Belt sizing suggestions

Just wondering if there’s some kind of rule of thumb or any suggestions anyone here can make on the right belt size for my DIY machine. My concern, (just because I worry about things), is that the 10mm wide GT2/2mm pitch belt I’m using is insufficient for the length of my axes, (1300 x 750ish). I’m using 59Ncm/84oz 2A NEMA 17 steppers which seem to be fine, and I’m not really seeing an issue with my belts at the moment, but I’m running conservative settings on speeds and accelerations because I don’t have a rigid frame yet. Since that’s what I’m working on I want to consider upgrading things I might have undersized previously as I progress through the re-build. When the frame is better and I can up the acceleration I want everything to be ready to go.

Can anyone tell me what they use in commercial systems of a similar size? I just looked up a Boss 3655 and they spec NEMA 34 motors, (other models just NEMA with no indication of size), but no belt information. That’s a pretty big motor so I’m guessing they have belts to suit. It also makes me think I should consider bigger motors, but I’m not trying to build a Boss. I am realizing just how badly my design sucks though. :roll_eyes:

For machines the size of yours NEMA 23 motors and 12-15mm belts are not uncommon - though in that size you are usually going to need HDT style belts - GT2 is tough to find that big and HDT is specifically designed for high torque applications. Boss uses NEMA 34 because their machines can pull more G’s than a fighter jet :slight_smile: But that also requires an incredibly rigid frame and very high end linear motion components.
I custom build a similar size machine last year and am using 2Nm (4x the torque of yours) NEMA 23’s and 12mm HDT belts and it works quite well. It can hit 1000mm/s at high acceleration (don’t remember the specific numbers at the moment).
Anyways, the general “rule” is the larger the machine (And therefore more weight being moved) or faster you want to go, the more torque you need. And more torque means bigger motors and fatter belts. But there’s no “this size == this motor” - all depends on what your goals are.
However - there’s obviously a limit based on your frame… too big of motors and you could literally break the frame or something connected to it. Also, once you jump to NEMA 23’s you will prob need larger external stepper drivers that can do 4A or more.

Thanks Adam. Just the info I’m looking for.

I’ve decided to take an incremental approach to this, (well carry on with the incremental approach I started with I guess.) First order of business is switching out to proper linear rails. I’ve finally found a good deal on a HIWIN set, (shipping is always the sticking point). Even if the frame is more rigid my linear shafts would be inadequate, (better described as a joke).

Next I’ll address the motors and belts together. I was looking at 15mm wide stuff last night, just have to decide on what to go with. From there I can pick pulleys and motors to suit. I actually have some very nice NEMA 34 servo motors with step/direction input which I’m tempted to use because they’re sitting idle and I have everything I need with them. But they just seem like overkill to me, and they have a 1/2" shaft, (not a show stopper). Even looking at the Boss pictures, although it’s difficult to tell, I can’t see them using a NEMA 34 driving the X-axis and carried by the Y-rail. It’s certainly possible though. I have a single NEMA 23 and 5.6A driver I could use on the Y-axis, and continue to use the NEMA 17 on the X. Then switch out the X for another NEMA 23 at a later date. But man, those NEMA 34’s would be a thing to behold!

Finally, if all this works well I’ll consider switching to a Rudia controller. I bought most of my stuff from Cloudray and I see that they now list LightBurn along with the Rudia as a package. That was nice to see!

Even as my machine stands with all the issues it does have I still get good results from it so I’m happy that I’m heading in the right direction. Who knows, maybe I’ll end up with a semi-Boss. :smiley:

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Because the size of the belt is 2x the size of the axis move, you can use open-ended and a belt crimp/clamp, you just need to be accurate with your belt location so the join doesn’t end up under the pinion.

Most machines only have a toothed pinion on the drive with a smooth idler on the other end.

You can also join with a small offcut of belt facing the belt drive side and strong glue, if your gantry is relatively light.

Thanks Bo. I already use open ended belts with belt clamps. But I’m using toothed idlers the same size as the drive pinion. I assumed that the smooth idlers were just used if the non-toothed side of the belt was the contact surface. Is there a preference or a reason I should use smooth idlers?

No. Usually toothed idlers are used because it means only one part to stock. Theres no benefit at all. From an engineering perspective, a smooth idler should be spec’d for the back-side of the belt. A toothed idler will result in a bit of transmission vibration on the belt, from the teeth. It’s the same from the other side, except it’s the teeth of the belt inducing vibration.

You can make a toothed idler into a smooth one by using heatshrink, but it’s less good :slight_smile:

OK, so the incremental approach is out the window. In redesigning this beast I really need to decide on belts and pulleys upfront. It’s been a challenge to find a single source for what I want with reasonable shipping, especially with GT2 stuff being so plentiful in the hobbyist market. The downside being it won’t get here for 6 weeks. Anyway, I’ve settled on HTD 3M 15mm belts and 20T pulleys. I have one NEMA 23 and driver on hand and will be using a dual shaft NEMA 23 for the Y-axis. If all goes well I can get it all built and assembled before the belts arrive.

Just one more question - rubber or polyurethane? :smiley:

I haven’t seen real rubber belts for years, but rubber. They are invariably some plastic composite, with glass-fibre or kevlar reinforcements.

Polyurethane belts come with steel reinforcement - not needed with such a light gantry and a bit more expensive.