The Pokecart: a Therapeutic Hand Driven Cart

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The Pokecart: a Therapeutic Hand Driven Cart

Students: Deidra Garrett, Tara Harbin, Erin Williams
Client Coordinators: Dr. Gary Edwards, Marlese Delgado, United Cerebral Palsy of Birmingham

 

In this project, a toy car or cart is to be manufactured that the children at UCP can drive with only their hands. The desired outcomes of using the cart include the following: entertainment, increase of upper body strength, increase cardiovascular endurance, prepare for mobility and motorization, enhance cognitive abilities, encourage neutral hand grasp (opposed to hands turning in) and encourage a neutral upright position.

 

TECHNICAL DESCRIPTION

 

The general design is a tubular aluminum frame (length = 32 in., width = 28 in.) supported on either side by a 20-in. bicycle wheel. 6063-T4 aluminum tubing (o.d. = 1.66-in., thickness = 1/8-in.), has a higher strength-to-weight ratio than steel, is readily available, inexpensive, has good weldability and is ductile enough for bending. The sides are made of two lengths of tubing each, bent to 90° to make each of the four corners.  The two lengths of tubing on each side are cut at an angle and welded together to form an apex angle of 136° about 1-in. behind the wheel shaft. A triangular bracket made of 3003 1/8-in. thickness, aluminum sheet with a predrilled hole is welded to the frame at the bend so that the wheels can be bolted onto the frame at the wheel shaft. The two side portions join to a length of tubing that widens the cart with four 3-way 6061-T6 aluminum tees and sets of stainless steel hex head bolts and acorn nuts. The frame also has two straight pieces of tubing that join its front and the back.  This tubing forms the support for the seat.  These are connected to the outside frame by the perpendicular outlet of the four 6061-T6 aluminum 3-way tees—two for the front and two for the back lengths of tubing.  All of the fittings are bolted to the incoming tubing using three sets each of hex head bolts and acorn nuts.

The maximum load carrying requirement for the cart was 129 lb. Stress analysis showed that all of the stresses on the cart were very small (less than 150 psi) compared to the strength of the material, 13 ksi.  The maximum shear stress on the bolts is 118 psi, (< 30 ksi yield strength of 316 stainless steel bolts).  The stress at the bracket weld and the force required to move the cart were also calculated.  The shear in the weld was found to be 132 psi when fully loaded.  The allowable stress is 0.4 times the shear yield stress.  The shear yield stress of 6063 aluminum in the annealed condition has a value of 10 ksi.  Two 2-in. casters are attached to the frame, one on the front piece of tubing and the other on the back, to ensure stability and prevent tipping.  The cart generally rests on the front caster because the center of gravity is located in front of the wheels. The force required to move the cart was calculated as 1.5 lb.  This is acceptable since the average force that the children are capable of applying is 2.2 lb.

The driving mechanism of the cart includes two crank and gear systems, one for each wheel, similar to that of a bicycle.  It is made up of two gears and a roller chain. The handles, which were machined from bicycle pedals, screw into the end of the crank arms and face the inside of the cart.  The crank arm makes a 7-in. circle as the user turns it. A gear cover houses the gears and chain.  A prefabricated seat with the desired dimensions and seat belts already installed was purchased through the Abilitations catalog.  An additional seat belt is added that would sit on the children’s hips or the tops of their thighs.  The seat is attached to the frame with a series of two bolts and piece of aluminum sheet metal with two slits extend along the length of the frame, so the seat position can be adjusted.  The final desired position of the seat is locked into place using wing nuts.  Footrests (National Seating) are attached to the front tubing between the two central lengths of tubing that the seat rests upon.

The wheels are bicycle wheels, and therefore they require some type of covering to prevent the children’s hands from getting caught between the spokes.  Custom-made spoke covers, commonly used on wheelchair wheels for users that play sports, were purchased from Durable Medical [5].  The finished cart is painted bright yellow and a royal blue canopy is attached.  This canopy is made from flag material that will be easy to wash.  It is also easily removable, so the teachers can put the children in the cart more easily. Pokémon stickers add to the playfulness of the cart, which is shown in Figure 28.

28 - poke-cart
The Poke-cart hand propelled vehicle