Abstract:
To provide a two-wheel drive/four-wheel drive switching device for a vehicle capable of applying both rear braking and engine braking to front and rear wheels even when the vehicle is in a two-wheel drive mode. In a vehicle capable of running while switching between two-wheel drive and four-wheel drive, when the rear wheel speed becomes lower than the front wheel speed in the two-wheel drive mode, switching to four-wheel drive mode is carried out.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a two-wheel drive/four-wheel drive switching device for a vehicle.  
           [0003]    2. Description of Background Art  
           [0004]    A vehicle which is capable of running while switching between two-wheel drive and four-wheel drive has previously been known.  
           [0005]    [0005]FIGS. 4 and 5 illustrate an example of such a vehicle. In both figures, the vehicle, indicated by reference numeral  1 , is substantially composed of a body frame  3  with an engine  2  mounted centrally thereon. Front wheels  4  and rear wheels  5  are disposed on both sides of front and rear portions of the body frame  3 . A steering handle  6  is disposed at a front upper position of the body frame  3  to steer the front wheels  4 . A fuel tank  7  is mounted to the body frame  3  above the engine  2 . Furthermore, a seat  8  is mounted behind the fuel tank  6 .  
           [0006]    As shown in FIG. 5, the front wheels  4  are supported vertically movably by suspension systems  9  disposed on both sides of the front portion of the body frame  3 . Likewise, the rear wheels  5  are supported vertically movably by suspension systems  10  disposed on both sides of the rear portion of the body frame  3 .  
           [0007]    A final reduction gear  13  for the front wheels  4  and a final reduction gear  14  for the rear wheels  5 , respectively, are mounted in front and rear central portions of the body frame  3 . The final reduction gears  13  and  14  are connected to the engine  2  through propeller shafts  11  and  12 , respectively. The right and left front wheels  4  and the right and left rear wheels  5  are connected respectively to the final reduction gear  13  for the front wheels  4  and the final reduction gear  14  for rear wheels  5 .  
           [0008]    For example, between the final reduction gear  13  for the front wheels  4  and the propeller shaft  11  is disposed a two-wheel drive/four-wheel drive switching device which makes and breaks the transfer of power to the front wheels  4 , thereby switching between a rear-wheel drive mode and a four-wheel drive mode.  
           [0009]    Alternatively, the two-wheel drive/four-wheel drive switching device is disposed between the rear propeller shaft  12  and the final reduction gear  14  for the rear wheels  5  to switch over between a front-wheel drive mode and a four-wheel drive mode.  
           [0010]    This switching between a front-wheel drive mode and a four-wheel drive mode is conducted by the driver, as necessary, according to the state of the road surface and the running mode of the vehicle.  
           [0011]    For example, a switching device of such a structure as shown in FIG. 6 has been proposed to be used as the two-wheel drive/four-wheel drive switching device of the background art.  
           [0012]    Also in FIG. 6, a two-wheel drive/four-wheel drive switching device, indicated by reference numeral  15 , is made up of two input shaft halves  16   a  and  16   b . The input shaft halves  16   a  and  16   b  are formed by axially bisecting an input shaft  16  of the final reduction gear  13  for the front wheels  4  and butting the input shaft halves  16   a  and  16   b  in alignment with each other. Furthermore, a switching unit  17  is provided for connecting and disconnecting the input shaft halves  16   a  and  16   b  from and to each other.  
           [0013]    To be more specific, a cylindrical positioning protrusion  18  is formed centrally on an end face of the input shaft half  16   a , which is positioned on the final reduction gear  13  side. Furthermore, a positioning recess  19  for rotatably fitting with the positioning protrusion  18  is formed centrally on an end face of the input shaft half  16   b , which is positioned remote from the final reduction gear side. Both input shaft halves  16   a  and  16   b  are in butting relationship with each other and the positioning protrusion  18  and the positioning recess  19  are fitted together, whereby both input shaft halves  16   a  and  16  are aligned and connected with each other so as to be relatively rotatable with respect to each other.  
           [0014]    Outer peripheral surfaces of the abutting portions of both input shaft halves  16   a  and  16   b  are splined (not shown) and the switching unit  17  is disposed so as to surround the abutting portions.  
           [0015]    The switching unit  17  has a splined inner surface and is slidably fitted to the abutting portions of both input shaft halves  16   a  and  16   b . The switching unit  17  is made up of a switching ring  20  adapted to be engaged with and disengaged from each of the splines and a drive mechanism  21  constituted by a solenoid. The drive mechanism  21  causes the switching ring  20  to slide axially with respect to both input shaft halves  16   a  and  16   b  to selectively move to a position where the switching ring  20  is brought into engagement with the spline of only one input shaft halve  16   a  and a position where the switching ring is brought into engagement with the splines of both input shaft halves  16   a  and  16   b.    
           [0016]    In the two-wheel drive/four-wheel drive switching device  15 , the switching ring  20  is moved in one direction by the drive mechanism  21  and is brought into engagement with only one input shaft half  16   a  to cut off the transfer of driving force to the front wheels  4 , affording a two-wheel drive mode of driving only the rear wheels. Furthermore, the switching ring  20 , in an engaged state thereof with one input shaft half  16   a , is slid into engagement with the other input shaft half  16   b  to connect both input shaft halves  16   a  and  16   b  with each other, thereby transmitting the driving force to the front wheels  4  and thus affording a four-wheel drive mode in which the front and rear wheels  4  and  5  are driven simultaneously.  
           [0017]    In the two-wheel drive/four-wheel drive switching device  15  of the background art, there remains the following problems to be solved.  
           [0018]    In the case where a braking operation is performed with the rear brake in a two-wheel drive mode, for example, a braking force is exerted on the rear wheels  5  and not on the front wheels  4 , so that a difference in brake feeling between two- and four-wheel drive modes occurs. The same problem occurs under the action of engine braking as well.  
           [0019]    For solving such a problem with the background art device, it is necessary to construct a brake setting which is consistent in both two-wheel drive and four-wheel drive modes. This can be done by using a complicated control mechanism, which, however, results in an increase of cost.  
         SUMMARY OF THE INVENTION  
         [0020]    The present invention has been accomplished in view of the above-mentioned problems of the background art, and it is an object of the present invention to provide a two-wheel drive/four-wheel drive switching device capable of exerting both rear braking and engine braking on the front and rear wheels.  
           [0021]    According to a first aspect of the present invention, there is provided a two-wheel drive/four-wheel drive switching device for a vehicle which is capable of running while switching between two-wheel drive and four-wheel drive, wherein when the rear wheel speed becomes lower than the front wheel speed in a two-wheel drive mode, switching is made to the four-wheel drive mode.  
           [0022]    According to a second aspect of the present invention, there is provided a two-wheel drive/four-wheel drive switching device for a vehicle, wherein a switching unit is provided in a power transfer mechanism disposed between an engine and front wheels or between the engine and rear wheels. The switching unit functions to make and break the transfer of power in the power transfer mechanism, and comprises a drive shaft connected to a drive side, a driven shaft fitted to the drive shaft with an annular clearance therebetween, a plurality of engaging and disengaging members disposed within the clearance between the drive shaft and the driven shaft and brought into engagement with and disengagement from opposed surfaces of both drive and driven shafts to connect and disconnect the shafts from and to each other, a switching mechanism for positioning the engaging/disengaging members selectively at a connecting position and a disconnecting position for the drive shaft and the driven shaft, and an elastic member for biasing said engaging/disengaging member in the direction where the drive shaft is engaged to the driven shaft, wherein the engaging/disengaging members are moved in a direction to disconnect the drive shaft and the driven shaft from each other against the elastic force of the elastic member, with rotation of the drive shaft in a two-wheel drive mode.  
           [0023]    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  
       [0024]    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:  
         [0025]    [0025]FIG. 1 is a sectional view illustrating an embodiment of the present invention;  
         [0026]    [0026]FIG. 2 is a vertical sectional view of a principal portion in the embodiment;  
         [0027]    FIGS.  3 ( a ) and  3 ( b ) are enlarged sectional views of a principal portion for explaining the operation of a two-wheel drive/four-wheel drive switching device according to the embodiment;  
         [0028]    [0028]FIG. 4 is a side view showing an example of a vehicle provided with a two-wheel drive/four-wheel drive switching device;  
         [0029]    [0029]FIG. 5 is a plan view for explaining a body configuration of the vehicle shown in FIG. 4; and  
         [0030]    [0030]FIG. 6 is a sectional view of principal portions, showing an example of structure of a two-wheel drive/four-wheel drive switching device according to the background art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    An embodiment of the present invention will now be described with reference to FIGS.  1  to  3 . In the following description, the main vehicular components are common to those shown in FIGS. 4 and 5. Therefore, they will be identified by the same reference numerals, and explanations thereof will be omitted.  
         [0032]    A two-wheel drive/four-wheel drive switching device (hereinafter referred to simply as the “drive switching device”) according to this embodiment, which is indicated by reference numeral  30  in FIG. 1, is provided in a power transfer mechanism disposed between the front wheels  4  and the engine  2 . The drive switching device  30  includes a switching unit  31  which makes and breaks the transfer of power in the power transfer mechanism. The switching unit  31  is substantially composed of a drive shaft  32  connected to the drive side, a driven shaft  33  which is fitted to the drive shaft  32  through an annular clearance, and a plurality of engaging/disengaging members  34  disposed in the clearance between the drive shaft  32  and the driven shaft  33 . The engaging/disengaging members  34  are brought into engagement with and disengagement from opposed surfaces of both shafts to connect and disconnect the shafts from and to each other. Furthermore, a switching mechanism  35  is for positioning the engaging/disengaging members  34  selectively at a connecting position and a disconnecting position for the drive shaft  32  and the driven shaft  33 . In addition, a casing  36  surrounds the above components of the switching unit.  
         [0033]    A more detailed description will now be provided. In this embodiment, a cylindrical outer ring  37  which projects toward the engine side is rotatably provided within the casing  36  through a bearing  38 .  
         [0034]    An inner peripheral surface of an engine-side end portion of the outer ring  37  is splined at a spline  39  and the drive shaft  32  is inserted into the outer ring  37  so as to engage the spline  39 , whereby the drive shaft  32  and the outer ring  37  are connected together.  
         [0035]    A cylindrical inner ring  40  is disposed within the outer ring  37  so as to leave a predetermined annular spacing between the cylindrical inner ring  40  and an inner peripheral surface of the outer ring  37 .  
         [0036]    An inner surface of the inner ring  40  is splined at a spline  41  and the driven shaft  33 , which is inserted into the casing  36 , is connected to the inner ring  40  through the spline  41 .  
         [0037]    A longitudinally intermediate portion of the driven shaft  33  is supported rotatably by means of a bearing  42  which is mounted to the casing  36 .  
         [0038]    A bevel gear  43  is integral with an end portion of the driven shaft  33  and is engaged with a ring gear  44  which constitutes the final reduction gear  13  for front wheels.  
         [0039]    As shown in FIGS. 1 and 2, the engaging/disengaging members  34  which constitute the switching unit  31  are constituted by plural rollers disposed in parallel with the axis of the outer ring  37 . The switching mechanism  35  is composed of a retainer  45  which holds the engaging/disengaging members  34  rotatably and which is fitted to the outer ring  37  in a relatively rotatable manner (relatively movably about the axis) and a cam  46  which is formed on the surface of the inner ring  40  and which causes the engaging/disengaging members  34  to move radially as a relative movement with respect to the retainer  45  proceeds.  
         [0040]    Between the retainer  45  and the inner ring  40  is disposed an elastic member  53  which biases the retainer  45  in a direction opposite to the direction in which the drive shaft  32  rotates during forward movement of the vehicle  1 , thereby biasing the engaging/disengaging members  34  in a direction in which the outer ring  37  and the inner ring  40  are connected together.  
         [0041]    The elastic member  53  is formed in a generally C shape, having end portions engaged with the retainer  45  and the inner ring  40  respectively in the rotational direction. The elastic member  53  is mounted in a contracted state of its diameter.  
         [0042]    At an end portion of the outer ring  37  positioned inside the casing  36  is mounted an electromagnetic clutch  47  which fixes and disconnects the retainer  45  and the outer ring  37  from and to each other and which constitutes the switching mechanism  35 .  
         [0043]    The electromagnetic clutch  47  is made up of a clutch disc  48  disposed between the retainer  45  and the outer ring  37  and an electromagnetic coil  49  for engagement and release of the clutch disc  48 .  
         [0044]    The electromagnetic coil  49  energizes the clutch disc  48  to bring the clutch disc into an engaged state, whereby the retainer  45  and the outer ring  37  are fixed so as not to permit relative rotation.  
         [0045]    The electromagnetic coil  49  is formed in an annular shape and is received within a housing  50  which is also formed in an annular shape and which constitutes an iron core. The housing  50  is fitted to the casing  36  so as to surround the driven shaft  33  and is secured to the casing  36  thereby.  
         [0046]    The outer ring  37 , inner ring  40  and switching mechanism  35  are installed into the casing  36  and are unitized thereby. As shown in FIG. 1, in a mounted state of the driven shaft  33 , the casing  36  is bolted to a case of the final reduction gear  13  for front wheels  4  and is thus mounted to the casing  36 .  
         [0047]    A control unit  51  for controlling ON-OFF of the electromagnetic coil  49  and a power supply  52  for supplying a driving power to the electromagnetic coil  49  are connected to the coil  49 .  
         [0048]    In the drive mode switching device  30  of this embodiment thus constructed, for canceling the transfer of driving force to the front wheels  13 , the supply of electric power to the electromagnetic coil  49  is interrupted to let the electromagnetic clutch  47  disconnect the retainer  45  and the outer ring  37  from each other.  
         [0049]    In this state, as shown in region (b) of FIG. 3( a ), the engaging/disengaging members  34  are biased to one top portion of the cam  46  and are abutted against both outer ring  37  and inner ring  40  elastically by the elastic member  53 . However, since the rotational force of the outer ring  37  is large as indicated with arrows in regions (a) and (b) of FIG. 3( a ), the engaging/disengaging members  34  are moved by the outer ring  37  in a direction to disconnect the outer ring from the inner ring  40  against the biasing force of the elastic member  53 .  
         [0050]    As a result, the outer ring  37  and the inner ring  40  are disconnected from each other, so that the rotation of the drive shaft  32  is prevented from being transmitted to the driven shaft  33 , that is, the rotation of the front wheels  13  is stopped.  
         [0051]    In such a two-wheel drive mode, if a braking operation is applied to the rear wheels  5 , the rotational force of the outer ring  37  decreases, whereby the engaging/disengaging members  34 , which are biased by the elastic member  53 , are moved toward one top portion of the cam  46 , as indicated with arrows in regions (d) and (e) of FIG. 3( b ).  
         [0052]    Consequently, the front and rear wheels  4  and  5  are connected and brakes are applied to the front and rear wheels.  
         [0053]    This is also the case with the application of engine braking.  
         [0054]    For switching to the four-wheel drive mode, the electromagnetic coil  49  is energized to engage the electromagnetic clutch  47 , thereby fixing the retainer  45  to the outer ring  37 .  
         [0055]    As a result, the engaging/disengaging members  34  held by the retainer  45  are moved together with the outer ring  37  up to the other top portion of the cam  46  formed on the inner ring  40  and is abutted against the inner surface of the outer ring  37 , as shown in region (c) in FIG. 3( a ).  
         [0056]    As a result, the outer ring  37  and the inner ring  40  are connected together through the engaging/disengaging members  34 , the drive shaft  32  and the driven shaft  33  are connected together, the rotation of the drive shaft  32  is transmitted to the driven shaft  33 , and rotation of the front wheels  13  is started.  
         [0057]    If the rear wheels  5  are braked in this four-wheel drive mode, the inner ring  40  is the first to rotate, whereby one top portion of the cam  46  formed on the inner ring  40  is brought into abutment against the engaging/disengaging members  34 .  
         [0058]    Consequently, the four-wheel drive mode is maintained and brakes are applied to the front and rear wheels  4  and  5  in the same way as above.  
         [0059]    In the drive mode switching device  30  of this embodiment constructed as above, the outer ring  37  and the inner ring  40  are connected together under the action of the elastic member  53  when brakes are applied and the braking forces of rear brake and engine brake can be applied to the front and rear wheels  4  and  5 .  
         [0060]    The above function can be attained by such a simple configuration as the elastic member  53  being disposed between the retainer  45  and the inner ring  40 .  
         [0061]    The shapes and sizes of the components described in the above embodiment are only an example and may be changed in accordance with requests in design.  
         [0062]    According to the present invention, as set forth above, no matter in which of two- and four-wheel drive modes the vehicle may be, the braking forces of rear braking and engine braking can be exerted on the front and rear wheels  4  and  5  when brakes are applied.  
         [0063]    Furthermore, the above function can be attained by such a simple configuration as the elastic member  53  being engaged with the engaging/disengaging members  34 .  
         [0064]    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.