Page 17 - Mobility Management, January 2018

Mighty Materials
As is so often the answer in CRT: It depends. All three materials offer wheelchair engineers (and, eventually, consumers) advantages and relative disadvantages.
For example, Jesus Ibarra, product manager for manual wheelchairs, Sunrise Medical, said of carbon ber, titanium and high-end aluminum, “The main advantage of all of them is that they’re lightweight materials. Each of them has pros and cons: e.g., carbon ber will get you the lightest weight, but overall, all of them are considered light weight.”
All three materials are also very strong.
“Each material has different strength values,” Ibarra said, “therefore giving you the ability to use different amounts of each material in order to achieve the desired strength.”
As an example, he compared a quantity of titanium with a quantity of steel that weighs the same amount.
“You have a titanium block that can weigh the same as a block of steel, but if you reduce the amount of material in that titanium block, you will not compromise the strength by going to less material. So for each material (carbon ber, titanium or aluminum), its strength-to-weight ratio is very high. This allows you to use less of each material and get the same equivalent strength.”
Engineering Equations
The three materials also differ from
an engineering perspective, said Eric Simoneau, CEO of Motion Composites.
“Not all materials are equal when comparing their density, mechanical strength and cost,” Simoneau pointed out. “A material might be highly ef cient for certain designs, and could be just the opposite if used for another purpose.”
He described aluminum, for example, as a commonly available material, “offering good mechanical strength and easy to machine. It has one of the lowest
densities of the metal alloys family. It needs less knowledge and experience to design and for fabrication than other compared materials.”
By comparison, Simoneau said titanium “is considered an exotic alloy and is a high-capacity material. It has a lower density than steel, and also offers extremely high mechanical strength. Its fatigue strength makes it a good material for wheelchair applications.”
And what about carbon ber, the mate- rial that Motion Composites is perhaps best known for?
“Carbon ber is also considered exotic due to its special manufacturing process and high material cost,” Simoneau said.
with — bending, cutting, welding — so all of the aspects of manufacturing get more expensive, and [titanium] is more expensive than aluminum.”
Carbon ber is lightweight, Ibarra acknowledged, “but it’s dif cult to work with in tooling and manufacturing. You can’t make a carbon ber tube and then bend it into shape. The tooling and mate- rial costs are higher than titanium.”
Finding Material Balances
Those practical issues, whether it’s the cost of a certain material or the materi- al’s inherent strengths and weaknesses, currently mean that ultralightweight wheelchair manufacturers typically
MobilityMgmt.com
MOBILITY MANAGEMENT | JANUARY 2018 17
A chair can be lighter, but if it is poorly tted to the user, it will feel less ef cient than a heavier chair correctly tted to the user —Prashant Srinivasan
“However, its density is lower than both titanium and aluminum, and its strength- to-density ratio makes it a top performer. Its characteristics can also lead to good energy absorption, making this mate-
rial a good candidate for a comfortable wheelchair. Carbon ber composites are highly resistant to fatigue and absorb high energy, which means it resists impacts. Moreover, carbon ber composites are appreciated for wheelchair designing because the manufacturing process allows complex and optimized shapes.”
The costs — not only the initial
costs of buying the materials, but also how much time and money needs to be invested to work with the materials — is another important factor that manu- facturers consider when making their ultralight frames.
“Aluminum is strong, it’s easy to work with, it’s easy to manufacture — to bend, to form, to weld — so it’s the lowest cost out of all three,” Ibarra said. “Titanium
is high strength, but is dif cult to work
work with multiple materials, even if they have a reputation for preferring one over the others.
Consider, for instance, Permobil. Its ultralightweight manual chair brand is TiLite, named for and famous for its work and success with titanium. But Permobil still works with additional materials in creating its ultralight chairs.
Prashant Srinivasan is Permobil’s Global Product Manager for Business Unit Manual.
“Titanium is a very durable wheelchair frame material that has proven its value over time,” he said. “It has a non-corro- sive, natural nish that doesn’t require paint or powder coat at all. It’s a nish that can look great for years and not show scratches from daily wear.”
A frame made of titanium, Srinivasan said, “retains shape and tension despite wear and tear over time due to better fatigue strength, and titanium’s damp- ening properties can result in a smoother ride for the client.”