a tale of two rails

My second Prusa i3Mk3S (Prusa B) had a couple of interesting restricted movement issues. Both the E-Axis and the Y-Axis needed diagnosis and treatment in the last few months. I found solutions for both ailments.

Update 2021-01-24: I contacted Prusa Customer Service with the details and photos of the bearing issues. Prusa sent me replacement bearings: good customer service!*

E-Axis

Symptoms

My print failed and it was clear that:

  1. Prusa B was lost: it didn’t know where home was.
  2. Re-calibrating failed.
  3. The E-Axis would move right to left freely but not easily left to right.

Diagnosis and Treatment

  • I checked the belt tension: OK
  • Problem getting worse.
  • Reluctant to take the E-Axis apart, and still puzzled, I exercised the E-Axis in both directions a little, or to put it another way, apply a little brute force and ignorance.
  • Some small bits (crescents) of black material fell onto the build plate. The E-Axis was moving better. I exercised it a little more until it was again moving freely in both directions.

The bits of plastic on the build plate obviously fitted together into a circular shape the same size as an bearing seal. So I presume a bearing seal, broke up, and the debris caused the problems. When I exercised the E-Axis the bits worked their way out of the bearing.

Prusa B printed correctly again and could again produce a decent Benchy. I will replace the bearing some other time when I have to work on the E-Axis anyway.

Bearing seal debris

Y-Axis

Symptoms

Two months later another print failed.:

  1. Prusa B was lost again!
  2. The Y-Axis would move but with difficulty.
  3. Belt jumping the pulley.
  4. Belt tension ok.

Diagnosis and Treatment

  • At first I thought that is was another bearing seal issue: but no debris.
  • Removed the bed from the Y-Axis.
  • Exercised the Y-Axis and the bearing on the right felt like that it might be the problem.
  • After loosening the u-clamps the bearing moved freely on the rod.
  • Retightened the u-clamps in stages checking that the Y-Axis still moved freely.
  • Replaced the bed.

In this case I presume that one or both of the u-clamps was loose so that the bearing was not running straight. Prusa B prints correctly again.

PrusaB with bed removed PrusaB test OK