Left Z-axis is loosing steps  

Try switching the trapezoidal nut out on your left side.

The kit comes with an extra trapezoidal nut.  Try switching it out.  If there is too much pressure from _anywhere_ that will mess with the Z axis turning smoothly what you describe is possible.  You may want to put in a chat session with Prusa support over on the E shop as well.  Z binding is not a common problem.  Remove any addons you have installed on the Z rods - no motion spinners, no clever replacement tops.  Get as close to stock as you can. 

Did you modify the extruder carriages in any way?  Too much weight?

x rods inserted correctly, or are they putting pressure on the z axis?

maybe try unscrewing the trapezoid nuts and check if you can move the x axis up and down smoothly.

are the connectors properly inserted on the controller board?

RE: Thank you very much for your quick replies!

I dis- and reassembled the x-axis and made sure the rods are inserted to the very end of the holes. I also switched the trapezoid nut. The X-axis is sliding smoothly all the way up and down Z-axis, when the nuts are not connected. All addons are already removed, the printer is back in stock and i checked on the board-connectors again. I also loosened the x belt even more, just to make sure (if its too loose, i'll see in my prints and re-tighten it).

Unfortunately none of these things worked out 🤔 

The chat-support at the E-Shop wasn't that helpful either. I contacted them, but after talking to three guys there without any real results I thought I'd give it a shot and ask the community.

After the reassembly i managed to capture a video of the jamming. Don't mind the noise, thats just me spinning the controller-wheel to raise Z.

Video

Well, since the embedding of videos doesn't seem to work out, here is the link:

https://forum.prusa3d.com/wp-content/uploads/2021/08/1627933548402.mp4

You can see it right off the get go.

Interesting.  One thing that can make this happen is misalignment or un-lubricated bearings on the Z guide rods.  These sit inside of the X axis left and right assemblies. If you removed the Z motors but left the guide rods in place, you can see if there are hesitations in the bearings on the left side.  Did you remember to lubricate those bearings well before you put them in?  While the manual says no lubrication required - that is no longer true.  The thin shipping oil that the metal linear bearings are packed in needs to be replaced with grease - the lubrication tool that comes with the little tube of lube is adequate for doing this task.  If you did not lubricate those bearings you will probably develop problems.

I just put a MK3S+ together last week - and remember how much of that clear fluid comes out of those bearings as you lubricate them.  

Also, the Z alignment rods can be "crooked" if they also are not fully socked in on top and bottom.  It's a bit more subtle than the X rods which have an inspection hole.

I totally stuck to the manual

When I first assembled the printer, I didnt use any lubrication. After the first issues, I sprayed some Ballistol (I dont know if thats a thing outside germany, but we use it for everything: cars, bikes, guns, leatherware) on the smooth rods.

To be sure I cleaned all rods with Isopropyl, before I started as described here: https://www.help.prusa3d.com/en/guide/maintenance-tips_23200 and just put some of the prusa-grease on the rods (while I was at it, I did so with x&Y rods too).

Unfortunately that did not help much, so I decided to disassemble rods and bearings and use the orange tip to also get grease into the bearings. I also made sure in reassembly, that the bearings are 45°rotated as shown here in step 3: https://www.help.prusa3d.com/en/guide/3-x-axis-assembly_27464

Once again - no real improvement, as you see in the images:

Specimen #3 was printed before I put grease on the Rods, #4+5 before I put grease in the bearings and #6 after that. L and R are each the difference in mm from top of the axis to the axis holders.

The blue dots indicate the seams, which almost certainly occur on specific heights (note, that between #5+#6 the x-axis was partly disassembled!). There was also some vertical seam in 4+5, but that was, because i loosened the x-belt.

Next step for me: I'll order a new left Z-axis - just to make sure, that it isnt the problem with the NEMA itself. And to make sure, some new bearings - even though I think that they are alright, as at no point I noticed that any of the balls were falling out.

Any more recommendations? I might have some time to kill, while parts are shipping 😆😆😆😪 

I guess its solved 🙂

So, I got my new axis today and since I changed it, the problem is gone 🙂

sample #7 has an even surface and #8 is already 50% on its way, showing no seams yet (most of them were in the lower half, so I dont expect them to come now)

As I was curious, what the problem could be, I took a close look at the Nema&Axis:

Wires and pins looks pretty OK, so I dont think its an issue with the cables. Then I opened the Nema and I think I found the Problem:

There were two springs (these black circles) pushing it upwards, but i guess its supposed to be only one? I opened another Nema I had from an earlier project (right side) and there was only one spring in it.

Is it normal for Prusa Nemas to have double springs, or could it be that I was just unlucky and there was one too much in my motor?

Same problem here.

IMG_9641New kit out of the box. See the video.

You just saved me a lot of time

Same issue here - got one spring out, the printer finally could calibrate!

Glad, that my workaround was helpful for you too!

But the question still is: Do prusa-Nemas usually have two springs or is this kind of a bad batch? 🤣 

The solution appears to be partial…

Actually the XYZ and first layer calibration went perfectly but the printer struggles to pass 5-6 mm height. The screw drive is still not rotating smoothly. My guess is may be there is no space for two Belleville springs under the bearing, and during assembly the bearing gets overloaded and damaged due to cover screws tension. It’s hard to believe that for that price Nema use high quality thrust bearings.

Well, there goes that warrantee

I would have asked support for a new Z motor set - I don't know how they will deal with "and I took mine apart"

spring issue

I suspect that is a question for Prusa, or LDO, who I believe make Prusa Motors. 

I have never taken a z motor apart, and I suspect there are not a large number of people who have had this experience, 
Certainly in the 6 years I have been on the forum I have not noticed this being a common issue!

regards Joan

Motor.

I would still contact support and see if you can get another motor.  I would tell them I replaced the motor and it works now.  They will probably replace it.