Layer Shifting Problem  

Re: Layer Shifting Problem

A photo of the failed print goes a long way in describing the type and magnitude of the shift(s).

Since you recently worked on the printer, belts and drive gears are an easy "check" ... make sure you attached the belt drive gears FLAT-FIRST, and the belts are tight and not able to slip over the gears.

Second place to look is for things hanging down that can snag the bed and extruder as they move. Improperly dressed cables can easily jam things, especially if the nylon filament supports aren't installed right.

Third place is to ensure the bearings were lubricated before they were installed, and that they move freely along the rods. And when assembled, there should not be any tight or catching spots in the travel. The bed and extruder must be able to move side to side back and forth without any resistance.

And another thing is if you are using a hot end sock. Make sure it hasn't dropped down and is catching on the part.

Re: Layer Shifting Problem

A photo of the failed print goes a long way in describing the type and magnitude of the shift(s).

Since you recently worked on the printer, belts and drive gears are an easy "check" ... make sure you attached the belt drive gears FLAT-FIRST, and the belts are tight and not able to slip over the gears.

Second place to look is for things hanging down that can snag the bed and extruder as they move. Improperly dressed cables can easily jam things, especially if the nylon filament supports aren't installed right.

Third place is to ensure the bearings were lubricated before they were installed, and that they move freely along the rods. And when assembled, there should not be any tight or catching spots in the travel. The bed and extruder must be able to move side to side back and forth without any resistance.

And another thing is if you are using a hot end sock. Make sure it hasn't dropped down and is catching on the part.

Thanks for the input.

Belts are new and tight. Bearings are lubed. Nothing hanging down. No sock.

The shifts are typically one single offset during the print. They aren't multiple shifts. Even when it has been an x and y shift. It is one shift in either direction. The single shifts are also never the same distance. The prints are usually perfect except for the single shift in one direction.
The shifts also, typically, happen after the first 2 hours, but have happened as late as the 6th hour.

It's driving me crazy.

Re: Layer Shifting Problem

A single offset doesn't really explain much since others can't see what you are seeing. Are these four inch shifts, sub millimeter shifts, a missing extrusion that looks like a shift. So many possibilities, not having a visual reference is problematic and you'll probably not get a meaningful answer.

Presuming you've tried the stock sample gcodes to make sure it isn't a slicing issue; you've pretty much identified everything that could cause a shift as being okay; You could try a DMM with min-max detection and monitor the power supply for drops, or replace the control board in case it's dropping bits. It could also be broken wires ... some printers cables fail after a few thousand hours ... wires would be suspect if they shifts are becoming more frequent. Heck, could even be a pet walking past the printer at night.

Re: Layer Shifting Problem

A single offset doesn't really explain much since others can't see what you are seeing. Are these four inch shifts, sub millimeter shifts, a missing extrusion that looks like a shift. So many possibilities, not having a visual reference is problematic and you'll probably not get a meaningful answer.

Presuming you've tried the stock sample gcodes to make sure it isn't a slicing issue; you've pretty much identified everything that could cause a shift as being okay; You could try a DMM with min-max detection and monitor the power supply for drops, or replace the control board in case it's dropping bits. It could also be broken wires ... some printers cables fail after a few thousand hours ... wires would be suspect if they shifts are becoming more frequent. Heck, could even be a pet walking past the printer at night.

Thank for your input. I've finally added images.

Re: Layer Shifting Problem

Here are some images. They are from different prints. There are two that shifted twice during the print.
Some shifted early, some later. These were during prints that included multiple items with print times up to 8 hours total, so even the early shifts happened far into the print.

Printing the same items, under the same conditions in groups of one or two, the shift does not occur.

Re: Layer Shifting Problem

More images.

Re: Layer Shifting Problem

Looks like a belt or drive gear issue. Torque the gear set screws following the FLAT-FIRST method:

1) Loosen both set screws until the gear spins freely and both screws are raised and can't touch the shaft.
2) Align a set screw on the FLAT of the motor shaft, and tighten it while wiggling the gear a bit to ensure the set screw end is parallel to the flat. Tighten but don't strip the set screw hex.
3) Tighten the jam set screw to lock the fitment.

Do NOT go back and tighten the flat set screw unless you loosen the jam set screw first ...

Belts should be tight, and deflect about 7mm when 250g force is applied, as a minimum. I adjust my belts to 7mm at 500g (about 8 lbs tension). The belt minimum spec is 2 pounds tension.

Re: Layer Shifting Problem

Looks like a belt or drive gear issue. Torque the gear set screws following the FLAT-FIRST method:

1) Loosen both set screws until the gear spins freely and both screws are raised and can't touch the shaft.
2) Align a set screw on the FLAT of the motor shaft, and tighten it while wiggling the gear a bit to ensure the set screw end is parallel to the flat. Tighten but don't strip the set screw hex.
3) Tighten the jam set screw to lock the fitment.

Do NOT go back and tighten the flat set screw unless you loosen the jam set screw first ...

Belts should be tight, and deflect about 7mm when 250g force is applied, as a minimum. I adjust my belts to 7mm at 500g (about 8 lbs tension). The belt minimum spec is 2 pounds tension.

Thanks. I'll double check everything.

Any idea why it would only happen on long prints?

Re: Layer Shifting Problem

Probably something to do with heat build up or the random nature of longer prints give more opportunity for failure. Belts do get warmer as they work; they get longer, thus looser. Improper tightening of the set screws can allow the gears to rotate on the shaft: the motor heats up, the screws wiggle loose, shaft spins; there's been a few videos of this posted.

In the end, best to troubleshoot the well known and simplest causes first. If those are not found to be causing the problem, then look for more unlikely issues like low drive current or bad motors.

RE: Layer Shifting Problem

With the help of Prusa support, the problem seems to be solved.  I increased the Z lift by .2 - .4 mm, and my layer shifts are gone.

Maybe something was hanging up and causing the shift.  I hope this proves to be a long term fix.

Thanks for the help and suggestions.