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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
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
Spring-loaded NEMA 17 steppers / belt tensioner by MoonCactus is licensed under the Creative Commons - Attribution license.
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