Must pulley and idler sizes match?



  • When I started with 3D printing I read or heard somewhere that idler size does have to match the one of the pulley. I did not question this at that time.
    Talking about a simple Cartesian where the pulley is just used to change direction of the belt.

    Yesterday I came across a blog post by E3D that they now also offer genuine Gates belts, pulleys and idlers. For pulleys they have various tooth counts but for idlers they only have 20T (or equivalently sized smooth idlers).

    So I started to question this previous "rule" I had in my head. But the more I think about it the more I confuse myself. 🤦

    Right now I am at a point where I think idler size does not matter since it is just a passive component rerouting the belt. But I could not explain why this would be true (or not).

    So can please anyone help me out here? I'm going crazy on this one. 😠 Also an example as part of the explanation would be much appreciated.



  • @wilriker Quick answer - no they don't have to match. Longer answer - why should they have too? The only thing an idler does is change the direction of the belt path. As a general rule, idlers ought not to be too small in order to minimise the bend radius of the belt but that's the only criteria I can think of off hand.



  • @deckingman Thanks Ian! That's where I ended up, too. I think I just needed somebody to confirm.

    I can't even remember where this "rule" came from. Not having this restriction makes much more sense.


  • administrators

    The critical thing is that where a belt attaches to a carriage, the sections of belt leading into and out of the carriage must run parallel to the direction of motion. For example, if you have a length of belt running in the Z plane parallel to the X axis with a pulley at either end and a carriage in the middle, the pulleys at the two ends can have different diameters as long as their shafts have different offsets in the Y direction to allow for the different diameters. The rule "same size idlers as motor pulleys" would apply if the two pulley shafts were lined up with the same Y coordinate.

    I hope this makes sense without a diagram.



  • @dc42 It took me a while to visualize this in my head (mainly because I mixed two different representations of XYZ 🤦 ) but now I think I got it.

    That though means that I need to adjust how my Y belt is fixed to the bed carriage since the fixing point and the pulleys are on slightly different Z heights. That leads to the belt not being strictly parallel to the movement direction - even though I have equally sized pulley and idler.
    Or I need to lower the bed carriage a few mm.



  • @wilriker I guess one way to visualise it would be to think of a right angle triangle where the belts attach to a carriage. If the belt path forms a hypotenuse, then it's wrong.



  • @deckingman Now, that sent me back into confusion state again. So now it's diagram time. 😁

    In the following diagram I showed what I understood to be bad in the upper part and good in the lower part. Did I get this right? This is only regarding belt paths where the parts attach to the bed.

    0_1546850636947_belt_paths.png



  • your lower picture is the right one the main thing to aim for is that the driving part of the Belt path needs to be a straight line and paralell to the axis being moved.



  • @dougal1957 Thanks for confirming.

    Just to be totally sure I got it right this time another diagram with a totally crazy and exaggerated belt path. I believe it is still correct, isn't it?

    0_1546852382388_crazy_but_correct_belt_path.png

    P.S.: Only one of these is supposed to be driven by a motor.



  • @wilriker yes that's my understanding


  • administrators

    @wilriker said in Must pulley and idler sizes match?:

    @dougal1957 Thanks for confirming.

    Just to be totally sure I got it right this time another diagram with a totally crazy and exaggerated belt path. I believe it is still correct, isn't it?

    0_1546852382388_crazy_but_correct_belt_path.png

    P.S.: Only one of these is supposed to be driven by a motor.

    Yes, that's what i was trying to convey.


 

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