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Proximals and elastic

Posted: Sun Apr 05, 2015 6:37 am
by Jason M Bryant
One thing that bothers me about my test hand is that getting the fingers fully closed is close to impossible. This isn't a problem on the 175% hand that I made, but it is on the 100% hand.

One place where I thought I could eliminate some friction was at the joint between the fintertip and the proximal. I've watched the fingertip bend closely, and noticed that it rubs at that joint so much that it doesn't slide around it. The elastic catches on the proximal joint when the fingertip bends around it. The joint at the palm doesn't catch the elastic as much, so that joint gets to stretch almost all of the elastic. When the fingertip closes, it has to stretch the little bit of elastic between the tie-off on the fingertip and the joint at the proximal. Obviously, stretching such a small amount of elastic is a problem.

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There is a little friction at the palm joint. However, most of the elastic slips past that joint, allowing it to stretch the whole line.

I came up with an idea to reduce the friction at the proximal/fingertip joint. I changed the proximal a little, removing a bit of the plastic at the end. This is the plastic that separates the elastic from the snap pin. Without that plastic there, the elastic touches the snap pin when the fingertip rotates.

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Since the snap pin rotates with the fingertip, this causes the elastic to be pulled around the pin with the fingertip. So instead of pulling on just that small length of elastic from the fingertip to the proximal, it's pulling the entire length of the elastic.

I made two proximals like this and wired them up to my hand. They did, in fact, rotate better than the other two fingers. In fact, they rotated so well that the fingertips tended to fully close before the rest of the fingers. This is not very good for picking things up.

I think there's still a little friction as the palm joint. If I could reduce that and make the joints close equally, this might be a decent change. As it is, it works a little too well and the fingers close in an inconvenient fashion. I may revisit this later, but right now it doesn't seem helpful.

This led me to realize that the problem is really the elastics themselves. My original test hand used rubber bands instead of elastics. I didn't have elastic bands yet. That hand closed super easy, while still snapping the fingers back up. I rewired that hand with elastics, and sure enough, it was just as hard to close the fingers as my current test hand.

A little testing determined that rubber bands are able to stretch to 500% of their original length. The elastics that I'm using stretch to 200% of their original length. The hand won't close easily because the elastics just won't stretch far enough.

This explains why my 175% hand closes so much more easily than the 100% hand. I don't mean relatively easier, it takes less absolute force to close the fingers on the big hand than the little one. It's all because the elastic has room to stretch on the big hand.

Side note: I'm not suggesting that we all start using rubber bands. Wouldn't they deteriorate faster? I think? The elastic on a small hand is regularly being pulled to close to it's full capacity, while the rubber bands never come close to fully stretched. I want to say that the rubber bands I got at the grocery store won't hold up as long as the nice elastic cord I bought, but I'm not completely sure.

At any rate, I feel confident in saying that my experiment in changing the shape of the proximal wasn't bad, just unnecessary. The proximals are 1% of the problem when the elastics are really 99% of the problem. Some people have done good research on hands that don't use elastics. These experiments make me think they're on the right track.

Re: Proximals and elastic

Posted: Sun Apr 05, 2015 2:50 pm
by jonschull
This is great systematic work.

We've explored Dental elastics as a standardized readily available product, but no one has taken the prospect across the finish line. I'm hoping present company will!

Re: Proximals and elastic

Posted: Mon Apr 06, 2015 2:13 am
by Jason M Bryant
Oh right, the Falcon hand. The thing I like best about the dental elastics design is that you could hand the recipient a little plastic bag of rubber bands and say, "If the elastics break, you can fix it yourself in about 30 seconds."

Re: Proximals and elastic

Posted: Fri Apr 10, 2015 3:23 am
by AdamArabian
Sorry folks - been lazy about keeping engaged here but the falcon hand dental elastics work really well. I think they're great for field maintainability and long term support.

The falcon hand was on haiatus for the last few months but we're driving it home over the next 10 weeks and will hopefully be fitting folks come May or so. Anyone who wants to use the dental elastics on a raptor variant I'd be glad to help out too.

Re: Proximals and elastic

Posted: Fri Apr 10, 2015 5:56 am
by Jason M Bryant
Hi Adam.

I think I'm going to start work on adapting the elastic bands to the Raptor Reloaded. Unfortunately, I can't get Fusion 3D and work with the original version of the files. So instead of making a nice version that maintains all the parametric features of the RR, I'll have to make a simple STL. However, I think that will be good enough for testing. If the tests go well, then we can submit it to be added to the official model.

Actually, I wouldn't mind it if you started a thread on the Falcon hand. You can update put up the new information on v2.0, plus what you're planning to do. I printed out a part of the gauntlet earlier today so I could see how the crimping system works. I wouldn't mind a dedicated thread to talk about that and a few other things.

Re: Proximals and elastic

Posted: Sat Apr 11, 2015 2:47 pm
by AdamArabian
New thread created... Feel free to comment on where we're going.

Re: Proximals and elastic

Posted: Mon Sep 12, 2016 8:27 am
by johnwest12
AdamArabian wrote:New thread created... Feel free to comment on where we're going.

Where is the new thread???

Re: Proximals and elastic

Posted: Mon Oct 10, 2016 10:49 pm
by jesusflorencio
hello. i need one like this one.
im from Venezuela
my name is: Jesus Colmenarez , 58 years old, needing the right hand.