INTRODUCTION
RESULTS AND DISCUSSION
REFERENCES

DESIGN OF AN INTERNAL THREE-SPEED HUB FOR A  MANUAL WHEELCHAIR
Rory A. Cooper, Ph.D.^*; Alan Nakahara, M.S.^**; James F. Ster III^**; Rosemarie Cooper, M.P.T.^*

From the *Department of Rehabilitation Science and Technology, University of Pittsburgh and Human Engineering Research Laboratories, VA Pittsburgh Healthcare System,**School of Engineering and Computer Science, California State University at Sacramento. Address reprint requests to: Rory A. Cooper, Human Engineering Research Laboratories (151-R1), VA Pittsburgh Healthcare System, 7180 Highland Drive, Pittsburgh, Pennsylvania 15206, Tel: (412) 365-4850, Fax: (412) 365-4858, E-mail: rcooper@pitt.edu

The low efficiency of manual wheelchairs makes them difficult or impossible for some individuals to use effectively. The purpose of this study was to develop a prototype three-speed wheelchair hub that could potentially increase wheelchair propulsion efficiency and increase mobility for people who have difficulty effectively propelling a classical manual wheelchair. The design criteria were based upon the development of a proof-of-concept prototype. Given the design criteria it was decided to pursue a planetary type gear system. We were able to identity a commercially available bicycle hub that met most of our design criteria. Therefore, we decided to modify a three-speed bicycle hub for wheelchair use. The total cost of the parts and supplies for this project were about $160. Approximately, 40 hours of machine shop time were required. Upon using the hubs, we discovered an unexpected problem that needs to be overcome in future design work. The pushrim moved about 15> with each stroke prior to engaging. This problem occurred in both the forward and reverse directions. Studies should continue to search for viable alternatives to pushrim wheelchair propulsion.

Key Words: Wheelchair, Assistive Device Design, Geared-Hubs, Mobility

INTRODUCTION :
          The common manual wheelchair is an effective, but at times inefficient means of conveyance¹. Moreover, there are many people who have difficulty effectively propelling a manual wheelchair due to pain, low cardiopulmonary reserves, insufficient arm strength or inability to maintain a posture effective for propulsion².
          Previous studies have shown manual wheelchair propulsion efficiency to be between 5% and 18%^3-5. The low efficiency of manual wheelchairs makes them difficult or impossible for some individuals to use effectively. In order to address this problem, several alternatives have been explored, including lever-drive units⁶, crank-drives⁷, and geared hubs⁸. However, none of these solutions have proven themselves practical or widely commercially accepted. Manual wheelchair users also experience a high degree of upper extremity joint degeneration and pain. The incidence of injuries to the wrist, elbow, and shoulder among manual wheelchair users to be between 25% and 80%^9,10. There is also some indication that the incidence of pain increases with the length of wheelchair use¹¹, and that cardiopulmonary fitness tends to decrease with age¹². These issues are compelling reasons to be investigating alternatives to pushrim propulsion.
          The purpose of this study was to develop a prototype three-speed wheelchair hub (see Figure 1) that could potentially increase wheelchair propulsion efficiency and increase mobility for people who have difficulty effectively propelling a classical manual wheelchair.
 

Figure 1. Photograph of prototype 3-speed pushrim

Design Criteria
          The design criteria were based upon the development of a proof-of-concept prototype. The following criteria were developed:

• The gear ratios should approximate three to one, one to one, and one to two for fast propulsion outdoors, indoor and standard use, and hills or ramps, respectively.

• The additional width of the wheelchair should not exceed 75 mm in order to maintain the function of the wheelchair in indoor environments.

• The wheel must be quick release (i.e., simply removable without the use of tools).

• The gearing system must be internal to the hub in order to keep maintenance to a minimum, and to prevent the user from coming in contact with the gears.

• The mechanism is to be lightweight, durable, and use as many original equipment manufacturer parts as possible.

Overview of Design
          Given the design criteria it was decided to pursue a planetary type gear system. We were able to identify a commercially available bicycle hub (Sturmey Archer, Chicago, IL) that met most of our design criteria. Therefore, we decided to modify a three-speed bicycle hub for wheelchair use. The gear ratio design criteria was not achievable with the bicycle hub. For this stage of the prototype development, we compromised for the 1.33 to one increase (i.e., 33% increase), one to one, and one to 0.714 (i.e., 28.6% decrease). The change in gear ratio is achieved by altering the gears that are turned, driven or held stationary. A ring gear encases the planetary gears, and a carrier encases the sun gear. With the sun gear held and the planet carrier turned, the ring gear becomes the output or driven member providing a gear ratio increase. With the sun gear held and the ring gear turned, the planet carrier becomes the driven member resulting in a gear ratio decrease. Low gear and high gear paws are used to hold the appropriate gear to achieve the desired gear ratio. When the clutch is moved to its extreme left position, the planet cage is turned and the gear ring becomes the driver. The high gear paws drive the hub and hence the wheel. At the same time, the clutch disengages the low gear paws. When the clutch is moved to the extreme right, the gear ring is turned and the planet cage becomes the driver providing the low gear. The high gear paws are retracted by the clutch. When the clutch is placed in the mid-position, both sets of paws drive the hub creating the one-to-one gear ratio.
          In order to be suitable for wheelchair use, we eliminated the free wheel function used to allow bicyclists to coast without the pedals moving. This was done to allow the wheelchair user to control the wheelchair via applying braking forces to the pushrims. To eliminate the freewheel, square slots were cut in the inner ball cup and the outer ball cap, see Figure 2. In addition, one of the low-gear paws was reversed to engaged when a braking force is applied to the pushrim. One of the high gear paws was also reversed and a new hole was drilled into the ring gear to accommodate the change in paw
 

Figure 2. Internal assembly drawing of the hub

orientation, see Figure 3. These changes resulted in removing freewheel in either low or high gear.
          The sprocket, that normally drives the bicycle chain, was replaced by a disk used to attach the pushrim. The pushrim adapter plate was made from aircraft quality aluminum. Three aluminum bars were bolted to the adapter plate to extend out to the radius of the pushrim. Through these modifications, the pushrim drives the hub, see Figures 2 and 4.
          A new shaft was machined with the sun gear attached 

directly to the shaft. A hole was drilled axially through the shaft. Another hole was drilled perpendicular to the axis of the shaft. A shaft was turned to fit inside the axial hole. A short section of the inner shaft was turned down 

proximal to where the cross-axis holes in the shaft were drilled. Two hardened ball bearing were inserted inside the axle to partially protrude from the cross-axis holes in the axle. A spring and steel plug were used to complete the ball-lock pin quick release axle, see Figure 5. Shifting was accomplished by adding a shifting knob with a graduated shifter to the out portion of the axle, see Figure 6.
          The axle must support the mass of the user and wheelchair during a variety of driving conditions. In addition, the hub is modified to use a cantilever axle to attach to the side of the wheelchair. The original equipment manufacturer (OEM) bearing was retained for the outer race of the hub. The inner bearing was replaced by a sealed ball bearing with a 12 mm inner 

diameter, see Figure 7. The ball cup of the inner hub cap had to be machined to accommodate the new bearing.
          The OEM parts were heated treated to a Rockwell hardness of 59 which is nearly that of machine tools. To modify the OEM parts, they first had to be annealed between 1450^o Ð 1500^o F and oven cooled for 12 hours. After the parts were modified they were oil hardened by heating them to 1450^o Ð 1500^o F and then quenching them in oil. These same parts were then reheated to 500^o F and oil quenched. The resulting hardness was around Rockwell 50.

RESULTS AND DISCUSSION :
          Stress analyses were done on the shaft and pushrim

 

Figure 7. Sealed bearing assembly

connectors. For the shaft analysis, we assumed a force of about 1000 N to account for the weight of the user and wheelchair distributed equally over both axles¹³. Since the force will increase while traversing common obstacles, a factory of safety of three was assumed. With this factor of safety, the assumed load becomes about 1500 N per axle. The approximate moment at the point of attachment of the hub to the wheelchair was about 69 N%m. With these values the maximum shear stress was calculated to be about 22267 kPa, and the maximum bending stress was 21561 kPa. Given these values, 4140 steel alloy was used.
          For the pushrim connectors, we estimated a static force of about 445 N. Using a factor of safety of three and the fact that there are three connectors, the force at each connector remains 445 N. The pushrim connector acts like a cantilever beam. The supported end will have bending moment of about 9 N%m. These values yield a shear stress of about 8274 kPa, and a bending stress of about 52954 kPa. We selected 1060 H6 aluminum for the pushrim connectors. Galvanic corrosion was not a problem because the area ratio of aluminum to steel was high.
          The total cost of the parts and supplies for this project were about $160. Approximately, 40 hours of machine shop time were required. However, this should be reduced by about half since the first prototypes have been completed. We have conducted minimal testing with the hubs.
          We were able to achieve all of the design criteria with the exception of the gear ratios. The cost would have been orders of magnitude higher to design a custom hub to achieve the desired gear ratios. This did not prove to be a problem, as the gear ratio appeared sufficient for a variety of driving activities. Upon using the hubs, we discovered an unexpected problem that needs to be overcome in future design work. Since the hubs were design for bicycles, and designed for driving only in the forward direction, there was too much free play in the pushrim. The pushrim moved about 15> with each stroke prior to engaging. This problem occurred in both the forward and reverse directions. As wheelchair propulsion is cyclic, the backlash in the gears and paws must be overcome with each stroke or change in direction. This movement is present, but negligible for bicycling, but it is troublesome for wheelchair driving. Studies should continue to search for viable alternatives to pushrim wheelchair propulsion. However, wheelchair driving has unique requirements that require specialized designs.

REFERENCES :

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2. Cooper RA, Quatrano LA, Axelson PW, Harlan W, Stineman M, Franklin B, Krause JS, Bach J, Chambers H, Chao EYS, Alexander M, and Painter P: "Physical Activity and Health Among People with Disabilities" J. Rehabil. Res. & Dev., 1999;36(2):142-154.

3. Lakomy HA: "Treadmill performance and selected physiological characteristics of wheelchair athletes" British J. Sports Med, 1987;21(9):87-133.

4. Glaser RM, Sawka MN, Laubach LL, Suryaprasad AG: "Metabolic and cardiopulmonary responses to wheelchair and bicycle ergometry" J. App..Physiol.: Respir., Env.& Exer.Physiol.. 1979;46:1066-1070.

5. Sawka MN, Glaser RM, Wilde SW, von LC: "Metabolic and circulatory responses to wheelchair and arm crank exercise" J. App..Physiol.: Respir., Env.& Exer.Physiol 1980;49:784-788.

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7. Arabi H, Vandewalle H, Pitor P, de LJ, Monod H: " Relationship between maximal oxygen uptake on different ergometers, lean arm volume and strength in paraplegic subjects." Eur. J. Appl. Physiol. & Occup. Physiol. 1997;76:122-127.

8. O'Connor TJ, DiGiovine MM, Cooper RA, DiGiovine CP, Boninger ML,: "Comparing a Prototype Geared Pushrim and Standard Manual Wheelchair Pushrim using Physiological Data" Saudi J. Disabil. & Rehabil., 1998;4(3):215-223.

9. Gellman H, Chandler DR, Petrasek J, Sie I, Adkins R, Waters RL: "Carpal tunnel syndrome in paraplegic patients" J. Bone & Joint Surg. (Am). 1988;70:517-519.

10. Nichols PJ, Norman PA, Ennis JR: "Wheelchair user's shoulder? Shoulder pain in patients with spinal cord lesions." Scand. J. Rehabil. Med., 1979;11:29-32.

11. Sie IH, Waters RL, Adkins RH, Gellman H: "Upper extremity pain in the postrehabilitation spinal cord injured patient" Arch. Phys. Med. & Rehabil., 1992;73:44-48.

12. Cooper RA, Baldini FD, Langbein WE, Robertson RN, Bennett P, and Monical S: "Prediction of Pulmonary Function in Wheelchair Users", Paraplegia, 1993;31:560-570.

13. Cooper RA: "Rehabilitation Engineering: Applied to Mobility and Manipulation" Institute of Physics Publishing, Bristol, U.K., 1995.