Belts, Pulleys, and Motion Checks

TL;DR

If you get layer shifts or repeating ripples, power off and hand-move X/Y to feel for binding, then check belt tension and verify every drive pulley set screw is tight on the motor shaft flat. Belts should be firm (not guitar-string tight) and the belt path should run straight without rubbing flanges.

What These Problems Look Like (and Why)

Start with the motion system when you see: sudden X/Y layer shifts (the whole print jumps sideways), periodic ripples or “echoing” that repeat every few millimeters, corners that vary from one side to the other, or parts that measure differently in X vs Y. Loose belts can let teeth climb and skip under acceleration. A pulley that is not locked to the motor shaft can slip suddenly, causing a clean, dramatic shift. Binding (dirt, misalignment, over-tight wheels, cable tugging) creates tight spots that overload the motor and can also cause missed steps.

Fast Safety and Prep

• Power off the printer before touching belts, pulleys, or moving the axes by hand.
• Let the hotend and bed cool; belt paths often require reaching near hot parts.
• Clear scraps, strings, and dust from belts, wheels/rails, and the idler areas.
• Have the correct hex key/driver for pulley set screws so you don’t strip them.
• Good light helps: you’re looking for alignment, rubbing, and tiny gaps.

Hand-Feel Test First (Find Binding vs. Tension)

• With power off, move X and Y through full travel slowly.
• Feel for tight spots, grinding, or “notchy” motion; smooth should feel consistent end-to-end.
• If motion is inconsistent, fix the cause (debris, wheel preload, misalignment, cable drag) before cranking belt tension.
• Check near travel limits: cable chains, wire bundles, or Bowden tubes can tug and create fake “binding.”

Belt Inspection (X and Y)

• Look for slack, frayed edges, missing teeth, cracking, or a belt that walks up onto a pulley flange.
• Check that belt clamps are secure and not creeping; a slipping clamp can mimic under-tension.
• Confirm the belt runs centered on pulleys/idlers (not rubbing a flange or bracket).
• Pluck/press comparison: belt feel should be similar across the axis; big differences can indicate misalignment or one side pulling.
• After any tension change, repeat the hand-feel test to ensure you didn’t introduce binding.

Pulley and Set-Screw Checks (Common Layer-Shift Cause)

• Inspect each drive pulley: it should be in line with the belt path so the belt runs straight.
• Confirm the pulley is not sliding along the shaft; the belt should not be forced to track sideways.
• Tighten set screws firmly (use the correct tool). If there are two set screws, tighten the one over the motor shaft flat first, then tighten the second as a lock.
• Do a slip check: mark a thin line across pulley and shaft. After a test print, verify the marks still line up.
• If a set screw repeatedly loosens, remove it and re-seat it cleanly (debris/oil can reduce grip), then re-tighten onto the flat.

Idlers, Wheels/Rails, and Alignment

• Spin idler pulleys/bearings: they should spin freely without wobble or gritty feel.
• Inspect V-wheels/linear bearings/rails for dust, chips, or flat spots that create periodic bumps (often seen as repeating artifacts).
• Check for hardware rubbing: belt teeth contacting a flange, belt rubbing a printed guard, carriage rubbing a frame member.
• Verify the gantry isn’t racked: left and right sides should be at the same height before tightening brackets; a twisted gantry increases friction and missed steps.

Quick Verification Print (20–30 Minutes)

1. Print a simple part like a 20 mm cube or small calibration block.
2. Use moderate acceleration/jerk (avoid your fastest profile for the check).
3. Inspect for any layer shifts and look for repeating ripples on flat walls.
4. Measure X and Y dimensions; note if one axis is consistently worse.
5. If problems persist: re-check the pulley mark test and re-do the hand-feel test to find a tight spot (binding) instead of increasing belt tension again.