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Pulley assembly - shaft or bolt?

Posted by Shachar85 
Pulley assembly - shaft or bolt?
June 30, 2015 05:14AM

I'm new around, so I hope no problems with the question. Searched the net pretty deep with no good answer.
I started designing a printer.
I got stuck in the pulley assembly stage.
The side that attached to the motor is obvious of course.
What about the other side? Do I have to assemble it on 5mm smooth rod, or will 5mm bolt do the job?

Re: Pulley assembly - shaft or bolt?
June 30, 2015 05:27AM
Mounted on a bolt youl`ll get a wobbling belt. This can have a visible effect on the print surface.
I´d look for a coupling for a 4mm shaft diameter and drill it to match the M5 rod ( 4.7 - 4.8mm ) => no more wobble smiling smiley
Re: Pulley assembly - shaft or bolt?
June 30, 2015 05:44AM
Or just fix the 5mm shaft and slip the pulley on it? smiling smiley
sorry, I don't understand why the coupling is for?
Re: Pulley assembly - shaft or bolt?
June 30, 2015 06:26AM
Bolts/threaded rod works fine for me. You have to put nuts on either side of the bearings to keep them from moving around.

This is one place where threaded rods are acceptable...
Re: Pulley assembly - shaft or bolt?
June 30, 2015 10:22AM
Do you have a lathe? If so, you can easily turn flanged bushings to match the ID of the bearing to whatever fixing you want to use. The thickness of the flange acts as a spacer to keep the bearing centered in whatever space it is running in.

edit - I assumed that you were using a bearing as a smooth idler pulley. If you want to run a toothed pulley for the idler then you'll need an axle to match the bore of the pulley, and a bearing on each end of the axle to allow it to turn smoothly.

Edited 1 time(s). Last edit at 06/30/2015 10:25AM by JamesK.
Re: Pulley assembly - shaft or bolt?
June 30, 2015 02:12PM
Bottom line is zero lateral side to side movement of pully on shaft.
Re: Pulley assembly - shaft or bolt?
June 30, 2015 03:57PM
Shoulder bolts work well for this application.
Re: Pulley assembly - shaft or bolt?
July 01, 2015 02:47AM
Thanks guys for the tips
BTW the construction is based on aluminum profiles (20X40) if it matters

I considered shoulder bolts indeed, but eventually I bumped into this:

which will sit perfectly on the profile. I'll put 5mm shaft between 2 bearings and lock the pulley with its M4 on the shaft

How important is it that the pulley won't move to the sides? (let's say 1mm max)
As I see it - 1mm to one side or another won't make much difference, but I'm pretty new to this stuff, so I'll be happy to have your opinion about it
Re: Pulley assembly - shaft or bolt?
July 01, 2015 08:08AM
Not quite sure I follow how you will use those rod ends. Those bearings are not designed for rotation, just mild swivelling. If you put a pulley on an axle you will need a pair of ball bearings like these (in whatever size is appropriate). Obviously you'd need to build some sort of mounting to hold them. Most people take the simpler route of using a single bearing that the belt runs directly on.

A small amount of sideways play in the pulley doesn't seem like a big problem to me, provided that the belt remains clear of any contact with the sides.
Re: Pulley assembly - shaft or bolt?
July 01, 2015 10:16AM
I figured because of the relatively slow rotation - the rod ends will act fine as bearings.

I might be wrong though
What do you think?
Re: Pulley assembly - shaft or bolt?
July 01, 2015 10:26AM
So I'm no expert, but I don't think that will work well. You want the belt to move with very little resistance, and the belt also needs to be under a reasonable amount of tension. I know, that's a completely useless term, but it's difficult to quantify - somewhat twangy, don't go nuts or you'll stress the stepper bearings, but too loose and the belt will jump pulley teeth. If you don't have a decent bearing for the idler then you risk a fair bit of friction and wear due to the belt tension.
Re: Pulley assembly - shaft or bolt?
July 01, 2015 10:37AM
I think that might have worked, but you can easily be right about that.
Experience thought me that when you can fix potential problem during design - fix it then smiling smiley

As you pointed that most people use bearing as idler instead of another pulley - I can't think of a reason not to use that method.
I think I'll go that way
Re: Pulley assembly - shaft or bolt?
July 01, 2015 10:49AM
One reason that a toothed pulley might be better than a smooth bearing is that as each tooth on the belt comes into contact with the pulley there is a small change in belt tension. It's thought that this can give rise to ripples in the surface finish of the prints, so using a toothed pulley should be technically better. The effect with a smooth bearing will be greater the smaller the diameter of the bearing. It seems intuitive to use a bearing that's the same diameter as the pulley, but I don't think there is any negative to using larger sizes, provided you can physically fit them into the space available. The side of the belt that connects to the carriage should be level with the same side of the driving pulley and the carriage connection point to minimise off-axis forces. The other side of the belt will then run at an angle because of the different diameters, which looks weird but as far as I can see has no real disadvantages. I'm using the 608 bearings with 22mm OD and it's difficult to imagine there's much issue as the belt arrives and leaves that size bearing.
Re: Pulley assembly - shaft or bolt?
July 01, 2015 11:03AM
Thanks a lot for all the answers and explanations!

Just took another step to a printer
Re: Pulley assembly - shaft or bolt?
July 01, 2015 01:09PM
My experience with cheapo GT2 pulleys is that errors in the tooth profile and even center hole position probably cause bigger print defects than using a smooth bearing surface. I went through three of those pulleys looking for one that had satisfactory center hole position for the X axis drive motor. They were literally so bad that you could see the thing wobbling as the motor turned.

My X axis has a GT2 belt riding on a pulley that is just 3 stacked bearings mounted on a threaded rod. I can detect no print surface anomalies that match the belt pitch in the X axis. Using threaded rod doesn't matter (except for its convenience) because the belt is kept under tension. The pulleys aren't going anywhere. For a while I tried twisting the belts so the flat side would contact the bearing surface but eventually returned to untwisted belt because the twisted belt didn't want to stay aligned with the pulleys.

Edited 1 time(s). Last edit at 07/01/2015 05:24PM by the_digital_dentist.
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