Z axis M8 nuts riding up / sometimes no movement on Z axis

On my Prusa Mendel the x axis smooth rods are going non-level.
Of the 4 M8 nuts, 2 of them have ridden up & one down, slightly out of their sockets in the motor holder & idler. Not fully out , but protruding by up to maybe 4mm. Seems worse on the top than the bottom.

Also an intrmittent problem on z axis , motors whirr & maybe one in 10 times, no movement. Seems
less of a problem when moveing down.

Nuts & rods from the reprapkitstore, assumed to be steel. Using Z axis couplings by Nophead's design.

After some searches, my plan is to dismantle the rods, run die on them to remove any burrs, then keep them lubricated with WD40. I've read that brass nuts not such a good idea , due to wear over time.

Am I on the right lines? Anyone else had this ?

(Alternative could be to drill/tap small holes in the motor mount & the idler a put small pieces of plastic to
retain the nuts, however that doesnt have the right feel to it).

Thanks in advance.

Mike

Edited 1 time(s). Last edit at 03/05/2012 01:30AM by 3D2B.

I have sort of the same issue with my Z-axis, if i tighten the nuts there is too much resistance in the axis and the motor can't turn it, if i don't tighten them they move like they are trying to escape.

Can they just be replaced with adjuster rings? the ones on the inside of the frame at least, the top and bottom probably needs to stay

I suspect you are both missing the springs and/or you don't have them compressed enough.

Put one nut on the rod, then put the spring on, then push the rod with spring and nut into the X-end from the top (spring first) and put the second nut on from the bottom. Now push the rod all the way in the top so the spring is compressed and turn the bottom nut until it touches the bottom. Once you let it go the spring should pull the bottom nut up into the cavity.

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I don't have any springs on my z-axis, but none where mentioned in the assembly files either. unless i missed it? mine is an orca 0.4

[www.mendel-parts.com]

Does not look like it, but I am willing to adapt that design if it will solve my issue, though i have an excess of the adjuster rings so if they work i might as well use them instaed of having to go out and find/buy springs

Edited 1 time(s). Last edit at 03/05/2012 03:25AM by Asger.

I don't know anything about the orca but you can see here in section 8 step 17 how the springs are used in a Prusa.

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FFF Settings Calculator	Gcode post processors	Geometric Object Deposition Tool Blog
Tantillus.org	Mini Printable Lathe	How NOT to install a Pololu driver

oh like that... i don't see how that would work with my orca, since i don't have that housing to lock the nuts

it looks like the orca is based off the sells mendel in terms of construction. It could be the nuts on around the bearings are binding

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Kreativ-e
Tinkerine Studio

they are all fine, no binding on the rods, i bougth quality rods cause i had that problem with the ones supplied with the kit

could self-locking nylon nuts be the solution? loctite? adjuster rings? drilling a hole into the nuts tapping a thread and inserting a lock-pin ? not through the rod, but just the nut, like on the adjuster rings.

Edited 1 time(s). Last edit at 03/05/2012 04:21AM by Asger.

WD40 is not lubrication (or at best only a light lubrication), it dries out and doesn't help much, if at all. Try using something like the PTFE based Super Lube, or similar.

The idea behind using a soft brass nut on a hard stainless steel rod, is letting the nut wear out first since it's the cheapest to replace. With proper lubrication, it's been said the soft on hard metal actually might lead to less to no wear due to lower friction. I've used this combination on my latest Prusa for several months now, and there's no sign of the black "dust" I got on my Sells with steel on steel.

We've had many interesting discussions about this in the past, just search the forums.

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-Nudel
Blog with RepRap Comic

I think both of my lathes use brass nuts on a steel lead screws, so I think it is common engineering practice.

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[www.hydraraptor.blogspot.com]

Yes brass nuts on steel is a good idea. I have just ordered some for my machine.

Traditionally phosphor bronze nuts were used on lathes which is better. You can still get them if you shop around. They were generally replaced by brass due to the rising cost of bronze.

i had the same issue with my aleph objects x-ends until i placed a properly sized spring above the nut. an extra side benefit is that it give you a few mm of upward travel in case you smash the nozzle into the bed. not a replacement for bed springs, though it could be if you were so inclined.

Just an update on this:
I replaced the original nuts (steel?) with brass nuts.
Before fitting I ran an M8 die down both threaded rods and a tap in the brass nuts.
For lubricaiton I used what I had to hand, some lightish machine oil.

Early tests, but it looks ok.

Mike

Update:
Well the problem seems to have 99% gone away, but occassionally recurs, typically after printer just turned on. Might just be a need for more/better lubrication.

I had a similar problem and found the answer after looking carefully at what I had.
I found that the spring between the nuts was getting caught in the hole of the printed plastic part that retains the nuts and spring.
To overcome the problem, I took the assembly apart one side at a time, and with a small pair of pliers I opened up the spring diameter at each end a little, so now the spring has larger ends than the middle. This stopped the spring from entering the hole in the plastic part and also acted as a funnel shape at each end of the spring for the thread to travel in. No more binding and everything runs smoothly now.
I also changed the stainless nuts for brass nuts while I was at it.