Best timing belt and pulleys type for minimum backlash

[birchy]

This is concerning. I'm working on my first CNC conversion of a milling machine and opted for belt drives in order to make the build more compact. And now, halfway through the build, I'm reading that using T5 belt drives will significantly increase the amount of backlash - something that I am trying my best to eliminate!

[Greeny]

I'm looking into belts at the moment.
I have just got a sample AT5 pulley & belt from Beltingonline.com, Supplier of power transmission and conveying products

It seems ok, but a lot of people here seem to use HTD pulleys and belts so i might order a sample one of these to have a look at too,

Photo of AT5 belt on 44 tooth pulley

[Boyan Silyavski]

This is concerning. I'm working on my first CNC conversion of a milling machine and opted for belt drives in order to make the build more compact. And now, halfway through the build, I'm reading that using T5 belt drives will significantly increase the amount of backlash - something that I am trying my best to eliminate!

Sometimes reading the manufacturers general guide lines on the first page is better than reading 100 posts on forums. Its all there, the good and the bad in bold letters:


Are you too far to go back now? Belt and pulleys are so cheap. I would not be worried if i were you. As Jonathan confirmed, backslash is eliminated if tightened properly. However i would be worried if some of the pulleys you intend to use are the smallest for the belt size. Up to 18-20 tooth i mean. Implementing smaller than 18 tooth pulley is a call for problems with any belt.

Generally that's why people use so much HDT timing belt. Stronger than XL, better than the T for linear motion and requiring smaller pulleys than the stronger AT.

As far as i know Belting Online is the cheapest place, i buy from them so its worth a look comparing pulley sizes and so.

[birchy]

Damn, I wish I'd seen that chart before! I only chose T5 belts because someone on here recommended them when I started the build. I know you say pulleys are "cheap" but I've only recently bought two T5 belts from beltingonline at a cost of £9 each and 4 pulleys at just over £10 each with boring and grub screws. So that's almost £60 of "cheap" parts for the scrap bin. In hindsight, I can see that HTD5 pulleys and belts would of been a much better choice for cost, availability AND backlash reasons. Oh well, we live and learn.


EDIT: Just a FYI, http://www.bearingstation.co.uk/ are generally a bit cheaper and have a "price match guarantee", however they don't offer a machining service.

[Boyan Silyavski]

Damn, I wish I'd seen that chart before! I only chose T5 belts because someone on here recommended them when I started the build. I know you say pulleys are "cheap" but I've only recently bought two T5 belts from beltingonline at a cost of £9 each and 4 pulleys at just over £10 each with boring and grub screws. So that's almost £60 of "cheap" parts for the scrap bin. In hindsight, I can see that HTD5 pulleys and belts would of been a much better choice for cost, availability AND backlash reasons. Oh well, we live and learn.


EDIT: Just a FYI, Bearing Station - Adhesives, Bearings, Belts, Chains, O Rings, Oil Seals, Pulleys & Sprockets are generally a bit cheaper and have a "price match guarantee", however they don't offer a machining service.

Do not throw anything in the bin! And sorry for that "cheap" . I meant relatively compared to other linear movement.

If somebody told you is maybe because it works. In practice there is a belt backslash only if you have open belt clamped from both sides and the linear movement is transmitted by only one pulley

In a closed system where 2 pulleys are engaged, when you tension the belt enough / but not too much!/ the teeth on the opposing belt touch tightly the teeth of the opposing pulleys and preload the system, the way 2 ball screws are preloaded.

So whatever specification say, i believe you did nothing wrong and the belt will work without backslash.
And talking about backslash and various types of belts manufacturers refer more to the tooth strength and theoretical situations with one pulley only.

So cheers up!

[birchy]

OK, OK! I'll put the lump hammer down for now!

I have got one 14T pulley to give me a 3:1 ratio (42T) on the Z axis, so might have to change that to an 18T or bigger. My thinking is that even if we had 1 or 2 degrees of rotational backlash in the pulleys, this would still not be enough to cause a problem if using ballscrews?

[Boyan Silyavski]

OK, OK! I'll put the lump hammer down for now!

I have got one 14T pulley to give me a 3:1 ratio (42T) on the Z axis, so might have to change that to an 18T or bigger. My thinking is that even if we had 1 or 2 degrees of rotational backlash in the pulleys, this would still not be enough to cause a problem if using ballscrews?

Here is a simplified, why backslash in reality will not happen with these belts
Imagine 2 pulleys on a horizontal plane with a belt mounted. When tightened the belt, all the teeth from 6 to 12 o'clock on the left and from 12 to 6 on the right will preload.

Now imagine the belt start to move clockwise driven by the left pulley. From 9 to 12 o'clock is where the backslash will happen, but from 6 to 9 will be still preloaded. At the right pulley the same. From 12 to 3 backslash and from 3 to 6, tooth engaged.

So at any time at least half of the engaged teeth are without backslash.

Hence the tooth strength together with the tension plus the pulley size , if enough will not permit backslash.

18t pulley is what i use on my machine, changed from 14, he he. For other reasons though.

Yeah, for the Z it shouldn't be a problem.

When i changed belts even when the pulleys were tightly fitted they gave me problems, especially the small ones connected directly to the motor shafts. They were backslashing by slipping on the shafts even if i used 3 grub screws on each. And only under heavy loads so it took me a lot of time to figure the problem. A little "super" glue eliminated this.

[Clive S]

EDIT: Just a FYI, Bearing Station - Adhesives, Bearings, Belts, Chains, O Rings, Oil Seals, Pulleys & Sprockets are generally a bit cheaper and have a "price match guarantee", however they don't offer a machining service.

I have used them before a few times and to be fair they have been ok. BUT this last time they let me down badly with regards to delivery times in that I ordered 8 20t pulleys on the 5th August and despite 10 -12 phone calls which they promise to return with an answer, they never do. I have now got 7 of the 8 and they are telling me that none of their supplies have got any more and its not their fault. I now have rung Belting-on-line and they have plenty.
I personally will not be using them again. Clive