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Added springs to Prusa Z threaded rod...

Posted by Mazaw 
Added springs to Prusa Z threaded rod...
February 14, 2012 03:52PM
I was switching from regular M8 nuts (part of the vitamin kit from MixShop, so I'm not sure if they're zinc coated or if it matters in the slightest) to brass nuts and I talked myself into installing springs for the Z axis while I was in there anyway. Previously, I just used two nuts on the bottom of the X ends.

Now that the springs are installed, it seems to be binding a bit more and the threaded rods seem to have increased wobble. So, my questions:

1. Are these commonly available couplings on ebay any good? I'm using these but I wonder if there's a trick to alignment that I stumbled upon the first time around that I didn't accidentally perform this time.

2. Is it normal for the Z threaded rods to be harder to rotate? I'm guessing yes, because now it needs to overcome friction on two sets of threads from two nuts and the force of the spring.

3. Right now there are certain spots in the threaded rod that, when the X ends reach them, I get loud clicking indicative of not enough torque. I'm guessing it's probably preferable to reduce the spring tension than increasing current, but is it the correct thing to do? If the tension is so light that it doesn't actually perform it's function, I might as well just removed the spring and second set of nuts.

4. Since the Z motors are in parallel, does current get shared evenly between them? That is, is it safe to crank that pot on the stepstick with the assumption that it's less risky than doing the same on single-motor drivers?
Re: Added springs to Prusa Z threaded rod...
February 14, 2012 04:33PM
1) I wouldn't recommend them because they will stretch under the weight like springs. They are designed to be used between bearings to transfer rotational torque, not support axial loads. I designed these to solve the wobble problem: [hydraraptor.blogspot.com]

2) Yes much harder. The springs I added needed to be stiff because the weight of the x-axis was not enough to overcome the backlash, so they had to provide more force than that weight.

3) Assuming you have lubricated the rods you probably need more torque. BZP is not ideal as the thread can be patchy. I prefer stainless steel and it is generally straighter.

4) Yes they get half the current each but the driver struggles to provide twice the current. I changed mine to being in series to get more torque but that reduces the top speed as they get the full current each but half the voltage.

Cranking the current up on a Stepstick should be no risk as it is limited to 1A so unlikely to damage the driver or the motor. Pololus will overheat above 1A unless you use a heatsink or a fan.

Edited 2 time(s). Last edit at 02/14/2012 04:35PM by nophead.

Re: Added springs to Prusa Z threaded rod...
February 14, 2012 08:10PM
I think most of the problem relating to Z-axis on the Prusa is detailed in this github posting. which I don't think has been fixed yet: [github.com]

Basically, the distance between the Z-bar and motor shaft centre is different on the z-motor-mounts (30mm) to the x-ends (29.5mm). Additionally, NEMA17 motors can't be properly accommodated in the z-motor-mount (at least, mine can't), so end up even further away from the z-bar. Also, if you do the nuts up too tight that hold the z-motor-mount on, with the version that has reduced material in the sides, the back between the motor and the z-bar can bend outward causing, again, more space between the motor and z-bar.

Result: z-leadscrew runs eccentrically. I have made a version of the z-motor-mount that corrects these errors, and allows the motor position relative to the z-bar to be adjusted, which I'm still testing, which I will upload to thingiverse and submit to the Prusa github in due course. I can bolt down the motors, and while I don't use two nuts and a spring in the x-ends, the z-leadscrew turns linearly. By being able to align the z-leadscrews correctly and solidly bolt down the motors, I have beautifully straight vertical walls now.

I agree with nophead that stainless steel is a better leadscrew material. And I use his motor couplers, which are the best out there.
Re: Added springs to Prusa Z threaded rod...
February 15, 2012 12:54AM
I must not have noticed the eccentricity before. Or, perhaps coupled with the metal couplers it's just amplified.

Before picking up some stainless threaded rod I'm definitely going to try nophead's coupler design. My steppers are notched from the factory (maybe it's called a D shaft?) but I don't think that smaller arc of tubing making contact will affect it too much. smileys with beer
Re: Added springs to Prusa Z threaded rod...
February 15, 2012 02:37PM

In my Mendel variant (Simplified Ball Bearing Mendel, please excuse shameless plug), I used stainless M8 rod from McMaster, and brass nuts from an auto supply. I found suitable springs at my hardware store. The springs were "Servalite 84XAU". They should also work on a Prusa. The result was butter-smooth motion without any lube. See my documentation for the preload setting I used, and for the McMaster part number for the rod.

Re: Added springs to Prusa Z threaded rod...
February 15, 2012 05:00PM
Why no lube? Was that just a test or do you run it without lube, if so why?

Re: Added springs to Prusa Z threaded rod...
February 21, 2012 02:09PM

I ran it a lot without lube before final assembly. Then I forgot to add any later. It really seems to work very well without. I really should add some, but then it will attract dust and such... The brass nuts and high quality stainless rod make all the difference. Also, my design has a belt on z, so it is geared down, as in the Sells Mendel.

Re: Added springs to Prusa Z threaded rod...
February 21, 2012 03:40PM
I would be worried that the steel would wear away the brass pretty quick without lube, but I don't know much about it.

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