Having an issue with my Z-axis coupler stretching...

[tmbomber]

Hiya Everybody,

I'm having a bit of an issue with my MendelMax 1.5. It seems the z-axis couplers are stretching a bit when moving down towards the bed.

As you can see in the pictures, I'm getting a variation of over a millimeter when I home my Z axis. The Z axis was homed several times between shots and I provided views of the coupler showing the measurement on my calipers as well as shots of the top of the threaded rods showing a gap between the nut and bearing in one and no gap in the other.

I'm unsure exactly why this is happening. I've done a few dozen prints before this started becoming a problem, but it seems to be getting worse.If I move the Z axis a lot using Pronterface I can actually see the X carriage moving down in little jumps as the coupler expands then springs back together. Moving the carriage up isn't as much of a problem seeing the ball bearing at the top of the rod is acting as a thrust bearing, keeping the layer to layer travel smooth on the way up. The issue is particularly prevalent at the start of a print. (my hot end starts out too far from the print surface and my first layer doesn't stick. I've also had the issue while attempting to set Z0 and wound up with hot end too close to the bed causing it to plow through previously extruded plastic. It's very inconsistent)

I think part of my problem might be that the 8mm smooth rods for my X carriage are a bit short. (you can see the ends of them recessed down in the X idler. This may be causing the tension on my X axis belt to rotate the X idler on the Z axis smooth rod, putting a sideways load on the threaded rod. (I'm not sure about this, but those rods being short isn't helping anything.)

I shot a bit of video showing what I'm talking about here. (At the time I didn't realize my X smooth rods were so short)
https://www.youtube.com/watch?v=pOZJaruzslo

Anybody have any thoughts or suggestions? Maybe a good source for 8mm smooth rods that are the correct length???

[Misha]

In my experience you don't need the thrust bearing on top, just let the ends of the rods be free.

[Sean Mitchell]

What I did to resolve issues like this is ...break open a bearing that you have and borrow some steel balls out of it.  I used 608ZZ but I don't think the size is particularly important, so long as the ball is tiny.  If you have a BB or something around, those would be good too (and you could save the bearing!)

Put the ball on top of the motor shaft, and then set your leadscrew directly on top of this ball.  This all goes inside the coupler, so the coupler doesn't take any of the weight.

I tried to capture it in an image here, 

https://lh4.googleusercontent.com/-3uj6GxuNSlc/UX0ah5G0QMI/AAAAAAAAEEI/MjJrXqf6EKk/w935-h623-no/IMG_1902.JPG

but it's hard to see.  Atop the motor is a bearing (and then the shaft would sit on the bearing directly, with the shaft coupler to hold it all in place)

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Sean Mitchell

echo "zvgp...@tznvy.pbz" | tr '[a-m][n-z][A-M][N-Z]' '[n-z][a-m][N-Z][A-M]'

[tmbomber]

That's..... brilliant....

What your describing would keep the coupler from compressing. My issue is more a problem with stretching. Inserting a BB as a spacer then stretching the coupler down before tightning it down should prevent it from stretching up any more...

I'll be give that a shot tomorrow.

Thank you :)

[Lee]

I like it Sean! Nice hack.

[peabody1929]

Are the smooth z rods 8mm or 10mm? When I had 8mm rods, they would bow or deflect if the x carriage belt was too tight. I rebuilt with 10mm rods and problem went away.

[Sean Mitchell]

I wish I could take credit for the idea, but I'm not that smart ;)  It came to me via the forums.

I don't understand how the coupler would stretch during operation, as that would require something to pull the leadscrew up.  All the weight of the axis should be sitting on the bearing, which at the same time should prevent the coupler from stretching.

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[Kai Dupke]

Usually the z-axis does not start at zero but above. If there is no
free/smooth move of the lead screw then it might become stretched or
pressed. Or if the stepper for one side does not do all steps etc. etc.

In such scenarios you need to fix the main issue (non-smooth move) first
instead of fasten something.

BTW, free rotation of the z-axis head is a good idea, not sure if I
would fasten this. If you block z-screw wobbling then it will happen
somewhere else (or break something).

There are special bearings for that which are intent to limit but allow
z-wobbling plus ensuring the high. So it's better to have a loose
coupling between the stepper and the lead screw and keep the position of
the lead screw not by the coupler but by other ways.



greetings kai

Kai Dupke
Senior Product Manager
Server Product Line
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[Larry Knopp]

This definitely is symptomatic of z-binding.

I don't believe the smooth rod length is an issue, as both ends are firmly constrained (it appears).

I would be inclined to investigate the whole x-carriage assembly / free movement on the smooth rods.

Could be one side binding as the z-home command attempts to "pull" the x-assembly downward - then sticking (coupler stretches) - then overcoming the binding (coupler "springs" back, jerking the x-assembly down).

I have moved completely away from flex couplers on all my builds.

On Fri, Sep 20, 2013 at 4:26 PM, Trevor Ajamil <taj...@gmail.com> wrote:

If that top part is for a 10mm rod Im the one who posted it on thingiverse. My aluminum couplers looked kinda bent. I use them on my prusa (top z motors) but i use the PLA printed clamp style couplers on my mendelmax with the top of the leadscrew constrained in the 608 bearing. It took a little bit of work to get it aligned but its perfect and has been for a year now. I think most people have had better luck with the aluminum coupler though. 

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