Belt stretched - how to prevent this?

Hi all,

while preparing my next job I noticed that my X-belt got overly stretched at one point:

Replacement is on its way, but does anyone have an idea what could have caused this and how to prevent this in the future?

I don’t think that there was too much tension on the belt because I could easily slip a finger underneath it.
It must have happened during the last three weeks where I didn’t use the machine it was really hot here in Germay.


I have seen this reported on the forum a couple of times, with no obvious root cause, so it might just be a random quality issue with the belt (the heatwave we got in Europe two weeks ago may not have helped indeed)

I have come to think of belts as another consumable item, two years ago I got a roll of steel-core belts (like those), swapped the three belts on my SO3 a few times, and have not yet used up the whole roll.

n.b.: in case you missed it, you may be interested in trying @LiamN’s methods for precise belt tensioning when you install the new ones


There aren’t many wires in the steel core belts — if just one has a kink or manufacturing flaws it puts stress on the other wires along that section of belt resulting in this sort of failure.

Contact us about this at and we’ll do our best to work through this with you.


Thank you @Julien and @WillAdams.

Wasn’t aware of Liams thread, this looks like a really complete description of the process!
I decided to go with these belts that were mentioned in the wiki.

Its also a good timing to install them along with the bit-zero that will soon arrive (looking at the German customs office that held back my parcel for 3 weeks without notifying me :face_with_monocle:)


Those are the belts I currently have, they seem pretty good to me.

Remember the hint (I cannot remember who posted it sorry) about taping one end of your new belt to the old one to pull it through to avoid the “fun” of trying to thread the new belt through the pulleys.


@Microwave_Monkey did:



p.s. I am really enjoying my 10mm wide steel core belts so far. I haven’t tried “pushing” them the way I did the 9mm, so we shall see how they hold up.


Done it. Crashed the carriage and it stretched and failed in one spot just like this.


Out of interest, how much static tension did you have on the belts when the crash stretched them to failure?

Hard to answer - not something I was measuring closely. Here’s the thread though: What a failed steel belt looks like

I do think the steel work hardens with use, I think the bends are a little tight, but that’s my gut talking, not data.



It’s not just your gut BTW, the SDP documentation lists steel reinforced belts as;
Operate Over Small Pulley - Poor
High Speed Pulley - Poor
High Intermittent Shock Loading - Good
Vibration Absorption - Poor
Low Belt Stretch - Excellent

(and a load of other characteristics)

But I think it’s safe to say your findings match the selection criteria of “don’t flex steel cored belts rapidly round small radii”


Did that make it onto your list of “include in the e-book” or sticky thread yet?


“include in the ebook” it is, thanks for the reminder.


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