25 July 2017
25 July 2017
Frog Legs, of Ottumwa, Iowa, US, partnered with PlastiComp when it switched from machined aluminium to a carbon fibre composite for a new generation of its wheelchair caster wheels.
Frog Legs got its start in 1997 when a chance encounter with a quadriplegic rugby team led founder Mark Chelgren to develop an alternative to the traditional rigid casters used for the front wheels on wheelchairs that allow them to make tight turns. Instead of fixed forks that require the wheel to move up and over obstructions – bouncing the rider in the process – Chelgren designed forks with a patented pivot point and wedge shaped shock absorber that allows them to move over impediments in an arc path, functioning much like airplane landing gear, smoothing out the ride.
“An arc is a much more efficient movement,” says Chelgren. “But its compression zones are not linear, so instead of a traditional cylinder shaped shock absorber, Frog Legs uses a wedge shape to better handle the differential loadings from front to rear.”
People with physical disabilities who require wheelchairs for mobility literally feel every bump in the road as their chairs encounter obstacles as seemingly insignificant as cracks and uneven surfaces. A day filled with constant vibration and jolts from these hindrances causes fatigue and can produce spasms in riders that require treatment with medication. Testing performed by the Centre for Industrial Research and Service at Iowa State University showed that 80% of the vibrations wheel chair riders experienced were created by the front caster wheels and that Chelgren’s design improvements reduced those vibrations from being transmitted to the wheelchair frame by 76%. But Chelgren knew Frog Legs could do better.
“We are always being pushed to lighten our product,” he says. “People with disabilities are greatly affected by any additional weight.”
“This time we looked at a different type of manufacturing process than machining aluminium,” explains Chelgren. "Injection moulding allowed us to have a much more complex shape and carbon fibre composites really gave us advantages in what our design parameters could be.”
“If the idea is to take an aluminium part and make it using carbon fibre, I think people are missing the boat,” he continues. “A carbon fibre part and an aluminium part shouldn’t look identical, instead you need to design for the material and process combined. It would be almost impossible to machine or forge the bottom part of our new fork design.”
Frog Legs’ second generation caster wheels make use of two different long carbon fibre reinforced composite materials from PlastiComp. Machined aluminium has been replaced by a long carbon fibre reinforced nylon 66 composite in the wheel forks and a long carbon fibre reinforced thermoplastic polyurethane in the wheel hub. PlastiComp custom developed the reinforced polyurethane composite to bond with the urethane used for the outer rolling surface of the wheel.
“Using thermoplastic polyurethane for the hub allows us to obtain a chemical bond,” says Chelgren. “With an aluminium hub there are dissimilar materials that never fully bond and can slip.”
Changing from aluminium to long carbon fibre reinforced composites reduced the weight of a pair of Frog Legs caster wheels by 33% – over a 0.5 lb (280 g) lighter – a reduction that is appreciated by riders.
The new design and materials passed RESNA (Rehabilitation Engineering Society of North America) industry standard testing that put them up against million cycle impact and drop forces. Frog Legs went on to test them to failure to have data to compare with traditional metal caster wheels.
“Then of course you have real world testing which effectively puts the product through the most extreme circumstances you can find to try and induce failure,” says Chelgren. “You are never 100% sure if something will work in the field differently than expected with regards to dynamic loading.”
Frog Legs took advantage of PlastiComp’s application design and performance analysis services throughout its product development cycle.
“Overall the efficiency of our product is now much higher,” says Chelgren. “We could see from the computer modelling what the benefits were going to be. I don’t think we could have developed a product nearly as good as the one we got without that assistance.”
Frog Legs’ caster wheels are not just a solution for wheelchairs. They can decrease vibration in all types of rolling equipment where there’s a weight versus performance equation.
Photo provided by PlastiComp
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