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It is common setup to have two lead rods that carry a platform that needs to move on linear bearing. The platform itself is controlled by a separate lead screw connected to a stepper.

I'm looking for very lightweight and small setup and wonder what kind of problems should I expect if instead of two rods + lead screw I would use one rod and a lead screw that would use as second rail for the device.

I understand that the end of the lead screw (maybe start as well) need to be put into some kind of bearing to provide structural support and accuracy.

Question:

  1. What kind of problems should I expect if any? In terms of control accuracy stability, etc.
  2. What kind of support recommended for such a lead screw (one bearing, two bearings) and what kind (since standard bearings for example 8mm are actually bigger than 8mm lead screw)?

Edit: It is for movement of optical system. I mostly for accurate slow movement - i.e. the speed should be constant and as accurate as possible.

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    $\begingroup$ That common setup is a bad practice in most cases. Cheap rods and screws (sold for 3D printers etc) are usually curved, corrugated and soft, not suitable for small and precise setup. You should clarify, what are you building because there is no universal solution for all circumstances. $\endgroup$
    – Vladimir
    Jul 7, 2022 at 19:05
  • $\begingroup$ @Vladimir I added clarification about the purpose. For me the critical is smoothness. Now why is this considered a bad practice? $\endgroup$
    – Artyom
    Jul 9, 2022 at 17:21
  • $\begingroup$ Leadscrew may not be your best bet. Anyway @user287001 is on the money its because of jamming. Anyway how long is the planned travel. $\endgroup$
    – joojaa
    Jul 9, 2022 at 22:31
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    $\begingroup$ After the question was edited: Optical systems may not at all stand vibration during the imaging or focusing process. Stepper as an idea has something to be compensated or attenuated with serious mass and fluid friction. If you happen to design something which applies laser interference the needed accuracy nor precision cannot be got this way. Can you possibly reveal your actual goal? $\endgroup$
    – user33233
    Jul 10, 2022 at 11:35
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    $\begingroup$ Precise optical mover is especially required to be rigid, self-aligning and gap-less mechanism. Such problems require investigation and discussion, this is not in format of this site. We can go to PM then, also, you can surf through factory mechanisms drawings and photos to see and learn, how they are made and read something about practical mechanical engineering. Problem of 2 rods+screw is that it is practically impossible to maintain parallelism and precise distance of them and such platforms either dangle or jam. $\endgroup$
    – Vladimir
    Jul 10, 2022 at 17:06

1 Answer 1

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You may plan something like this:

enter image description here

The green screw rotates and the blue block moves along the purple rod.

You may remember or be able to imagine this very real situation:

enter image description here

There's no screw, only the rod is in the hole of the blue block. The gravity cannot make the block to slide down, because the weight of the block generates enough torque to make the lower edge of the hole to bite the rod.

The screw can cause the same, but it also tightens the block to a tilted position which surely causes damage.

Long sleeve with rounded edges around the rod and tight tolerances may prevent the unfortunate tighted tilted position to develop, but 2 rods at the opposite sides of the screw would allow less tight tolerances and shorter sleeves.

There exists numerous single rod solutions in actuator mechanisms. Study them.

For more advanced answers insert more known data of the conditions that your system must stand and limitations, too.

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  • $\begingroup$ What I can do is to put two linear bearing over the rod at large distance between. In fact since the object I want to traverse is round I can easily have distance between the rod and the lead screw to be the same as the distance between two bearings - so they will be far apart at least as the distance between the rod and the screw. Thanks $\endgroup$
    – Artyom
    Jul 7, 2022 at 8:18
  • $\begingroup$ So as long as the friction is low and the coefficient of friction is much lower than the ratio between the lever created between two linear beatings to the lever created by the distance between the lead screw and the rod - it should work. I hope I got my mechanics right $\endgroup$
    – Artyom
    Jul 7, 2022 at 8:26
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    $\begingroup$ We unfortunately have no idea of the possible stresses of your system. You should reveal what the actuator should stand. A custom adapter between the screw and a standard ball bearing or lathing the screw is needed. But does your application really need ball bearing? $\endgroup$
    – user33233
    Jul 7, 2022 at 9:14
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    $\begingroup$ What about accidental thumps and thuds? Something which works well being in a certain position on a sturdy table may well get crushed if it's for ex transported in a car. G-force spec is essential. Insert all these details to the question. BTW remove the accept sign. It pushes off more competent persons. I'm only a hobbyist. You can do it a while after I have edited the answer. $\endgroup$
    – user33233
    Jul 7, 2022 at 9:35
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    $\begingroup$ Still one hobbyist grade suggestion: Check if you could scale a long movement to a short one with a sliding lever mechanism . Using it to scale down reduces also the absolute errors. $\endgroup$
    – user33233
    Jul 12, 2022 at 9:09

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