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  • Hi, I want to know the best way to make pivot pins.

    Discussion in 'The main mechanical design forum' started by SavedByTheLamb, Aug 20, 2021.

    1. SavedByTheLamb

      SavedByTheLamb Member

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      Hi,
      I thought of a design for a wind spinner, but I want to know the best way to connect things so they will spin freely, and not rust (because it's for outside). Preferably using a simple design.
      2021.08.20. - Help for pins 01.jpg
      I expect one of the simplest solutions is using metal rods, tubes, and some sort of spacer.
      It would be good to only need to drill holes with the same diameter.
      2021.08.20. - Help for pins 02.jpg
      That is just one side, and would be the same on the other side, but reversed in direction, so it would all stay together.

      I'm sure there are many ways to do it. That is the best I can think of so far. Does anyone have a better idea?.. It's not that important, I don't even know if I'll bother trying to make it. I just want to know how easy or not it would be. There may be things in existence already that would do it; like a 3mm thick rod bearing type thing, but I have no idea what they are called if they do exist. The rings will probably be made of wood. These pivots/bearings need to be able to spin freely no matter their orientation, the force on them will going in different directions at different times, not just vertical. I realise this is well below most of your capabilities BTW, but I don't need to know about fluid dynamics at the moment, I just want to know how to get a bit of wood to spin freely within another bit of wood :)
       
      Last edited: Aug 21, 2021
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    3. SavedByTheLamb

      SavedByTheLamb Member

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    4. SavedByTheLamb

      SavedByTheLamb Member

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      Can someone at least tell me what type of metal would be best for making a bearing type thing out of tubing and rods (like in the diagram)?.. It needs to be rust resistant, but also resistant to wear....
       
    5. Dana

      Dana Well-Known Member

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      What kind of bearing is appropriate depends on a number of factors; what is the material the bearing is going into, how much does it weigh (different kinds of bearings are suitable for different amounts of loading), how fast is it spinning, how long does it need to last, how much must it cost? You also have to consider how you will assemble it; it's possible to design an assembly that can't, well, be assembled.

      For low speed and low loads, no bearing at all may be appropriate, especially if you're using a hard wood. Your glass bead as a spacer would work fine. You can also buy plastic bearings meant to press into a housing, with a flange to separate the two parts. Like these:

      https://www.igus.com/product/66?artNr=JFM-0306-10

      Of course, that requires a different size hole in the two pieces, but that's normal. Don't compromise the design and add an extra part just to have the same size hole in both pieces.

      You could glue the shaft into one side, or just make the hole a little undersize and press it in, like a nail. For that matter, an ordinary nail will probably work just fine for the shaft if you can find one whose diameter matches the bearing used.
       
    6. SavedByTheLamb

      SavedByTheLamb Member

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      Hi, thanks for replying.

      I think I would use wood or plastic. To get perfect circles they need to be laser cut I think, and some places do wood and some do perspex (too brittle probably) and some other plastics.

      Not sure of the weight; I intend for it to be about 30 cm wide, made out of 9 rings and a centre circle – 4mm thick.
      2021.08.23. - Tech drawing.png

      Not sure how fast it would spin. Depends on the wind and how good the bearings are.

      I’d want it to last as long as possible; being left out side in the wind and rain.

      I want the bearings to be pretty cheap I think. If I could get some really good bearings for about £10 I probably would.

      I found these the other day, but I think a 2mm rod would probably be too thin to deal with strong winds without bending. I tried bending a 2mm thick stainless steal nail and it was pretty easy.

      https://smile.amazon.co.uk/Atoplee-...&keywords=tiny+bearings&qid=1631286867&sr=8-5

      If they were 3mm inner diameter and 5mm outer I would probably use them, and make the rings thicker, to 6mm. They might rust quickly though.

      I think it would need to be assembled from the inside out, doing the centre disk first. I would get one shot at getting it right. Trying to disassemble it after it all being glued in place probably wouldn’t work.

      Not sure what type of wood they use, it’s specifically for laser cutting I think.

      Thanks for the suggestion of the plastic pieces with the flange; they are well expensive though it seems. Think I’d need 36; = £49.68. That is way more than I want to spend.

      The reason I wanted the holes all the same size is because I have no way to drill proper holes, no pillar drill, so wanted to ask the people doing the laser cutting to drill them. I don't know if that’s something they would do though. If I was doing the drilling myself I wouldn’t mind doing different sizes.Trying to do it with a hand drill probably wouldn't end well.

      “You could glue the shaft into one side, or just make the hole a little undersize and press it in, like a nail. For that matter, an ordinary nail will probably work just fine for the shaft if you can find one whose diameter matches the bearing used.“

      Yeah it’s a good idea. I did think if it but they would need to be perfectly straight and round though - the people that make them don't tend to care too much about them being perfect. I'm sure there are some that are good enough.

      Was looking at stainless steel tube and rod the other day; would that be the best metal for it, if I use that?.. Least likely to rust?... It's pretty cheap.

      Oh yeah; would 0.1mm difference between the rod and tube be enough for it to spin freely?..

      Cheers,
      Lloyd
       
      Last edited: Sep 17, 2021
    7. Dana

      Dana Well-Known Member

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      As they have no seals, those ball bearings you linked to won't last long outdoors. Plastic bearings like the ones I posted would work much better.

      The igus bearings I linked to are pretty cheap as bearings go, but if that's still too much you could make your own by cutting short lengths of plastic tubing. You wouldn't get the flange, but you could use your original idea of a glass bead or washer.

      You can laser cut the disks, but they probably won't be able to laser the bearing holes; the kind of laser system typically used can only cut out flat parts. There isn't any kind of wood "specifically for laser cutting"; typically they can cut any kind of wood within the size limitations of the machine.

      If you make the disks out of plastic, then you won't need bearings, the stainless pin riding in the hole in the plastic will do fine at the light loads this will see.
       
    8. SavedByTheLamb

      SavedByTheLamb Member

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      OK thanks.

      Yeah, I know they can’t cut the bearing holes with a laser :D
      I meant getting them to drill them with a pillar drill; they might have access to one.

      Ah yeah. I couldn't remember what they said about wood types but now I remember they use MDF a lot for laser cutting and it would probably be cheapest. I think some place called it laser cutting board or something, that's why I though it might be specially made for it.

      What type of plastic would be good for it?.. Lots of places do perspex but I think it’s too brittle (too hard to drill holes in). Plastic would be better, because it won’t need weatherproofing and repainting.

      I asked for a quote from a wood place and a plastic one and neither bothered replying.

      Here is how kind of how it would move, except it'd look very different with the wind pushing it instead of it being uniform. That is why it needs to be able to spin freely no matter which direction the force is applied to the bearings.
      https://imgur.com/a/0uqamBV

      It would be a lot simpler just using a pin in plastic. Would a 0.1mm gap work for the pin to spin freely?..
      2021.09.17 - Pins.jpg
      I might have to wait and get a pillar type holder for a Dremel to do the holes.
       
      Last edited: Sep 17, 2021
    9. Dana

      Dana Well-Known Member

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      You wouldn't want to use MDF; although it cuts well and is very stable when kept dry it has no moisture resistance at all and even if painted it won't last long outdoors.

      Perspex, also called "Lucite" or "Plexiglass" in the US, is acrylic, and probably your best choice. It's not difficult at all to machine and drill, though it's best if you use a [readily available] special drill bit optimized for it. Lexan (polycarbonate) is another good choice; it's softer but more impact resistant (they make bulletproof windows out of it) than acrylic. Both are good for outdoor use; many plastics aren't as UV light degrades them.

      0.1mm would be a reasonable clearance in this application.
       
    10. SavedByTheLamb

      SavedByTheLamb Member

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      OK thanks for the help. I'll try and find somewhere that laser cuts one of those 2; and find somewhere to do the holes or wait until I can do them myself.
       
    11. SavedByTheLamb

      SavedByTheLamb Member

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      Oh dear. Plexiglas are not as confident about being able to drill it. They say in this video the minimum distance to the edge needs to be 1.5 times the diameter of the borehole.

      My design is nowhere close to that. I want a 3mm hole in a 4/5mm thick bit of plastic. Looks like Polycarbonate is the only option then?..

      Links not going to the right spot; it's 1:20 time-stamp.


      I think I'm going to need to take it to a machine shop and get some professionals to drill it.

      Lol, he says the same thing about polycarbonate; "stay away from the edge":
      (Time-stamp 2:02)
       
      Last edited: Sep 19, 2021

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