z stabilizers can sieze when traveling

Hi everyone,
I installed some z-stabilizers on my prusa mendel the other day:
[www.thingiverse.com]

It wobbles less but sometimes when going up and down the motors will stall and make a loud sound. Have other people run into this? It seems like something isn't exactly centered but how can I fix it? I use nophead z couplers instead of the official ones so I hoped it would help.

The whole point of using flexible couplers is that the screws can wobble and it does not matter. They flex at the motor shaft so don't impart any wobble to the X ends that are constrained by the Z bearings.

If you add the stabilisers it becomes over-constrained and will bind. I can't see the point of them.

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[www.hydraraptor.blogspot.com]

I cannot see any reason why the stabilisers are a good idea, though I did notice improvements (probably just more care assembling the second time.) if you're going to use them, attach them do you can just slide them up and down the smooth rod, and then take off and reattach the couplers. This helps make sure the couplers are on straight.

I'm also puzzled why people are putting this constraint at the end of the z lead screws. I think they confuse the wobble at the end of the rod for wobble of the carriage.

There shouldn't be any wobble, even without the z stabiliser. You get wobble because the coupler isn't holding the leadscrew centrally, or the leadscrew isn't straight, or because the distance between the smooth rod centre and the leadscrew centre at the motor mount is different from the distance on the x-end. You can test and fix the first two problems, and hopefully adjust out the third. There is a 0.5mm error in the x-ends from the Prusa repository, though I have bought up the issue with spacing with Josef Prusa on github, but it is not fixed yet: [github.com]
It's easy to fix yourself and recreate the stl. If all of these are correct, you should be able to bolt down the motors and have more accurate Z-axis moves, with no leadscrew wobble.
The other reason to have the stabilisers is to support the Z-axis on a thrust bearing, like on the MendelMax. This takes the weight of the whole X-axis off the bearings inside the motor and stress off the motor to leadscrew coupling, both of which are a good idea.

There is inevitably some eccentricity with printed couplings and studding leadscrews. However with my couplings (that allow angular flex only) it doesn't impart any wobble to the axis because that is constrained to follow the Z rods. Where the studding goes through the nut there is only slight angular movement but the far end of the leadscrew will harmlessly describe a small circle, especially when the axis is close to the motor end.

If you constrain a third point on the studding the motor coupling would need to allow radial movement instead of angular movement, like an Oldham coupler. In that case it can't also provide axial support and that would have to be provided by the stabilizer and the lead screw would have to be precisely straight and parallel to the Z rod. That would be the normal arrangement with leadscrews and couplers made on a lathe and precision bearing mounts. It isn't appropriate for printed parts.

The motor bearings are ball bearings, so taking the stress off them by adding another ball bearings doesn't achieve much, especially since the Z axis moves so little.

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[www.hydraraptor.blogspot.com]

These things might be an alternative, decoupling any eccentricity of the Z leadscrew from the X axis: [www.thingiverse.com]

nophead Wrote:
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> There is inevitably some eccentricity with printed
> couplings and studding leadscrews. However with my
> couplings (that allow angular flex only) it
> doesn't impart any wobble to the axis because that
> is constrained to follow the Z rods. Where the
> studding goes through the nut there is only slight
> angular movement but the far end of the leadscrew
> will harmlessly describe a small circle,
> especially when the axis is close to the motor
> end.

I have had some trouble with motors with two factory-made flat sides in the shaft. They are don't really want to center in the couplers no matter how you turn the shaft. I can still confirm that this does seem to be just a cosmetic issue since the wobble in the shafts isn't causing any lateral movement in the X axis ends.

Z stabilizers only make sense if your z screws are perfectly straight - in which case, they're there to fix the bad alignment in your coupler.

A better approach is to use a more flexible coupler, and thinner (more flexible) leadscrews.

I have two printers, both of which use aluminum flexible couplers. One of them uses 1/4" acme leadscrew, and the other uses 5/16 threaded rod.

The one which uses 1/4" acme shows about 1mm of wobble at the bottom end of the screw (farthest away from the motor), but that wobble is not transmitted to the X axis, and instead is eaten by the flexible coupler, and the acme rod.

The one which uses 5/16 rod shows about .5mm of wobble at the top end of the screw (nearest the motor). That wobble is transmitted to the X axis, and affects print quality.

Other than the rods, the major difference between the two printers is the drive shaft on each motor. On the "good" printer, the drive shaft is 23mm long, and round. on the "bad" printer, the drive shaft is 12mm long, and has two 10mm long flats ground into it at 90° angles to each other. I suspect that the flats are causing misalignment of the couplers.