Z-axis pole misalignment into motor jam

Hi there, ran into the issue with my new TAZ 5. The problem was easily fixed after some troubleshooting, but I figured would share this, as it has the potential to damage the unit (probably):

The problem:

Uneven Z-position on extruder bar anchor points.

The extruder position on each of the two threaded rods that are driven by the Z-axis stepper motors became unaligned. This was strange to me when it occurred. It happened mid-print early in the process (first 5 minutes). I noticed because of the loud straining sound of a motor locked up - the Z axis stepper motors could no longer turn (and were trying) because the height disparity on the thread bearings for the carriage support rods. This was even more strange to me because I had meticulously calibrated my bed z-axis stopper, bed level, and x-axis carrier level prior to my first print and had actually double checked (with the supplied ruler) that my bed was still squared up to the carriage rods (that is what I call them, the support rods for the extruder to ride along for x-axis movement).

The Fix:
I immediately turned off the TAZ5 when I heard the sound. It took me a bit to diagnose the issue because the carriage rods were not that far from horizontal. When I finally noticed this, I unbolted the right side mounting plate (the plate with the four bolts in a square pattern) from the thread-riding bearing/nut unit. I then manually turned the right most threaded rod until the carriage rods were about square/level to the bed. I did fine-tuning with the ruler so that the heights on both sides of the rods were within 0.5mm of each other. The printer has been doing a lot of work ever since and the issue has not reappeared.

The Cause
My guess is that either the stepper motors came out of sync, or one of the stepper motors is not securely fastened to its threaded rod. But since the problem has not happened again I’m at a bit of a loss. I thought maybe someone here might have experienced this or have other ideas for the cause? My biggest fear with this issue is that if it occurs when away from the printer it could warp/bend the carriage support rods, given sufficient time, strain, and temperature.

Yeah this can happen sometimes with two separate motors on one axis.

I wonder if you could put a belt between the screws or couplers to keep them synced and prevent this issue. I’ve attached a super technical drawing to illustrate my idea.