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Old 04-25-12, 09:52 AM   #1
Gyro_T
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Willing to build a custom walker?

I work in the wheelchair industry. Lots of aluminum alloy frames, some titanium. Walkers, or gait trainers are usually cumbersome and rather thick walled aluminum. I have an active 31 year old with cerebral palsy that I wanted to put in contact with someone that has the capability of bending and TIG welding titanium tubing in a configuration like the picture. The picture is an **** Bock Nurmi Neo, just shown for general design idea. Any leads on that would be highly appreciated.
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Old 04-25-12, 12:12 PM   #2
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Is there a design you are copying in Ti? Ti is an interesting, and high tech metal, but not the sure fire solution. It is twice as heavy a Al, and 2/3rd as strong as steel. So you can make some stuff for which al is too soft, and steel is too heavy. On the other hand. It is flexy, and can end up providing less stiffness for a given weight that al would at far lower cost.

One interesting structure is the Bike Friday suspension boom. It is a little controversial due to concerns about fatigue, but it goes to show an application where Ti's flexibility is in play. So think carefully of what kind of characteristics you want from this thing, and whether Ti is the solution, which if you are copying some existent product, should be easy to resolve. But Ti isn't a magic bullet.

One other material is carbon fiber. Since it doesn't require welding, it is relatively easy for people to work with, and these days fairly cheap. It can be difficult to build light since very little goes a long way for stiffness, but the structure can turn out understructured in places. It can take one or two tries to get a working proto, but it is a home building option.
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Old 04-25-12, 10:37 PM   #3
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Is there a design you are copying in Ti? Ti is an interesting, and high tech metal, but not the sure fire solution. It is twice as heavy a Al, and 2/3rd as strong as steel. So you can make some stuff for which al is too soft, and steel is too heavy. On the other hand. It is flexy, and can end up providing less stiffness for a given weight that al would at far lower cost.

One interesting structure is the Bike Friday suspension boom. It is a little controversial due to concerns about fatigue, but it goes to show an application where Ti's flexibility is in play. So think carefully of what kind of characteristics you want from this thing, and whether Ti is the solution, which if you are copying some existent product, should be easy to resolve. But Ti isn't a magic bullet.

One other material is carbon fiber. Since it doesn't require welding, it is relatively easy for people to work with, and these days fairly cheap. It can be difficult to build light since very little goes a long way for stiffness, but the structure can turn out understructured in places. It can take one or two tries to get a working proto, but it is a home building option.

The only equipment made of titanium that I know of are wheelchair frames some bits and pieces, and fore-arm crutches. The wheelchair guys are very specialized and bend each tube for each specific client. Each chair built by hand, much like a bike. They have some master TIG welders, but won't branch out from wheelchairs.

I agree, it is very interesting. My industry s used T-6000 and T-7000 heat treated and hydroformed aluminum alloy to try to match the strength to weight ratio of titanium. It has been only partially successful. I have seen some hydroformed heat treated aluminum alloy frames fail. This industry use to use 4130 chrome moly but it has been largely abandon. Aluminum may be lighter by volume, but for a thin walled tube, it hasn't matched the durability and lightness of titanium. I would love to see some of the really thin walled alloy steel be used in adaptive equipment, but the industry lags behind bicycles and ties its own hands with the viscous cycle between funding agencies, manufacturers and suppliers. I think the higher end gait trainers are made of something below T-6000 aluminum. If I cannot find someone to take on a custom project like this, I would like to explore using ultra-thin walled alloy steel and try it myself, but my gut feeling is that I would not achieve the lightness of titanium tubing there. I personally would not know where to begin with carbon fiber, although I built some surfboards as a kid. Thanks for your input. It is helpful

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Old 04-26-12, 01:14 AM   #4
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Maybe check in with Steve Garro in AZ?

Framebuilder in a wheelchair.
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Old 04-26-12, 08:52 AM   #5
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Steve only works with steel.

I would think you could build a fairly lightweight walker out of steel. Ti has issues in smaller sections, that's why you don't see Ti forks. This may actually be a good application of steel, because the sections would be smaller. Steel, Ti, and aluminum have virtually the same specific modulus, i.e. 'stiffness' per weight. This means that Ti and aluminum only beat steel if the wall thickness would get too thin with steel. I don't see this as the case with a walker.
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Old 04-26-12, 08:22 PM   #6
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Thanks for that information. I will see if I can get a message to Steve and ask him about it. I understand that True Temper makes a very thin walled tubing. This lady is only about 105 pounds. Any other suggestions on steel tubes? The aluminum alloy ones I have seen are around 1 1/8" O.D.
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Old 04-26-12, 09:15 PM   #7
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I don't understand what you mean by "issues with smaller sections". If you are talking about shorter sections of tubing then they must have solved that in these wheelchair which support the whole caster assembly with a short oval curved tube about 5 inches long.
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Old 04-26-12, 09:28 PM   #8
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Check out Ticycles in Portland. They make other titanium things besides bikes and components. They do stuff from ti wheel chairs to ti canes. Ticycles.com
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Old 04-29-12, 02:11 AM   #9
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Good call, some serious eye candy there!
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Old 04-29-12, 05:08 AM   #10
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I am sort of interested in what the design objectives are. If there are any.
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Old 04-30-12, 10:17 PM   #11
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Hello Frank, thanks for the response, The objectives are to make something very light weight for someone that has very limited ability to ambulate. In addition to that, it has to be very easy to fold. Currently, she has to push a couple of buttons to fold the front and back legs together and lift it over her head into the passenger seat. A unit like the one in the thumbnail above goes for about $730 retail. It has those yellow knobs that she has difficulty with when it is folded. The pushbutton folding is easier for her. Thanks also to H the III. The Ticycles site is pretty awesome. Back to the idea of carbon fiber...if I understand it correctly, the carbon fiber frames are layed up in molds of some kind, is that right? Polyester resin with carbon fiber fabric?
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Old 04-30-12, 10:25 PM   #12
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Originally Posted by Gyro_T View Post
Hello Frank, thanks for the response, The objectives are to make something very light weight for someone that has very limited ability to ambulate. In addition to that, it has to be very easy to fold. Currently, she has to push a couple of buttons to fold the front and back legs together and lift it over her head into the passenger seat. A unit like the one in the thumbnail above goes for about $730 retail. It has those yellow knobs that she has difficulty with when it is folded. The pushbutton folding is easier for her. Thanks also to H the III. The Ticycles site is pretty awesome. Back to the idea of carbon fiber...if I understand it correctly, the carbon fiber frames are layed up in molds of some kind, is that right? Polyester resin with carbon fiber fabric?
Carbon also comes in pre made tubes which I expect would be used to make a walker. Cheeper and easier than laying up over a foam core.
But yes, carbon also comes in a cloth not unlike fiberglass cloth and one uses the same resin to make it stiff.
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