Prusa i3 X/Y-Axis Belt Adjustment Guide  

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nick.m23
(@nick-m23)
New Member

Prusa i3 X/Y-Axis Belt Adjustment Guide

Properly-tensioned belts are essential if you want the best print possible.

Fortunately, the belt tensioning process is a simple one. There's a few steps involved, but trust me, it's not difficult.

This guide was made for the MK3 but should work on any newer models in the near future.

Anything before the MK3 cannot use the Selftest and must use more primitive testing methods to determine tightness (a decent guide for older printers can be found HERE).

1. HOW DO I CHECK MY BELT TENSION?

On any model newer than the MK3, belt tension can be checked in the menu (LCD Menu --> Support --> Belt Status).

However, this number is only updated when a Selftest is performed! To get the most current numbers for your belt tension, you need to perform a Selftest. (LCD Menu --> Calibration --> Selftest).

If you have recently adjusted your belt tension, your belts may loosen over the next few days as they stretch slightly.

2. WHAT DO THE NUMBERS MEAN?

This may be a little confusing, so I'm gonna put this part on its own line in big letters:


*** LOWER NUMBER = HIGHER BELT TENSION ***


If the belt tension numbers are too high (LCD Menu --> Support --> Belt Status), then your belt is too loose.

If the belt tension numbers are too low, then your belt is too tight.

3. ARE MY BELTS TOO TIGHT? TOO LOOSE?

Chances are that your belt is too loose rather than too tight.

The Prusa manual (PDF) recommends 240-280, although this guide recommends 255 - 270. It took enough force to get my belts to 250 that I believe that overtightening is less likely to be a significant problem than undertightening.

Just don't go too crazy getting your belts tight. A loose belt can be tightened, but an overtightened belt may need to be replaced.

Tension on both belts should be as close as possible. They don't have to be exactly the same, but closer is better.

Remember that belt tension will decrease over time. In 4 days, my tension decreased about 10 points on each axis.

If you bought your printer after February 2018, your x-axis should have a built in tensioner near the motor. Adjusting this can affect your belt tension by around 20 points depending on how tightly it is adjusted (Please reply with your numbers if you decide to adjust your x-belt tension using the built-in tensioner, we need more data!)

If your y-axis is a little too tight, you can simply loosen the screws on the printed idler pulley holder (behind the LCD display).

4. HOW MUCH DO I NEED TO ADJUST MY BELTS?

My personal experience indicates that tightening a single notch will increase your belt tension by about 25 points (remember, this means the number in the Belt Status menu will go DOWN).

- Tightening a single notch on my y-axis increased my tension by 25 (tension increased from 280 to 255)

- Tightening 2 notches on my x-axis (plus releasing the x-tensioner screw which was previously tightened all the way) increased my x-axis tension by 25.

*** PLEASE REPORT YOUR BEFORE AND AFTER NUMBERS SO THAT OTHER USERS CAN GET AN IDEA OF HOW MUCH TO ADJUST THEIR OWN BELTS. THANK YOU!!! ***

5. ADJUSTING THE Y-AXIS BELT

Adjusting the y-axis belt is pretty easy. The printer should upside-down so you can easily access the belts. There are a couple models you can print to make it easier to invert your machine:

- Prusa Maintenance Stand - low material use (This one is bigger and takes longer to print but is very stable)

- Work stand upside-down for original Prusa MK3 / 6mm Frames (This one prints faster but isn't as stable; I printed 3 and it worked well but was a little rickety)

Tools needed:

- 2.5 mm allen key (The one that came with your printer that has a little ball on the end)

- Something to hold the printer while it's upside-down (see printed stands above)

1. Unplug your printer and turn it upside down.

2. Loosen printed belt holders using the allen key. You do not need to remove them, you just need enough slack to remove the belt .

3. Take note of how many spaces are visible in the belt holder (or, how many belt notches are sticking out)

4. Remove the belt from y-belt holder, insert or remove belt so that the desired number of notches are inserted into the belt holder (1 notch will yield about 25 points of tension).

5. Re-tighten the pulley holders (They can be released slightly later if tension is too high).

That's it for the y-axis! If your belt is too tight after running a Selftest (LCD Menu --> Calibration --> Selftest), you can slightly loosen the screws holding the printed idler pulley holder (on the front of the machine, behind the LCD Display).

6. ADJUSTING THE X-AXIS BELT

This process is done with the printer upright. It may help to rotate the printer for easier access to the back of the x-axis carriage. Don't forget to recalibrate the XYZ axis afterwards (LCD Menu --> Calibration --> XYZ Calibration).

Tools Needed:

- 2.5 mm allen key (The one that came with your printer that has a little ball on the end)

- The needlenose pliers that came with the printer, or a small pair of side cutters, scissors, knife, etc. (something to cut the zip ties securing the cables onto the back of the x-axis)

- Extra zip ties (For securing the cables to the back of the x-axis after cutting the zip ties)

1. Turn off the power to the printer. You may want to remove the power plug.

2. Cut all 5 zip ties holding the cables.

3. Detach the optical filament sensor cable from the top of the extruder.

4. Remove the back of the printed rear x-carriage plate (Remove 2 screws on top and 2 screws on the bottom, as well as the one in the center (these 5 screws form a sort of x-shape).

- There two screws at mid-height that are deeper, these don't need to be removed.

4. You may wish to take note of how many spaces are in the belt holder on the side nearest to the x-axis motor. Remember that for each notch the belt is moved, you will get about 20-25 points difference in the belt tension. Also, remember that the x-axis tensioner can adjust the tension by 15-25 points if needed.

5. Remove the belt on the same side as x-axis motor.

6. Remove screws holding x-axis motor in place (release angled x-axis tensioner screw first if applicable)

- HOLD ONTO THE MOTOR WHEN RELEASING FINAL TENSION ON THE SCREWS OR IT WILL FALL!

7. Remove the belt from the toothed pulley/motor, set the motor aside.

8. Re-adjust the belt. (20-25 points per notch)

9. Re-attach the x-axis motor. I needed to apply a good bit of tension to get the screws to line up (I started the bottom screw first, then the outside top screw, then the inside top screw, then tightened them all evenly).

- Ensure that the motor cables are pointing down and the writing is readable (without having to tilt your head).

10. Re-attach the printed rear x-carriage plate.

11. Re-insert the optical filament sensor cable.

- At this point, you may turn the printer on and run a Selftest (LCD Menu --> Calibration --> Selftest) to ensure proper tension before you attach the zip ties.

12. Attach the zip ties.

DON'T OVERTIGHTEN THE ZIP TIES!!

- Ensure the zip ties won't hit the power supply when the x-axis moves all the way to the right.

7. RE-CALIBRATING THE PRINTER

- After everything is re-assembled and the printer is back in its place, you can do a Selftest to ensure that the belts are properly tensioned (LCD Menu --> Calibration --> Selftest) (My zip ties were hitting the power supply, so I needed to rotate them out of the way to pass the x-axis test.)

- It is a good idea to recalibrate the XYZ axes any time the printer is moved (LCD Menu --> Calibration --> XYZ Calibration).

- I had issues with the Selftest even after ensuring free travel on my x-axis. Re-running the Wizard (LCD Menu --> Calibration --> Wizard) did the trick for me, so give that a shot if all else fails.

8. THE END

You're done! Please comment with your before and after numbers so that others can get a better idea of how much of an adjustment they may need!

I hope this guide was helpful for you! I made this since I was unable to find a step-by-step guide to tensioning my belts (particularly on the x-axis). If anyone is aware of a better guide, please link to it and I can edit my post to include it.

Posted : 19/02/2019 6:39 am
Tim
(@tim-m30)
Illustrious Member

If I used that guide I'd break my motor shafts trying to get to 240 ... I went two notches and went from 283 to 278 even though it was difficult to get the motor attached: and belt was way too tight according to Gates' method of checking. Dropped back a notch, and used the tension gauge method to get to about 6 lbf, and my last selftest still reports 278.

It is always wise to get more than one opinion......
Posted : 19/02/2019 8:24 am
nick.m23
(@nick-m23)
New Member


If I used that guide I'd break my motor shafts trying to get to 240 ... I went two notches and went from 283 to 278 even though it was difficult to get the motor attached: and belt was way too tight according to Gates' method of checking. Dropped back a notch, and used the tension gauge method to get to about 6 lbf, and my last selftest still reports 278.

I was right around 278 before I decided to get my numbers closer to the "recommended" amounts, and I don't think I'd have had much luck getting another notch on the belt.

My goal in making this guide is less about the specific numbers than to fill a gap I found when I was unable to find a similar guide.

Posted : 19/02/2019 9:44 am
Tim
(@tim-m30)
Illustrious Member

So you managed to reduce your numbers by 25 or more, so from 280 to 255? I'd really like to know how. I can adjust the X-Axis screw end to end and not see real change on the self test; same for moving the Y-Axis idler a quarter inch; maybe a few points, but not even 5. And according to the motor specs, I am already very close to the limits for shaft bearing load with my present belt tensions.

One thing I did notice is that ensuring my frame was square and replacing the bearings with higher quality reduced the motor load from mid 270s to 280s. No change in belt tension required. But it makes sense because the numbers are derived from torque required (current in), and moving the belt is the same torque requirement at 1lbf tension as it is at 10lbf tension (except for bearing forces, which should be very low). The numbers just don't make sense as a reasonable indication of belt tension.

That said, Gates says to use deflection as the means for tension measurement. So that's what I do.

It is always wise to get more than one opinion......
Posted : 19/02/2019 8:22 pm
nick.m23
(@nick-m23)
New Member


So you managed to reduce your numbers by 25 or more, so from 280 to 255? I'd really like to know how. I can adjust the X-Axis screw end to end and not see real change on the self test; same for moving the Y-Axis idler a quarter inch; maybe a few points, but not even 5. And according to the motor specs, I am already very close to the limits for shaft bearing load with my present belt tensions.

One thing I did notice is that ensuring my frame was square and replacing the bearings with higher quality reduced the motor load from mid 270s to 280s. No change in belt tension required. But it makes sense because the numbers are derived from torque required (current in), and moving the belt is the same torque requirement at 1lbf tension as it is at 10lbf tension (except for bearing forces, which should be very low). The numbers just don't make sense as a reasonable indication of belt tension.

That said, Gates says to use deflection as the means for tension measurement. So that's what I do.

Yes, my numbers went from 280/278 (x/y) down to 254/255, simply by adjust a single notch on the y-axis and two notches on the x-axis (while also releasing the tension on the x-axis tensioner, which made a few points difference after I originally tightened it). However, they already crept back up 10 points a couple days after doing this, and it's likely a little higher now (it's been about 5 days since I tightened the belts plus about 20 hours of printing). I'll get you a set of numbers once my print is done since I'd imagine it's stretched a little further yet.

Would you mind linking to a belt tensioning guide using this deflection process? Sounds like it may be useful to have.

Posted : 19/02/2019 10:00 pm
Tim
(@tim-m30)
Illustrious Member

Here's the Gates info... useful tension info starts at T-29 of the SDP book;Page 184 of the GT2 manual; 180 of the GT3 manual (most recent info, but no GT2 tables). I treat pitch and span lengths the same since the idler pulley is free wheeling. But it's debatable; though the difference is a maximum of about 1.7x in my interpretation using the lower belt deflection. The SDP book is probably the best resource of the three, though it is not from Gates; equations on page T-31.

http://www.sdp-si.com/PDFS/Technical-Section-Timing.pdf

http://dpk3n3gg92jwt.cloudfront.net/gates.pt/pdf/drivedesign_powergrip.pdf

https://assets.gates.com/content/dam/gates/home/resources/resource-library/catalogs/powergripdrivedesignmanual_17195_2014.pdf

And other Belt Tension discussions:

https://shop.prusa3d.com/forum/general-discussion-announcements-and-releases-f69/how-exactly-to-increase-tension-on-x-and-y-belts--t26094.html

https://shop.prusa3d.com/forum/general-discussion-announcements-and-releases-f61/belt-tensioning-belt-status-t26599-s10.html

The important detail is all this is the belt is tensioned to maintain tooth to gear engagement, nothing more; with a slight bias to the higher end of the range to reduce backlash because the application is bi-directional.

It is always wise to get more than one opinion......
Posted : 19/02/2019 10:19 pm
Jakub
(@kubajs)
Active Member

Are we sure the lower the number is, the higher the tension is? I'm getting pretty much opposite results. When tightening the X axis belt in a small increments, I'm starting on 220, tightening more, getting 235, tightening more, gettin 240, then 245. 

I started with extremely loose belt when the test was failing at the beginning so I'm pretty sure I'm not wrong.
I was also checking the number is approximately the same when the test is re-run with no tightening update.

Prusa MK3S

I3 MK3S, FW 3.9.0, no closed box, fusion 360, Blender, PrusaSlicer, Windows 10...
Posted : 03/08/2020 3:04 am
Jakub
(@kubajs)
Active Member

Nevermind, in my case the situation is really opposite as described but only in case of loose belt. When I add a few half turns to the trimmer screw of the X axis motor, I start on about 250 (the best number I was able to achieve) and with every turn the number goes down. Tried up to 220.

For me there seems to be no way to get around 275. This is probably why the help page says the number is for raw orientation purposes.

I also tried to disassemble the extruder in the way I was able to change the postition of the belt by 1 teeth, still no way to get above 250 on X axis.

Guys please could anybody who is able to print benchy perfectly (no ghosting at all) record a sound of both belts when resonating it by a finger?

This would really help a lot. I know there might be some deviations as the belt production tolerances may vary a little bit but still it would give some good estimate for newbies where to start.

This post was modified 3 months ago by Jakub
I3 MK3S, FW 3.9.0, no closed box, fusion 360, Blender, PrusaSlicer, Windows 10...
Posted : 03/08/2020 11:33 am
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