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Y-belt holder and tensioner for MK3 and Bear MK2, MK2.5, and MK3

by zbrozek Jul 9, 2018
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As you may know, they are combining the MK2/MK2.5 BEAR onto the MK3 BEAR frame which is a bit longer than the MK2 BEAR frame. As a result your system will not work as the belt holder will not trigger the endstop. It needs to be a bit longer to the rear. Would you look into this and post a modified part please?

Hi Zbrozek - I really like this design and I would like to use this....do you plan to post some instructions please?

I had hoped it was pretty clear how to put it together from the transparent line-art. Maybe I should make an assembly video; did you find something particularly troublesome when trying to put yours together?

Maybe I'm just being dense this morning. I'm trying to print out the parts for a Bear MK2.5 V-Slot.

Do I use:

Thanks for the help

If you're using an random idler and bressing out the bearings, yes. If you're using an E3D one, substitute the slide for the E3D-specific slide. And you'll use a 5 mm shaft and no 608 bearings.

dears another question, i printed the y belt holder, any installation instruction?
first screw the main part on ? direction docent matter ? tooth of belt holder looks to the bed right ?
i would install belt first to y belt holder then the tensioner correct ?

sry many stupid questions but i don't want to do anything wrong

I would install the main body first, but the direction does matter. If I recall, it should go on the printer with "up" in the same orientation as the STL. Otherwise the belt line will be wrong. Then I would attach one half the belt to the carriage, route it through the pulley and idler (still unattached to the body), and attach the other side to the carriage. Now insert the slider into the body and start tightening each screw a little at a time until you get the tension that you want.

dears im a little confused.
is the dowel pin with m3 x 18 stock ? and do i really need to have a gt2 16 toothed pulley or does a toothless pulley also his job, really hard to get in germany therefore i ask :D

It's not a stock part, which is why I link it. There are other approaches - like finding a longer shaft and chopping it down. I'm not sure who your best suppliers are in Germany, but it should be possible to find. Worst case you can use a shoulder screw and cut the ends off.

On my stock MK2, I don't have a toothed pulley, but a "623h Bearing Housing", both for X and Y. That's just for remark about you wrote that Prusa stock is 20T. Then, I bought 15.8x3 dowel pins (pack of 100) for building the "official" X Bear and Extruder. It's disappointing that I can't reuse them for your tensioner. Do you need absolutely that length?

You can certainly use a shorter one; it just won't fully engage the housing. Try it and see if anything goes wrong.

Hi @zbrozek
could you please upload the source files in .step format? Because Fusion 360 can't import the Solidworks 2019 files. I'm getting the Error "Version of the file format is not supported".
I want to modify the slide part and make the space for the idler a little bit narrower, so it couldn't slide left and right so much on the shaft.
Thanks for your great work.

STEP isn't source, and I don't want to support an infinite number of file formats. You could use the same tool used to make the parts. Cost isn't a barrier any more.


Sure cost isn't a barrier now but I never used Solidworks and won't install and learn to use it, only for for the tiny modification I want to do.
Don't know why it's such a problem to also upload universal step files, so everybody can easily modify them with their prefered tool.
Will stick with your stl then :(

The tensioner looks great, but could we see a diagram or picture of how the belt holder is supposed to work? Seems like it would just slide around on the belt.

Maybe this helps to clarify?

Perfect, I didn't see the teeth on the bottom before. Thank you.

I don't get how the belt holder should work - it has no teeth.

It does have teeth! They're on the inside of the channel.

Totally didn't see them :) Thanks - and sorry for the noise.

It seems impossible to get a 16T idler with bearings that don't fail very quickly. Is there a version of this that works with a 20T idler (on Bear frame)?

Take a look here: https://www.thingiverse.com/thing:3122625

It's maybe a step further than you want to go, but the "half-serious" build might be perfect for you. It uses the teeth from a 20T, but has you use external 608 bearings to make a super-beefy idler. I've been running mine now since before that post went up and it's been great.

Serious business Bear MK2, MK2.5, MK3 y-axis replacement
by zbrozek

Thanks, will take a look at that. What do you recommend to press the old bearings out of the idler?

I used an arbor press and an appropriately sized punch. Folks without arbor presses have had luck with a vice and a bit of steel rod.

I am thinking my drill press and a punch would work

Will this work on Bear Full Mk3 upgrade? If yes then which files do we need to print. Its a bit confusing

Yes it does. There's no difference between any of the bears wrt/this model. Here are the files you need:

Now get back to work and ship my order already. :)

do i reale need a toothed gt2 16 pulley ? or is a toothless also okay?

Hi mate, nice model. I just printed this for the mk2.5, but found that the V-slot guide is a little too wide for the aluminium extrusion. It causes the main body to sit proud of the aluminium and causes it to pivot.

It's exactly the nominal size, though maybe I should consider shrinking it. If you're over extruding, not running linear advance, or not using the as-exported part orientation, you might have a dimensional accuracy error large enough to cause issues. Also make sure you're not bottoming out the M5 screws. 10mm screws are too long and will hit the metal before clamping down the plastic. Let me know what you learn!

Hi, I had the same issue with the MK3 part. I calibrated my MK3 before printing the parts for the bear upgrade. All of the bear parts fit perfectly. So I think making the part which goes inside the v-slot just a tiny bit smaller should take care of that. I had to hone down mine a little bit, because I didn't check carefully enough before disassembling my MK3. But that was clearly my fault and now every thing is fine :D

Do you have a picture of the printed part? Or if you have a spare bit of extrusion it would be super-great to have a picture with it mated at the end of the extrusion so I could see the interference. It was perfect on my print and extrusion, but I have really sharp printed corners on my setup. I just want to know how far to go.

I'm sorry I can't help with a picture. I already scraped the printed part on the sharp edge of the extrusion to make it fit. It's also already installed on the printer frame.

I posted an updated part over the holiday break!

Any chance of getting the stp files for the MK3 version? I'd like to remix it to incorporate a stop similar to the rear motor stop. Thanks!

*Edited to remove the words my phone decided should be in the sentence.

Oh whoops, I guess I packaged those up a while ago and haven't updated since I added new material. Yeah, I'll make a new set tomorrow.

Solidworks source is up!

Hi, just out of curiosity. Is there a reason for the middle screw hole in the MK52 belt holder, not going all the way through? What's the purpose of this "hole" anyways? It has space for a hex nut at the back side, but I cant see for what it's used.

It's there in case you made your own y-frame and don't have a threaded hole, and therefore need to use a nut. Most people won't use it.

Hi, just confirming, you meant "Stock-MK3 tensioner" and not "Stock MK3 tensioner", right? The tensioner isn't stock, right?

It's a tensioner for a stock MK3, but is not the idler that comes stock with a MK3. Yeah I should be more clear.

Thank you kind sir. The hyphen makes all the difference.

Anyone tried the Mk3 one yet, any feedback?

I've found one confirmed user so far. Claims it worked nicely for him, but didn't say much more than that. I don't have a MK3, so it's hard for me to do better than test fit in software. The MK3 flavor is less good than the Bear flavor because of the offsets to avoid the stock screws. I'm considering remaking that variant with the tensioning screws on another pulley plane of symmetry to restore the lost travel. But that may also be over-polishing something that is already good enough.

Thanks for your very good work!

Kind words are much appreciated; thank you!

Enough ego stroking.. get back to work! :)