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Double belt slack

Discussion in 'CNC Mills/Routers' started by nate campbell, Feb 14, 2016.

  1. nate campbell

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    Anyone else with a double belt setup fighting with slack on the top belt?

    On my machine with gt3 belts I can see the top belt moving as the carriages change direction. It will pop up off the bottom belt but some times not really become unaligned the the teeth on the bottom belt. Other times I can follow it from under the wheels of the and it is interlocked but at some point it will there will be slack in it. Hoping it's not related to my bottom belts.
     
  2. David the swarfer

    David the swarfer OpenBuilds Team
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    the top and bottom belts must be at the same tension.
     
  3. nate campbell

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    for me that seems easier said than done...
     
  4. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Did you put anything beneath the lower belt like foam tape to tighten up the gap under the wheels?
     
  5. nate campbell

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    no, used fairly thin double sided tape. I was thinking about this today. Have people had success with this?
     
  6. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Yes, and no. My ox is still a work in progress but I figured out fairly quickly that unless the gap was filled, holding the teeth in the belts together beneath the wheels the upper belt would slip relative to the lower one which is what it sounds like yours is doing. The problem came however in trying to get the belts to stick to the foam tape. Something about the belts having low surface energy causes them not to want to stick to the acrylic 3m foam tape adhesive. I finally worked out a system of applying adhesive to the back of the belt, letting it dry and then sticking the tape to the dried adhesive. This created a much more stickable surface. But this is where I stalled out and haven't gotten back to it....

    The tape should be in the 0.040" to 0.045" thickness range for the gt3 belt.
     
  7. Jonny Norris

    Jonny Norris Well-Known
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    I had the same problem with slack in a single belt. In the end I layed my hands on a buckle arrangement like adjusters on ruck sack straps, one could use one of these at each end of axis. One for top belt and one for the other belt.

    They are simply a 1.5mm 5mm wide strip of steel with two squares cut out that make up the buckle and a fixing hole.
     
    GrayUK likes this.
  8. gibson

    gibson Well-Known
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    can some one do a video on a doulble belt system
     
  9. Rick 2.0

    Rick 2.0 OpenBuilds Team
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  10. lucascorato

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    I have just finished my upscaled OX and came here for the same problem: slack on the belts are giving me ovals instead of circles, in a 45º axis. Being familiarized with 3D printers I know that this is a simptom of slack in one or both axis.
    My question: why use a double belt? Wouldn't one be enough? If the top one is tensioned on both ends, why use the another?
     
  11. Giarc

    Giarc OpenBuilds Team
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    It then becomes a rack and pinion. The second is glued down to the rails so it can't stretch. If the teeth are meshing then you should get good circles. Or switch to lead screws. How large is your ox?

    Another option if you have a large system would be to get wider belts so you have less issue with stretch.
     
  12. lucascorato

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    That makes sense now for me, I mean gluing it to the rails. My cutting area is 105x160cm. Lead screws are not a option because that would mean substancial physical changes to my machine, and with those lenghts, the screws would surely arch. I'll try gluing the bottom belts, and if it doesn't work I'll have to use metallic rack and pinion I guess.

    I think, though, that at least a portion of the slack is on the section of the top belt that goes up to the motor and down back to the pulley. Could it be it?

    Thanks for replying Giarc!
     
  13. Giarc

    Giarc OpenBuilds Team
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    I believe that the 3M double-sided tape is the way to go so the belt is slightly elevated as Rick mentions above. Double belt slack
     
  14. David the swarfer

    David the swarfer OpenBuilds Team
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    tension your belts with a spring balance. I pull to 12kg. and make sure they are all the same.
     
  15. pauna

    pauna New
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    hi
    Have you ever seen this GT3 timing pulley? The GT2 timing pulley is shown in the photo.
    I couldn't find the GT3 dual timing pulley anywhere.
    Do you think a double belts won't work on the "worker bee" X axis? Screenshot_20240119-011159~2.png
     
  16. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Not quite what is meant by dual belts. The dual/double belt system refers to two belts facing into each other with the teeth interlocking to prevent belt stretch. See video above.
     
  17. pauna

    pauna New
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    Of course, I know this, now I was going to install two belts with the above system, but as it turned out, one of the old belts will not fit, I need both new ones and I bought them on the Internet.
     
  18. pauna

    pauna New
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    While I'm waiting for a new set of ramen, I decided to go back to just one ramen. But I noticed something similar. If you tighten the belt, its movement is uneven, as if it gets stuck, and then weakens... I don’t know how much this is visible in the video, but from the sound it seems so. Is the motor having problems with the axle? Or even the need for motor maintenance?

     
  19. David the swarfer

    David the swarfer OpenBuilds Team
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    DO NOT push the carriages around by hand unless you have disconnected the motors from the drivers (with power off).

    cyclic stiffness may be due to the pulley being off center. they should be a very close fit, not a 1/4" hole on a 6mm shaft, that woudl give 0.35mm off center which you would see and feel in cyclic stiffness.
     
  20. pauna

    pauna New
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    Of course, the motors are switched off and have no contact with the driver.
    ....

    You think that the problem is not in the engine, but in the GT3 timing pulley. These are the originals that came from the very beginning.
     
  21. pauna

    pauna New
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    Has anyone changed stepper motor bearings? I think it's his fault that he gets stuck every now and then.
     
  22. David the swarfer

    David the swarfer OpenBuilds Team
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    cyclic stiffness is an off center pulley
    not the bearings, they will outlive us all unless you pour sand in them
     
  23. Gary Caruso

    Gary Caruso OpenBuilds Volunteer
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    This is totally normal, if you disconnect the motor from the driver it will run smoother. Also some fear back voltage damaging the drivers so not advised to move so quickly with motors connected, yes even when not powered. (edit looking at the video again looks like the motor is disconnected)
     
  24. pauna

    pauna New
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    I removed the pulley and put it back. Nothing helped. What came with the head was the same, 0.35 mm, but I couldn’t measure it. I think one and then 6 mm.. :(
     
  25. pauna

    pauna New
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    I already have three pairs of belts. I will attach a double belt to the X and Y axis. If only the X axis?
     

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