are larger in height (projection amount) than the four low male dowels  51 (B). The four high male dowels  51 (A) and the four low male dowels  51  (B) are alternately arranged. 
     FIG. 1 is an enlarged view of neighboring ones of the high and low male dowels  51 (A) and  51 (B). As shown in FIG. 1, each male dowel  51  has a substantially elliptical shape as viewed in elevation, and has a radially inner end  70 , a radially outer end  71  wider than the radially inner end  70 , and opposite sides  72  and  73  connecting the radially inner end  70  and the radially outer end  71 . Each of the opposite sides  72  and  73  is formed by a curved surface C. 
     The four corners of each male dowel  51  are chamfered to form round surfaces. In FIG. 1, reference symbol O denotes the center of the radial width of each male dowel  51  (i.e., the distance between the radially inner end  70  and the radially outer end  71 ), and reference symbol P denotes a contact point between the high male dowel  51 (A) and the female dowel  51   a  in their engaged condition. It should be noted that the contact point P is set at a position radially outside of the center O by a distance d. 
     As shown in FIG. 4, the four female dowels  51   a  are equally spaced in the circumferential direction of the first gear  21 . Each female dowel  51   a  is a substantially sectorial dowel hole. Any neighboring ones of the female dowels  51   a  are partitioned by a substantially elliptical partition wall  80  whose radially outer side is wider. Each partition wall  80  has opposite side wall surfaces  81  exposed to the neighboring female dowels  51   a . As shown in FIG. 1, each side wall surface  81  is formed by a flat surface D, and the four comers of each female dowel  51   a  are chamfered to form round surfaces. Each female dowel  51   a  has a size large enough to engage the neighboring male dowels  51 (A) and  51 (B) with play. Thus, the number of the female dowels  51   a  is half the number of the male dowels  51 . 
     The third shifter  5  is formed by forging, and the male dowels  51  and  52  are integrated with the third shifter  5  by forging. The surface of each high male dowel  51 (A) is machined. However, the surface of each low male dowel  51 (B) is not machined after forging. 
     FIG. 5 is a cross section taken along the line  5 — 5  in FIG. 3, showing the difference in height between the high male dowel  51 (A) and the low male dowel  51 (B). For the purposes of illustration, the height differences are exaggerated. In FIG. 5, reference symbol H 1  denotes an actual height of the high male dowel  51 (A), which actual dowel height is the sum of an effective dowel height in which the high male dowel substantially comes into contact with the female dowel on shifting and the height of a rounded dowel root E. 
     Reference symbol H 2  denotes an actual height of the low male dowel  51 (B), which actual height is smaller by ΔH than the actual height HI of the high male dowel  51 (A). The tolerance of the actual height H 2  of the low male dowel  51 (B) is set as a maximum working tolerance by forging, and the amount ΔH is set as a difference which can ensure an absolute engagement amount necessary for reliable engagement of the dowels  51 (A) and  51 (B). That is, the difference obtained by subtracting this difference ΔH from the actual dowel height is equal to an effective dowel height of the low male dowel  51 (B). 
     The operation of the preferred embodiment will now be described. FIG. 5 shows a condition where the male dowels  51  come into engagement with the female dowels  51   a . Each high male dowel  51 (A) first comes into engagement with the opposing female dowel  51   a , and then comes into abutment against the side wall surface  81  of the corresponding partition wall  80 . The distance between the neighboring high male dowels  51 (A) is twice the distance F between the neighboring high and low male dowels  51 (A) and  51 (B). Accordingly, the chance of engagement can be increased to thereby obtain quick abutment of each high male dowel  51 (A) against the corresponding side wall surface  81 . 
     After each high male dowel  51 (A) abuts against the corresponding side wall surface  81 , each low male dowel  51 (B) also comes into engagement with the opposing female dowel  51   a , so that a proper play can be ensured. Furthermore, the side surface  73  of each male dowel  51  abutting against the corresponding side wall surface  81  is a curved surface, and the contact point P between the curved side surface  73  and the flat side wall surface  81  is set at a position radially outside of the center O by the distance d, so that it is possible to avoid the contact between a weak portion of the male dowel  51  near the radially inner end  70  and the side wall surface  81 . Additionally, since the side surface  73  is a curved surface, the contact of the side surface  73  with the side wall surface  81  becomes smooth. Accordingly, the position of the contact point P defines the radius of a pitch circle. 
     Since the contact point P is offset radially outward from the center O, torque transmission can be performed by a thick-walled portion of each high male dowel  51 (A), thereby improving the durability. Further, the contact point P can be arbitrarily set by setting the pitch circle. Moreover, since the surface of each low male dowel  51 (B) is not machined, but it is a forged surface, the machining of the male dowels  51  can be reduced by half, thus effecting cost reduction. 
     On the other hand, when reverse torque is applied to the counter shaft  2 , each low male dowel  51 (B) comes into contact with the nearest side wall surface  81 . Therefore, as compared with the case that only the high male dowels  51 (A) are provided, the timing of contact of the male dowels  51  and the female dowels  51   a  can be made quicker, thereby reducing noise upon gear shifting. According to the preferred embodiment, the chance of engagement of the male and female dowels can be increased with a reduced play, and the degree of freedom of design can be increased. As a result, both manual and automatic gear changing can be carried out by the same gear. 
     The present invention is not limited to the above preferred embodiment, but various modifications may be made. For example, the surface of each high male dowel may also be left as a forged surface. In this case, the production cost can be further reduced. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.