Belt Test - X-Axis Fail - solved (stepper motor)  

Belt Test - X-Axis Fail - solved (stepper motor)

TLDR : Printer suddenly developed an X-axis problem. After replacing the linear bearings, pulleys, and belt, the problems was fixed by replacing the X-axis stepper motor.

After running my MK3S printer frequently over a period of three years, the printer suddenly failed. Running the self test showed there was an X-axis length error. I attempted to resolve this by adjusting the belt tension, making it looser, tighter, and various stages in between. The belt test continued to fail.

I had recently done a firmware update, so to make sure this was not causing the problem I downgraded to the prior release. Still failed.
I reset the printer to clear all the data - still failed.

In discussion with Prusa support, I checked the following :

• Checked the X-axis rods for scratches, gouges, roughness, etc. No problems found.
• Tried various belt adjustments again - no luck.

It was suggested that the linear bearings might need replacement. I ordered a set of uxcell LM8UU Linear Ball Bearings on ebay (not because the bearings were cheaper than Prusa, but because the shipping was much less). Installing the new bearings did not correct the problem. I observed that when the print head was disconnected from the belt, it moved smoothly with no discernible sticking or roughness. When the belt and X-axis stepper motor were connected, moving the print head felt "sticky"; in spite of this, when moving the X-axis via the panel settings, I was able to move the x-axis fully left and right. When running the belt test, however, the print head would stop at random positions, and would rarely move fully to the right.

I again went through various adjustments with the help of Prusa support, but the problem was not resolved. It was suggested that other possible sources of the problem might be a bad belt pulley or a bad belt, or in very rare cases a bad stepper motor.

I ordered replacement parts from Prusa (pulley, belt, additional LM8UU bearings just in case, and an X-axis stepper motor. The parts were delivered promptly.

Initially, I replaced the belt pulley and belt and re-tested the printer. The X-axis again failed. Adjusting the belt tension as before did not resolve the problem.

Finally, I replaced the X-axis stepper motor (see note below). This immediately resolved the problem and the printer passed the belt test.

With the old X-axis stepper motor removed, I was able to give it a crude test by spinning the axle by hand. The axle turned roughly and "stuttered" instead of turning smoothly.

I would like to add here that Prusa Support was very helpful and patient in working with me. I understand why Support would focus on potential belt problems as this is the likely problem in most cases. However, even though my problem was a rare failure of the X-axis stepper motor, I fell it would have been worthwhile to explore this possibility further. For example, comparing the rotation of the X-axis motor to one of the other stepper motors on the printer would have shown a clear difference. In short, while I think Prusa Support did a good job, there is an opportunity here for improvement.

NOTE - A final note on replacing the X-axis stepper motor: Replacing the motor itself is easy, but replacing the connection inside the control box is a little bit tricky. That being the case, I initially replaced the axis motor but left the old cable in place (only switching out the connector). This was sufficient to show that the X-axis motor corrected the problem.  I could then loosen one screw holding the back of the control box in place, so that I could pry the back open enough to remove the old cable and connector, and then replace it with the new cable/connector (after switching the cable cover). Also note that the connector has a latching mechanism - squeezing the connector gently with needle nose pliers will open the latch.