Stephen J. Mraz
After starting an aerospace component- manufacturing company in 1997, Keith Entz and his company Entz Aerodyne, Valley Center, Kans., got caught up in one of the aerospace industry’s downturns. So around 2004 his partner David Kirkwood and Production Manager Matt Cochran started looking for another line of business. “We wanted to branch out to another market that wasn’t so cyclical and where we could be an OEM instead of a contractor,” says Cochran.
After some discussion, the three men decided to take advantage of two facts: the aging Baby Boom generation would soon be needing a vast array of medical devices, including wheelchairs. And David Kirkwood, the firms’ vice president and an engineer, had extensive, first-hand experience with wheelchairs, having survived an attack of bacterial meningitis that left him a triple amputee.
“Wheelchairs had been made pretty much the same way for the past 50 years, like patio furniture with bent and welded tubing and fabric,” says Cochran. “We thought wheelchairs were screaming for new technology. So we applied the same technology to wheelchair design that we had learned while building aircraft parts.”
The result? A new company, Aero Innovative Research Inc. (AIR) and the development of a strong, lightweight chair that is durable and comfortable. And when folded up, it measures only 9.5-in. wide, making it the tightest-folding wheelchair on the market.
Starting from scratch
The goals of the AIR’s design were strength and light weight, especially on the wheelchair components they would be manufacturing, including frame panels, caster fork, rear-axle plate, and folding foot plate. Other parts, such as the standard wheels, hand rims, and hubs, they decided to purchase from suppliers. But whether parts were made or outsourced, they also needed to have a built-in safety factor.
The team modeled and analyzed every part that would go into their Flight Ultralight wheelchair using Alibre Design, a CAD package, and DesignCheck, FEA software from Algor.
“Every part went through many design iterations,” says Cochran. “We analyzed 20 different versions of some parts, seeing where material was stressed, where we could remove material or add it. Basically we fine-tuned every part. It was laborious, but the software let us do it quickly.”
The caster flange, for example, was created and modeled in Alibre and then analyzed for linear stress, the only type of analysis this design project required. They subjected the flange to a 1,620-lb load to simulate the chair with a 250-lb occupant falling 3 ft onto one wheel. “It’s a load we wouldn’t expect a competitor’s tube chair to survive,” says Cochran, referring to the bent-tube construction commonly used on other chairs. “We tested the caster at this load as a way of overdesigning it, to ensure it could take the abuse, and to boost our confidence in its durability.”
FEA results showed stress concentrations on the flange, which suggested a design revision. “We added a fin to better distribute the load, and we ended up with a part that was stronger and also about 15% lighter than our first effort.”
FEA also helped the design team overcome the dreaded wiggling wheel, a problem well known to almost anyone who has pushed a grocery cart. “The front wheels mounted in casters on wheelchairs, like those on grocery cars, can flutter and turn sideways once they go above a certain speed,” says Cochran. “Simply adding a Delrin bushing, which acts as a damper, prevents the wheels from fluttering.”
Analysis also suggested they beef up the caster’s bearings if they wanted a really durable chair. “Most other chairs rely on plain bearings that can withstand about 300 lb of radial load. We use combination needle-roller bearings rated for 2,000 lb of radial load and 6,000 lb of axial load, and equipped it with a single-point grease fitting. Such fittings are common on aerospace components and make the bearings easy to lube. So the bearing should last the life of the chair.”
In the medical industry, wheelchair life is somewhat artificially set at between three and five years because that is how often thirdparty payers such as Medicare and private insurance companies will pay for a new chair. People usually hold on to the older chair and use it as a back up if it is not too worn.
AIR also designed their chair to fold up, but unlike other collapsible chairs with fabric-based sling seats, the Flight chair has a solid seat. Sling seats, like a hammock, are suspended from two sides and dip in a slight arc.
“Seats in traditional wheelchairs eventually sag, limiting internal rotation and abduction of the hips, plus they increase the weight put on bony prominences (in the pelvis),” says Dr. Kathy Lewis a professor of physical therapy at Wichita State University. “The resulting poor posture, increased risk of contracture (permanent shortening of muscles and tendons) and skin breakdowns (bed sores) increase health-care costs and decrease (patient) function. These problems are not an issue with a sold-seat design.”
The standard Flight seat is rigid and lets occupants sit up straight, preventing their hips from rotating. People with sling seats can get the advantages of a hard, straight seat by placing a lightweight carbon-fiber plate on their wheelchairs, but such inserts can cost up to $500. “And though a flat, rigid seat sounds uncomfortable, all chairs are used with some kind of cushion, and that can be anything from a 2-in.-thick air bladder to a 4-in.-thick air and foam combination,” says Cochran. “The Flight chair is quite comfortable, but you might have to sit in it to believe it.”
The flat seat has another advantage. If a person falls out of the chair, the rigid seat gives them something to grab hold of and lift themselves up onto. Sling seats, on the other hand, move side to side and don’t feel stable.
Wheel locks on the Flight chair are also somewhat different, especially in the way they mount to the chair. In traditional chairs, the locks, one per side, are held on by a double-tube clamp gripping a round tube. “Eventually, such clamps loosen or the lock components loosen,” says Cochran. “Our locks mount to a flat panel, so they stay put. We can also mount the locks in a lower position down closer to the casters, again by firmly attaching them to a flat panel. This lets more athletic people use a longer stroke in turning the rear wheels without running their hands or thumbs into the locking mechanism,”
Finally, the Flight chair folds into a compact, easily transportable package with a one-hand motion on the seat handle. There are no latches, and the spring-loaded footplate automatically folds out of the way. Other chairs take a least a second step to fold away the footplates. And there are no locks that keep the Flight chair open. “Our chair relies on its box structure to give it rigidity and keep it from folding.”
After a year and half of design work, which included building nine prototypes, the chair endured four months of FDA-required testing. One of the more rigorous tests involves a doubledrum device. Technicians strap a weighted dummy into the chair then run it on two double-drum rollers, one for each wheel. But each drum has a thick strap around it that runs perpendicular to the chair’s wheels. So as the drums spin, which make the chairs wheels turn, the chair is subject to a jarring impact each time it rolls over the strap. The test lasts for 200,000 cycles or impacts and is meant to simulate three years of wear. The Flight chair passed all its FDA testing, including the double-drum test, on its first try and without a single fastener coming loose. In fact, Kirkwood uses the chair that survived the life-cycle tests. “And it still performs like new,” he says.
When it comes to choice of materials and assembly techniques, AIR’s aerospace heritage shines through. The frame is CNC machined out of aircraft-grade aluminum that is bonded and riveted together. And the rivets are wet shot, meaning they are coated with epoxy when they are installed. “Because we don’t weld, which calls for softer metals, we can use stronger alloys with twice the tensile strength to make smaller and lighter parts,” says Cochran.
Most traditional chairs have bent and hand-welded hollow-tube frames made of 6000 Series aluminum alloys. “When those chairs come out of the jig, the welds cool and warp,” notes Cochran. “Welds are also inconsistent and manufacturers never know exactly what’s in the weld or how the heat used in welding changed the material properties of the aluminum. And hollow tubing eventually bends, kinks, and possibly breaks.”
With a solid, damage-tolerant frame that will take abuse, AIR then adds precision-machined components such as sandwich panels and gun-drilled hinges. The aluminum sheet metal for the panels gets CNC machined and are then bonded to a solid core of high-density foam with aircraft- grade epoxy. Each panel has threaded inserts that go though both skins to create a solid member through the panel wherever there is a fastening point. Companies making conventional chairs add components to a tube frame with clamps or by drilling holes through the hollow tubing. “And every time they drill a hole through the frame, they are making the chair weaker,” says Cochran. “Whereas every time we add a fastening point to our chair, we make the chair stronger.”
The hinges are machined from a solid piece of aluminum with a 1/8-in. hole drilled through all of the knuckles. “It’s not a rolled hinge in which metal is rolled over to form a tube. Our hinge is smaller and stronger, which helps save weight and makes the chair more reliable,“ Cochran says.
In keeping with aircraft standards, AIR holds many of its parts to 0.001-in. tolerances. Is that overkill on a wheelchair? “We don’t think so,” says Cochran. “It’s all about building a quality product, and it gives us several advantages.”
For example, the frame is precise, with squared, parallel sides. This means the rear wheels will be well aligned and stay that way. “Every 1.5° of misalignment between the rear wheels doubles rolling resistance,” says Cochran. “So our chairs roll easier, which translates into less shoulder fatigue, and over time, users will appreciate that fact.”
“The machined frame also means users can add, change, or replace components because all parts are made to the same demanding specs,” says Cochran. So if a young user grows or an older one loses weight, they can purchase individual parts to modify their chair, confident the new parts will fit and function as well as the original. They aren’t forced to buy entirely new chairs.”
Currently, AIR has 12 employees making about 80 chairs/month and the goal is to work up to about 1,000 chairs/year. The design team, meanwhile is working on using FEA and simulation software from Algor to improve their chair and to develop accessories that could be used on wheelchairs from competitors.