The Wheelchair Trainer

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The Wheelchair Trainer

Client: Marliese Delgado, UCP Hand in Hand

 

Marlese Delgado proposed a device that would allow children with CP a means of locomotion by way of a hand powered cart, for children from ages three to five diagnosed with spastic CP.  More specifically, the device would focus on kids with insufficient muscle tone to mobilize a wheelchair.  The device would also increase endurance, enhance muscle tone, promote peer interaction, and improve perceptual motor skills.  The occupant would be in a seated position, with both legs extended in front.  The design constraints included: 1) The height and weight for the 25th percentile three year old to the 75th percentile five year old were used – weight not exceed 50 lbs. and height should not exceed 47 in (NCHS, 2003). 2) The device should require minimum arm strength to provide propulsion. 3) The children should have the ability to turn and stop the device with relative ease. 4) Design should permit staff members to manually propel the occupants.

TECHNICAL DESCRIPTION

 

For the frame, the team chose to use 3/8” carbon ASTM A36 tubular steel, purchased in sections of threaded tubing, along with t-junctions, elbows, and sleeves.  A modular frame was developed and welded into place to accommodate the rear bicycle wheels as well as the seat.  The dimensions of the frame were based on the Head Start Design Guide (Olds, 1987).  The device is approximately 25” from the seat back to the footrest, and has a 30° knee flexion incorporated into it. The height of the seat back is 22”, so this will allow variability with children.  The width of the frame (15.5”) was designed to accommodate the width of the seat, which was 14”.    A push handle was welded onto the back of the device, approximately three feet high (35”), for control and access by the staff. The gear shifters were located on this push handle in such a way as to provide easy access.

The seat consists of a molded plastic body bolted onto a metal supporting frame, and was taken from a children’s stroller. The seat is vertically adjustable from 55° to 125°.  A degree of declination was built into the frame by mounting the seat at a constant 15° angle.  For trunk restraint, a 3-point harness was attached to the seat.  To restrain movement of the pelvis, another buckle was mounted parallel to the seat. The upholstery consists of two 1-inch thick padded cushions covered with black vinyl.  The padding material was attached to the seat by strips of Velcro, which allows the seat to be removed at the staff’s convenience for cleaning.  For the front leg/feet support, a diamond-plated sheet of aluminum was bolted in the horizontal plane of the frame.  A second plate was added at an angle of approximately 70° from the horizontal, to serve as the footrest.

Two large (1 in.) aluminum caster wheels were purchased, complete with bearings, to serve as the driving wheel.  The driving wheels were coated with a red polyurethane surface and were threaded into the existing tubing using a 1” sleeve. The rear wheels were custom-made 20” bicycle wheels, complete with spokes necessary to hold the gearing hubs. Two Shimano Nexus 4-speed internal gearing hubs were selected. A sprocket was mounted on the rear wheels that was approximately ½ the diameter of the sprocket on the driving wheel, thus decreasing the gear ratio by a factor of two.  The final gear ratios were 1-.5, 1-.61, 1-.75, and 1-.92.  The coaster brake version of the Nexus hub was chosen for its turning ability and it provides a means to abruptly stop the device. The propulsion system was covered by a sheet of aluminum bent at a 90° angle, and bolted to the frame of the seat. The rear wheels and spokes were not covered, similar to many bikes already present at Hand-In-Hand.  The output Force Ratio, F2/F1 = 1.4, gave a 40% mechanical advantage. For the front wheels, free-swiveling casters, each with brake apparatus were attached to the frame.  The frame, driving wheels and the footrests were painted with a red Krylon paint.  Motion activated beacon lights were added to the rear tires.  The final product is shown in Figure 6, and cost $1069.

6 - Wheelchair trainer
Figure 6.  The wheelchair trainer.  While seated, the child turns the yellow driving wheels which independently drive the rear wheels at a mechanical advantage to the child