Ah, *THAT* was what was coming

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Re: Ah, *THAT* was what was coming

Postby Nicholas Seward » Tue May 06, 2014 2:48 am

http://forums.reprap.org/read.php?14,350879

What do you guys think about this idea?
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Re: Ah, *THAT* was what was coming

Postby Dpharris » Tue May 06, 2014 1:14 pm

You could mount the additional pulleys on the same axle as the previous ones. In fact buying such might be cost effective. Many sailboats use such pulley system to give mechanical advantage. I did buy some bearings with grooves, so I may try it out. Stretch might be a problem.

David
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Re: Ah, *THAT* was what was coming

Postby Nicholas Seward » Tue May 06, 2014 3:50 pm

You technically have to keep the string in 1 plane to keep the length of it constant. If I go with 625vv's, it should be doable. I think what I might do is keep this arrangement but trade in the 608 stacks for 625vv's to make it lighter and more compact. It will only be slightly more expensive. I calculate that it will only be $8 more expensive to go with the specialty bearings.
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Re: Ah, *THAT* was what was coming

Postby Nicholas Seward » Tue May 06, 2014 5:26 pm

corexz.png
corexz.png (73.72 KiB) Viewed 71394 times


I am thinking this might just be enough to let the static friction of the system prevent a crash when power is lost. Unfortunately, I won't know this until I build it. Looks like I will have to prototype another version before I go to the final design.

I know that the 608 stack was my innovation and motivation to make this design but what do you guys think about the move to a 625vv. It will be lighter and more compact.

Judging only by the pictures, some 625vv's have different groove depths. 0.5mm seems to be in the majority. That could be a problem. I can just put a note on the BOM that indicates the groove depth should be .5mm. (I suppose a machine designed for a .5mm groove will work without complaint with a .75mm groove but the misalignment will hurt my soul.)

Side note: I am thinking that the 625vv may be used for my new GUS arm.

Side side note: There are even cheaper 623vv's. I don't like them for GUS but they might be great here. Thoughts?
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Re: Ah, *THAT* was what was coming

Postby cdsteinkuehler » Tue May 06, 2014 6:49 pm

Nicholas Seward wrote:Judging only by the pictures, some 625vv's have different groove depths. 0.5mm seems to be in the majority. That could be a problem. I can just put a note on the BOM that indicates the groove depth should be .5mm. (I suppose a machine designed for a .5mm groove will work without complaint with a .75mm groove but the misalignment will hurt my soul.)

Side note: I am thinking that the 625vv may be used for my new GUS arm.

Side side note: There are even cheaper 623vv's. I don't like them for GUS but they might be great here. Thoughts?


I like the idea of smaller and lighter bearings, but a quick search shows that the groove depth on these parts seems to be anything but standard. I sense recipe for disaster (or at least mass confusion, and much pain for Nicholas' soul). :)

What about instead of using a grooved bearing, you laser-cut some string guides out of the waste panel stock and use plain bearings? The OD appears to be substantially more "standard" than the groove depth!

It will probably be easier to string this version, as the guide slots in the flat-stock will likely hold the string in-place with no tension better than the grooved bearings. You could even make "L" or "S" shaped slots to discourage the string from wandering about until you tension it.

Overall this ought to be cheaper than grooved bearings, and you could use the guide pieces as part of the structure to make more of a keyed box truss if you need to support the bearings on both sides.
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Re: Ah, *THAT* was what was coming

Postby Nicholas Seward » Tue May 06, 2014 7:03 pm

@cdsteinkuehler: I don't know if I fully understand but I feel that it will be harder to string. A diagram may be needed.

I will do a full box structure for the next go around.

I could do a double stack of a smaller bearing but every other bearing is more expensive than a 608.

If I go with the 625vv's, the grooves seem to be anywhere between .5mm and .75mm. It probably won't matter which ones they get. However, I can put a big warning on the BOM with suggest suppliers. I haven't looked at the others to see what I range of groove depths I get.
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Re: Ah, *THAT* was what was coming

Postby cdsteinkuehler » Tue May 06, 2014 7:31 pm

BearingGuide.png
BearingGuide.png (17.01 KiB) Viewed 71387 times

OK, perspective drawing and I don't get along well, but something like this:

The round thing is a standard bearing.
The rectangular thing is a laser-cut string guide.
The red thing is the string.

The string guide slots I have drawn as straight, but they could have angles or curves to help keep the string from coming out of the slot until tension is applied to keep everything in place.
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Re: Ah, *THAT* was what was coming

Postby Nicholas Seward » Tue May 06, 2014 9:12 pm

I see. I think that could work. The downside is having a lot more laser cut parts. Grinding could happen that could weaken the string and cut unintended grooves in the wood. Any misalignment would eventually end in failure. While I think this would take a long time to fail, I think I want to stick with the grooved bearings or the double stack 608s to remove these failure modes.
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Re: Ah, *THAT* was what was coming

Postby tommythorn » Tue May 06, 2014 9:14 pm

$8 or so is trivial compared to making it easier to build.

All things equal, I really like the deep groves of the 623vv. I would find one or more good sources and design around their part(s). If need be, you could parametrize the design files or offer a couple of variants for the most common grove dimensions.
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Re: Ah, *THAT* was what was coming

Postby Nicholas Seward » Tue May 06, 2014 10:25 pm

@tommythorm: One bad thing about the 623vv is that I can't just throw a M3 washer between the bearing and the mount board. I have seen others use laser cut spacers out of thin acrylic. That really isn't a show stopper but it offers some barriers to DIYers. You can always print a spacer.

I have also found that the depth of the groove is of no real importance. The one and only time that you would care would be during stringing and that is infinitely easier than with GUS. (For the proto I made my life hard by not leaving an access hole for tying on the strings. I will fix that.)

I am leaning towards the 625vv because I can just purchase M5 washers and be done with it. Additionally, they are also what I currently want to use for GUS. I am also playing around with the idea of using 628vv's because they could match my M3/M8 scheme and GUS would benefit from having a bearing with a large ID.
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