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Some quick questions regarding idler pulleys

Posted by asdasd123 
Some quick questions regarding idler pulleys
September 19, 2017 02:08AM
Hi all

In the attached image, assuming number 1 is the motor's toothed gear. On which of the others (2-6) should I use smooth pulleys, and which should be toothed? The crudely drawn dots are the teeth on the belt.

The second question, Number 6 is being made to be moved out to tension the belt. Is that an ok method to do so, or not?

Lastly, are the smoothed pulleys such as the ones here hot-swappable with the toothed ones, without causing my belt to be out of alignment, or will I have to move the bearing to get the belt back to parallel to the axis?

Many thanks

Jack
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Re: Some quick questions regarding idler pulleys
September 19, 2017 06:12AM
Yes, moving 6 left-right would adjust tension of the belt, but with very few teeth in contact with the pulley, it could be noisy, and could create print artifacts. It might be better to move 3 left/right or forward/back- it will have more contact with the belt so less likely to create print artifacts. Moving the motor back and forth will tension the belt, too.

Without knowing the dimensions of the existing pulleys or the smooth ones linked, it is impossible to say whether the axles will have to be shifted.

The problem with any of those pulleys is that the bearings they use are small and the bore is very small. You can't just stand those bearings on a screw because the belt tension will make the screw flex and then you'll have alignment problems with the belts. You have to support the screw's top and bottom in order to use those pulleys. That's why many people use stacked, flanged ball bearings for pulleys instead. They cost less and typically have larger bore that will allow a post-type axle (but still best if the axle is supported at its top and bottom).

You don't have to have teeth on any of the pulleys, provided they are of sufficient diameter. Some piece of literature from Gates recommends that smooth pulleys be at least the diameter of a 40 tooth pulley, which for 2 mm pitch belt is about 25 mm. I use F608zz bearings that are 22 mm dia and they deliver excellent print quality. The other way to use smooth pulleys is to twist the belt either at 6 or between 3 and 4 in your diagram, provided the length is sufficient (not sure what the spec is for that minimum distance for a 180 degree twist).


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Some quick questions regarding idler pulleys
September 19, 2017 11:10AM
What about a smooth pulley between 3 and 4 that pushes the belt inwards?

Quote
the_digital_dentist
The problem with any of those pulleys is that the bearings they use are small and the bore is very small. You can't just stand those bearings on a screw because the belt tension will make the screw flex and then you'll have alignment problems with the belts. You have to support the screw's top and bottom in order to use those pulleys.
Listen to the good dentist. He speaks the truth. smiling smiley

I have placed my pulleys on a 3mm screw and they do not look too good at all. This is the first thing I want to change when I finish the rest of the printer.
Re: Some quick questions regarding idler pulleys
September 19, 2017 12:03PM
Thanks guys, That's some solid advice!

I was planning on supporting top and bottom by using some square tube aluminum/steel and mounting them in the middle.

If I move number 3 up and down on the picture for tension, can I remove 6? it will allow 3 to have more teeth in contact too.

I didn't know about the larger diameter requirement for the smooth ones, but I can certainly swap to them. I'm still in the planning stage at the moment anyway
Re: Some quick questions regarding idler pulleys
September 22, 2017 02:11AM
What about removing 6 and tension the belt by moving the motor like it is done in many machines for simple ink printer to heavy farm machinery ?
The less pulleys the best.


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