*plucks belt making high pitch twang*
What do you set your 3d printer belt tension to? A note? This note?
*another slightly lower*
This note?
Well, maybe you're wrong. Let's look into it....
Ok, so in this episode my aim was to really explore two main ideas, as I usually have some
expectations before I go in as to what the outcome will be, and what advice I can give you. First
up I wanted to really prove that people think about belt tension far more than they ought to,
and secondly, the advice to "tighten them" is misleading.
It's probably very worth noting that in this video I am very specifically talking about synchronous
belts on bed slinger style printers, where the belts are used for locating the parts only. I
am not talking about delta printers where the belts are used in a different way,
and I *REALLY* am not talking about coreXY because it's..well..I'm just not.
Also a trigger warning for any engineers, I will be misusing several engineering terms
in this video, and conflating weight and force a fair bit.
So anyway - these belts are called timing belts or synchronous belts. They are so named
because they are meant for exactly that. They have teeth, but the teeth are shaped to mesh
perfectly with the gear that drives them. They are designed specifically to not stretch and
to be able to cope with a range of tensions within which they will perform correctly. If
your printer has a genuine gates belt then you can expect that, but if it has a random brand, well,
don't assume it won't stretch over time. Most of my printers do have gates belts, which is good.
So we'll get to my first set of tests. Synchronous belts are designed to essentially grip to their
teeth, if you want to think of them as rubber chains, the analogy is pretty good,
they don't slip at all until you have a LOT of slack.
Of course, this isn't the whole story, belts this loose, just as chains, will have enough slack to
effectively dampen any directional change, in fact thats where your real failure mode ought to be,
logically, if your belt is too loose. The first thing you will get, in theory, is lagged movement
of the axis on change of direction. But, it really doesn't take much tension to stop that happening.
Anyway. Next demonstration. I needed a way to tension the belts to a very specific...tension,
and after trying a fair few things, I discovered a very obvious solution
to this. Rather than plucking, using displacement, or various other options,
I discovered that I can hook up a cheap luggage scale to the end of the belt tension mechanism
and see the load applied, and this is actually a pretty direct way of tensioning the belts.
My theory here is that increasing the tension on the belts too much will stress not only the belt,
but the belt can probably largely handle it, what will suffer though, is
the bearings. However, it's hard to test an affect on bearings in a short space of time
and I don't want to ruin the printer, so I thought it would be worth considering the
temperature of the stepper. When you increase the tension of the belts, you're increasing friction,
and this has to manifest as heat, which is probably observable in the stepper.
And this turned out to be true. In a 30 minute test of different tensions, I observed a
measurable increase in the stepper temperature when loading the belt with 12kg of force,
as opposed to 1kg. Around 2 degrees C difference. But what I also observed, that I didn't expect,
was a huge increase in noise, which I can't easily convey over the video. The
tighter belts caused the steppers to really scream, I guess due to the increased load.
So I'm really not convinced that "tighter is better" when it comes to belt tensioning.
The problem is, that while it's hard, ish, to achieve these tensions with the old style ender
3 tensioners, what has become popular in recent years is these tensioners like on Steve. You are
totally decoupled from the tensioning process here, you can't get by with pulling fairly hard
while you tighten, it becomes both really hard to judge the tension, and also impossible to feel it.
All this is very well though, but at the end of the day - the system is only as good as
its weakest link. While there's an assumption that the weakest link is the belt, as evidenced
by at least some accounts of broken belts, there's actually a part of the system that has a spec
sheet, well, not the creality ones obviously but you can find generic spec sheets for the
steppers. And they specify exactly this force that we are applying to them - it's called "radial
force", and it's usually in the range of 25 to 30 newtons at 2cm away from the body of the motor,
ie 2cm down the shaft, which is usually where the gear sits give or take. 25 newtons
represents around 2.5kg on our scale, and the assumption is that when there's no other forces
acting on the system and the screws are loose on the tensioner, a force...weight...whatever
of 2.5kg on this end of the belt will impart a force of 25 newtons radially on the motor.
So assuming you want to drive the steppers in spec, literally nothing else matters, that's
what dictates your tension. Why? Because I'll show you what 25 newtons looks like in practice.
I feel like there is a counter argument that could claim that tightening beyond the on paper spec of
the stepper motors will give you better quality so I did try that too. Personally I'm not seeing it
but there are a lot of factors at play, weight, speed, and bearing in mind this is a direct
drive with a huge stepper on it that would really exhibit a lot more loss in quality on the x axis
but I'm not seeing it. If you choose to run your steppers out of spec if it gets you better
quality then that's up to you, they're not that expensive I guess, and you might get away with it,
in fact it seems like most people do, certainly within reasonable loads. If you find you get
better results at 30 newtons than 20 Newtons then I think that's a very reasonable tradeoff.
I guess if you want me to sum this up with advice, well, the spec sheet for the stepper motor speaks
for itself. Tightening the belts to more than 25 newtons may not wear the stepper immediately,
but it will wear the stepper motor and shorten its life. I've also found that it can increase noise,
and increase temperature, and logically increase power usage. If you're one of the people who tunes
the belts to a middle C you might however want to look into that. It doesn't make a lot of sense.
So that's my advice. Get a cheap luggage scale, and tighten to the stepper spec.
What do you think? Too loose for you? Am I wrong?
Will you change how you tension the belts? Did you have them right anyway?
Let me know in the comments, and as always check the description for any corrections,
come join us on discord - 150 members now so hi to anyone who joined from the last video!
Otherwise, see you next time!