Thread

1. Perz1val · 1786034311 · focus · HN ↗

   That center pin looks fragile, especially if it's printed along Z axis. How about printing teeth that'd mesh with the belt?

   1. CarVac · 1786035343 · focus · HN ↗

      Printed teeth are weaker than the belt's own teeth.

      Conversely, with this design the center pin could break loose and the belt would still hold, because the pin just has to stop the loop from fitting through the slot.

      1. tonyarkles · 1786035483 · focus · HN ↗

         That’s exactly it… if you tolerance the mouth reasonably tight, the centre pin doesn’t actually carry much, if any, of the load. Friction in the mouth carries a bunch of it, especially if you add the zip-tie clamp from my sibling post.
      2. altcognito · 1786035993 · focus · HN ↗

         Timing belts are crazy strong. From my FRC work the amount of abuse and weight they will deal with is handy.

         Belts will burn through polycarb if it isn't tracking properly. They are strong enough to deform any pulley mount on a tight fit.

         Only minor complaint is that the teeth will slip if you don't have a ton of them engaged (or they are excessively loose) but otherwise they are amazingly useful.

         1. wildzzz · 1786113762 · focus · HN ↗

            For FRC 2010, we built four custom gearboxes using timing belts. It was a pod with two wheels driven by a CIM motor to get a crazy amount of torque to get over the humps in the middle of the field. At any time, whether on flat ground or when tackling the humps, we would always have four wheels touching the surface. The most forward and rearward wheels would only touch the inclined . We couldn't go through the tunnels because our endgame mechanic was a big hook on top that would pivot out, latch onto a vertical pole, and pivot the bot sideways using a winch. It made for an amazing photo since few teams at that first regional event were even attempting the endgame.
      3. ErroneousBosh · 1786049659 · focus · HN ↗

         The pin doesn't even need to be captive, if you tug on the belt it'll jam in the slot kind of like a climbing nut in a crack.
   2. tonyarkles · 1786035376 · focus · HN ↗

      I agree that it looks fragile, but my experience with a similar kind of design in practice has shown that it’s not actually as big of an issue as it seems it would be.

      A few other things that can help:

      - for the “it can work itself out” problem, I’ve had pretty good luck on other projects by making a standardized “dual dovetail” (bowtie) male/female slot with clearance tuned so that the male piece fits tight into the slot. Printing the male part flat on the bed means that the filament is oriented correctly for strength

      - integrated zip tie slots. One design I worked on that was similar to what OP did but needed better grip… I made the slot where the folded over timing belt meshes longer, removed the printed centre pin, and put two integrated zip-tie slots: one near where the centre pin was, so I can use the zip-tie ratchet to actually pull the belt backwards and put tension on it; the other was near the mouth of the jaws, so that after tensioning it I could use a second zip-tie to “lock” the jaws onto the belt

      - a third one: you make a little block with dovetails that looks like the “circle plus mouth” piece from this post, but also include a pocket for a nylok that you can use to add tension to the belt after getting the belt secured inside the clamp.

      I have tried printing teeth that would mesh with the belt but didn’t get particularly good results from that. Possible that newer printers have better resolution and could print the teeth better, but my test with that just ended up with a little bit of a rougher surface and the teeth couldn’t actually mesh.

      1. 1e1a · 1786035761 · focus · HN ↗

         I have printed GT2 teeth on a prusa mk3s+ that mesh perfectly

         1. tonyarkles · 1786036409 · focus · HN ↗

            Awesome! I’ll have to try it again. Last time I tried was on some Wanhao i3 clone like a decade ago and it was not great, but the printer itself was not great all around other than the price :).
   3. _spduchamp · 1786040931 · focus · HN ↗

      It looks like the pin doesn't take much load. It looks like the pin holds the teeth open, while the load is on the shoulders of the entryway.
   4. shawnz · 1786044214 · focus · HN ↗

      If you printed teeth, you'd have to make sure the teeth in the top loop are precisely aligned in relation to the teeth in the bottom loop based on the length of the belt, right? That seems like it could be limiting

      1. kevin_thibedeau · 1786063406 · focus · HN ↗

         You don't need a loop, just a toothed slot.
   5. moebrowne · 1786055077 · focus · HN ↗

      A small bolt could easily replace the pin, this would also allow a washer to be added preventing any chance that it working loose.
   6. lelanthran · 1786091819 · focus · HN ↗

      You dont actually need the pin

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