Since the advent of manual wheelchairs with rear push wheels and front casters in 1869, not much has changed. Except for newer materials, most handrim and wheel designs remain about the same, using a single gear drive regardless of the terrain. One manufacturer, however, Seattle-based Magic Wheels Inc., is attempting to redefine the personal mobility landscape with a novel two-speed transmission, borrowing on technology used in bicycle design. Although the additional gears add a little more weight, the payoff in ease of operation is worth the tradeoff. Reduced shoulder pain, a problem among 80% of wheelchair users, makes the multi-gear mechanical drive concept all the more magical.
Back story
Steve Meginniss, an inventor and design engineer, came up with the idea of a two-gear wheelchair drive based on a technology transfer project at the University of Washington's mechanical engineering department. Meginniss had a hunch he could improve on UW's approach. Ten years and many iterations later, he succeeded.
Testing of a two-speed belt drive wheelchair prototype at Seattle University convinced Meginniss that a lower (high-torque) speed wasn't very useful without some sort of hill-holding arrangement. To prove his point, he used a floor broom as a brake, showing that hill holding made it possible to take extreme inclines at the lower (2:1) ratio. Six months later, Meginniss hit on the idea of designing an automatic hill-hold mechanism with override by adapting a self-locking hypocycloidal drive he built a decade earlier for a wrenchless, ratchetless swimming lane gate.
He then spent several months using CAD software to figure out if it was feasible to incorporate the hypocycloidal winch mechanism in an envelope that fit within the wheel hub, and could be shifted by moving levers on the rotating wheels. With the introduction of standard quick release axles on higher end wheelchairs in the 1980s, it seemed possible to make two-gear wheels that could be easily installed on existing wheelchairs.
Design dilemmas
“The biggest challenge was configuring a robust hypocycloidal drive to fit in a lightweight wheelchair wheel where every gram counts,” says Meginniss. That's because very thin parts must keep the gears engaged with high unbalanced drive loads. The next big challenge was shifting — getting gears to shift in a planar mechanism and making a wheel that could be shifted at any 360° rotary position, easily and with one hand. This took several attempts. Gears had to be shiftable by both strong paraplegics as well as quadriplegics with limited grip strength. Another challenge was getting the friction linings (that provide the hill-hold override capability) to hold a 250-lb. person on a 15° hill, but still override easily when the handrim moved in the reverse direction.
“It often took months to figure out how to resolve each design issue,” recalls Meginniss. “The biggest surprise was how difficult this project was from beginning to end. I felt like Edison trying his 1,000 light bulb filaments.”
Design impact
Magic Wheels two-gear drive wheels are now in use by a growing population of disabled people. The company expects the product to become a new standard for wheelchair drive wheels over the next 10 years as costs come down, weight is reduced, and more sizes and options become available.
For more information, visit magicwheels.com.