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How should i deal with the vertical force of my Z-axis screw drive?

Posted by DerEchteLanger 
How should i deal with the vertical force of my Z-axis screw drive?
July 18, 2017 07:28AM
Im currently planning a small CoreXY machine and want to use a screw drive for my Z-axis. Since i want to use 2 screws and keep them synced with belts (using one motor) i need to use something else to "carry" the load (whats the right word for this? there doesnt seem to be an appropiate translation for the german verb "lagern").

I found those flangle bearings:
[www.dold-mechatronik.de]
and want to combine them with this screw:
[www.dold-mechatronik.de]

Now im not sure if the fittings inside the bearing are able to carry the load of the heatbed, they would only have those 2 grub screws as contact points on the threads. It wont be heavy (only 200*200mm), even with big prints i dont expect it to exceed 2kg (spread over 2 of those screws). Are the bearings enough or should i add something else?
Re: How should i deal with the vertical force of my Z-axis screw drive?
July 18, 2017 12:29PM
I would think those bearings would be OK. It's not ideal because the weight of the Z axis parts will be side loading the bearings, but you're probably well within the bearing's spec. If you want to get the perfect part, you want a thrust bearing like this: [www.ebay.com]. You'd need some sort of stop on the lead screw, and a mount for the bearing. The stop would usually be in the form of a shoulder cut into the lead screw on a lathe.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: How should i deal with the vertical force of my Z-axis screw drive?
July 18, 2017 11:33PM
In my new printer i use thrust/axial bearings to hang the rods from the frame. At the lower end i fix them with axial bearings since this is where the pulleys, for the belt that connects my two z rods with the motor, are.
In the end i have axial bearings where there is axial load, radial bearings where there is a sideways load and the rods are not compressed but stretched, which all helps minimizing any bending forces.


[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]
Re: How should i deal with the vertical force of my Z-axis screw drive?
July 20, 2017 11:55AM
There are many strategies for mounting leadscrews.

Since a 3d printer bed is preloaded by gravity, a single bearing is sufficient to eliminate axial play. You should be most concerned about introducing wobble if the screws sit off-center in the bearing, or if the unsupported end is free to whip around.

The extent to which your bed is vulnerable to wobbling will depend on how it is connected to the frame. If the bed is supported only by leadscrews you should be very concerned about keeping the screws centered and stable. If the bed is constrained by linear guides you can be more casual about mounting the screws.

Load is basically irrelevant when selecting bearings for this application since even the smaller 8mm bearings can survive 70Kg of static load (and a typical 8mm bearing can survive closer to 150Kg).

Fixing the screws to their bearings with two grubs screws will probably work, but is not particularly elegant because they will be pushing the screw off-center. The major diameter of a TR8 thread is toleranced as 7.850-8.000mm so there is a pretty high chance the screws will not be centered. The ideal solution would be to turn the screw ends on a lathe and use smaller diameter bearings. Other options include clamping the bearing to the screw with a pair of thin TR8 nuts, and/or ensuring the screw has an 8.000mm outer diameter via a shim or sleeve.

Edited 2 time(s). Last edit at 07/20/2017 11:59AM by 691175002.
Re: How should i deal with the vertical force of my Z-axis screw drive?
July 21, 2017 04:03AM
Oh, never thought about the screws not being centered. I will use linear guides, so i hope the wobble wont be to bad. The screws are toleranced 7.9mm to 8mm, with a micrometer i can check which end is closer to 8mm and mount that one in the bearing, maybe together with a carefully selected shim. Im not sure what to choose there, maybe folded aluminium foil.
Re: How should i deal with the vertical force of my Z-axis screw drive?
July 21, 2017 06:39AM
Aluminum foil or shim stock wrapped around the shaft will work well. I've started using Kapton tape to shim non-critical fits because it is much easier to work with and doesn't creep as much as other tapes.
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