Abstract:
A mechanism, e.g. for driving a wheelchair, converting oscillating motion of a lever ( 15 ) into rotary motion of an output disc (capstans  34 ) by means of a flexible member (belt  26 ). In one direction of the lever ( 15 ), the belt ( 26 ) drives the output disc (capstans  24 ). In the other direction, the belt ( 26 ) slips. This is achieved by a (rotary) drag element ( 22 ) providing a (rotary) resistance force(=counter force) upstream the output disc ( 34 ) thereby tensioning the belt ( 26 ) such that slip is avoided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application 61/518,269 filed May 4, 2011 and hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present application relates to a drive mechanism that coverts reciprocating linear motion of a lever into rotary motion in a single selectable direction and, further, to a wheelchair or other manually propelled vehicle incorporating such a drive mechanism. 
         [0003]    The mechanics of a traditional wheelchair have long remained unchanged. Typically, an operator uses her hands to roll the wheels that are located on either side of a seat. The rotation of the wheels in turn propels the wheelchair. However, this form of operation requires the operator to possess both the necessary upper body range of motion and strength to reach and push the wheels. Unfortunately, many persons such as elderly, young children, and those afflicted with ailments that limit both upper and lower body function are not capable of using a traditional wheelchair. 
         [0004]    One approach to simplifying the operation of a wheelchair uses levers that may be gripped by the user instead of the wheels. Reciprocating motion of the levers then serves to propel the wheelchair forward. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides an improved drive mechanism that coverts reciprocating linear motion of a lever into rotary motion using a belt passing over one or more capstans. A drag element selectively tensions the belt to cause it to grip the capstans with one direction of belt motion and release the capstan with another direction of belt motion. The rotatable capstans in turn engage an output shaft, which may be a wheel axle. 
         [0006]    Specifically, one embodiment of the invention provides a mechanism for converting a reciprocating action to unidirectional rotary motion having a frame with a lever attached to the frame to pivot about a pivot point and an output shaft attached to the frame to rotate about a shaft axis. A first flexible belt communicates with the lever to move in a first and second direction along the length of the first flexible belt with reciprocating of the lever, and a first rotatable capstan is coupled to the output shaft and provides an outer wall contacting a surface of the first flexible belt with movement of the first flexible belt. A first drag element provides an outer wall contacting a surface of the first flexible belt with movement of the first flexible belt, the first drag element adapted to apply a drag tension against the first flexible belt so that the first flexible belt tightens against the outer wall of the rotatable capstan to rotate the same with a first direction of reciprocation of the lever and so that the first flexible belt loosens against the outer wall of the rotatable capstan with a second direction of reciprocation of the lever to not rotate the same. 
         [0007]    It is thus a feature of at least one embodiment of the invention to provide a belt mechanism for converting lever action into rotary motion eliminating the noise and complexity of ratchet mechanisms and the like. 
         [0008]    The first drag element may provide an operator switching the drag element between a drag mode applying the drag tension against the first flexible belt and a non-drag mode not applying the drag tension against the first flexible belt. 
         [0009]    It is thus a feature of at least one embodiment of the invention to provide a simple mechanism for enabling and disabling the belt drive. 
         [0010]    The mechanism may include a second drag element providing an outer wall contacting a surface of the first flexible belt with movement of the first flexible belt, the second drag element being positioned on an opposite side of the first rotatable capstan with respect to the first drag element, the second drag element adapted to apply a drag tension against the first flexible belt so that the first flexible belt tightens against the outer wall of the rotatable capstan to rotate the same with a second direction of reciprocation of the lever and so that the first flexible belt loosens against the outer wall of the capstan with a first direction of reciprocation of the lever to not rotate the same. 
         [0011]    It is thus a feature of at least one embodiment of the invention to permit the direction of motion to be easily changed. 
         [0012]    The rotatable capstan may communicate with the output shaft through a gear linkage. 
         [0013]    It is thus a feature of at least one embodiment of the invention to permit the introduction of an arbitrary mechanical advantage into the drive chain. 
         [0014]    The mechanism may include multiple rotatable capstans each having outer walls contacting the surface of the first flexible belt with movement of the first flexible belt wherein the first drag is adapted to apply a drag tension against the first flexible belt so that the first flexible belt tightens against the outer wall of each of the multiple rotatable capstans to rotate the same with a first direction of reciprocation of the lever and so that the first flexible belt loosens against the outer wall of each of the multiple rotatable capstans with a second direction of reciprocation of the lever to not rotate the same. 
         [0015]    It is thus a feature of at least one embodiment of the invention to provide a method of flexibly scaling the total contact force between the belt and capstans for a given belt tension. 
         [0016]    The drag element may be selected from the group consisting of a hydraulic dash pot providing an operator-controlling bypass of hydraulic fluid through the dash pot and an electric generator wherein the operator is a switch-controlling bypass of electrical current through the electric generator. 
         [0017]    It is thus a feature of at least one embodiment of the invention to provide readily switchable drag elements. 
         [0018]    The flexible belt may be a polymeric composite material. 
         [0019]    It is thus a feature of at least one embodiment of the invention to provide a lightweight, strong, and quiet drive element. 
         [0020]    A second flexible belt communicates with the lever to move in a first and second direction along the length of the belt with reciprocating of the lever and a second rotatable capstan coupled to the output shaft, wherein the second rotatable capstan provides an outer wall contacting a surface of the second flexible belt with movement of the second flexible belt. A second drag element may provide an outer wall contacting a surface of the second flexible belt with movement of the second flexible belt, the second drag element adapted to apply a drag tension against the second flexible belt so that the flexible belt tightens against the outer wall of the second rotatable capstan to rotate the same with a second direction of reciprocation of the lever and so that the second flexible belt loosens against the outer wall of the rotatable capstan with a first direction of reciprocation of the lever to not rotate the same. 
         [0021]    It is thus a feature of at least one embodiment of the invention to capture both the forward and reverse action of the lever in providing a given direction of motion to the output shaft. 
         [0022]    The mechanism may include a wheelchair and the output shaft may be mechanically coupled to a wheel of the wheelchair. 
         [0023]    It is thus a feature of at least one embodiment of the invention to provide an improved wheelchair using lever action. 
         [0024]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a simplified exploded view of a drive mechanism according to an embodiment of the present invention showing a drive belt attached to a lever, each communicating with the capstan assembly; 
           [0026]      FIG. 2  is a side elevation view of the capstan assembly of  FIG. 1  showing passage of the belt around multiple capstans communicating with an output shaft and further passing around drag elements; 
           [0027]      FIG. 3   a  is a side elevation view of a capstan of  FIG. 2  with a corresponding upstream drag element deactivated to allow the belt to slide in a first direction along the capstan; 
           [0028]      FIG. 3   b  is a side elevation view of the capstan of  FIG. 2  with the upstream drag element activated to cause the belt to grip the capstan as it moves in the first reaction; 
           [0029]      FIG. 3   c  is a side elevation view similar to  FIG. 3   a  with the belt moving in the second direction; 
           [0030]      FIG. 3   d  is a side elevation view similar to  FIG. 3   b  with the belt moving in the second direction; 
           [0031]      FIG. 4  is a simplified exploded view of the drive mechanism shown in  FIG. 1 , when the lever is activated to produce clockwise rotation of the output shaft in response to the forward stroke of the lever; 
           [0032]      FIG. 5  is a simplified exploded view of the drive mechanism shown in  FIG. 1 , when the lever is activated to produce clockwise rotation of the output shaft in response to the backward stroke of the lever; 
           [0033]      FIG. 6  is a simplified exploded view of the drive mechanism shown in  FIG. 1 , when the lever is activated to produce counterclockwise rotation of the output shaft in response to the forward stroke of the lever; 
           [0034]      FIG. 7  is a simplified exploded view of the drive mechanism shown in  FIG. 1 , when the lever is activated to produce counterclockwise rotation of the output shaft in response to the backward stroke of the lever; 
           [0035]      FIG. 8  is a partial schematic view of an electrical generator drag element in accordance with an alternative embodiment of the present invention; 
           [0036]      FIG. 9  is a partial schematic view of a hydraulic pump drag element in accordance with an alternative embodiment of the present invention; 
           [0037]      FIG. 10  is a side elevation view of a capstan engaging two flexible belts in accordance with an alternative embodiment of the present invention; 
           [0038]      FIG. 11  is a partial schematic view of an electrical circuitry and lever actuated switch for selectively activating the tensioning devices of the drive mechanism shown in  FIG. 1 ; 
           [0039]      FIG. 12  is a perspective view of wheelchair including the drive mechanism shown in  FIG. 1 ; 
           [0040]      FIG. 13  is a side elevation view of a portion of a drive mechanism assembly according to an alternative embodiment of the present invention; 
           [0041]      FIG. 14  is a side elevation view of a portion of a drive mechanism assembly according to yet another alterative embodiment of the present invention; and 
           [0042]      FIG. 15  is a simplified exploded view of the drive mechanism shown drive mechanism assembly according to yet another alterative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0043]    Referring now to  FIG. 1 , a drive mechanism  10  for converting linear reciprocating motion of a lever  12  into rotational motion of a rotary output shaft  14  may include a paired first and second drive assembly  16 ,  18 , which operate alternatingly when the lever  12  is reciprocated (an “extension” stroke or a “retraction” stroke) to rotate the output shaft  14 . As will be discussed in detail below, a first drive assembly  16  is activated during one stroke of the lever  12 , while the alternate second drive assembly  18  is deactivated. Subsequently, when the lever  12  is moved in the opposite direction, the first drive assembly  16  is deactivated while the alternate second drive assembly  18  is activated. The lever  12  may be mounted to pivot about an axis  13  defining a pivot point with respect to a frame  11 . One end of the lever  12  removed from the pivot point and axis  13  may include a handgrip  15  for gripping by a human hand. 
         [0044]    Through this alternating activation of the drive assemblies  16 ,  18  the rotary output shaft  14  may maintain rotation in a consistent direction. As will also be detailed below, the particular drive assemblies  16 ,  18  that are activated for a particular stroke of the lever  12  may be switched to allow the rotary output shaft  14  to rotate in the opposite direction, i.e., reverse. 
         [0045]    Referring still to  FIG. 1 , the first and second drive assemblies  16 ,  18  each comprise a capstan array  20 , a first and second drag element  22  and  24 , and a first and second flexible belt  26  and  27 . The first flexible belt  26 , for example formed of a polymeric material possibly with reinforcing fibers of glass or the like, engages the lever  12  at engagement point  28  and extends to the capstan array  20  for the first drive assembly  16 . The first flexible belt  26  engages a drag element  22  positioned between the lever  12  and the capstan array  20 . As will be discussed below, the drag element  22  provides for a cylindrical wheel frictionally engaging the belt  26  to provide a resistive drag to movement of the belt  26 . 
         [0046]    The first flexible belt  26  then returns to the lever  12  at the second engagement point  30  from the capstan array  20  after passing by drag element  24 . Drag element  24 , like drag element  22 , engages the flexible belt  26  with a cylindrical wheel that frictionally contacts the first flexible belt  26 . 
         [0047]    Likewise the second flexible belt  27  engages the lever  12  at engagement point  28  and extends to a capstan array  20  for the second drive assembly  18  passing by drag element  22  (independent of the drag element  22  associated with the first belt  26  but identically constructed) and then returns from the capstan array  20  to the engagement point  30  after passing by the drag element  24  (independent of the drag element  24  associated with the first belt  26  but identically constructed). 
         [0048]    These engagement points  28 ,  30  may be a fixed point on the lever  12  or, as shown in  FIG. 1 , may be points on a drive wheel  32  which rotates in conjunction with the reciprocal movement of the lever  12 . The drive wheel  32  may present an outer cylindrical surface for frictionally engaging a smooth surface of the belt  26  or  27 . 
         [0049]    Referring now to  FIG. 2 , for each of the first and second drive assemblies  16 ,  18 , the belt  26  and  27  is threaded through the capstan array  20  such that it at least partially engages an outer cylindrical surface of each of the capstans  34 . Generally, the interface between the belt  26  and  27  and the outer cylindrical surface of the capstans  34  is smooth without teeth or the like to provide only frictional engagement. In one embodiment, the flexible belts  26  and  27  may also be threaded through rotating bushings  36  located between adjacent capstans  34  in the array  20 , as to redirect the flexible belt  26  and increase the engagement of the flexible belt  26  around the capstans  34  by increasing the angular contact area between the two. 
         [0050]    Referring momentarily to  FIG. 10 , in an alternative embodiment, the first and second assembly  16 ,  18  may utilize a single common capstan array  20 , with the flexible belts  26  and  27  of each assembly  16 ,  18  engaging opposing sides of the capstans  34  within the array  20 . 
         [0051]    Referring again to  FIG. 2 , the capstans  34  within the array  20  may co-rotate with corresponding attached spur gears  40  that are axially parallel with axes of both other capstans  34  in the array  20  and the rotary output shaft  14 . The teeth of the spur gears  40  may engage corresponding teeth of a rotary gear  44  driving the output shaft  14 . 
         [0052]    Referring momentarily to  FIG. 14 , in one embodiment the teeth of alternate spur gears  40  of the capstans  34  may engage spur gears of adjacent capstans  34 , which rotate in an opposite direction within the array  20 , rather than directly engaging the rotary gear  44 . The remaining spur gears  40  may communicate directly with rotary gear  44 . Alternatively, the capstan array  20  and rotary output shaft  14  may be rotatably engaged to one another by means other than teeth such as through frictional contact, belts, chains or the like. This orientation eliminates the need for rotating bushings  36  to be placed between adjacent capstans  34  to redirect the flexible belt  26  and any associated energy loss therewith. 
         [0053]    Referring now to  FIG. 3   a , when the flexible belts  26  and  27  move in a first direction  31  corresponding to movement of the lever  12  in an retraction stroke (for belts  26 ) and in an extension stroke (for belt  27 ), the flexible belts  26  and  27  slide over the capstans  34  when the upstream drag element  24  of the assembly  16 ,  18  is deactivated. This sliding does not cause the capstans  34  to rotate about their axes. 
         [0054]    Alternatively, as shown in  FIG. 3   b  when the upstream drag element  24  is activated and the lever  12  is moved to cause movement of the flexible belts  26  and  27  in the first direction  31 , sufficient tension or drag is placed on the flexible belt  26  such that the flexible belt  26  will engage and grip the outer wall  38  of each capstan  34 , thereby forcing the capstans  34  to rotate about their axes. This rotation of the capstans  34  will in turn rotate the rotary gear  44  and output shaft  14 . 
         [0055]    When activated, the force on each capstan  34  can be substantially larger than the force of tension provided by the drag element on the flexible belt  26  according to equation (1) below: 
         [0000]      ln( T 2/ T 1)=μμ  (1)
 
         [0000]    where T 1  equals the tension on the flexible belt  26 , T 2  equals the force rotating the capstan  34 , μ equals the friction between the flexible belt  26  and the outer wall  38  of the capstan  34 , and β equals the number of revolutions the flexible belt  26  makes about the outer wall  38  in radians. The total force imparted to the rotary gear  44  of the output shaft  14  will of course be a summation of T 2  times the number of capstans  34  within the array  20 . The array  20  may include any combination of two or more capstans  34 . 
         [0056]    Referring now to  FIG. 3   c , when the flexible belts  26  and  27  move in a second direction  33  corresponding to movement of the lever  12  in an extension stroke (for belts  26 ) and in a retraction stroke (for belt  27 ), the flexible belts  26  and  27  slide over the capstans  34  when the upstream drag element  22  of the assembly  16 ,  18  is deactivated. This sliding does not cause the capstans  34  to rotate about their axes. 
         [0057]    Alternatively, as shown in  FIG. 3   d  when the upstream drag element  22  is activated and the lever  12  is moved to cause movement of the flexible belts  26  and  27  in the second direction  33 , sufficient tension or drag is placed on the flexible belt  26  such that the flexible belt  26  will engage and grip the outer wall  38  of each capstan  34 , thereby forcing the capstans  34  to rotate about their axes. This rotation of the capstans  34  will in turn rotate the rotary gear  44  and output shaft  14 . 
         [0058]    Again, the force on each capstan  34  can be substantially larger than the force of tension provided by the drag element on the flexible belt  26  according to equation (1) above. 
         [0059]    Referring momentarily to  FIG. 12 , in one embodiment, the drive mechanism  10  described above may be incorporated into a wheelchair  54 . The wheelchair  54  may provide a seat  55  resting on the frame  11  and flanked by wheels  58  placed approximately beneath the center of gravity of the seated individual on the seat  55 . Guide rollers  59  may be positioned in front of the seat to provide stability. Flanking the seat  55 , on each of the left and right sides of the seat  55 , may be levers  12  communicating with the drive mechanism  10  discussed above. 
         [0060]    In use, the operator of the wheelchair  54  may select the desired direction of wheel rotation, i.e., forwards or reverse. This selection may be made by use of a switch  56  or button, as seen in  FIG. 11  and as will be discussed below, which controls which of the drag elements  22  or  24  is activated. Each wheel  58  of the wheelchair  54  may be actuated independently by moving the associated lever  12 . Independent control of movement of the wheels  58  and their direction provides a high degree of maneuverability and control. 
         [0061]    Referring now to  FIG. 4 , we will consider first the case where the operator has elected to reverse the wheel  58 , as designated by the clockwise direction of an arrow shown on the rotary gear  44 . With the reverse direction selected, drag elements  22  will be activated and drag elements  24  will be deactivated. When the operator extends the lever  12  through the forward stroke, linear movement of the lever  12  will translate into counterclockwise rotational movement of the drive wheel  32  associated with corresponding counterclockwise rotational movement of the flexible belt  26  (as indicated with arrows) for a drive assembly  16 . Simultaneously, the flexible belt  27  in the second drive assembly  18 , which is counter wound relative to the first assembly around the drive wheel  32 , will travel in the clockwise direction. 
         [0062]    As noted, given that the operator has elected to reverse the wheel  58 , the first drag element  22  of the second assembly  18  will activate to apply a tension to the flexible belt  26  of the second assembly  18 . This drag element  22  is upstream of the capstan array  20  and thus will activate the rotation of the capstan array  20  with the movement of the taut flexible belt  26 , and subsequently rotate the rotary gear  44  and output shaft  14  in the clockwise direction. The second drag element  24  of the second assembly  18  will remain deactivated to allow the taut flexible belt  26  to be pulled over the second drag element  24  towards the second engagement point  30 . 
         [0063]    On the other hand, the second drag element  24  of the first drive assembly  16 , which is upstream of the belt movement for the second drive assembly  16 , will remain deactivated to keep the flexible belt  26  of the first drive assembly  16  loose, such that it will pass over the capstan array  20  without inducing rotation thereof. Activation of the first drag element  22  of the first drive assembly  16  will have no effect because it is downstream from the capstan array  20 . 
         [0064]    Referring now to  FIG. 5 , when the operator retracts the lever  12  through the reverse stroke, linear movement of the lever  12  will translate into clockwise rotational movement of the drive wheel  32  with corresponding clockwise rotational movement of the flexible belt  26  (as indicated with arrows) for a drive assembly  16 . Simultaneously, the flexible belt  27  in the second drive assembly  18 , which is counter wound relative to the first assembly around the drive wheel  32 , will travel in the counter clockwise direction. 
         [0065]    Under the above conditions in which the operator has still elected to reverse the wheel  58 , the first drag element  22  of the first drive assembly  16  will be active to apply a tension onto the flexible belt  26  of the first drive assembly  16  as it passes through the capstan array  20 . This tension will cause the rotation of the capstan array  20  under the pressure of the taut flexible belt  26 , and will rotate the rotary gear  44  and output shaft  14  in the clockwise direction. The second drag element  24  of the first drive assembly  16  will remain deactivated so as not to interfere with passage of the belt  26 . At the same time, the second drag element  24  of the second assembly  18  will remain deactivated to keep the flexible belt  27  of the second assembly  18  loose, such that it will pass over the capstan array  20  without inducing rotation thereof. The first drag element  22  of the second assembly  18  will be activated but without practical effect. 
         [0066]    Turning now to  FIG. 6 , the case where the operator has elected to rotate the wheel  58  forward will now be considered such as will provide a counterclockwise direction of the rotary gear  44  shown by the arrow. When the operator extends the lever  12  in a forward stroke, linear movement of the lever  12  will translate into counterclockwise rotational movement of the drive wheel  32  associated therewith to drive the flexible belt  26  in the first drive assembly  16  to travel in counterclockwise direction (as indicated with arrows). Simultaneously, the flexible belt  27  in the second assembly  18 , which is counterwound relative to the first drive assembly  16 , will travel in the clockwise direction. The second drag element  24  of the first drive assembly  16  will be actively applying a tension onto the flexible belt  26  of the first drive assembly  16  as it passes through the capstan array  20 . This tension will rotate the capstan array  20  with the movement of the taut flexible belt  26  and subsequently rotate the rotary gear  44  and output shaft  14  in the counterclockwise direction. The first drag element  22  of the first drive assembly  16  will remain deactivated to allow the flexible belt  26  to pass freely over the first drag element  22 , towards the first engagement point  28 . 
         [0067]    Simultaneously, the first drag element  22  of the second assembly  18  will remain deactivated to keep the flexible belt  27  of the second assembly  18  loose as it passes over the capstan array  20  without inducing rotation thereof; and the second drag element  24  of the second assembly  18  will be activated to prevent pulling force from the lever  12  from being translated through the capstan array  20 . 
         [0068]    As seen in  FIG. 7 , with the forward direction still selected, the operator may retract the lever  12  through the backwards stroke. Linear movement of the lever  12  will translate into clockwise rotational movement of the drive wheel  32  associated therewith and move the flexible belt  26  in the first drive assembly  16  to travel in clockwise direction (as indicated with arrows). Simultaneously, the flexible belt  26  in the second assembly  18  will travel in the counterclockwise direction. Having elected to rotate the wheel  58  in the forward direction, the second drag element  24  of the second assembly  18  will be activate to apply a tension onto the flexible belt  26  of the second assembly  18 . This tension will activate the rotation of the capstan array  20  with the movement of the taut flexible belt  26 , and subsequently rotate the rotary gear  44  and output shaft  14  in the counterclockwise direction. The first drag element  22  of the second assembly  18  will remain deactivated to allow the taut flexible belt  26  to pass freely toward the first engagement point  28 . The first drag element  22  of the first drive assembly  16  will remain deactivated to keep the flexible belt of the first drive assembly  16  loose such that it will pass over the capstan array  20  without inducing rotation thereof; and the second drag element  24  of the first drive assembly  16  will be activated to prevent pulling force from the lever  12  from being translated through the capstan array  20 . 
         [0069]    Accordingly, by selective engagement of the drag elements  22 ,  24  on the belts  26  and  27 , the operator will be able to rotate the output shaft  14  and an associated wheelchair wheel  58  in either of two desired directions regardless of the direction of lever movement. By independently controlling both the direction and amount of movement, the operator will be able to both propel and steer the wheelchair  54  via the two drive mechanisms  10 . Furthermore, if the operator wished to brake the rotational movement of one or both wheels  58  on the wheelchair  54 , she would simply need to engage both drag elements  22 ,  24  in at least one of the paired drive assemblies  16 ,  18  on the given wheel  58 . Similarly, by deactivating all of the drag elements  22 ,  24  in the paired drive assemblies  16 ,  18 , the operator would be able to place one or both wheels  58  in a neutral position. This neutral position may be helpful when the operator is positioning the wheelchair  54  during transfers, or if another person were to assist in pushing the wheelchair  54 . 
         [0070]    Referring now to  FIG. 8 , drag elements  22  or  24  may each be an electrical generator  46 , as seen in  FIG. 8 , which may be activated by shorting the windings of the generator  46  through a load resistor so as to dissipate mechanical energy into resistive heating. Activating or deactivating the drag elements  22  or  24  may then be accomplished simply by throwing a switch. 
         [0071]    Alternatively, as seen in  FIG. 9 , the drag elements  22 ,  24  may be a hydraulic pump  52  (generically a hydraulic dashpot) which extends an arm  50  engaging a pulley  47  with the flexible belt  26  or  27  thereby increasing the drag on the flexible belt  26  or  27  sufficient to activate the capstan array  20  when hydraulic fluid from the pump  52  is forced to pass through a restrictive orifice  57 . 
         [0072]    Opening a bypass valve  61  around the orifice  57  allows the hydraulic pump  48  to turn freely thereby releasing the tension in the flexible belt  26  sufficiently to allow the belt to slip freely through the capstan array  20 , without inducing rotation of the capstans  34 . 
         [0073]    Referring now to  FIG. 13 , in an alternative embodiment of the drive mechanism  10 , the drive assemblies  16  and  18  discussed above may be combined in a single drive assembly  60 . In this embodiment of drive assembly  60 , the capstan array  20  constitutes a first capstan  62  and a second capstan  64  flanking a single drag element  66 , wherein the first capstan  62  is located closest to the first engagement point  28  of the flexible belt  26  and the second capstan  64  is located closest the second engagement point  30  of the flexible belt  26  while the flexible belt  26  is counter wound around the two capstans  62  and  64 . 
         [0074]    By locating the drag element  66  between the two capstans  62 ,  64 , the drag element  66  will always be upstream of one of the single capstans  62  or  64  for each direction of the flexible belts  26  causing alternating engagement of the belt  26  with a proper one of the capstans  62  or  64  depending on the direction of movement of the belt  26  to provide unidirectional motion of the output shaft  14  and associated wheelchair wheel  58  with reciprocation of the lever  12 . A second drive assembly (not shown) with the belt wound around the capstans in the opposite direction may be used for reversal of the wheel simply by activating or deactivating the drag element  66  for one of the two drive assemblies. 
         [0075]    Referring now to  FIG. 14 , in yet another embodiment of the drive mechanism  10 , a drive assembly  68  (corresponding to one of drive assembly  16  or  18 ) may provide that the lever  12  be received by a toggle arm  70  which in turn reciprocates a sprocket  72  over a rotation of approximately 180°. The sprocket  72  operates analogously to drive wheel  32  in the previous embodiments but instead of directly engaging the flexible belt  26 , the teeth of the sprocket  72  engage a chain  74 , which is connected to a first and second engagement point  28 ,  30 . These engagement points  28  and  30  are also connected to the ends of the hydraulic arm  50  of a first and second hydraulic cylinder  51  and  53  providing the first and second drag elements  22 ,  24 . This embodiment eliminates some of the slipping which may occur when the flexible belt  26  engages the lever  12  via a smooth drive wheel  32 . 
         [0076]    The hydraulic cylinders  51  and  53  may be activated and deactivated by use of an orifice and switch arrangement as described generally with respect to  FIG. 9  above. Again, direct connection of the arms  50  to the engagement points  28  and  30  eliminates slippage that would otherwise be possible between the first and second drag elements  22  and  24  and the belt. 
         [0077]    Referring now to  FIG. 15 , in yet another embodiment of the drive mechanism  10 , a drive assembly  76  (corresponding to one of drive assembly  16  or  18 ) may provide that the lever  12 , which is pivoted about pin  78 , be rotatably received by a toggle arm  80  which in turn reciprocates a sprocket assembly  82  over a rotation of approximately 180°. The sprocket assembly  82  includes a first and second partial sprocket  84 ,  86  located on opposite sides of a centrally located sprocket hub  88  having an elongated slot configured to receive a sprocket axel  90  therein. The first and second partial sprockets  84 ,  86  are positions in a directionally opposed orientation relative to each other. The sprocket assembly  82  operates analogously to sprocket  72  in the previous embodiment as illustrated in  FIG. 14 , wherein the teeth of the first and second partial sprockets  84 ,  86  engage a first and second chain  92 ,  94  respectively, which are fixed to an end of the partial sprockets  84 ,  86  at first and second engagement points  96 ,  98  respectively. These chains  92 ,  94  extend from the engagement points  96 ,  98  to the linkage  100 ,  102  located on the ends of the hydraulic arm  104 ,  106  of the first and second hydraulic cylinder  108 ,  110 . The hydraulic cylinders  108 ,  110  provide the first and second drag elements  22 ,  24 , as was described in the previous embodiments. The hydraulic cylinders  108  and  110  may be activated and deactivated by use of an orifice and switch arrangement as described generally with respect to  FIG. 9  above. Again, connection of the arms  104 ,  106  to the engagement points  96  and  98  via chains  92  and  94  eliminates slippage that would otherwise be possible between the first and second drag elements  22  and  24  and the belt. 
         [0078]    Still referring to  FIG. 15 , the linkages  100 ,  102  also receive the belt  112  extending in an opposite direction from the first and second chains  92 ,  94 . In the present embodiment the belt  112  includes at least three segments: a first capstan chain  114 , a capstan coil  116 , and a second capstan chain  118 . The first capstan chain  114  engages the linkage  100  and extends in a direction opposite the lever  12 , where it is engaged by the teeth of a first capstan gear  120  and fixed to the first capstan gear  120  at an engagement point  122 . The capstan coil  116  may be a helical torsion spring that is wrapped around the output shaft  124 . The capstan coil  116  includes a first and second end  126 ,  128 . The first end of the capstan coil is fixed to the first capstan gear  120 , while the second end of the capstan coil  128  is fixed to the second capstan gear  130 . In its passive or disengaged orientation, the capstan coil  116  is biased away from the outer surface of the output shaft  124 , such that the output shaft  124  can rotate freely, i.e., in a neutral or free-wheeling position. The second capstan gear  130  also received the second capstan chain  118  at the engagement point  132 . From the engagement point  132 , the second capstan chain  118  engages the teeth of the second capstan gear, as it extends back towards the direction of the lever  12  where it is fixed to the linkage  102  on the side opposite the second chain  94 . In combination the belt  112  includes the first and second capstan chains  114 ,  118  and the capstan coil  116 ; wherein the first capstan chain  114  is indirectly fixed to the capstan coil  116  by way of the first capstan gear  120  at engagement points  122  and  126  respectively, and the capstan coil  116  is indirectly fixed to the second capstan chain  118  by way of the second capstan gear  130  at engagement points  128  and  132  respectively. As such, the movement of the belt  112  may be transferred through all three sections the first and second capstan chains  114 ,  118  and capstan coil  116 , by way of the intervening capstan gears  120 ,  130 . In this embodiments, the capstan  134 , is composed of both the first and second capstan gears, and a capstan housing (not shown), with the belt  112  engaging the capstan  134  at both the teeth of the gears  120 ,  130  and the various wrappings of the capstan coil  116 , located within the capstan housing (not shown). 
         [0079]    As the drive assembly  76  seen in  FIG. 15  corresponds to one of drive assembly  16  or  18 , it is understood that drive mechanism  10  would further include at least one additional capstan and drive assembly disposed about output shaft  124 , which would be directionally opposed to the drive assembly  76 . This directionally opposed drive assembly (not shown) may include a sprocket assembly which rotates opposite the rotation of the sprocket assembly  82  by way of a directionally opposed toggle arm, or may alternatively include a counter wound capstan coil. 
         [0080]    In use, when a tension is exhibited by one of the first and second drag elements  22 ,  24 , as was described in the previous embodiments, the motion of the belt  112  will cause the capstan coil  112  to compress around the outer circumference of the output shaft  124 , and thereby force the output shaft to rotate in the direction consistent with the movement of the belt  112  and capstan gears  120 ,  130 . That is to say, when the sprocket assembly is rotated clockwise about axel  90  and drag element  22  is engaged, i.e., extended, the capstan  134  and output shaft will rotate clockwise. When the sprocket assembly is rotated counter-clockwise about axel  90  and drag element  24  is engaged, i.e., extended, the capstan  134  and output shaft will rotate counter-clockwise. The paired directionally opposed drive mechanism (not shown), as was described above, will similarly produce directionally opposed motion in response to lever  12  movement, thereby allowing continuous rotational movement of the output shaft  124 . Disengagement of both drag elements  22 ,  24  releases the tension from the capstan coil  118  to allow free-wheeling, while engagement of both drag elements  22 ,  24  compresses the coil around the outer surface of the output shaft  124  without exhibiting a rotational force, thereby breaking the movement of the output shaft  124 . As with the embodiment illustrated in  FIG. 14 , the introduction of a belt  112  having a first and second capstan chain  114 ,  118  and capstan coil  116  further reduced slippage within the drive mechanism  10 . 
         [0081]    It will be appreciated that, this drive mechanism described above may be placed in other manually operated, self propulsion vehicles in addition to wheelchairs. 
         [0082]    The term belt as used herein should be broadly construed to cover functional equivalents that can frictionally engage a drum or similar surface with a tensioning of the belt that provides a force between the belt and surface normal to that surface thereby moderating the frictional force. As such, a belt may be constructed of a variety of materials sized and configured to provide the necessary flexibility, support the necessary tension and promote the necessary frictional engagement, including polymers, metals, composites, woven and nonwoven materials and the like. The term engaged as applied to the action between the belt and the capstan means a releasable attachment arising primarily from frictional contact between interface surfaces of the belt and the capstan. 
         [0083]    Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0084]    When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0085]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications are hereby incorporated herein by reference in their entireties.