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View Full Version : Belt drive vs direct coupling pro's and cons, Poll



Desertboy
28-08-2017, 11:36 AM
I see a lot of you are belt driving your ballscrews so you can get more speed or torque out of the screws (Seems mostly speed). I've gone with direct couplings simply because it seemed slightly easier for my first router although I now realise due to way I made the nema mounts I could convert to belt drive with a couple of pulleys at least my Y axis the X not so easy would need to remake one of the gantry mounts. my Z axis would need completely rebuilding with new aluminium to enable belt drive but I think the Z axis will also gain the least from being belt driven and speed and torque shouldn't be an issue.

As far as I can tell
Direct coupling is more accurate
Belt drive is quicker
Belt drive can give more torque (Although then it will be slower than direct drive) I guess potentially accuracy as well but the possiblity of a slipping belt might offset this.

I have 1605 ballscrews is it even worth belt driving them? Due to screw whip.

Which way do you drive your cnc? And why?

Neale
28-08-2017, 11:59 AM
I belt-drive 2 X ballscrews as well as Y and Z. One thought was that it would give me the option of changing ratios if experience suggested that this might improve things. However, given that X ballscrew critical speed is roughly the same as the Nema 23 corner speed (not a coincidence!) this seems unlikely. The other main reason was mechanical simplicity - on my machine this made the motors easier to mount, and meant that I could tuck them into the overall machine envelope and avoid them sticking out. To get the best out of direct drive, even with flexible couplings, you need to align motor and ballscrew fairly accurately. I suspect that it's easier to adjust a belt. With the right belt and pulleys, slip is not an issue. Cost of decent flexible coupling and belt and pulleys is not that different, although I did make my own flexible couplings for my first budget router, so I have tried both approaches.

Desertboy
28-08-2017, 12:42 PM
LOL that's funny because I went with direct drive because I felt it was more simple to implement lol, but I had all the parts made on a CNC so relied on the accuracy of both the machine it was made on and my cad models to get the alignment correct.

Having the motor's in the chassis is a very good point and it worries me that mine aren't. But am going to put it on legs so if I orientate it to the wall correctly the chance of damaging it is remote.

This is 1 of my Y axis motor mount (Notice the nema isn't bolted in)
22687

I think it would be simple to convert this mount to belt drive with 2 pulleys as it bolts into the T slots horizontally which means I can adjust.

My other axis would mean remaking plates so if I ever do change those to belt drive I will have to remake the plates (On my machine of course)

In case of confusion I'm classing the bottom twin ballscrew axis as my Y & Gantry as my X.

Am open to advice though on any point of the build as I'm only vaguely sure at anyone time of what I am actually doing lol.

Boyan Silyavski
28-08-2017, 09:12 PM
It was done long ago to eliminate vibration when drives were crappy. Now drives are digital so that will not matter so much. Now is done for the sake of hiding the motor, gearing it and so on. And yeah, for the sake of alignment and simplicity, as 2 things bolted to the same plate/plane is easier to align than 2 thing bolted on a same axis.

Anyway i don't like spring couplings and having in mind the cheap price of ball screw mounts, i would always use both sides angular contact bearings. The so called rigid mounting, or scenario D.

At the end of the day machines look more tidy and all is more rigid when you don't see the motors. Protruding motors will need a good mount, not just 4 bolts and spacers, so its up to you.

routercnc
29-08-2017, 01:48 PM
I agree with the above. My current machine uses
1:1 belt drive on Z (to hide the motor)
Direct drive on Y through spring coupler (although I had plans for belt drive to hide the motor, but never implemented them)
Direct drive on X and A through spring couplers

My comments:
Choice of which to use on DIY machines is mainly to hide motors, mounting simplicity, and if you want options to gear the drive train up/down
Not noticed errors due to belt play or stretch so don't think that is a factor on DIY machines (commercial machines I've seen are all direct drive though)
Have had those spring couplers break (I don't think they are a good idea on a cnc machine), and next machine will use 'oldham style' couplers which look more robust (although cost a little more)

Zeeflyboy
29-08-2017, 06:18 PM
It was done long ago to eliminate vibration when drives were crappy. Now drives are digital so that will not matter so much. Now is done for the sake of hiding the motor, gearing it and so on. And yeah, for the sake of alignment and simplicity, as 2 things bolted to the same plate/plane is easier to align than 2 thing bolted on a same axis.

Anyway i don't like spring couplings and having in mind the cheap price of ball screw mounts, i would always use both sides angular contact bearings. The so called rigid mounting, or scenario D.

At the end of the day machines look more tidy and all is more rigid when you don't see the motors. Protruding motors will need a good mount, not just 4 bolts and spacers, so its up to you.


Angular bearings on both sides!? Am I misunderstanding what you said here and you actually mean one set of back to back angular bearings on one end and a floating bearing on the other or are you actually suggesting angular contacts on both ends (fixed/fixed)?

I don't personally think fixed/fixed is a good idea in general unless you really need that increase in critical speed, but especially so on machines built from aluminium extrusion like Desert's due to the difference in thermal expansion between steel and aluminium.

Re the topic, If you use proper high torque belts (bearing boys is a good source of HTD belts) then slip is a non issue. Theoretically you might have a bit less accuracy due to belt flex and a teeny tiny extra backlash but in reality I don't think it's an issue on a hobby level machine... they are pretty much backlash free and the re-inforcement strands mean that flex is very minimal.

Personally for me I prefer direct with decent couplings, but it's really just a question of what better suits your machine design I think.

Boyan Silyavski
29-08-2017, 10:07 PM
My machine has fixed to fixed on all axis and generally having in mind the low price of the bearings i am even doing my foam cutter like that. The only problem with that arrangement that you will ever encounter is if something is not well aligned. I am milling up to steel with my router, so excuse me if i do not trust a fixed/floating arrangement. Plus FYI Chinese sell the many times the BK and other mounts with deep groove bearings, not angular. Sometimes you will have to specifically specify that you like to have angular bearings. Its in the small letters usually if you pay attention. Hence that's why i don't change providers if i can help it.

Buy HTD pulleys from reputable sellers only or you will have a box of faulty pulleys, like the one i have.