Did belt tension break my stepper shaft?

Hello, we were recently printing a large object (19 hours)on our german reprap X400 3d printer... I'm not sure when it happened, everything was going well, stepped out for 10 minutes and when I returned the extruder was "printing" on air, in a part of the plate that had nothing to do with the print... Long story short, disasembled the Y axis carriage and discovered that the shaft for one of the Y axis steppers had broken off!

Here are some pictures:




I'm guessing this was caused by too much belt tension creating a constant radial stress on the shaft? I had looked for tips on belt tensioning and had recently tightened it in such a way that it would sound like a low note when plucked (this seems like a very subjective way of knowing if the belt is properly tensioned btw)

Can anyone confirm this was cuased by incorrect tension? Or should we be looking at another issue?

Thanks

... most commone cause with this sort of failure are faults in the material of the shaft ...

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Viktor
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The belt pulley should be placed nearest to the stepper housing with the grubscrew-skirt on the far side of the shaft. That way, the belt tension has the lowest lever on the bearings and shaft.

I also think, this was a production failure.
-Olaf

Thanks for your replies, I guess we won't worry about it happening again and it being our fault. Mounting the pulley as close as possible to the stepper housing makes sense though, thanks

-Hugo

I would bet that the stepper shaft was made in China. They are not known for high quality steel.

Either there are too many impurities in the steel or it was not tempered to the correct hardness after machining / hardening process, that would leave a lot of tension in the crystalline structure. Try to run a file across it and see what it does.

If it won't bite and remove material, then it is too hard and brittle.

If it does remove material, then it is just poor quality steel.

As a rule, don't use PRC steel for critical load bearing equipment, bridges or buildings... and possibly 3d printers.

Edited 1 time(s). Last edit at 08/07/2015 03:49PM by Kryoclasm.

"had recently tightened it in such a way that it would sound like a low note when plucked."


Hard to say, but that tells me that the tension was too tight.
Could be cheap,low grade, metal shaft material.


It's not good on other components, if you are plucking too many notes.

That's coming from a Machinist and Musician.

You should witness the slack/slop, in belts of large equipment; especially on the humongous old machines, such as VTL's and Planers.
Naturally, small equipment usually requires higher belt tension.

Quote
Metalmann
You should witness the slack/slop, in belts of large equipment; especially on the humongous old machines, such as VTL's and Planers.
Naturally, small equipment usually requires higher belt tension.

Large equipment almost never use belts for positioning, just for the spindle/hydraulics and other power transfer applications.

Belts used in low backlash applications must be extremely tight. If you go through the application notes, the recommended tension is enough to fold most 3d printers in half. Larger belts require higher tension - up to several thousand pounds.

Some stepper motors include radial load ratings. Generally roughly 3-5Kg for a Nema17, 10-15Kg for a Nema23 and 20Kg+ for a Nema34.

Quote
Metalmann
"had recently tightened it in such a way that it would sound like a low note when plucked."


Hard to say, but that tells me that the tension was too tight.
Could be cheap,low grade, metal shaft material.


It's not good on other components, if you are plucking too many notes.

That's coming from a Machinist and Musician.

You should witness the slack/slop, in belts of large equipment; especially on the humongous old machines, such as VTL's and Planers.
Naturally, small equipment usually requires higher belt tension.

Thanks, the low note thing was the only real reference I could find as to how tight the belts should be. Not too sure how it would be done in an industrial setting. I'l loosen them a bit just to be sure.

Do you mind running a file on the shaft as mentioned earlier? But actually the key curiosity is to run near where it was broken, and also on the other end of the shaft where unbroken.

If neither side bite with the file, then they are both hardened, and the shaft came hardened to you.
If the break side doesn't bite, but the other side bites, then the metal came to you and was soft but because hardened by perhaps a repetitive bending motion.

I don't know, it's just a curious failure it's really hard for a small printer machine to warrant busting a 5mm steel shaft :p.

So I ran the file across the shaft as suggested. I didn't really feel too much bite, but it definitely removed a bit of material. Took a picture after a few passes.

Did you have a chance to check out the other end of the shaft, like where the pulley is on? So the shaft you've just tested gives you a baseline from which you can tell if the other end of the shaft is much softer or the same/similar.

My reasoning being is that the shaft is almost certainly hardened. It broke in a clean fracture, and it also probably broke after a lot of repeated motions so it could have hardened a bunch from that.

But don't worry too much about this since it's just idle curiosity and I don't think there's anything profound to learn from this good experimenting.

Quote
Tomek
Did you have a chance to check out the other end of the shaft, like where the pulley is on? So the shaft you've just tested gives you a baseline from which you can tell if the other end of the shaft is much softer or the same/similar.

My reasoning being is that the shaft is almost certainly hardened. It broke in a clean fracture, and it also probably broke after a lot of repeated motions so it could have hardened a bunch from that.

But don't worry too much about this since it's just idle curiosity and I don't think there's anything profound to learn from this good experimenting.

It's a bit more difficult to test that at the moment since the shaft broke really close to the bearing and I would have to separate them somehow. I'll give it a shot when I find some time, I'm curious as well.

Nono, I haven't been clear; I'm sorry I'm not asking that much as to get into the bearing side.

In the first post photos, you are pinching the shaft at the point of the pulley, to show the fracture break. The point where you are pinching, is where I'm looking to see if it's soft or not.

Not anything internal to the motor.

Is it just me seing the "air bubble" that must have been in the shaft?

Like a (kind of big) half moon on the inside, right side, of the broken shaft?