Issue with the Y axis: is the belt worn out?

check that the pulleys are not rocking a little relative to the ballscrews?

That seems to be ok: no backlash, but the pulleys are running freely (I can turn the two axis easily by hand).

I've noticed some serious worn on the belt at some points though.

So I think that all my Y axis issues (strange sounds, damaged pulley as it touched a screw at the backplate, and now the belt) are related.
I have to replace all the worn and damaged parts, and I hope that will be enough to make it run again.

Maybe the belt didn`t have proper tension or the preload assembly got loose over time (if SC 2 has it). On my machine when Y axis jammed or hit something there was this weird "grrr" sound. Took the small tooth belt cover off and saw how servo wheel kept on spinnig when the axis was mechanically stopped, creating the weird noise.

That's possible indeed... the famous "grrr" sound while the unit is not moving...
I don't know where the issues have started, but I need a lot of spare-parts right now to correct the issue.

Weird thing is that the machine was working just fine till a few days ago.

I'll have to wait for an invoice from StepCraft because right now, I'm stuck in the middle of a project... :dry:

Same problem here too. My SC2 840 works very well and boom, y axis loosing positions. And second belt have damaged. How much this belt cost? And maybe would be better if y axis was on 2 motors. Sorry for bad english.

Photo is from first belt. But now is the same.

Picture looks like belt has been overloaded and has failed between teeth. If the belt is too tight this will happen.

Maybe is chance to install 2 motors on y axis? Then the belt is not needed.

The same thing happened to me today. I sent StepCraft Support a message, along with a video of what is happening with my belt. During the initial install, I watched their video on setting the proper belt tension so I feel like my belt wasn't set too loose or too tight. What is happening with mine, is that the belt slides back and forth on the 3 bearing tension piece. Often, it slides too close toward the machine and the edge of the Y axis drive pulley. During one of my jobs today, the machine lost all Y axis movement during a diagonal cut, leaving only X moving, which completely messed up my job. The side of the belt closest to the machine is shredded and is no longer reliable. I have only had the machine for about a week so this is a little concerning. I asked them about replacement belts since now I feel as if I should have at least one for backup purposes. It didn't take long for this to happen to me, so hopefully after watching the video I sent them, they can give me some pointers or guidance as to what I did wrong or what I can do to prevent this in the future. Like you all said, everything was working just fine one minute, and then then next, this happens. I hope to be able to get a replacement belt as quickly as possible as this is a showstopper and cannot move forward with my work.

I would inspect the drive pulleys to see if there is any free play and inspect the drive pullys themselves if there is any sort of wobble in them when you turn them i think that would cause the belt attached to move and wear on the edges and teeth.

I also had problem where the belt was moving sideways on the tension support, and this was caused because the pulleys were not aligned. I.e. the Y axis ones were one mm closer to the machine then the Y motor one. This meant that when the belt went from the closest Y axis pulley to the Y motor pulley it was angled slightly and this caused the belt move even further out for the tension support pieces. If I tried to run machine at all without the belt cover the belt immediately fell off from then tension support, thus proved that in normal operation it was hitting the belt cover.

I could not fix this by moving the pulleys, as immediately when I tightened the locking screws the pulley moved back to original position. I did fix this by adding washers between the Y-motor and the frame, i.e., moving the Y-motor about 1 mm away from the frame, which moved the Y-motor pulley 1 mm inwards, and then it aligned with the other pulleys. After that I could run it without belt cover, and the belt stayed in place.

You might want to check that your pulleys are aligned, meaning each of them are out from the frame for equal amount.

I don't know if this is the problem but... could be the pulley not correctly mounted on the motor axis?
Loosing steps on the Y-axis that was happening to me too (long time ago) and that was the reason.

When I built my machine a year ago I noticed that when you tighten those things the wurm drive screws through to mower y axis there is play and if you don't get them perfect the y axis can't move freely and will stall your stepper motor. I run Linux cnc and if the computer is not good enough problems like this occurs from missing steps. To be safe I always run part by part in big projects so I can abort if it glitches in y or x axis. Takes more time but it saves expensive carbon fibre plates from being completely wasted. If the machine misses steps from tension or computer problems.

Just chiming in to share my Y axis issues, which are now resolved. A little off this thread topic but may help someone looking to solve a Y axis jumping issue and maybe save a headache.

After first assembling my 420/V2, the Y axis motor was groaning at 80%+ jog speed. After back tracking the assembly instructions, I missed out a washer on one of the rollers!

After replacing the washer, I adjusted the angles of the Y axis screws to make sure they were central to the bearings either end, adjusted the tension in the rollers and the tension in the belt.

I did a few test cuts which were fine, but today moved onto a larger job. The Y axis was jamming up in the back to center of the machine.

Although it jogged through fine at 100%, the jamming was occurring when stopping and starting.

Turns out the inside (of the three) pulley tension bearings was damaged and wasn't turning, this was causing enough friction to cause the jamming. I put the damaged bearing in between the good two and ran a test - it s now fine, although I need to replace the damaged bearing as the belt is not central. I may have used the wrong washer causing it to knacker the bearing.

I am new to this, but anyone with a similar problem, just be persistent, go through one thing at a time, don't change too many variables at once. Run the machine on a slow jog speed and observe the mechanisms moving, check every part that moves. If it gets too frustrating, walk away get some sleep and look at it again with a fresh pair of eyes.

I had the same problem with belt moving left and right on the three small ball bearings. With belt cover i wasn't able to determine the problem and my belt became unusable thus all steel cords in the belt broke one by one always flipping on pulleys sides and my 840/2 started loose steps on Y. Had to order replacement belt, but the problem was the same with new belt. I don't know how, but the problem with belt sliding disappeared after i changed the middle ball bearing with another one on the side. All three tension bearings turned smoothly, so i don't actually know what caused the problem. Assembly was done exactly as prescribed with washers in it's place. As I understood, belt tension should be aligned very carefully, so what i did - I started with very low tensioned belt with speed 50 and Y axis moving I put the pressure on Y axis by holding all the moving mechanism with my hand till the machine started no pull herself on the floor while not loosing any steps. Of course this is not the best solution, but my machine was without the T-slot panel which weight is another 22kg's. First what I did, to align ballscrews was to loosen four corner screws in the front and four in the back of the machine, then moved (screwed) with my hands both ballsscrews till the end, tightened first four screws. After that moved along the Y axis till the other end and tightened another four screws. Now everything works like a charm.