Abstract:
A semi-automatic transmission comprises an input shaft; a countershaft; constantly meshed gears arranged between the input shaft and the countershaft; and an electrically controlled dog clutch operatively disposed on one the input shaft and the countershaft. The dog clutch is of a type which is free of a synchronizing mechanism and fastens one of the meshed gears to a corresponding one of the input shaft and the countershaft when operated, thereby to establish a given torque transmission path from the input shaft to the countershaft. An electromagnetic multiple disc clutch is employed, which has an input part adapted to be driven by an engine and an output part connected to the input shaft. A control unit is further employed, which controls the dog clutch in accordance with information signals applied thereto.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to transmissions of wheeled motor vehicles, and more particularly to the transmissions of a semi-automatic type which provides the transmission with a synchronized gear change operation by the work of an electromagnetic multiple disc clutch. 
     2. Description of Related Art 
     In order to clarify the task of the present invention, transmissions shown in Japanese Patent Second Provisional Publication (Tokkou-sho) 54-28894 and Japanese Patent First Provisional Publication (Tokkai-hei) 5-99333 will be briefly described in the following. 
     In the former publication, there is shown a so-called constantly gear meshed transmission equipped with a synchronizing system which is provided at one end of a countershaft of the transmission. A first gear tightly disposed on an output shaft is constantly meshed with a second gear which is rotatably disposed on the countershaft. The synchronizing system is constructed to decelerate or accelerate the second gear to establish rotation synchronization of the two shafts before a selected dog clutch effects engagement of a corresponding speed gear with the output shaft. 
     In the latter publication, there is shown another constantly gear meshed transmission equipped with a synchronizing system which is constructed to establish a rotation synchronization of the countershaft and output shaft (viz., main shaft) only in the lowest and highest gear changes. More specifically, only the dog clutches for the lowest and highest operation gear speeds are equipped with a synchronizing mechanism. 
     SUMMARY OF THE INVENTION 
     However, due to the following reasons, the transmissions of the above-mentioned publications have failed to provide users or car makers with a satisfaction. 
     That is, in the transmission of former publication, the synchronizing system provided at one end of the countershaft induces enlargement of the transmission in an axial direction and increase in weight of the transmission, which inevitably bring about a difficulty with which the transmission is mounted onto a given limited space of the vehicle body. 
     While, in the transmission of latter publication, when a gear change other than the lowest and highest gear changes is needed, the synchronizing mechanism of either one of the dog clutches for the lowest and highest operational gear speeds has to be indirectly used, which however increases the time needed for establishing the rotation synchronization between the countershaft and the output shaft. If the needed time for the synchronization is excessively large, an engine roar tends to occur. 
     In general, the manual transmissions of the above-mentioned constantly gear meshed type are equipped with a dry type clutch for connecting the transmission input shaft with an output member of the engine, and the dry type clutch is actuated by a hydraulic actuator. Thus, operation of the clutch (viz., engagement/disengagement operation) is inevitably affected by a temperature of hydraulic oil in the actuator and by a surge pressure appearing upon engagement of the clutch, which tends to make a responsive operation of the clutch difficult. 
     Accordingly, it is an object of the present invention to provide a semi-automatic transmission which is free of the above-mentioned shortcomings. 
     According to a first aspect of the present invention, there is provided a semi-automatic transmission which comprises an input shaft; a countershaft; constantly meshed gears arranged between the input shaft and the countershaft; an electrically controlled dog clutch operatively disposed on one the input shaft and the countershaft, the dog clutch being of a type free of a synchronizing mechanism and fastening one of the meshed gears to a corresponding one of the input shaft and the countershaft when operated, thereby to establish a given torque transmission path from the input shaft to the countershaft; a control unit which controls the dog clutch in accordance with information signals applied thereto; and an electromagnetic multiple disc clutch having an input part adapted to be driven by an engine and an output part connected to the input shaft. 
     According to a second aspect of the present invention, there is provided a semi-automatic transmission which comprises an input shaft having a first group of gears rotatably disposed thereon; a countershaft having a second group of gears tightly disposed thereon, said second group of gears being constantly and respectively engaged with said first group of gears; an electrically controlled dog clutch operatively disposed on said input shaft, said dog clutch functioning to fasten one of said first group of gears to said input shaft when operated; an electromagnetic multiple disc clutch having an input part adapted to be driven by an engine and an output part connected to said input shaft; and a control unit which controls both said dog clutch and said electromagnetic multiple disc clutch upon receiving a gear change instruction. 
     According to a third aspect of the present invention, there is provided a semi-automatic transmission which comprises an input shaft having a first group of gears rotatably disposed thereon and a third group of gears tightly disposed thereon; a countershaft having a second group of gears tightly disposed thereon and a fourth group of gears rotatably disposed thereon, the second and fourth groups of gears being constantly and respectively engaged with the first and third groups of gears; an electrically controlled first dog clutch operatively disposed on the input shaft, the first dog clutch functioning to fasten one of the first group of gears to the input shaft when operated; an electrically controlled second dog clutch operatively disposed on the countershaft, the second dog clutch functioning to fasten one of the third group of gears to the countershaft when operated; a wet type electromagnetic multiple disc clutch having an input part adapted to be driven by an engine and an output part connected to the input shaft; and a control unit which controls the first and second dog clutches and the electromagnetic multiple disc clutch upon receiving a gear change instruction, wherein the wet type electromagnetic multiple disc clutch comprises an input drum adapted to be driven by the engine; an input clutch hub connected to the input shaft to rotate therewith; an annular main clutch selectively assuming an engine condition wherein the input drum and the input clutch hub are engaged and a disengaged condition wherein the input drum and the input clutch hub are disengaged; an annular pilot clutch which selectively assumes an engaged condition and a disengaged condition; and an annular cam mechanism which forces the annular main clutch to assume the engaged condition when the annular pilot clutch assumes the engaged condition, wherein the annular pilot clutch and the annular cam mechanism are concentrically received within the annular main clutch with respect to an axis of the input shaft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic view of a semi-automatic transmission according to the present invention; 
     FIG. 2 is a partial sectional view of an electromagnetic multiple disc clutch employed in the present invention; 
     FIG. 3 is a flowchart showing operation steps executed by a control unit for controlling the transmission of the invention; and 
     FIGS. 4A and 4B are time charts showing operation manner of various elements with respect to elapsed time. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, the present invention will be described in detail with reference to the accompanying drawings. For ease of understanding, various directional terms, such as, right, left, upper, lower, rightward and the like are used in the description. However, such directional terms are to be understood with respect to only a drawing or drawings on which the corresponding part or portion is illustrated. 
     Referring to FIGS. 1 and 2, particularly FIG. 1, there is schematically shown a semi-automatic transmission according the present invention. 
     Denoted by numeral  1  is an internal combustion engine which drives or powers the transmission. Denoted by numeral  2  is a throttle actuator which actuates a throttle valve (not shown) of the engine  1 . 
     Denoted by numeral  20  is a multi-gear transmission proper which is housed in a transmission case  21 . Denoted by numeral  23  is a clutch housing which is mounted to a front open end of the transmission case  21 . 
     Within the clutch housing  23 , there is defined a first container chamber  23   b  in which a torsional damper  80  is installed. As will become apparent as the description proceeds, for defining the first container chamber  23   b,  a front cover  28  (see FIG. 2) is bolted to the clutch housing  23 . The interior of the first container chamber  23   b  is communicated with the open air. 
     Referring back to FIG. 1, within the transmission case  21 , there is defined a second container chamber  21   a.  For defining the second container chamber  21   a,  a part of the clutch housing  23  and a part of the front cover  28  are secured to the transmission case  21 . 
     Denoted by numeral  5  is an input shaft which extends longitudinally in the transmission case  21 . As shown, the input shaft  5  has a front end portion projected into the clutch housing  23 . 
     The front end portion of the input shaft  5  is incorporated with a wet type electromagnetic multiple disc clutch EMDC which is installed in the clutch housing  23 . 
     As will be described in detail hereinafter, the electromagnetic multiple disc clutch EMDC comprises generally an annular main clutch  30 , an annular pilot clutch  40  installed within the annular main clutch  30  and an annular cam mechanism  50  installed within the annular pilot clutch  40 . 
     As shown, to the input shaft  5 , there are mounted a reverse drive gear  216 , first to fifth drive gears  201 ,  202 ,  203 ,  204  and  205 , a third/fourth gear switching dog clutch dc 2  and a fifth gear switching dog clutch dc 3 . As shown, the reverse drive gear  216  and first and second drive gears  201  and  202  are secured to the input shaft  5  to rotate together therewith like a single unit, while the third, fourth and fifth drive gears  203 ,  204  and  205  are rotatably disposed about the input shaft  5 . The reverse drive gear  216  is engageable with a reverse counter gear  216   a  which can be axially shifted. 
     A countershaft  200  extends in parallel with the input shaft  5  in the transmission case  21 . To the countershaft  200 , there are mounted first to fifth driven gears  211 ,  212 ,  213 ,  214  and  215 , a first/second gear switching dog clutch dc 1 , a reverse driven gear  216   b  meshed with the reverse counter gear  216   a,  and a speed reduction gear  217 . As shown, the first and second driven gears  211  and  212  are rotatably disposed about the countershaft  200 , while the third, fourth and fifth driven gears  213 ,  214  and  215 , the reverse driven gear  216   b  and the speed reduction gear  217  are secured to the countershaft  200  to rotate together therewith like a single unit. 
     The first/second gear switching dog clutch dc 1 , the third/fourth gear switching dog clutch dc 2  and the fifth gear switching dog clutch dc 3  are moved by a shift drum  3   a  driven by a pulse motor  3  which is controlled by a control unit  150 . Although not shown in the drawings, into the control unit  150 , there are inputted various known information signals for controlling the dog clutches dc 1 , dc 2  and dc 3  in a well-timed manner. 
     It is to be noted that the multi-gear transmission proper  20  has not conventional synchro-mesh units, and thus the transmission proper  20  can be reduced in axial length. 
     Referring to FIG. 2, there is shown the detail of the electromagnetic multiple disc clutch EMDC and its surrounding parts. 
     As shown, the electromagnetic multiple disc clutch EMDC is arranged to receive an engine power from the torsion damper  80  through an input hub  70 . 
     The annular main clutch  30  of the disc clutch EMDC comprises a first input drum  31  which is a cylindrical bottomed member. The first input drum  31  has a projected center portion  13   b  to which the input hub  70  is connected by means of a spline connection. The first input drum  31  has further a smaller diameter cylindrical portion  13   d  which is slidably held by an oil seal member  28   a  and a cylindrical bearing supporting portion  13   e  which receives a bearing  28   b  in cooperation with the front cover  28 . 
     The annular main clutch  30  further comprises a second input drum  32  which has a double wall construction. An outer cylindrical wall of the second input drum  32  is engaged with the first input drum  31  by means of a spline connection  37 , so that the second and first input drums  32  and  31  rotate together about a common axis like a single unit. Due to the spline connection  37 , the second input drum  32  can axially move relative to the first input drum  31 . 
     Within an outer annular groove of the second input drum  32 , there are installed drive plates  33  and a retainer  36  which are axially movable in the annular groove. For this axial movement, a spline connection is employed between the drive plates  33  and an inner surface of the annular groove. A snap ring  36   a  is fixed to an open end of the annular groove to keep the drive plates  33  and retainer  36  in place. 
     Within an inner circular recess of the second input drum  32 , there is installed the annular pilot clutch  40  which comprises a rotor  41 , outer plates  44  and an armature  43 . As shown, the annular main clutch  30  and the annular pilot clutch  40  are concentrically arranged about a common axis of the input shaft  5 . 
     The rotor  41 , the outer plates  44  and the armature  43  are connected to the inner surface of the inner circular recess through a spline connection, so that these parts  41 ,  44  and  43  can rotate together with the second input drum  32  about the common axis like a single unit. A snap ring  43   a  is fixed to a right end of the inner surface of the inner circular recess to keep the parts  41 ,  44  and  43  in place. 
     The rotor  41  is formed with an annular groove for spacedly receiving therein an annular electromagnet  42  which is held on a bearing  46 . ON/OFF operation of the electromagnet  42  is controlled by the control unit  150 . 
     A plurality of holding brackets  42   a  (only one is shown) are secured to the annular electromagnet  42 , which have respective leading ends engaged with grooves  102   c  formed in an annular connecting block  102  tightly disposed on a fixed or immovable sleeve member  27  in which a right end of the input shaft  5  is rotatably received. Thus, the annular electromagnet  42  can keep its immovable state even when the rotor  41  rotates. The sleeve member  27  is integrally connected to the clutch housing  23 . 
     Into a front portion of the inner circular recess of the second input drum  32 , there extends an input clutch hub  60 . The input clutch hub  60  has a cylindrical wall to which driven plates  34 , which are alternately mated with the above-mentioned drive plates  33  of the annular main clutch  30 , are connected through a spline connection. Each driven plate  34  has both surfaces lined with a frictional material. The cylindrical wall of the input clutch hub  60  has an annular press portion  60   a  which abuts on a floating plate  35  located just behind the rearmost one of the driven plates  34 . Thus, when the input clutch hub  60  is shifted rightward in the drawing, the annular press portion  60   a  can press the floating plate  35  and the drive and driven plates  33  and  34 . If the rearmost one of the driven plates  34  has a left surface which is free of the frictional material, the floating plate  35  may be removed. 
     The annular cam mechanism  50  comprises a first annular cam member  51 , a second annular cam member  52  and cam balls  53 . The first annular cam member  51  has a radially outwardly raised wall welded to a radially inwardly projected wall of the input clutch hub  60 . The first annular cam member  51  has a splined bore through which a splined end portion  5   e  of the input shaft  5  is operatively received. As shown, a thrust needle bearing C is operatively disposed between the first annular cam member  51  and a radially inside portion  31   a  of the first input drum  31 . Accordingly, rightward axial movement of the first annular cam member  51  is restrained by a combination including the thrust needle bearing C, the radially inside portion  31   a,  the bearing  28   b  and the front cover  28 . 
     The second annular cam member  52  has inner plates  45  of the annular pilot clutch  40  which are axially movably mounted on an outer cylindrical surface through a spline connection. The inner plates  45  are alternatively engaged with outer plates  44  and the armature  43  which are axially movably disposed on an inner cylindrical surface of the second input drum  32  through a spline connection. A thrust needle bearing A is disposed between the outer cylindrical portion of the second annular cam member  52  and a radially inner portion of the rotor  41 , so that the second annular cam member  52  and the rotor  41  can axially move together. A needle bearing B is operatively disposed between the sleeve member  27  integrally connected to the clutch housing  23  and an inner cylindrical surface of the rotor  41 . 
     With the above-mentioned construction of the electromagnetic multiple disc clutch EMDC, there can be constituted a power transmission path which comprises generally a driven plate  81  of the torsion damper  80 , the input hub  70 , the first input drum  31 , the second input drum  32 , the annular pilot clutch  40 , the input clutch hub  60  and the input shaft  5 . 
     As will become apparent hereinafter, when the annular pilot clutch  40  assumes ON condition (viz., engaged condition), the power transmission path is established thus the engine power is transmitted to the input shaft  5 . While, when the annular pilot clutch  40  assumes OFF condition (viz., disengaged condition), the power transmission path is not established and thus the engine power is not transmitted to the input shaft  5 . More specifically, when the annular pilot clutch  40  assumes ON, the annular main clutch  30  becomes engaged, so that the engine power is transmitted to the output shaft  5  through the annular main clutch  30 . 
     As is seen from FIG. 1, the torsion damper  80 , more specifically, a drive plate  82  of the torsion damper  80  is driven by the engine  1 . Between the drive and driven plates  82  and  81 , there is produced a damping action during rotation of the torsion damper  80 . 
     The power transmission manner in the disc clutch EMDC will be clarified from the following description. 
     That is, when the annular pilot clutch  40  assumes its ON condition, the engine power transmitted through the above-mentioned power transmission path is applied to the annular cam mechanism  50 . Upon this, the cam followers  53  are forced to run in and along respective cam grooves formed on the first and second annular cam members  51  and  52 , so that the second annular cam member  52  is shifted leftward in FIG. 2 against a biasing force of a return spring  16  incorporated with the second annular cam member  52 . With this leftward shifting of the second annular cam member  52 , the second input drum  32 , the rotor  41  and the electromagnet  42  are shifted leftward. The leftward shifting of the second input drum  32  brings about the engaged condition of the annular main clutch  30 . Thus, under this condition, the engine power on the first input drum  31  is transmitted to the input shaft  5  through the annular main clutch  30  and the input clutch hub  60 . As shown, the return spring  16  is held on the first annular cam member  51  by a snap ring  15 . 
     In the following, a lubrication system of the electromagnetic multiple disc clutch EMDC will be described with reference to FIG.  2 . 
     As shown, the sleeve member  27  integrally connected to the clutch housing ( 23 , see FIG. 1) is formed with an oil opening  27   a.  On the sleeve member  27 , there is tightly disposed the connecting block  102  which is formed with both a radially extending oil passage  102   b  mated with the oil opening  27   a  of the sleeve member  27  and an axially extending oil passage  102   a  exposed to a left end portion of the radially inside portion of the rotor  41 . The connecting block  102  is positioned above a right end of a receiving chamber in which the countershaft  200  is installed. 
     The oil passage  102   b  of the connecting block  102  is connected to an oil pump  100  through an oil pipe  101 . The oil pump  100  is driven by an electric motor  100   a.  The input shaft  5  is formed at its right end portion with an axially extending oil passage  5 ′ which is connected with the oil opening  27   a  of the sleeve member  27  through an oil inlet opening  5   c  formed in the input shaft  5 . The oil passage  5 ′ has two oil outlet openings, one being a radially extending opening  5   a  exposed to an inner area of the second annular cam member  52  and the other being a right open end  5   b  exposed to a recess defined behind the shaft portion  13   b  of the first input drum  31 . The right open end  5   b  is equipped with an orifice  4  for controlling the amount of oil flowing therethrough. 
     The shaft portion  13   b  of the first input drum  31  is formed with an oil passage  31   b  through which lubrication oil flows from the recess of the shaft portion  13   b  to a clearance defined between the front cover  28  and the first input drum  31 . Thus, when the oil pump  100  is driven by the motor  100   a,  lubrication oil is forced to flow in the oil passages in such a manner as is indicated by the arrows. With this oil flow, various elements of the electromagnetic multiple disc clutch EMDC are lubricated by the oil as is seen from the drawing. 
     Although not shown in the drawing, the first and second input drums  31  and  32  are formed with a plurality of openings through which the lubrication oil is led to a clearance defined between the clutch housing  23  (see FIG. 1) and the front cover  28 . As is seen from FIG. 1, the lubrication oil contained in the clearance is then led to an oil pan of the transmission case  21  through a drain opening  23   a  formed in a partition wall of the clutch housing  23 . 
     In the following, operation of the semi-automatic transmission of the present invention will be described with reference to operation steps shown by the flowchart of FIG.  3  and time charts shown by FIGS. 4A and 4B. The operation steps are programmed in the control unit  150 . 
     At step S 300  of the flowchart of FIG. 3, the gear ratio “i0” assumed by the transmission proper  20  and the existing engine speed “Ne0” are read. Then, at step S 301 , judgement is carried out as to whether the transmission proper  20  is in the condition of D-range or not. If NO, the operation flow goes back to step S 300 . While, if YES, that is, when the transmission is in the condition of D-range, the operation flow goes to step S 302 . At this step, judgement is carried out as to whether an instruction of gear change from the operational gear speed of gear ratio “i0” to a new operational gear speed of gear ratio “i1” has been issued or not. If NO, the operation flow goes to END. While, if YES, that is, when such instruction has been issued, the operation flow goes to step  303 . At this step, the annular pilot clutch  40  is turned OFF to make the disc clutch EMDC OFF (viz., disengaged), and then the engine speed “Ne0” is increased or decreased to a target engine speed “Net” (=Ne0×(i0/i1)) by operating the throttle actuator  2  and the transmission proper  20  is forced to assume a neutral condition by shifting corresponding one of the dog clutch dc 1 , dc 2  or dc 3  to its neutral position. As has been mentioned hereinabove, shifting of such dog clutches dc 1 , dc 2  and dc 3  is carried out by the pulse motor  3  which is controlled by the control unit  150 . Then, at step S 304 , judgement is carried out as to whether an absolute value of a difference between an actual engine speed “Ne” actually provided after the engine speed control and the target engine speed “Net” is smaller than an allowable value “ε” or not. If NO, the operation flow goes back to step S 303 . While, if YES, that is, when the absolute value is smaller than the allowable value “ε”, the operation flow goes to step S 305 . At this step, by operating the pulse motor  3 , a corresponding dog clutch dc 1 . dc 2  or dc 3  is shifted to cause the transmission proper  20  to take the operational gear speed of gear ratio “i1”. Then, the operation flow goes to step S 306 . At this step, a slip control is carried out by the disc clutch EMDC, so that, based on the difference between the engine speed “Ne” appearing after the engine speed control and an existing rotation speed “No” of the input shaft  5  of the transmission proper  20 , the input shaft  5  is controlled to have a rotation speed equal to that of the engine  1 . The slip control is carried out by alternately turning the annular pilot clutch  40  ON and OFF. Then, the operation flow goes to step S 307 . At this step, judgement is carried out as to whether the speed “Ne” of the engine  1  and the speed “No” of the input shaft  5  are the same or not. If NO, the operation flow goes back to step S 306 . While, if YES, that is, when both the rotation speeds “Ne” and “No” are the same, the operation flow goes to step S 308 . At this step, the annular pilot clutch  40  is turned ON to cause the disc clutch EMDC to assume its engaged or ON condition. In the above-mentioned manner, an up-shift gear change and a down-shift gear change are smoothly made. 
     As is seen from FIGS. 4A and 4B, the time needed for carrying out the steps from S 300  to S 308  is about 0.3 to 0.4 seconds. For achieving up-shift gear change, the engine speed “Nt” is decreased, while for achieving down-shift gear change, the engine speed “Nt” is increased. The disc clutch EMDC is controlled to start its slip-control in response to engagement of the dog clutch with a desired gear (higher gear in up-shift or lower gear in down-shift). 
     As will be understood from the above description, in the semi-automatic transmission of the present invention, the gear change is carried out very quickly due to the effective operation of the electromagnetic multiple disc clutch EMDC. In fact, the response speed of this clutch EMDC to ON/OFF instruction (viz., engine/disengage instruction) is quite higher than that of a conventional hydraulic disc clutch. Furthermore, because the disc clutch EMDC is not actuated by a hydraulic force, the disc clutch EMDC has no disadvantages which the conventional hydraulic disc clutches usually have. 
     In the semi-automatic transmission of the present invention, the electromagnetic multiple disc clutch EMDC is used as a start clutch. Accordingly, there is no need of using a hydraulic pump for actuating the disc clutch. The disc clutch EMDC can transmit an engine power with a time-lag of first order upon receiving an instruction. That is, the disc clutch EMDC is protected from a serge torque that is inevitably produced in a hydraulic disc clutch. 
     As has been described hereinabove, when the annular pilot clutch  40  becomes ON, the engaged condition of the annular main clutch  30  is induced due to function of the annular cam mechanism  50 . Once the annular main clutch  30  is turned to assume the engaged condition, the engine power is assuredly and smoothly transmitted to the input shaft  5  of the transmission proper  20 . 
     Since, as is seen from FIG. 2, the annular main clutch  30  is concentrically disposed around the annular pilot clutch  40 , the axial length of the electromagnetic multiple disc clutch EMDC can be reduced. 
     Because of provision of the oil passages  102   a,    102   b    5 ′,  5   a,    5   b  and  5   c,  various elements of the disc clutch EMDC are effectively lubricated by a lubrication oil that flows in the oil passages. Under rotation of the input shaft  5  (see FIG.  2 ), the lubrication oil in the passage  5 ′ is thrown radially outward through the opening  5   a  due to a centrifugal force applied to the oil from the input shaft  5 . This brings about an assured oil lubrication of the elements of the disc clutch EMDC. 
     The annular connecting block  102  mounted on the immovable sleeve member  27  serves as a holding means for holding the annular electromagnet  42  by the function of the holding brackets  42   a  engaged with the grooves  102   c  of the block  102  as well as a lubrication oil passage defining means which defines therein the oil passages  102   a  and  102   b.  As shown in FIG. 2, the annular connecting block  102 , the input shaft  5  and the countershaft  200  are overlapped in a radial direction, and thus, the reduction in axial length of the disc clutch EMDC is much assured. 
     As is seen from FIG. 1, due to provision of the drain opening  23   a  in the wall of the clutch housing  23 , the elements of the disc clutch EMDC and the elements of the transmission proper  20  can be lubricated by the same lubrication oil. 
     The entire contents of Japanese Patent Application 2001-224901 filed Jul. 25, 2001 are incorporated herein by reference. 
     Although the invention has been described above with reference to the embodiment of the invention, the invention is not limited to such embodiment as described above. Various modifications and variations of such embodiment may be carried out by those skilled in the art, in light of the above description.