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using non-idler pulleys as idlers directly on the shaft - no bearings

Posted by ag123 
using non-idler pulleys as idlers directly on the shaft - no bearings
December 02, 2018 04:17AM
i'm searching for parts on ebay/amazon/aliexpress for gt2 pulleys

and as it turns out gt2 idler pulleys are expensive often double and more the prices of the pulleys that is directly coupled to the motors

can you share your thoughts about using non-idler pulleys directly on metal shafts without the bearings as idlers?
would that work after all?
i'm thinking one way to reduce the friction is perhaps to use thin shafts say 3mm rather than say 5mm and a wider diameter on the belts say > 10mm, that would provide some torsion multiplier to overcome friction at the shaft

Edited 1 time(s). Last edit at 12/02/2018 04:18AM by ag123.
Re: using non-idler pulleys as idlers directly on the shaft - no bearings
December 02, 2018 07:58AM
The idler pulleys are more expensive because they have two (usually crappy) bearings pressed into them.

Sloppy fit between pulley and shaft is no way to build a high precision/accuracy machine, and a steel shaft will chew up the aluminum in no time at all.

Pulleys don't have to have teeth. You can stack flanged ball bearings to use as pulleys- F608 pulleys stacked gives a nice 11 mm wide flat surface for the belt. Use an 8mm bolt for the axle. If that's too big, there are many other sizes of flanged bearings available.



Since you're new to this, I can save you a lot of frustration later on. Belts are usually used in loops. The loop is divided into two halves by the motor pulley and the idler pulley. Usually only one half is coupled to whatever is sliding along the guide rail. That working-half of the belt loop has to be parallel to the guide rail, and the other half doesn't. Once you have your pulleys and belt selected, you have to figure out where and how to position the shafts (i.e. the motor and the idler shaft) to ensure that the working half of the belt is going to be parallel to the guide rail. You can use different sized pulleys on the motor and the idler, but you have to position them properly. Thickness of the belt matters, too, especially if you go with toothless pulleys.

Here's an example- look at the X axis belt in the picture of the Taz 6 printer on this page: [www.lulzbot.com] Notice that the top half of the belt loop is parallel to the guide rails and the bottom half isn't. The top half is the side that is connected to the extruder carriage. Also notice that the pulleys are different diameters, which is why the bottom half of the belt loop isn't parallel to the X axis guide rails. That's all OK. It would have been a huge error if they had connected the extruder carriage to the bottom half of the belt loop. If the working half of the belt isn't parallel to the guide rail, when you tell the machine to move 5mm, it will move less than 5 mm- how much less will depend on how close the moving part of the machine is to the pulleys. Think about what that would mean for the dimensions of your prints- the size/shape of the print will change depending on its location on the bed.

I know, you can find a lot of examples where the working half of the belt isn't parallel to the guide rails. It's a mistake, it's wrong, don't do it. You can easily ruin a perfectly good printer design by using different diameter pulleys from the ones the designer specified. If you're building someone else's design and you're going to change the pulley diameter, you probably have to move the shaft which means redesigning the mount.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: using non-idler pulleys as idlers directly on the shaft - no bearings
December 02, 2018 02:59PM
Everything the digital dentist said! Using Pulleys without bearings will only work for a very short time and badly at that.
If you have a lathe it isn't very difficult to fit bearings into a motor pulley, i did that a couple of times and one of them is still running after 8 years.
Depending on the situation it might be possible to use the backside of the belt and just use bearings, especially if you are only guiding the belt not inverting the direction.
Some belts can run directly on bearings or smooth idlers, HTD for example. In worst case a well brinted pulley that you fit with bearings can also work.


[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]
Re: using non-idler pulleys as idlers directly on the shaft - no bearings
December 02, 2018 04:26PM
That'll save you about 1% of the total cost and work very badly. It's the typial money saving scheme that costs you more over time.

Idlers cost like 60 cents. A pair of F695 will cost between 57 and 80 cents and in my opinion performs better

Edited 1 time(s). Last edit at 12/02/2018 04:26PM by ruyvieira.
Re: using non-idler pulleys as idlers directly on the shaft - no bearings
December 02, 2018 05:26PM
Is there any negative to using F695 idlers on the toothed side of the belt? Im concerned that it will damage the teeth or is it not an issue?
Re: using non-idler pulleys as idlers directly on the shaft - no bearings
December 02, 2018 06:25PM
Quote
Ed3D
Is there any negative to using F695 idlers on the toothed side of the belt? Im concerned that it will damage the teeth or is it not an issue?

People think it will give you extra-ripply vertical surfaces and flatten your belt and or shred it, but in practice it's fine, lots of commercial printers do that (Lulzbot Taz, Makergear M2 and most of the Creality 3d printers)
Re: using non-idler pulleys as idlers directly on the shaft - no bearings
December 03, 2018 01:59AM
hi all,

thanks much for the responses
i decided to buy a few pieces of the (crappy) idler pulleys and a bunch of small ball bearings
[www.ebay.com]
[www.aliexpress.com]

i checked prices, the prices of gt2 idler pulleys : small ball bearings can be as much as 10:1 i.e. a single piece of idler pulley buys 10 pieces of ball bearings.
and it is likely those ball bearings perform better than the idler pulleys

i think the ball bearings can be used to make improvised idler pulleys and they are useful in other applications e.g. linear rails etc

@the_digital_dentist's suggestion is even better which is to use flanged ball bearings, flanged ball bearings tend to be priced a little more than the simple ball bearings but they are still well below the prices of 'hacked' gt2 idler pulleys. my guess is that the gt2 idler pulleys are handmade in small work shops. while ball bearings are mass produced by large manufacturers possibly in the thousands per day

Edited 2 time(s). Last edit at 12/03/2018 02:12AM by ag123.
Re: using non-idler pulleys as idlers directly on the shaft - no bearings
December 03, 2018 02:33PM
Quote
ruyvieira
Quote
Ed3D
Is there any negative to using F695 idlers on the toothed side of the belt? Im concerned that it will damage the teeth or is it not an issue?

People think it will give you extra-ripply vertical surfaces and flatten your belt and or shred it, but in practice it's fine, lots of commercial printers do that (Lulzbot Taz, Makergear M2 and most of the Creality 3d printers)

You see that's what a lot of people say - I cant say I can see it giving you ripples in your print but it would be interesting to see what kind of vibration it makes. I feel the need to test the effect on belt life now - anyone interested?

I have seen some people using a 3D printed sleeve with the tooth profile between two F695 which is interesting - I wonder how well they work.
Re: using non-idler pulleys as idlers directly on the shaft - no bearings
December 05, 2018 03:40PM
Flanged bearings work better than (cheap) pre-made idlers but not because the bearings are better quality. The idlers have the two bearings pressed in with the outer races contacting the idler, but the inner races free. When you tighten the bearing axially, the inner races load up and make it hard to turn.

Flanged bearings can be stacked with a small washer in between (contacting inner races), and can be snugged down as tight as you want without loading them axially.
Re: using non-idler pulleys as idlers directly on the shaft - no bearings
December 05, 2018 03:54PM
Quote
gmedlicott
Flanged bearings work better than (cheap) pre-made idlers but not because the bearings are better quality. The idlers have the two bearings pressed in with the outer races contacting the idler, but the inner races free. When you tighten the bearing axially, the inner races load up and make it hard to turn.

That can be avoided, you just need to make sure that you have nuts or spacers next to the idler whose diameter is small enough so that they don't contact the outer part of the bearing; and don't over tighten them (just the same as for flanged bearings).

I changed from flanged bearings to toothed idlers on my delta, because having the belt teeth running over the smooth flanges was making the belts vibrate. So I recommend toothed idlers if the idler radius is small. For large radius idlers, I suspect that it makes less difference.

Edited 2 time(s). Last edit at 12/05/2018 03:56PM by dc42.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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