Abstract:
A wheelchair propulsion system provides a wheelchair user with a continuous input drive path of linear or elliptical shape. The propulsion system includes a Cardan gear system defining a linear reciprocating or elliptical input path over which propulsive force may be continuously applied to the wheelchair wheels. Because the input path is continuous, efficiency is increased and impact on the user&#39;s body is reduced thereby reducing discomfort and physical demands on the user&#39;s body. The angle of the input path may be altered to suit the particular needs of a user by rearrangement of the relative angular orientations of the gears in the propulsive system. The present propulsive mechanism may be integrated into a wheelchair or retrofitted to existing wheelchairs.

Description:
RELATED DOCUMENTS 
   Priority is claimed from the U.S. provisional patent application filed Jul. 10, 2002 and having application Ser. No. 60/394,785. 

   BACKGROUND OF THE INVENTION 
   The present invention pertains to wheelchairs for human travel and methods of manual propulsion of such wheelchairs. In particular, the present invention is a propulsive device and wheelchair including a mechanism with a continuous reciprocating linear or elliptical input path. 
   A large number and portion of the population in the U.S. use wheelchairs to cope with mobility impairments. The limitations imposed on these persons as a result of their reliance on wheelchairs significantly reduces their quality of life. In addition, many wheelchair users experience musculoskeletal injuries of the shoulder, elbow and wrist. These injuries are caused by the high force exertions, repetitive motions and awkward postures inherent in use of standard wheelchairs. In addition, the basic propulsive mode with a conventional wheelchair is one of asymmetric muscle use (push mode) leading to unbalanced muscle development. These problems may be complicated by limited upper extremity dexterity of those wheelchair users having spinal cord injuries. Fifty to 70 percent of wheelchair users experience some sort of shoulder pain, rotator tear, or impingement problem that can permanently hinder mobility and increase medical costs. The incidence of carpal tunnel syndrome among wheelchair users has been reported to be as high as 63 percent and research indicates that this may be a result of nerve compression from forceful exertions of the hand and wrist during wheelchair operation. Repetitive strikes of the heel of the hand against the wheel rim of conventional wheelchairs may cause pain and numbness of the thumb and fingers. The interaction of the user&#39;s body and their wheelchair is a direct consequence of the basic design of the conventional wheelchair and the associated necessary mode of propulsion. 
   The design of the typical manual wheelchair has endured, largely unchanged, due to its simplicity. Relatively recently, advances to wheelchair design have addressed the need for improved propulsive mechanisms. Various wheel cranks, geared hubs and the like have been proposed in the prior art to reduce the necessary force for propulsion. However, these prior art devices do not effectively allow for altering the user&#39;s posture nor modify the basic propulsive motion itself the primary factors contributing to injury. What is needed is a mechanism for propelling a wheelchair which allows for increased use of large muscle groups in symmetric operation, improved posture and reduced contact pressure to the user. 
   SUMMARY OF THE INVENTION 
   The present invention is a novel wheelchair propulsion system and a wheelchair incorporating the propulsion system. The propulsion system provides a human user with a continuous reciprocating input drive path in operation of which the user&#39;s hands may remain on an input handle. The propulsion system includes a gear system defining a linear reciprocating or elliptical input path for the user&#39;s handle. Because the input path is continuous, efficiency is increased and impact on the user&#39;s body is reduced thereby reducing discomfort and physical demands on the user&#39;s body. The angle of the input path may be altered in any embodiment to suit the particular needs of a user by rearrangement of the relative angular orientations of the gears in the propulsive system. 
   In a preferred embodiment, a propulsive system is mounted on both sides of a wheelchair body. Each propulsive system is connected to a respective weight-bearing wheel of the wheelchair through gears and chains. In operation, each propulsive system may be driven, independently or in concert, in forward or backward directions to propel or turn the wheelchair in any direction. Because the propulsive system is directly connected to the wheels of the wheelchair, the propulsive system may also be used to slow or stop the wheelchair by resisting the movement of an input handle. In the preferred embodiment, the gears of the propulsive system are linked by roller chains. 
   In alternative embodiments, a single propulsive system is incorporated on one side of a wheelchair body and is linked to drive wheels on both sides of the wheelchair. 
   One benefit of the present invention is the ability to retrofit a propulsive mechanism according to the invention to many standard wheelchairs. Due to the simplicity of the propulsive system and wheelchairs using the propulsive system, the invention provides a relatively low cost alternative to conventional modes of wheelchair operation. Other characteristics and advantages of this invention will become apparent from the following drawings, detailed descriptions, and claims. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded perspective view of one embodiment of a wheelchair propulsion system according to the present invention. 
       FIG. 2  is a top view of the assembled elements of the embodiment of FIG.  1 . 
       FIGS. 3A and 3B  depict two different alternative drive input paths of various embodiments of the inventive propulsive mechanism. 
       FIG. 4  is a perspective view of a wheelchair according to the present invention including the novel propulsive mechanism. 
       FIGS. 5A and 5B  are partial side views of the wheelchair shown in  FIG. 4  with a propulsion mechanism at different relative angular orientations. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is an exploded perspective view of one embodiment of a wheelchair propulsion system  100  according to the present invention.  FIG. 2  is a top view of the assembled elements of the same embodiment. The propulsion system  100  is based on a Cardan type gear arrangement. A sun gear  12  is rigidly secured to a main shaft  14  such that the sun gear  12  does not rotate. A planet gear  16  is held coplanar with the sun gear by an orbit lever  18  and a back connecting lever  20 . The orbit lever  18  is rigid and is connected through needle bearings to the end of the main shaft  14 . At the opposite end, the orbit lever  18  supports, also through needle bearings, a planet gear shaft  22 . The planet gear shaft  22  is parallel to the main shaft and may be either rigidly secured through the center axis of the planet gear  16 , or in similar configurations, may be integral to the planet gear body. The back connecting lever  20  is rigid and rotates freely on bearings on the main shaft  14 . The outward end of the back connecting lever  20  supports the planet gear shaft  22  in needle bearings, opposite the orbit lever  18 . The back connecting lever  20  is rigidly connected to a drive gear  24  which is centered on the main shaft centerline. In this arrangement, the planet gear  16  freely rotates about its center axis and about the sun gear  12  and main shaft centerline. The planet gear  16  may be supported, alternatively, solely by the back connecting lever in a cantilevered configuration without an orbit lever. However, such a configuration is less rigid and has less strength than the design shown in the figures. 
   To provide the desired propulsive functions, the planet gear  16  is rotationally connected to the sun gear  12  through a roller chain  19  (shown schematically). For simplicity, the details of the teeth of the gears in the figures are not shown. Construction of the needed gears or chain sprockets will be known to those skilled in the art from the functional requirements provide herein. To provide the desired drive path, the effective size ratio of the sun gear  12  to planet gear  16  must be 2:1 (two to one), respecting the effective pitch diameter or other like functional size characteristic. A rigid input lever  26  is rigidly secured to the end of the planet gear shaft  22 . The input lever  26  includes a perpendicular grippable handle  27  that preferably is rotatably mounted on the input lever  26 . 
   In operation, the handle  27  is used to forcibly move the planet gear  16  which consequently is driven about the sun gear  12 . The orbit lever  18 , back connecting lever  20  and drive gear  24  in turn rotate to follow the movement of the planet gear  16  about the main shaft  14 . The input motion of the handle  27  to continue this motion of these elements is, in part, determined by the relative lengths of the orbit lever  18  and input lever  26 . If the length of both levers, from bearing the centerline to opposing end bearing centerline, is the same, the input motion of the handle will be reciprocating on a straight line defined by the line through the longitudinal center axis of both levers when mutually parallel. If the input handle  26  is shorter, the handle motion path will be an ellipse with a major axis through the longitudinal center axis of both levers when parallel. Herein the term “reciprocating”, with respect to the handle input path, is meant to indicate both an alternating forward and backward motion on a linear path and also continuous motion on an elliptical path as motion parallel to the ellipse major axis is effectively reciprocating for the purposes here. 
     FIGS. 3A and 3B  depict two different constructions of the inventive propulsive mechanism providing for linear and elliptical input motion, respectively. For simplicity of illustration and discussion, only the sun and planet gears and attached levers are shown. In  FIG. 3A , the orbit lever  18  and the input lever  26  have equal lengths LL. The resulting input path PP of the handle  27  is a straight line passing through the longitudinal centerline of both levers. In  FIG. 3B , the input lever is relatively shortened to a reduced input lever length SL less than LL. The resulting elliptical path of the handle  27  is shown. 
     FIG. 4  depicts a wheelchair  40  having a typical wheelchair body including a propulsion mechanism  100  as described above. The propulsion mechanism  100  is mounted to the side of the wheelchair body in a location easily operated by a seated user. The propulsion mechanism drive gear  24  is connected to a wheel sprocket gear  42  by a drive chain  43 . The wheel sprocket gear  42  is rigidly attached directly to the chair wheel  44  or to a wheel axle (not shown) to directly impart drive force to the wheel  44  (wheel spokes not shown). A second similar propulsion mechanism and associated wheel sprocket gear are provided on the opposing side of the wheelchair to function in a like manner to drive the opposing wheel. In the configuration shown, propulsion is directed to the main supporting wheels of the wheelchair. However, in alternative embodiments the propulsion force may be likewise directed to any wheels bearing on the supporting traveled surface. In further embodiments, the same propulsion mechanism may be attached to, or integrated into, other wheeled chairs. For example in one alternative, a single propulsion mechanism as described herein is attached to one side of a wheelchair and linked to a single cross-axle to drive both wheels. Use of the present propulsion mechanism may be combined with other prior propulsion and braking systems. Note that the configuration shown in the figures is “direct drive”. That is, the motion of the input handle  27  directly and continuously drives the rotation of the wheels. Reversal of the handle motion results in reversal of wheel motion as is preferred in typical wheelchair operation. If the user stops imparting input force to the handle while the wheelchair is in motion, any continued wheelchair motion due to momentum will drive the handle along its input path. In alternative, and less preferred embodiments, a coast function may be obtained by introducing a clutch or similar mechanism to disengage the input handle at selected times. 
     FIGS. 5A and 5B  are partial side views of the wheelchair  40  shown in  FIG. 4  with the propulsion mechanism  100  at different relative angular orientations. The mechanism may be adjusted to change an input path angle  50  by changing the angular position with respect to the sun gear—at which the input lever is oriented radially outward from the sun gear. The input path angle  50  is the angle between the major axis  51  and the horizontal axis (with respect to the normal attitude of the chair on a traveled surface). In  FIG. 5B  the input path angle is zero. By changing the input path angle  50 , the nature of a user&#39;s muscular exertions used in propulsion may be changed. In inventive methods of use, the propulsive mechanism path angle is adjusted to match particular muscular strengths of a user. 
   The embodiment and configurations shown in the figures provide a means of either linear or elliptical input motion for propulsion. Alternative constructions providing the same function are contemplated. For example, for simplicity and low cost, a roller chain is used to link the planet and sun gears. Alternatively, an idler gear may be employed to link the planet and sun gears in the same manner. The form of the gears, their teeth and associated roller chains is typical of a variety of standard elements used for similar purposes. Alternative forms, such as toothed fabric belts and matching gears are also contemplated in alternative embodiments. 
   Prototype propulsion mechanisms were constructed and mounted on a wheelchair to test the effectiveness and efficiency of the propulsive mode provided by the invention. The following Table 1 provides characteristic parameters of the prototype. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Prototype Mechanical Parameters 
             
           
        
         
             
                 
               Description 
               Units 
             
             
                 
                 
             
           
        
         
             
                 
               Length of orbit lever 
               3.15 
               inch 
             
             
                 
               Length of input lever 
               1.5 
               inch 
             
             
                 
               Planet gear pitch 
               12.5 
               teeth/in 
             
             
                 
               Planet gear pitch diameter 
               1.598 
               inch 
             
             
                 
               Number of planet gear teeth 
               20 
               teeth 
             
             
                 
               Sun gear pitch 
               12.6 
               teeth/inch 
             
             
                 
               Sun gear pitch diameter 
               3.187 
               inch 
             
             
                 
               Number of sun gear teeth 
               40 
               teeth 
             
             
                 
               Drive gear pitch 
               8.4 
               teeth/inch 
             
             
                 
               Drive gear pitch diameter 
               3.11 
               inch 
             
             
                 
               Number of wheel sprocket teeth 
               26 
               teeth 
             
             
                 
                 
             
           
        
       
     
   
   The above parameters result in an elliptical input path having a major diameter of 9.3 inches and a minor diameter of 3.3 inches. This form was selected, for test comparison purposes, to approximate a typical input path for a user of a conventional wheelchair using a “rim push” mode of propulsion. However, in the use of conventional wheelchairs in prior “rim push” modes, propulsive power is only delivered during forward motion of the hand. Due to the fact that the inventive mechanism provides continuous drive force throughout the input path, the inventive mechanism is more efficient with respect to a user&#39;s hand travel. In addition, the continuous reciprocating motion, requiring both generally pulling and pushing exertion by the user, results in symmetric muscle use and development. 
   Note that the mechanical advantage of a wheelchair with the inventive propulsion system may be modified by selection of different sizes (pitch diameter or number of teeth) for the drive gear and wheel sprocket. The relative size of these elements defines the relative rotational speed and torque in operation of each. Due to the compact design of the propulsive mechanism generally, and of the individual elements, and the nature of its attachment and connection with the wheelchair, the geometric envelope of the wheelchair is minimally increased. In particular, the side-to-side width of the wheelchair, at the outer edge of the wheels increased by only 2 inches. 
   A comparative test of the inventive propulsive mechanism and a conventional wheelchair was performed to evaluate the performance of the invention. A number of human able-bodied subjects performed identical timed exercises in both a conventional wheelchair using rim-push propulsion and in a wheelchair including the inventive mechanism. Subjective evaluation of the test subjects discovered no complaints of muscle or joint pain or discomfort due to use of the inventive device. However, shoulder pain typical of wheelchair users was reported during use of the conventional wheelchair with rim-push propulsion. 
   The preceding discussion is provided for example only. Other variations of the claimed inventive concepts will be obvious to those skilled in the art. Adaptation or incorporation of known alternative devices and materials, present and future is also contemplated. For example, the above description is with respect to one typical design of a wheelchair while it is obvious that the present inventive propulsion mechanism and system may be applied to a great variety of different wheelchair designs to obtain similar benefits. The intended scope of the invention is defined by the following claims.