Abstract:
Many vehicles designed for running on rough terrain include an automatic transmission for performing speed change by connecting or disconnecting an oil hydraulic clutch provided alongside each speed change gear. However, depending on the posture of the driver, it may be impossible to step on the brake. As a result, it may be impossible to achieve creep prevention with a creep preventive mechanism dependent on a brake signal. Therefore, a creep preventive mechanism which operates independently from a brake signal is required. The present invention provides an oil pressure control system operating independently from a brake signal. The system includes an oil pressure supply source with a linear solenoid valve for supplying working oil or interrupting the supply of the working oil to the oil hydraulic clutches. The supply of the working oil to the clutches is interrupted when the vehicle is stopped under an idling condition of an engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2001-336510 filed on Nov. 1, 2001, the entire contents of which are hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an oil pressure control system for automatic transmission of a saddle ride type four-wheel vehicle (buggy car) for running on rough terrain.  
           [0004]    2. Description of Background Art  
           [0005]    An automatic transmission of a general four-wheel vehicle, in most cases, does not have a mechanical neutrality maintaining mechanism, and interruption of transmission of power is effected by releasing the connection of a clutch. In the case where an oil hydraulic clutch is used, a primary oil pressure is disconnected forcibly by a manual valve. As for creep preventive control, for improvement of power consumption, a system is used in which the oil pressure of the clutch is interrupted based on a brake signal when a brake is applied, only for the first gear position, so that creep is generated when the control mechanism is in trouble. Where it becomes impossible to start the vehicle under a creep preventive condition, running at another position is possible.  
           [0006]    In the case of a vehicle expected to be used on rough terrain, depending on the posture of the driver, it may be impossible to step on the brake, and thus it may be impossible to prevent creep with a creep preventive mechanism dependent on a brake signal. Therefore, a creep preventive mechanism which acts independently of a brake signal is required.  
           [0007]    In addition, in the case where it becomes impossible to start the vehicle under a creep preventive condition, it is necessary to start from another position, so that a creep preventive mechanism only for the first clutch position is unsatisfactory.  
         SUMMARY AND OBJECTS OF THE INVENTION  
         [0008]    The present invention has solved the above-mentioned problems.  
           [0009]    A first aspect of the present invention is based on an oil pressure control system for automatic transmission for performing speed change by connection and disconnection of an oil hydraulic clutch provided alongside each speed change gear, wherein an oil pressure supply source for the oil hydraulic clutch is provided with a linear solenoid valve for supplying a working oil and interrupting the supply of the working oil, and the supply of the working oil to the clutch is interrupted when an internal combustion engine is in an idling condition and a vehicle is at stop.  
           [0010]    According to this first aspect of the present invention, prevention of creep can be effected under all clutch conditions, and a neutral condition can be obtained by interrupting the supply of the working oil under a stand-by condition for starting (an idling stopped condition).  
           [0011]    A second aspect of the present invention is based the oil pressure control system for automatic transmission, wherein the linear solenoid valve supplies the working oil when a power source is OFF, and interrupts the supply of the working oil when the power source is ON.  
           [0012]    According to this second aspect of the present invention, since the power source is OFF during running, the problem of having running being impossible due to failure of the linear solenoid valve during running can be prevented.  
           [0013]    A third aspect of the present invention is based on the oil pressure control system for automatic transmission, wherein the linear solenoid valve increases gradually the amount of the working oil supplied to the clutch, with increases in throttle opening and engine revolution frequency as parameters.  
           [0014]    According to this third aspect of the present invention, abrupt starting due to abrupt connection of the clutch can be prevented.  
           [0015]    A fourth aspect of the present invention is based on the oil pressure control system for automatic transmission, wherein the linear solenoid valve is provided with a communication passage for equalizing the pressure of the working oil supplied to the clutch and the oil pressure on the back side of the valve. The pressure of the working oil and the oil pressure on the back side of the valve are equalized when the working oil is supplied, and the pressure on the back side of the valve is released when the supply of the working oil is interrupted.  
           [0016]    According to this fourth aspect of the present invention, when the output oil pressure is increased due to the balance of the thrust force of the solenoid and the pressure on the back side of the valve, the pressure on the back side of the valve is also simultaneously increased to close the valve, whereby the oil pressure is disconnected. With the oil pressure disconnected, the pressure on the back side of the valve is lowered, so that the valve is opened. These actions are repeated, whereby a constant oil pressure is outputted.  
           [0017]    In addition to this, the thrust force of the solenoid is varied by a current value, whereby the output oil pressure can be varied linearly. By this, it is possible to gradually increase the pressure of the working oil that is supplied to the clutch.  
           [0018]    A fifth aspect of the present invention is based on the oil pressure control system for automatic transmission, wherein a manual type emergency valve capable of forming an oil passage bypassing the linear solenoid valve is provided on the side of a discharge port of the linear solenoid valve.  
           [0019]    According to this fifth aspect of the present invention, it becomes possible to start the vehicle even when a stop or failure has occurred under the condition where the linear solenoid valve is interrupting the supply of the working oil.  
           [0020]    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  
       [0021]    The present invention will become more fully understood from the 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:  
         [0022]    [0022]FIG. 1 is a side view of a four-wheel buggy car (saddle ride type vehicle for running on rough terrain) on which an internal combustion engine is mounted;  
         [0023]    [0023]FIG. 2 is a front view of a power unit according to the present invention;  
         [0024]    [0024]FIG. 3 is a view of a front crankcase cover from the front side;  
         [0025]    [0025]FIG. 4 is a view of a front crankcase from the front side;  
         [0026]    [0026]FIG. 5 is a vertical sectional view of the inside of a crankcase, showing the relationship between a crankshaft and a main shaft;  
         [0027]    [0027]FIG. 6 is a vertical sectional view of the inside of the crankcase, showing the relationship among the main shaft, a counter shaft, an intermediate shaft, and an output shaft;  
         [0028]    [0028]FIG. 7 is an enlarged sectional view for illustrating the constitution and actions of a first gear position oil hydraulic type multiple disk clutch and a first gear position driving gear;  
         [0029]    [0029]FIG. 8 is an oil pressure control system chart of a valve body;  
         [0030]    [0030]FIG. 9( a ) shows an enlarged sectional view of a linear solenoid valve when the power source is OFF, and FIG. 9 ( b ) shows an enlarged sectional view of a linear solenoid when the power source is ON;  
         [0031]    [0031]FIG. 10 is an operation table of the solenoids of the present invention;  
         [0032]    [0032]FIG. 11 is an oil pressure control system chart in the neutral condition;  
         [0033]    [0033]FIG. 12 is an oil pressure control system chart in the case where the power source is OFF in the first gear position condition and the backward running condition;  
         [0034]    [0034]FIG. 13 is an oil pressure control system chart in the case where the power source is ON in the first gear position condition and the backward running condition;  
         [0035]    [0035]FIG. 14 is an oil pressure control system chart in the second gear position condition;  
         [0036]    [0036]FIG. 15 is an oil pressure control system chart in the third gear position condition; and  
         [0037]    [0037]FIG. 16 is an oil pressure control system chart in the case where the second gear position operation is conducted by setting an emergency valve when the linear solenoid valve is failed in the working oil interruption condition. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    [0038]FIG. 1 is a side view of a four-wheel buggy car (saddle ride type vehicle for running on wasteland) on which an internal combustion engine with an oil hydraulic type automatic transmission according to the present invention is mounted. The buggy car has a structure in which left-right pairs of front wheels  2  and rear wheels  3  are provided respectively at front and rear portions of a vehicle body frame  1 , and a power unit  6  integrally constituting an internal combustion engine  4  and a transmission  5  is supported on a central portion of the vehicle body frame  1 .  
         [0039]    The power unit  6  is disposed with a crankshaft  7  directed in the front-rear direction of the vehicle body. The rotation of the crankshaft  7  is transmitted through a main shaft  8 , a counter shaft  9 , and an intermediate shaft  10  (all shown in FIG. 4) of the transmission to an output shaft  11 . These shafts are all parallel to the crankshaft, and are disposed in the front-rear direction of the vehicle body. The front wheels  2  are driven by a front wheel drive shaft  12  connected to the front end of the output shaft  11 , while the rear wheels  3  are driven by a rear wheel drive shaft  13  connected to the rear end of the output shaft  11 . A steering handle  14 , a fuel tank  15 , and a saddle type seat  16  are provided, in this order from the front side, at upper portions of the vehicle body.  
         [0040]    [0040]FIG. 2 is a front view of the power unit  6  according to the present invention, in which the front surface of the power unit  6  is viewed from the front side. A main body portion of the power unit  6  is generally composed of four portions, namely, a cylinder head cover  20 , a cylinder head  21 , a cylinder block  22 , and a crankcase  23 , in this order from the upper side. In addition, the crankcase  23  is divided in a plane orthogonal to the crankshaft  7  into four portions, which forming of a front crankcase cover  24 , a front crankcase  25 , a rear crankcase  26 , and a rear crankcase cover  27  (these are partially shown in FIGS. 5 and 6), in this order from the front side.  
         [0041]    In FIG. 2, the front crankcase cover  24  is seen, and the front crankcase  25  is a little seen in the surroundings thereof. Various equipments and piping are fitted to a front surface of the front crankcase cover  24 . FIG. 3 is a view of only the front crankcase cover  24  from the front side, and FIG. 4 is a view of the front crankcase  25  from the front side.  
         [0042]    [0042]FIG. 4 shows the positions of the crankshaft  7 , the main shaft  8 , the counter shaft  9 , the intermediate shaft  10 , and the output shaft  11  of the transmission. FIGS. 5 and 6 are vertical sectional views of the inside of the crankcase passing through main shafts in the crankcase, in which FIG. 5 shows the relationship between the crankshaft  7  and the main shaft  8 , and FIG. 6 shows the relationship among the main shaft  8 , the counter shaft  9 , the intermediate shaft  10 , and the output shaft  11 . In these figures, arrow F indicates the front or forward direction.  
         [0043]    [0043]FIG. 5 shows a power transmission mechanism between the crankshaft  7  and the main shaft  8 . The crankshaft  7  is supported on the front and rear crankcases  25  and  26  through bearings. A front extended portion of the crankshaft  7  is supported on the front crankcase cover  24  through a bearing. The crankshaft  7  is divided into front and rear portions, which are connected by a crank pin  7   b  at a crank web  7   a.  An AC generator  28  for generating power by rotation of the crankshaft  7  is fitted to the rear end of the crankshaft  7 .  
         [0044]    The crankshaft  7  is provided with a primary driving gear  31  through a torque converter  30 . The primary driving gear  31  is rotatably supported on the crankshaft  7  through a needle bearing  32 . The torque converter  30  includes a pump impeller  33  fixed to the crankshaft  7 , a turbine runner  34  opposed thereto, and a stator  35 . The primary driving gear  31  rotatable in relation to the crankshaft  7  is connected to the turbine runner  34 , and power from the crankshaft  7  is transmitted to the primary driving gear  31  through the working oil. A primary driven gear  36  constantly meshed with the primary driving gear  31  is fixed to a front end portion of the main shaft  8  of the transmission. The rotation of the crankshaft  7  is transmitted to the main shaft  8  through primary speed reduction performed by the primary driving gear  31  and the primary driven gear  36 .  
         [0045]    [0045]FIG. 6 shows a power transmission mechanism among the main shaft  8 , the counter shaft  9 , the intermediate shaft  10 , and the output shaft  11  of the transmission. The main shaft  8  of the transmission is supported on the front and rear crankcases  25  and  26  through bearings. The main shaft  8  is provided with a first gear position driving gear  40 , a second gear position driving gear  41 , and a third gear position driving gear  42 , which differ in the number of teeth according to speed reduction ratios. The second gear position driving gear  41  and the third gear position driving gear  42  are fixed gears, which are fixed on the main shaft  8 , while the first gear position driving gear  40  is rotatably supported on the main shaft  8  through a needle bearing  43 .  
         [0046]    In the description below, generally, a gear rotatably supported on a rotary shaft through a needle bearing is called a floating gear. A first gear position oil hydraulic type multiple disk clutch  44  is intermediately provided between the main shaft  8  and the first gear position driving gear  40 .  
         [0047]    [0047]FIG. 7 is an enlarged sectional view for illustrating the constitution and actions of the first gear position oil hydraulic type multiple disk clutch  44  and the first gear position driving gear  40 . The first gear position oil hydraulic type multiple disk clutch  44  is composed of a vessel form clutch outer  46  fixed to the main shaft  8  through an outer cylinder  45 , an annular stopper  47  fixed to an open end of the clutch outer  46 , outside clutch disks  48  held on an inner peripheral surface of the clutch outer through splines so as to be displaceable in the axial direction, a pressure plate  49  fitted in a piston form adjacently to a bottom wall portion  46   a  of the clutch outer  46 , and a coil spring  51  provided between a locking portion  50  provided on the clutch outer  46  and the pressure plate  49  for pressing the pressure plate  49  toward the bottom wall portion  46   a  of the clutch outer  46 . Also included are a clutch inner  52  fitted integrally to the first gear position driving gear  40 , inside clutch disks  53  held on an outer peripheral surface of the clutch inner  52  through splines so as to be displaceable in the axial direction and arranged alternately with the outside clutch disks  48 , a working oil supply hole  54  in the first oil hydraulic type multiple disk clutch  44  provided so as to connect the main shaft  8 , the outer cylinder  45 , and the clutch outer  46 , and a lubricating oil supply hole  55  for the needle bearing  43  for the first gear position driving gear  40 . The main shaft  8  is provided with a center hole with an inside diameter varied steppedly on the center line thereof, and a steel ball  56  is press fitted in the narrowest portion of the center hole, whereby the center hole is partitioned into a front portion center hole  57  and a rear portion center hole  58 .  
         [0048]    As shown in FIG. 6, the working oil for the clutch  44  is fed from the side of the front crankcase cover  24  to the front portion center hole  57  through a working oil supply pipe  59 , and is supplied to the clutch  44  through the working oil supply hole  54 .  
         [0049]    As shown in FIG. 7, the working oil comes between the bottom wall portion  46   a  of the clutch outer  46  and the pressure plate  49 , the oil pressure causes the pressure plate  49  to move against an energizing force of the coil spring  51 , the inside and outside clutch disks are pressed in the axial direction, and the outside clutch disks  48  restricts the movement of the inside clutch disks  53 . By this, the clutch inner  52  is integrated with the clutch outer  46 , the first gear position driving gear  40  comes not to float but is fixed to the main shaft  8 , and the rotation of the main shaft  8  is transmitted to the first gear position driving gear  40 . A lubricating oil for the needle bearing  43 , which bears the first gear position driving gear  40 , is supplied from the side of the rear portion center hole  58  through the lubricating oil supply hole  55 .  
         [0050]    The counter shaft  9  composes a front portion counter shaft  9   a  and a rear portion counter shaft  9   b , which are integrated with each other to form the counter shaft  9 . The counter shaft  9  is supported on the front crankcase  25 , the rear crankcase  26 , and the rear crankcase cover  27  through bearings. The front portion counter shaft  9   a  is provided with a first gear position driven gear  60 , a second gear position driven gear  61 , and a third gear position driven gear  62 , which are constantly meshed respectively with the first gear position driving gear  40 , the second gear position driving gear  41 , and the third gear position driving gear  42  on the main shaft  8 .  
         [0051]    The first gear position driven gear  60  is a fixed gear fixed to a shaft, while the second gear position driven gear  61  and the third gear position driven gear  62  are floating gears, which are supported rotatably in relation to the counter shaft  9  through needle bearings  63  and  64  respectively. A second gear position oil hydraulic type multiple disk clutch  65  and a third gear position oil hydraulic type multiple disk clutch  66  are intermediately provided between the counter shaft  9  and these floating gears, respectively. In these clutches, a clutch outer is fixed to the counter shaft  9 , and a clutch inner is connected to the floating gear. The constitution and actions of these clutches are the same as those of the above-mentioned first gear position oil hydraulic type multiple disk clutch  44 . Therefore, detailed description thereof is omitted.  
         [0052]    Also in these clutches, a working oil is supplied through working oil supply holes  67  and  68  formed in the counter shaft, whereby floating of the floating gears is stopped, to enable transmission of power, thereby performing speed reduction at the second gear position or the third gear position. Lubricating oil supply holes  69  and  70  leading to the needle bearings  63  and  64  for bearing the second gear position driven gear  61  and the third gear position driven gear  62  are also formed in the counter shaft  9 .  
         [0053]    The rear portion counter shaft  9   b  is provided with a forward rotation driving gear  71  and a reverse rotation driving gear  72 . These are both floating gears, of which the one engaged with a manual dog clutch  73  provided at an intermediate position therebetween is fixed to a shaft, whereby transmission of power is enabled. Lubricating oil supply holes  76  and  77  for supplying a lubricating oil to needle bearings  74  and  75  respectively bearing the forward rotation driving gear  71  and the reverse rotation driving gear  72  are formed in the rear portion counter shaft  9   b.    
         [0054]    The counter shaft  9  composed of the front portion counter shaft  9   a  and the rear portion counter shaft  9   b  integrated with each other is provided with a center hole, which is partitioned into a front portion center hole  79  and a rear portion center hole  80  by a steel ball  78  press fitted in a narrowest portion thereof. The supply of a working oil to the second and third gear position oil hydraulic type multiple disk clutches  65  and  66  is conducted from the side of the front crankcase cover  24  through a double-wall pipe  81 . The double-wall pipe  81  composed of an outer pipe  81   a  and an inner pipe  81   b.    
         [0055]    The working oil for the second gear position oil hydraulic type multiple disk clutch  65  is supplied through an oil passage between the outer pipe  81   a  and the inner pipe  81   b  and through the working oil supply hole  67 . The working oil for the third gear position oil hydraulic type multiple disk clutch  66  is supplied through an oil passage inside the inner pipe  81   b  and the working oil supply hole  68 .  
         [0056]    The working oil for the needle bearing  63  for the second gear position driven gear  61  is supplied from the side of the front crankcase  25  through an oil passage between the front portion counter shaft  9   a  and the outer pipe  81   a  and through the lubricating oil supply hole  69 . The working oil supplied to the needle bearings  64 ,  74 , and  75  for the third gear position driven gear  62 , the forward rotation driving gear  71 , and the reverse rotation driving gear  72  is supplied from the side of the rear crankcase cover  27  through the rear portion center hole  80  of the counter shaft  9  and the lubricating oil supply holes  70 ,  76 , and  77 .  
         [0057]    The intermediate shaft  10  is supported on the rear crankcase  26  and the rear crankcase cover  27 . A first intermediate gear  82  constantly meshed with the reverse rotation driving gear  72  and a second intermediate gear  83  connected to the first intermediate gear  82  through a long sleeve portion  83   a  are rotatably held on the intermediate shaft  10 . A lubricating oil for sliding portions of the first intermediate gear  82  and the second intermediate gear  83  for sliding in relation to the intermediate shaft  10  is supplied from the rear crankcase  26  through a center hole of the intermediate shaft and a lubricating oil supply hole  84 .  
         [0058]    The output shaft  11  is supported on the front crankcase cover  24 , the rear crankcase  26 , and the rear crankcase cover  27  through bearings. The output shaft  11  pierces through the front crankcase  25  without making contact with the front crankcase  25 .  
         [0059]    An output shaft driven gear  85  constantly meshed with the forward rotation driving gear  71  and the second intermediate gear  83  is fixed onto the output shaft  11 . The output shaft driven gear  85  is driven for forward rotation or driven for reverse rotation through the gear with that the dog clutch  73  is engaged, whereby the output shaft  11  is rotated in a direction suitable for forward running or rearward running of the vehicle. Such a control is made that the reverse rotation driving is connected only when the counter shaft  9  is rotating at the first gear position.  
         [0060]    The gears in the above-mentioned transmission are all constantly meshed type gears, the speed change ratios of which is determined by which of the oil hydraulic type multiple disk clutches  44 ,  65 , and  66  is put into the connected condition. The oil pressure control for this is performed by a valve body  90  assembled as an integral oil pressure control system by collecting the solenoid valve and oil pressure changeover valves, and the position of the valve body  90  is as shown in FIG. 2; namely, the valve body  90  is fitted to a front surface of the front crankcase cover  24 .  
         [0061]    The valve body  90  is fitted to a fitting recessed portion  91  of the front crankcase cover  24  shown in FIG. 3, and is fixed to a fitting surface  92  in the surroundings of the fitting recessed portion  91  through a gasket. In the fitted condition, a front half of the valve body  90  is exposed to the exterior of the front crankcase cover  24 , and a rear half of the valve body  90  is embedded in the fitting recessed portion  91  of the front crankcase cover  24 . The fitting surface  92  of the front crankcase cover  24  is formed to be parallel with parting faces of the crankcase.  
         [0062]    Transfer of the working oil between the front crankcase cover  24  and the valve body  90  is performed between a plurality of working oil inlet/outlet ports provided in the fitting surface  92  and a plurality of working oil inlet/outlet ports provided in the fitting surface on the side of the valve body  90  at positions opposed to the former working oil inlet/outlet ports. The gasket intermediately disposed between the fitting surfaces of the valve body  90  and the front crankcase cover  24  is provided with oil passage holes at positions corresponding to the plurality of working oil inlet/outlet ports.  
         [0063]    [0063]FIG. 3 shows the working oil inlet/outlet ports provided in the valve body fitting surface  92  of the front crankcase cover  24 . These are a working oil supply port  93  leading from the front crankcase cover  24  toward the valve body  90 , a working oil inlet  94  leading from the valve body  90  toward the first gear position oil hydraulic type multiple disk clutch, a working oil inlet  95  leading from the valve body  90  toward the second gear position oil hydraulic type multiple disk clutch, and a working oil inlet  96  leading from the valve body  90  toward the third gear position oil hydraulic type multiple disk clutch.  
         [0064]    [0064]FIG. 3 also shows an extension position of a center line  100  of a pump shaft of an oil pump, which is not shown. The oil pump is provided between the front crankcase cover  24  and the front crankcase  25 . An oil fed from the oil pump is fed to an oil filter  103  through oil passages  101  and  102  formed in the inside of a wall surface of the front crankcase cover  24 . After foreign matter is filtered away from the oil by the filter, the oil is fed out through an oil passage  104 , and a portion of the oil is fed through the working oil supply port  93  into the valve body  90 . The other portion of the oil is fed out through an oil passage  105  and an oil passage  106  extending orthogonally to the oil passage  105  toward the front crankcase  25 , as a working oil for the torque converter or as a lubricating oil for bearing portions.  
         [0065]    [0065]FIG. 3 also shows an extension position of the center line  8   a  of the main shaft  8  of the transmission. The working oil fed out to the working oil inlet  94  leading from the valve body  90  toward the first gear position oil hydraulic type multiple disk clutch  44  is fed to the front portion center hole  57  of the main shaft  8  through the working oil supply pipe  59  provided bridgingly from a rear surface of the front crankcase cover  24  to the front portion center hole  57 , as shown in FIG. 6, and is supplied to the first gear position oil hydraulic type multiple disk clutch  44 .  
         [0066]    In addition, FIG. 3 shows an extension position of the center line  9   a  of the counter shaft  9 . This portion of the front crankcase cover  24  is provided with an oil passage connection portion  107 , and oil passages  108  and  109  are formed that lead respectively from the working oil inlet  95  leading toward the second gear position oil hydraulic type multiple disk clutch  65  and the working oil inlet  96  leading toward the third gear position oil hydraulic type multiple disk clutch  66  to the oil passage connection portion  107 . The working oil fed from the valve body  90  to the working oil inlet  95  or the working oil inlet  96  is fed from the oil passage connection portion  107  to the front portion center hole  79  of the counter shaft  9  through either of the inner and outer passages of the working oil supply double-wall pipe  81  provided bridgingly to the front portion center hole  79 , as shown in FIG. 6, and is supplied to the second gear position oil hydraulic type multiple disk clutch  65  or the third gear position oil hydraulic type multiple disk clutch  66 .  
         [0067]    [0067]FIG. 8 is a system chart of the above-mentioned oil hydraulic type multiple disk clutches and an oil pressure control system for controlling them. The portion surrounded by the dot-dash line in the figure is the portion provided in the above-mentioned valve body  90 . The working oil is supplied from the working oil supply port  93  leading to the valve body  90  shown in FIG. 3 to a working oil inlet  110  shown in this figure. Arrow P indicates the flow direction of the working oil.  
         [0068]    The working oil is controlled in the valve body  90 , and the destination of supply is determined. The working oil is supplied to the first gear position oil hydraulic type multiple disk clutch  44  through a working oil outlet  111  and the working oil inlet  94  leading to the first gear position oil hydraulic type multiple disk clutch. The working oil is supplied to the second gear position oil hydraulic type multiple disk clutch  65  through a working oil outlet  112  and the working oil inlet  95  leading to the second gear position oil hydraulic type multiple disk clutch. In a similar manner, working oil is supplied to the third gear position oil hydraulic type multiple disk clutch  66  through a working oil outlet  113  and the working oil inlet  96  leading to the third gear position oil hydraulic type multiple disk clutch. The positions of the working oil inlets  94 ,  95 , and  96  are shown in FIG. 3.  
         [0069]    In FIG. 8, a linear solenoid valve  114  is a valve for controlling the supply and the interruption of the supply of the working oil in the direction of an emergency valve  115 , and has the function of gradually increasing the amount of the working oil supplied, in an intermediate process ranging from an interruption condition to a full supply condition. The solenoid valve  114  supplies the working oil when a power source is OFF, and interrupts the working oil when the power source is ON.  
         [0070]    In normal circumstances, the emergency valve  115  simply provides an oil passage for the working oil leading from the linear solenoid valve  114  toward a first-second gear position changeover valve  117 . When a failure or stoppage occurs in the condition where a spool of the linear solenoid valve  114  interrupts the working oil, it would be impossible to start the vehicle. To avoid this situation, the emergency valve  115  forms an oil passage that bypasses the linear solenoid valve  114  so as to supply the working oil toward the first-second gear position changeover valve  117 . Upon the failure and stoppage of the linear solenoid valve  114 , a spool presser screw  112  is manually screwed in, to push a spool  123  into the inside, thereby forming the bypass oil passage.  
         [0071]    A first-second gear position changeover solenoid  116  is a valve for connection/disconnection of an oil pressure for driving a spool  124  of the first-second gear position changeover valve  117 . The first-second gear position changeover valve  117  is a valve by which the destination of supply of the working oil supplied from the side of the linear solenoid valve  114  is set to either of the first gear position oil hydraulic type multiple disk clutch  44  and a second-third gear position changeover valve  119 , according to the position of the spool driven by the oil pressure of the first-second gear position changeover solenoid  116 .  
         [0072]    The first-second gear position changeover solenoid  116  connects the oil pressure when the power source is OFF, and interrupts the oil pressure when the power source is ON. When the power source is OFF, the spool  124  in the first-second gear position changeover valve  117  moves leftward in the figure, against an elastic force of a coil spring  125  provided inside. When the power source is ON, the oil pressure is disconnected, and the spool  124  in the first-second gear position changeover valve  117  is moved in the direction reverse to the above by the elastic force of the coil spring  125 .  
         [0073]    A second-third gear position changeover solenoid  118  is a valve for connection/disconnection of an oil pressure for driving a spool of the second-third gear position changeover valve  119 . The second-third gear position changeover valve  119  is a valve by which the destination of supply of the working oil supplied from the side of the first-second gear position changeover valve  117  is set to either of the second gear position oil hydraulic type multiple disk clutch  65  or the third gear position oil hydraulic type multiple disk clutch  66 , according to the position of a spool driven by the oil pressure of the second-third gear position changeover solenoid  118 .  
         [0074]    The second-third gear position changeover solenoid  118  also connects the oil pressure when the power source is OFF, and interrupts the oil pressure when the power source is ON. The constitution of the spool and a coil spring in the second-third gear position changeover valve  119  is the same as in the first-second gear position changeover valve  117 .  
         [0075]    An oil pressure control valve  120  and an oil pressure control valve  121  are valves for controlling the discharge of a return oil from the oil hydraulic type multiple disk clutch that is not used. Each of them is provided therein with a spool and a coil spring, and is driven by the oil pressure.  
         [0076]    In FIG. 8, the oil passage ends denoted by symbol X are outlets for the return oil. The return oil is discharged through these outlets to the exterior of the valve body  90 , is mixed with the return oil from other routes, and a portion of the mixed return oil is fed through an oil tank, an oil pump, and an oil filter again to the valve body  90 , to be utilized in a circulated manner.  
         [0077]    [0077]FIG. 9( a ) and ( b ) show enlarged sectional views of the above-mentioned linear solenoid valve  114 . FIG. 9( a ) shows the condition where the power source is OFF (working oil supply condition), and FIG. 9( b ) shows the condition where the power source is ON (working oil interruption condition). In these figures, a valve main body  130  is provided with a working oil inlet  131  and a working oil outlet  132 , and a solenoid  133  is connected to an end portion of the valve main body  130 . A spool  134  is slidably fitted in a center hole of the main body  130 , and a rod-like body  135  connected integrally to the spool  134  is passed through a central portion of the solenoid  133 . The spool  134  is energized toward the solenoid  133  by a coil spring  137  fitted between an end portion of the spool  134  and a cover  136 .  
         [0078]    When the power source is OFF (FIG. 9( a )), the spool  134  and the rod-like body  135  connected thereto are retracted to the right side by an elastic force of the coil spring  137 . At this time, a little communication portion is present at an adjacent portion A of the working oil inlet  131  provided in the valve main body and an annular groove  138  formed at the outer periphery of the spool. The working oil flows through this portion into the annular groove  138 , is fed out through the working oil outlet  132  toward the emergency valve  115 , and is supplied to a predetermined oil hydraulic type multiple disk clutch. At this time, the oil pressure inside the annular groove  138  is exerted on the working oil filling a slant hole  139  and an annular small chamber  140 . The annular small chamber  140  is defined by an inside diameter stepped portion  141  of the main body  130  and an outer diameter step of the spool  134 , so that a thrust force directed leftwards in the figure is exerted on the spool  134 .  
         [0079]    When the power source is ON (FIG. 9( b )), a leftward thrust force is exerted on the rod-like body  135  by an electromagnetic force of the solenoid  133 . A resultant force of this thrust force and the thrust force generated in the annular small chamber  140  causes the rod-like body  135  and the spool  134  to move leftwards in the figure against the elastic force of the coil spring  137 . At this time, a small communication portion is generated in an adjacent portion B of the annular groove  138  of the spool  134  and a working oil discharge port  142  provided in the main body  130 . The working oil in the annular groove  138  and the return oil having passed through the working oil outlet  132  are discharged through the communication portion, whereby the oil pressure is lowered. The oil pressure in the annular small chamber  140  is also lowered, so that the thrust force pushing the spool  134  leftwards is also reduced.  
         [0080]    Since the linear solenoid valve  114  is constituted as above, when the output oil pressure is raised due to the balance between the pressure on the back side of the valve and the thrust force of the solenoid, the pressure on the back side of the valve is also raised, to close the valve, thereby disconnecting the oil pressure. With the oil pressure disconnected, the pressure on the back side of the valve is lowered, so that the valve is opened. These are repeated, whereby a constant oil pressure is outputted. In addition to this, the thrust force of the solenoid is varied by a current value, whereby the output oil pressure can be varied linearly. With this configuration, the pressure of the working oil supplied to the clutch can be increased gradually.  
         [0081]    The valve body  90  controls the supply and the connection/disconnection of the working oil to the oil hydraulic type multiple disk clutch by combinations of the ON/OFF conditions of the power sources applied to the three solenoids. FIG. 10 is an operation table of the three solenoids, and shows the ON/OFF conditions of the power sources applied to the solenoids, for the neutral position, the first gear position, the second gear position, the third gear position, and the back gear position in the operating conditions of the vehicle.  
         [0082]    The flow conditions of the working oil in each of the operating conditions are shown in FIGS.  11  to  15 . In FIGS.  11  to  15 , the oil passages in which a thick solid line is drawn are oil passages in which the oil pressure is exerted on the working oil. The oil passages in which a chain line is drawn are oil passages of the return oil having a lowered oil pressure. The blank oil passages are oil passages that are not active.  
         [0083]    When the vehicle is stopped in an idling condition, each of the solenoids is set in the neutral position in FIG. 10. At this time, the power source is ON at the linear solenoid valve  114 , and a current of 1 amp flows through the solenoid. Since the flow of the working oil is interrupted, as shown in FIG. 11, supply of the working oil to the oil hydraulic type multiple disk clutch is not conducted, so that creep would not be generated.  
         [0084]    When the operation of the vehicle is switched to the first gear position or the back gear position, the working oil is supplied to the first gear position oil hydraulic type multiple, disk clutch  44 . Particularly in the case of changeover from the stopped condition to the first gear position or the back gear position, a gradual clutch connection is required. Therefore, the power source applied to the solenoid repeats OFF/ON. The condition where the power source is OFF corresponds to the supply of the working oil as shown in FIG. 12, while the condition where the power source is ON corresponds to the interruption of the working oil as shown in FIG. 13. In this case, the current applied when the power source is ON is as little as 0.2 amp, which is ⅕ of the current value at the time of the neutral position, so that the thrust force due to the electromagnetic force exerted on the spool is weak. When the spool  134  is advanced with the power source being ON (FIG. 9( b )), the oil pressure is released from the communication portion B, and the pressure inside the annular small chamber  140  is also lowered, so that it becomes easier for the spool  134  to be retracted by the elastic force of the coil spring  137 .  
         [0085]    By repeating the OFF/ON conditions of the power source at minute time intervals, a gradual supply of the working oil and a gradual clutch connection can be achieved. For the gradual supply of the working oil, the ratio and the continuation times of the OFF/ON conditions of the power source are controlled, with increases in the throttle opening and the internal combustion engine revolution frequency as parameters.  
         [0086]    When the operation of the vehicle is switched to the second gear position or the third gear position, the working oil is supplied to the second gear position oil hydraulic type multiple disk clutch  65  or the third gear position oil hydraulic type multiple disk clutch  66 . The supply conditions of the working oil at these times are shown in FIGS. 14 and 15, respectively.  
         [0087]    In the present embodiment, the linear solenoid valve  114  is provided at the inlet portion of the oil pressure system. Therefore, a mechanism is needed to overcome a failed condition or stoppage of the linear solenoid valve  114  in the working oil disconnection condition (power source ON). The mechanism provided to overcome such a failed linear solenoid valve  114  is the spool presser screw  122  of the emergency valve  115 . The spool presser screw  122  can be manually screwed in to push the spool  123  into the inside, thereby forming an oil passage bypassing the linear solenoid valve  114 , and acting to restart the operation. FIG. 16 shows an example of the oil pressure system upon the setting of the bypass oil passage as above. Specifically, FIG. 16 shows the oil pressure system in the case of operating in the second gear position, and where the emergency valve  115  is set upon failure of the linear solenoid valve  114  in the working oil disconnection condition.  
         [0088]    In the present embodiment, in order to achieve creep prevention with respect to all the clutches, the linear solenoid valve for supplying the working oil and interrupting the supply of the working oil is disposed at a most upstream position in the oil pressure control mechanism. In the stand-by condition for starting, namely, in the idling stopped condition, the supply of the working oil to the clutches is interrupted. As a result, a creep preventive mechanism independent from a brake signal is obtained.  
         [0089]    In the present invention, a system is adopted in which the supply of the working oil is gradually increased with increases in the throttle opening and the internal combustion engine revolution frequency as parameters at the times of first gear position starting and backward running. Since the emergency valve is provided for mechanically causing the working oil to flow, bypassing the linear solenoid valve when the linear solenoid valve is stopped and failed in the working oil interruption condition, the operation can be restarted even when there is a stoppage or failure in the working oil interruption condition.  
         [0090]    Since the linear solenoid valve is provided at the most upstream position, the coping with the failure of the linear solenoid valve can be made independent from the running mode and be simplified. In addition, the operation of the solenoid valve is set for creep generation at the time when the power source is OFF, whereby the use of the emergency valve can be limited to the locking of the spool valve in the linear solenoid valve in the working oil interruption condition. As a result, the oil pressure system is simplified.  
         [0091]    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.