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Some RepRap models use only a single motor for the Z axis, others use two.

For example, there is the 3drag that has only one motor and a smooth rod on the other side. There are modifications that add a threaded rod on the other side that is connected to the motor axis with a belt - which seems to be a really good solution.

Other printers, like the Prusa i3 or the Mendel90 have two Z motors. And after playing around with a two motor model, I find it pretty annoying when they get out of sync and I need to calibrate the axis and the print bed again. So two motors seem more like an disadvantage to me.

Could someone please shed some light on why most RepRaps have two Z motors (nowadays)?

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There are three main options here for Mendel style ZX gantries:

  • One Z screw and motor, which is similar to a cantilevered design but somewhat more stable because of the opposite smooth rod
  • Two Z screws and two motors
  • Two Z screws and one motor, with belt synchronization of the two sides

Of all of these, running two screws off one motor is clearly superior in reliability and user-friendliness. There is no risk of the two sides of the Z stage going out of sync. One motor running at higher current will generally out-perform two motors splitting one driver's current via parallel wiring, because one motor with twice the torque can push much harder when one side of the gantry binds up or hits a rough spot.

The only real downside to the single motor, double screw approach is that it requires more engineering and parts. A closed-loop timing belt must be run between the two screws, with associated pulleys, tensioner, and support bearings. In comparison, using a separate motor for each screw is very simple. It adds a stepper and a shaft coupler, but saves a lot of vitamins and design complexity.

Two-motor, two-screw solutions are lower-cost and simpler to design. That's why they're used. End of story.

One-motor, one-screw Mendel style printers are quite rare. The passive side of the Z mechanism does add a little bit of stability to the X stage, but not a lot. It's possible to rack the X stage out of square with the bed and bind up the gantry. In order to work at all, they require a very wide/tall bearing footprint on the driven side to resist torque exerted on the driven side by the weight of the X stage and extruder carriage. So it's true that they don't have synchronization issues, but additional design challenges and undesirable flexure modes are introduced. It's much more common for one-screw designs to simply cantilever out the X stage, like a SmartRap or Printrbot Simple.

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  • $\begingroup$ Thank you for the good explanation, I'll mark this one as the solution, as I think that it answers my question best and matches my experience. So you think it could be good decision to replace the second z motor on a Mendel90 (or a Prusa i3) with a belt? Both have a smooth rod and threaded rod on either side anyway. Btw, the belt design I meant for the 3drag is this one, couldn't find it before. I thought I could use that as a rough template. $\endgroup$ – nnscr Mar 12 '16 at 13:53
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    $\begingroup$ I would add that having screws on both ends also lets you reduce the mass of the whole carriage, because of the much lower required stiffness. That should let you crank the speed/acceleration up higher, though that's less important for Z than for the other axes. $\endgroup$ – TextGeek Mar 14 '16 at 17:56
  • $\begingroup$ As an Ender 3 user, I can second that "one motor, one screw, two rails" is frustratingly error-prone if you mess anything up. It does work quite well if assembled correctly, though. $\endgroup$ – R.. GitHub STOP HELPING ICE Jul 1 '19 at 16:27
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The general concept is to provide additional stability during the print. In the case of the 3drag machine you mentioned, you could run the risk of the -X- axis sagging due to the weight of the print head and/or additional wear on the rod or bearings on the one side (smooth side) as a result of the off balance weight. However, you may find "hacks" like this can potentially help reduce the affects by providing a bit more stability.

Having two -Z- axis motors with the threaded rods can help ensure stability of the -X- axis during print and can, overall, reduce wear on the mechanical components.

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First up: I'm no expert. All of the below is guesswork.

I think that the main reason is that it makes for a simpler design.

If you hold the X-Axis only on one side, you have to keep it level through rigidity. (Especially if you have the extruder motor on that axis.) If you try to control the sagging, you have a problem because the torque changes depending on the position of the extruder.

For stepper motors to go out of sync, one has to miss steps. Regardless on whether you have one or two or seventeen motors, if you are missing steps, you need to debug it.

I got lots of missed steps in both X and Y, and I tried everything. Until I found out that I was actually missing steps in Z, and the nozzle was hitting the build.

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