Spring-loaded NEMA 17 steppers / belt tensioner

by MoonCactus, published

Spring-loaded NEMA 17 steppers / belt tensioner by MoonCactus Jan 2, 2013
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This is an improvement to precisely set and keep a given belt tension by pulling the stepper motor downwards with a spring.

Some people were asking for it in the original design, mainly to reduce the risk of over-tightening the belt.

So here it is: obviously you'll need an appropriate spring and a fews more nuts & washers.

Additionally, it might even compensate belt length variations during print if the stepper is kept slightly loose

Update: I've added a 4-screw version, probably required if you do not have the wooden plates but 4 cylindrical independent spacers.


This design incorporate an additional spring on the free end of the screw, that makes tensioning more secure. Adjust with the screw until the belt "twangs" as desired.

The other interesting feature (compared to the previous design), is the slightly thinner plates: if you omit the washers, the motor mounting screw reach the bottom of the stepper holes before they fix it completely onto the plywood. For this,, you'll need another pair of 25 mm screws + one nut and one washer for the top holes, so the motor is let loose just enough by its 4 screws so it can slide vertically.

See it live: http://www.youtube.com/watch?v=ow2jrRFdzb8

I hope that any belt length variation during a long print can be compensated by the spring. It may happen when it gets hots, or when if wears out, though I am a bit skeptical.

Now, the parent thing is simpler but also riskier for some people as it makes it easier to break something if you tighten the screw like a fool, check it here (with how to mount): http://www.thingiverse.com/thing:34785


  • a 3mm drill

Fixed plate:

  • one M3 nylock nut
  • two 10mm M3 screws (to attach the plate that holds the nylock)
  • two regular M3 nuts for the previous screws

Tensioning screw:

  • one long 30mm M3 screw (to tune the tension and hold the spring)
  • an M3 short but robust spring (those of the UM bed are almost too long but still ~OK: for this reason I made one part ~2mm longer -- to reduce the gap between the two parts--), but the holes are still in the same place so upgrade is possible without having to drill more holes. I used a shorter and stronger stainless steel spring that is about 10mm long and 4.8mm OD and 3mm ID (ie. it fits better the 30mm M3 screw). With this spring, I only need a few mm of compression to achieve a correct tension, sorry but no exact reference.
  • two M3 washers on each side of the spring (one is hidden within the top part hole, to prevent the spring from damaging the plastic in the long term -- it is probably overzealous, and you could "steal" some more length for a longer spring than mine if you omit the washers)

Sliding/motor plate:

  • four 25mm M3 screws to replace the stepper 20mm stock screws
  • two M3 simple nuts + two M3 washers for the top two screws

The latter two items are not compulsory, but they let the stepper motor gently slide freely along the plywood wall. As for me and my steppers, there were almost no freeplay left when I screw them at the maximum in the stepper motor, but still, the spring I use is strong enough to pull it downwards nicely back when I move the stepper. Really OK.

Note: I mounted one of these tensioners on my Ultimaker because I am curious to see how/if printing could provoke unwanted oscillations when the motor is kept lose. If it does, I will secure it by adding washers to the stepper mounting screws, once the belt tension is set up (or I'll even revert to the inital spring-less design).

Update: one month later, I cannot find any negative impact on the quality of my prints (see the remarks below btw). Also, I will add kapton tape between the moving part and the plywood wall to make it slide a bit better may be.

Final note for the curious: the dirty plug on the picture simply lets me unplug the filament feeder, more on http://betterprinter.blogspot.fr/2012/10/removable-filament-feeder-add-blug-to.html

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It's a good idea to bolt down the stepper once the belt tension is set. If you keep this contraption loose, it'll introduce flex into the belt drive, making for some really unsightly corners in your prints.

Yep, that's also what I need to check :-)

I suspect that brutal reverse moves will show in the print. IMHO, bolting completely the stepper defeats the pupose of a spring.

But, whether a spring-less design is better or useless was debated a lot, and Bertho + Owen + Jelle + Joergen together definitely cannot be all wrong :) https://groups.google.com/d/msg/ultimaker/EKThJ7xCVrw/yTh5-FY7704Jhttps://groups.google.com/d/ms...

Time will tell if it's worthwhile, but it cannot hurt in any case: if this version prevents just one printer to be damaged, I'll be happy :)

It now lacks a given & known spring reference may be.

If either your acceleration values are low enough or the spring has enough pre-tensioning, the spring will not compress a single bit from the head movements.

With a bit of applied mechanics, you could calculate the needed pre-tension - all you'd need is the weight of the printhead and the acceleration value set in the firmware. Multiply those two [acc in m/s², weight in kg] and you get the minimum spring force required to keep the motor from bouncing around. I'd say factor in a safety factor of 2, then divide through the spring constant (unit: N/m) to get the needed pre-compression of the spring.

Let's say the head weighs 200g, acceleration is set to 10000mm/s².

That's 0.2kg and 10m/s² (or N/kg), giving a needed pre-tension of 0.2*10=2[N] (4N with safety)

If your spring has a spring constant of 800N/m, the needed compression is 4/800=0.005[m].

So if you compress that exact spring by 5mm, you won't see any degradation of printing quality.

The Ultimaker uses 1/4" MXL belts afaik. The recommended pre-tension for those is 15lbs/1inch width - that's 1,7kg for a 1/4" belt (or 17N on earth) - much more than the required pre-compression to keep the motor in place. You can calculate the required spring compression to get 17N the same way as outlined above.

So to sum it up, my worries about flexing were unfounded - if you use a spring to correctly tension the belt, it will not give when the head moves around.

Wow, brilliant analysis!

Now, it could be awesome to add pods to the design for a small or removable luggage weight scale. This would set the tension up scientifically...