Robust and precise NEMA 17 steppers / belt tensioners

by MoonCactus, published

Robust and precise NEMA 17 steppers / belt tensioners by MoonCactus Nov 17, 2012
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I finally got rid of the very annoying and varying short belt sloppiness on my Ultimaker.


This two-piece tensioner is simple, robust and it lets you adjust the belt tension with both precision.

It also protects your case from abusive screwing/unscrewing of the stepper bolts.

Disclaimer: please make sure never to over tighten your belts as it becomes very easy to do so... It will not make your prints look better and you most probably would ruin the belts and/or your bearings! Simply proceed with care as usual!

Update: if you want a spring-loaded tensioner, this variant may better be what you want: http://www.thingiverse.com/thing:40041

Update: added a version to hold the stepper motor by its 4 screws, in case you use 4 independent cylindrical spacers on the other side of the wall. It will help pulling it straight.


What you did

The usual fix is to unscrew the 4 bolts, push the motor downwards with a thumb and tighten the screws while keeping pressure. It is inefficient and annoying, and it even damages the printer plywood in the end!

What you will now do

Loosen the 4 stepper screws, adjust the tension by screwing/unscrewing the long vertical M3 bolt. Tighten the 4 screws moderately. Done :)

You can pinch the short belt with a finger while you adjust the tension. Hearing the sound is a nice way to set the proper and identical tension for the two axes (i.e. as is done with the long belts). Make sure NOT to over-tighten the belts else you'll ruin them and/or the bearings!

What you will need

Here are the parts for one tensioner:

  1. a 3mm drill
  2. two 10mm M3 screws
  3. two regular M3 nuts for the previous screws
  4. one 30mm M3 screw
  5. one M3 nylock nut
  6. three M3 washers
  7. two 25mm M3 screws instead of the stock 20mm for securing the stepper motor

How to proceed

The nylock nut is inserted longitudinally in a slightly conic hexagonal hole. Hence, you can print at any temperature and it will be OK: no hassle with lateral holes here!

I recommend you first screw the two pieces together tightly once to lock the nut in place, "deep" in the conic shape. Make sure the screw can slide freely on the head side (else drill the plastic)

Then, unscrew the long M3 nut so that it still protrudes by 2-3mm out of the nylock nut.

Attach the part that goes on the motor lower two screws (you need 25mm screws, the stock ones may be too short). Let the "fixed" part hang flat on the case for now.

While pushing slightly the motor downwards, mark the two holes (see the picture with the pen).

Remove the tensioner.

Making absolutely sure that you do not damage the cables within the case, drill the two 3mm holes at the marks (by the way, check that they are horizontal!)

Re-attach the tensioner to the stepper lower two screws, and firmly fix the lower part with two 10mm screws and regular nuts in the inside of the case.

About the files

I ued PLA, 0.4mm nozzle, 0.15mm layer height, 218°.

There are variants (while the two pieces come together in each STL).

One "downward version" show here has bigger holes on the stepper plate. This was made to insert two homemade washers made out of PTFE tube (6mm OD), that act as thermal insulators. I printed one of each and tested them in the long term. Update: really no need to worry about temperature here, the screw is not able to propagate enough heat to the PLA through the thick plywood layers even on very long & tricky prints.

The "upside" STL variant has m3 holes only. It inverts the orientation of the long M3 screw, so that you will be working from above and not from below. Thanks @am001 for the remark! Check the pictures and your onwn printer setup to choose which one suits you best.

The scad file is preconfigured with an upside adjustment screw and M3 support holes.

This thing is Attribution - Share Alike - Creative Commons. I will not and I cannot waive my moral rights on this thing whatever the Thingiverse bozos asks for

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Great thing! As I posted in the picture I made, if I the plastic motor spacers are used, the tension is unevenly distributed between the upper and lower bolts, which tilts the motor axis a bit. I prefer the plastic spaces because they provide better aeration and view of the axis. I have tested replacing the spacers with a modified version of this tensioner on the inside wall (printed in PLA). But it gets deformed both by the pressure of the motor against the wall and the belt's tension that deforms the holes.
So could you add a longer version that also holds the top bolts? This should solve the issue.

Wow, are these cylinders part of the official/revised ultimaker kit? It really does not look like an improvement... do they need to spare some wood or what? :D

Anyhow I've just added a "stepper_tensioner_upside_4screws.stl" for you, tell me if it's ok!

They are! (http://wiki.ultimaker.com/Ultimaker_rev.4_assembly:_Frame)http://wiki.ultimaker.com/Ulti... And they even added an injection moulded (!!) part to the extrusion head.
Actually, using the wood assembly and washers you can screw the standard sized bolts only one or two turns into the motor at most. So the new cylinders are 11 mm long (1 mm shorter). You can't do that with any combination of wood. This in theory should reduce the tendency of these bolts to loosen and loosen the belt.
And thanks. I'll print it right away.

Uh. Well, why not... The UM is not a vitamin-free reprap anyhow.
Actually what should have been injection moulded first is the fan duct ihmo, and may be the air duct for the electronics!

This is so nice, works perfectly.
Why Dont Ultimaker include this in their designs? it is so flawless :D

hell yeah, it works like a charm!

I just mounted these on 4 ultimakers, and they are great!
Some things to keep in mind when using these:
-You need 25mm m3 screws for mounting the motor, otherwise the screws won't go deep enough into the motor.
-When your belts are really loose, keep in mind that you need to increase the length of the slots where the motor screws slide in. It's really easy to lengthen them by using a small file.

Nice remarks, I'll update the info. I forgot about the screws because mine were already 25mm (I had to add an nut b/c they were too long). Thanks for the info.

Good idea and nicely done.

Excellent idea. Looks like a good solution to the problem. I usually insert a screwdriver through the top of the opening in the side for the pulley, and lever the motors down to get enough tension then tighten the screws, this looks a lot neater.
Minor tweak but why not reverse the bolt, so it is accessable to a screwdriver from above rather than underneath?

Good remark, I added your "upside" variant. It was a sequel of my very first idea where I thought the stepper would be in the way... but hey, it's not on the same side of the wall!
I kept the holes in the same place so the tensioners can be swapped. 

have a spring below the vertical m3 and the lower fixed part!? would that work?

Spring-loaded NEMA 17 steppers / belt tensioner

Yes, with a very long m3 you could put one between the screw head and the washer against the lower fixed part. I think the belts have enough elasticity by themselves though