String configuration

String configuration

Postby Samuel » Mon Dec 02, 2013 8:42 pm

Just a general thread concerning the arm string, to continue from the reprap forum.

First question: has anyone had any difficulties with string friction / wearing around the bolts? From looking at it, it doesn't seem like it would be too much of a problem, but has anyone tried putting a plastic/metal sleeve over the bolt to wrap the string around instead? It could be an improvement, though maybe not enough to warrant an extra piece.

I could see it being more important on the pure-plastic Simpson though.
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Re: String configuration

Postby Nicholas Seward » Mon Dec 02, 2013 9:49 pm

I think a bearing would be preferred over a sleeve. The bolts/string combo is very slippery. Now that I have it down to one string sliding per bolt it wouldn't be crazy to switch to bearings. (You would need 12 bearings and a drastic redesign of the arms.)

BTW, I went from a 22mm pulley with 1.5 wraps to a 12mm pulley with 1 wrap. This is a 22% in increase in mechanical advantage plus the reduction in friction. I don't think we should have any problems.
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Re: String configuration

Postby Samuel » Mon Dec 02, 2013 11:12 pm

What about for the pure plastic Simpson? In that case, bearings would obviously be an issue. Would you try to print roller bearings, use a sleeve, or just let the string slide on a printed rod.

It would be nice to use a ptfe sleeve/rod, but ptfe can't be printed very easily...

Or would you just replace the string with a different mechanism in a pure plastic version?
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Re: String configuration

Postby Nicholas Seward » Tue Dec 03, 2013 3:48 pm

My plan is to use some kind of printed bearing. Something like this but with a cycloidal profile instead.

Now that I am down to only 1 loop around the two bolts this is very doable.
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Re: String configuration

Postby Dpharris » Wed Dec 04, 2013 12:47 am

Wow, this means the whole GUS can plastic. I sure wish I had a printer so I could get into this now .. Mine is expected in March. Amazing speed of development!
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Re: String configuration

Postby Ralphxyz » Sat Dec 07, 2013 5:14 pm

That bearing is definitely cool,but wouldn't you have to drastically reduce the size of it?

Ralph
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Re: String configuration

Postby Nicholas Seward » Sat Dec 07, 2013 7:13 pm

Nope. I can have the outer race integrated with sides. The inner race can extend out and be my "bearing".
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