Ideas for the next Wally.

Re: Ideas for the next Wally.

Postby realthor » Fri Apr 24, 2015 4:38 pm

@Nic: with the redirect pulley I would do a two step process, like I explained:
1) from motor shaft to free-wheeling pulley on shoulder: this would be identical with the first setup we have discussed; the free-wheeling pulley will have two sections, an upper half that will do the 1st job and a bottom half that will serve as the connection to the elbow drum.

2) from the free-wheeling pulley I would create two separate loops to the elbow drum like in the sketch below:

screenshot.155.png
screenshot.155.png (112.97 KiB) Viewed 15873 times


Actually they will not be closed loops, but they will be bolted to the drums so that 180deg of rotation will be available to each side. What do you think, will this work? Even the tensioning can be configured in the plastic bit that will press the string into the drum.
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Re: Ideas for the next Wally.

Postby Nicholas Seward » Fri Apr 24, 2015 5:52 pm

drums.png
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Here is one string arrangement. I couldn't figure out another arrangement that would keep the string between the drums always aimed in the same direction. If the string between the drums starts to change angle then you will lose accuracy.

I like all your solutions for the free wheel. They are in line with the direction I was/am thinking about going.
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Re: Ideas for the next Wally.

Postby realthor » Fri Apr 24, 2015 6:29 pm

I did some rough measurements and for our situation with 100mm diam drum it doesn't look quite that bad:

screenshot.158.png
screenshot.158.png (33.01 KiB) Viewed 15870 times


The free-wheeling drum would gain some 10 more mm perhaps. I was thinking for the free-wheeling drum to elbow drum straps to use poly strap (the plastic ribbons the big packages come strapped with, usually white). it has to be flexible enough to go round the drums, thin enough and strong.
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Re: Ideas for the next Wally.

Postby Nicholas Seward » Fri Apr 24, 2015 7:08 pm

I don't think the wraps can work like you have drawn. However, I agree with about how big the pulley/drum has to be.
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Re: Ideas for the next Wally.

Postby realthor » Fri Apr 24, 2015 8:16 pm

It is maybe a bit exagerated but I tested and it seemed to go like that. Anyway this is going to work in principle, can put this behind:).

On another pressing matter: The Z motor. I have to put it at the back of the SCARA Arm, so going up and down Z can be done in two ways:
1) imagine a Delta design and one of the 3 carriages is my whole bot.
2) a motor with a winch winding up the string as the bot goes upwards on Z.

This seems to be the only motor that begs a belt and although I try to avoid expensive parts I can't find a better solution. I don't want to use a screw. A string/belt solution only.

Also I believe this part must be the sturdiest of all since it will pull itself and the whole Scara Arm with the other steppers and extruders up.

Can you advice on a better one?
Last edited by realthor on Wed May 20, 2015 9:06 am, edited 2 times in total.
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Re: Ideas for the next Wally.

Postby Nicholas Seward » Fri Apr 24, 2015 8:51 pm

A block and tackle setup could do it cheaper than the belt reduction. (I would keep all the pulleys in one plane similar to the CoreXZ.) You could also use a differential pulley setup.
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Re: Ideas for the next Wally.

Postby realthor » Fri Apr 24, 2015 9:08 pm

dictionary time ...:D ... I would still need to wind up somewhere the cable i'm pulling right? A block and tackle would help with reducing load but the easier it gets to lift the more cable I have to wind up a winch, right? And while this would also increase the Z resolution, the only way I see the winding up of the cable on the winch is the way I have sketched it.

Image
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Re: Ideas for the next Wally.

Postby Nicholas Seward » Fri Apr 24, 2015 9:16 pm

Yep, spooling string without causing triangulation error would be challenging. The spool typically has to rotate as it also move axially. The screw nut trick applies here.

You can also use a gear reduction for your spool. Normally you need to worry about backlash but gravity will always load the gears from the same side.
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Re: Ideas for the next Wally.

Postby realthor » Fri Apr 24, 2015 9:43 pm

aha!!... so I can use gears instead of the belt? I was worried about backlash. So you are saying that because of the gravity the backlash would be less important because the teeth of the gears would always be pressing on one single side on the other gear (backlash is more evident when the rotation goes both ways).

Ok, that's one less problem then. I can get rid of the belt.

Next question would be if the gears themselves can be printed if the load on them is not very high (if I will use a compound pulley system I can achieve that reduction in load). Would any gear profile (ex: GUS Simpson's heringbone gears) work better than the others? If I can print them then it means I can use whatever profile without the added cost of manufacturing the metal specialized gears.
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Re: Ideas for the next Wally.

Postby Nicholas Seward » Fri Apr 24, 2015 10:36 pm

For this application, you will want herringbone gears with an involute profile. GUS uses a cycloidal profile specific to rolling contact. Herringbone is important for printed gears to average out a bad print. If you don't have a spiral (helical or herringbone) in your gear, you can see artifacts from the individual teeth hitting that aren't a perfect involute profile or the gears and imperfectly spaced.

You want herringbone instead of helical so you can balance the axial load on the gears. (This is a small concern.) If you do use herringbone, one side needs to be free axially so you don't force the gears into a misaligned position. (The Vs don't match up.)
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