Taking tension pulleys apart  

RE: Taking tension pulleys apart

Posted by: @bowtie6

Don't want to hijack the other thread...

What is the best procedure to follow to replace the tension pulleys on the Core One?  I realize this depends on whether the screws have seized or not.  It would be nice to have a simple guide and maybe document any tips/suggestions.  What is the best way to do this?

thanks!

It's really simple though. Just completely unscrew the tensioner, remove the screw holding in the pulley, put in new printed part and reverse the process. Just follow along with the build guide if you need photos. 

RE: Taking tension pulleys apart

To add to what @hyiger said:
Completely loosen the screws holding both tension pulleys.
Check that your gantry is square.
Check that the new pulley holder is able to move freely when slid into the original position. If it's tight, adjustment will be difficult.
Unscrew the screw holding the pulley, use it to attach the pulley to the new holder. Make sure the belt is wrapped around the pulley. 🙂
Slide it into place. 
Slowly - and I mean slowly tighten the two pulley holders, keeping the belt tensions as close to the same as you can so you avoid pulling the gantry out of square.

Once you get into range, use the app to check the tension, again equally moving each side. The Gates Carbon Drive is far easier than the other measuring tools.
When you near the right tension, move the extruder to each of the corners before the last tightening. You'll be tweaking it small fractions of a turn to get it spot on

If you let the belts pull unevenly, the gantry will oblige you by getting out of square. It's really important to not crank down the right then crank down the left. Go evenly as much as you can.

RE: Taking tension pulleys apart

The Prusa Belt Tuner web app works the best (in my opinion) with an iPhone. Make sure not to pluck the belt too quickly, pluck once, wait for a reading, then pluck again. Maybe every two seconds or so. 

RE: Taking tension pulleys apart

Posted by: @bowtie6

Don't want to hijack the other thread...

What is the best procedure to follow to replace the tension pulleys on the Core One?  I realize this depends on whether the screws have seized or not.  It would be nice to have a simple guide and maybe document any tips/suggestions.  What is the best way to do this?

My tensioning bolt seized against the nut which made disassembly more difficult. Here's what I did.

1. Removed the acrylic panels for easier access
2. Removed the door for easier access
3. Removed the nextruder & linear rail plate to release both belts
4. Removed the door sensor switch (difficult but doable with the allen key and side cutters for the zip tie)
5. Removed the three bolts holding the belt-tensioner plastic part onto the gantry (easy access with the switch and panels removed)
6. The belt-tensioner assembly now lifts out including the pulley and seized bolt
7. Threw the damn thing into the bin, cursed the demon who designed it
8. Printed a better designed belt-tensioner on another printer (!)
9. Reassembled and tuned the belts

I used the improved belt-tensioner design by Chris Hill which uses threaded inserts rather than an M3NS nut. I also lubricated the insert and bolt with anti-seize grease. I strongly recommend this upgrade and I hope Prusa incorporates it into future kits!

Also when reassembling its essential to get the linear rail square with ZERO belt tension. The belts aren't meant for adjusting the skew of the linear rail. The linear rail must be square before tensioning the belts. Tweak it gently to bend the right angle metal brackets square, testing each time until it hits both endstops at the same time. And tighten both belts at the same time. Ideally using two allen keys and turning them both together. Prusa does mention this in the instructions, but its not 100% obvious how important it is. If you tension one belt and not the other, you will throw the linear rail out of square. If you start with the linear rail skewed, it will stay skewed even after tensioning the belts.