Intermittent Rear Z-Motor Failure at Startup - Core One+
Hi everyone,
We're experiencing an intermittent issue with our Prusa Core One+ and could really use some ideas on what to check next.
Problem Description
The rear Z-axis motor occasionally fails to "wake up" correctly during startup, causing the bed to tilt dramatically as the two front motors move normally while the rear stays locked.
(see video ).
Symptoms & Pattern
- Timing: Only occurs immediately after powering on the printer, only first use of the day
- Intermittent: Doesn't happen every time, but when it does, it's always at startup
- Once past startup: If the printer starts successfully, it runs flawlessly all day without issues
- Temperature: Printer is in an unheated room (~15°C ambient)
What We've Checked
- Cable routing and connections - everything appears secure
- Physical obstructions - nothing seems to be blocking movement
- The dust cover is visible in photos, but we verified it's not interfering (plus the issue occurred frequently before installation)
Any known cold-start issues with Core One+ motors?
We're running out of ideas and would really appreciate any suggestions from the community.
Thanks in advance!
RE: Intermittent Rear Z-Motor Failure at Startup - Core One+
The common suggestion here is to do the following:
Lower the bed to the bottom.
Loosen the screws holding the trapezoid nuts on those z-axis screws.
Move the bed up and then back down.
Tighten the screws on the trapezoid nuts - not too tight, as you don't want to deform the nuts.
Also consider lightly lubing the z-axis screws using the Prusa lube.
RE: Intermittent Rear Z-Motor Failure at Startup - Core One+
I have similar but it's always the back and right that doesn't work.
Always happens on power up the same as you.
Happens even when selecting Preheat, only the left side of the bed goes down.
If using the control to move the access it seems to continue to do it until you move the axis back to 0mm then they all start working in sync again (see video)
If I know others are experiencing this I'll raise an issue on GitHub. I thought I was the only one.
RE: Intermittent Rear Z-Motor Failure at Startup - Core One+
The Z-axis motors are connected in parallel. It is impossible for them to move independently, which means that something is inhibiting the motors that aren't working. The common cause is that there's too much friction. That can happen because the trapezoid nuts are overtightened, they're improperly aligned, they're mis-shaped and adding too much friction, etc. That's why the earlier suggestion to loosen the screws holding those then gently retighten them.
I'd get onto a chat with Prusa support if that doesn't help you.
RE: Intermittent Rear Z-Motor Failure at Startup - Core One+
Thanks, I will try these steps today.
I can't work out why there is no stalling type noises on the other two motors? Once the working motors gets to a certain point it stalls and makes a very loud not happy noise.
I feel like I can replicate the issue, so I will report back in the next days if this fixes the issue for me.
RE: Intermittent Rear Z-Motor Failure at Startup - Core One+
Many users have reported very tight trapezoidal nuts due to poor manufacturing tolerances. I think this is most likely to be your problem. Lubrication might help, but it won't solve this problem.
RE: Intermittent Rear Z-Motor Failure at Startup - Core One+
I've had no luck so far, having followed all the instructions above and add some of the Prusa lub.
To recap, when cold switching on the back and right z axis motors do not work. Trying to do anything like Preheat attempts to lower the build plate resulting is some squealing sounds when the left motors tries to force the build plate down too far.
When it does work then z axis is what I would call really smooth operating.
I recorded this first when the printer was new last June (factory assembled) but it was very intermittent then, now it's almost every time (it's winter here - temperature related around 10c ?)
It's getting annoying because at first using the control to move the bed up and down a little seemed to get it going but that doesn't always work now and yesterday it couldn't get it going at all.
Today I'm going to check the splitter connections at the bottom, maybe I can use some connection cleaner and reseat the connections.
Any other ideas?
RE: Intermittent Rear Z-Motor Failure at Startup - Core One+
For added info
I tested this morning and it was still exhibiting the symptoms of only the left Z axis working.
With the C1 tilted backwards so I could see the steppers and the splitter board:
1) Warmed the right stepper, this did nothing.
2) Removed the splitter and used contact cleaner on all 4 connections, this did nothing.
3) Removed the working stepper, I could hear a vibration - at least one stepper was trying to move.
4) With the left stepper still not connected I removed the rear stepper also, the right stepper started working.
5) Removed the right stepper and connected the rear stepper instead, the rear stepper started to move.
6) Put all connection back in, only the left stepper would move.
7) Repeated 4 and 5. Individually connected all the steppers work.
8) Repeated 6, only the left stepper is working - when all connected only the left one works.
9) Grabbed my phone to video the findings and all started working normally 🙁
It would be interesting to know if anyone else has tried this ?
RE: Intermittent Rear Z-Motor Failure at Startup - Core One+
@bionicbone, have you done the step of "relaxing" the trapezoidal nuts, as suggested earlier by @k1mu?
It is important to do that with the bed in the bottom position. Then loosen all six M3 screws at once which hold the three black nuts (3 * 2 screws). That lets the nuts find positions which are in agreement with the lateral bed position defined by the smooth rods. Then tighten the M3 screws again, but do not overtighten to the extent where they may deform the nuts.
If you have done this and still have the problem -- it could be a case of bad trapezoidal nuts. Several users have reported this recently. Move the bed down again, remove the M3 screws entirely and try to rotate the black nuts by hand. Are one or two of them moving with much more resistance? If so, you will need to contact Prusa support to obtain replacement nuts. They seem to send them without much debate since the problem is well-known now.
RE: Intermittent Rear Z-Motor Failure at Startup - Core One+
@bionicbone, while you have the nuts loose, I would also take the screws out completely and see how well the nuts travel up and down the lead screws. They should freely move by hand. Then reinstall the nuts as suggested by k1mu & Jürgen.
It seems you've tried every electrical diagnosis. We're trying to help you eliminate mechanical problems now.
RE: Intermittent Rear Z-Motor Failure at Startup - Core One+
Hi All,
Thanks for all the feedback and sorry for the late reply.
I spoke with Prusa CS yesterday evening. Given I have had a number of issues with the printer from new, they almost immediately asked if I would be happy for them to collect it so they can give it a good look over / fully test to ensure everything is as it should be.
I have my fingers crossed it doesn't take too long 🙂