Machine Design

Simplifying The Wheelchair Wheel

The wheelchair uses a wheel with the handrail built right in, eliminating the need for fasteners and reducing the time it takes to assemble the chair.

Wheelchair maker Everest & Jennings Inc., Earth City, Mo., developed a wheel that combines two production processes: solid injection molding for the wheel itself (hub, spokes, rim, and track for the rubber treads); and gas-assist production for a hollow, 1-in.-diameter handrail. The move eliminated the eight bolts and spacers that once attached a chromed steel handrail to the wheels of its chairs.

The wheels are Capron 8331, a 14% glass-filled composite. It gives a smooth black surface and high strength and rigidity. Wheels come out of the mold ready for use. They don't have to be cleaned, sanded, or trimmed. The handrail affixes to the rim by eight permanent standoffs, creating a structurally true and round wheel. Gas-assist molding of the rail reduces production time by 30 sec and saves $1.46/part compared to old methods. Using less material and eliminating fasteners pushes savings to $3/wheel. Had solid-wall injection created the rail, manufacturing times would jump by 70 sec/part and cooling rate differentials could leave sink marks in its thicker areas.

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