S5 huge layer shift .5" or more on top 20% of print on Y axis

[xtrude64 0]

Hello,

What causes a huge layer shift (around .5") on the Y axis only, at some point in a print (top 20% in this case)? 

 

I have a UMS5, .8mm nozzle, printing PLA.

 

The belts all seem fine and tight, I do not detect an looseness anywhere.

 

I was not present during the print unfortunately, so did not have the opportunity to hear any sounds.

 

Picture attached. 

[gr5 1,610]

It's pretty much always a loose pulley.  Especially if it's just one shift and a large shift.  But there are other causes (overheating driver, too much friction).  90% of the time it's a loose pulley.  To prove it you can mark the pulleys and shafts with a sharpy and next time it happens see if the pulley slipped on it's shaft.

 

It's almost always the pulley on the motor (the hardest to get to).  2nd most common is the other pulley on that short belt.

 

Slide head back and forth in the Y direction and notice which stepper moves.  The printers often come with a green and black hex driver.  It's possible to get it into the set screw on the pulley if you slide the head around a bit until it lines up perfectly.  Without having to remove the metal cover or anything else.  Tighten the hell out of that set screw.  It should be scary tight - if you were to use a little L shaped allen wrench your fingers should have red marks and pain afterwards.   Very tight.  Then get the other pulley on the short belt.

[xtrude64 0]

Ok, I will try that, thank you gr5!