# How to solve this linear motion problem?

Context: I have a lead screw rod, nut and a pulley aligned vertically. The nut is attached on top of the pulley so their respective bores are aligned and the rod is arranged so that it is threaded through the nut but its diameter is smaller than the pulley bore so no contact is made between rod and pulley.

When the pulley spines the nut turns, because the pulley and nut are in a fixed position the rod moves up and down via the spinning nut and through the pulley bore. Essentially a linear actuator.

Problem: My problem is how to support the rod from lateral load. Please could you comment on the validity of my proposed solutions and propose your own if you have something more effective

Potential Solutions: A: The most obvious to me would be to have a linear guide rail with a linear bearing in parallel and a secondary nut on the threaded rod that is not fixed like the first nut so that as the rod moves up and down the secondary nut provides support via the guide rail. The issue with this is it predetermines the height of the mechanism as it will be has high as the linear guide rail, I would like the height to be as small as possible.

B: Put a linear bearing through the rod and fix it's position to the wall of the actuator so that as the rod moves up and down it moves through the linear bearing which provides support via the wall. The issue with this is I think linear bearings are designed for smooth rods and the gaps in the threaded rod could allow the ball bearings to flick out under load, is this a correct assumption ?

C: Have 3-4 roller bearings arranged around the outside of the rod, so that the rod moves between the bearings. The issue is I don't know if there is enough contact area to be effective.

Diagrams:

I appreciate what I am essentially looking for is a spline shaft, but that is not something I have access to so I am trying to mimic the linear aspect using a regular lead screw

All help appreciated.

• If you were to use a ballscrew in stead of a standard leadscrew and nut, then you could simply have two 'nuts', Imagine your Solution B, but with two nuts,and the nuts are full of balls so they can take lateral force without binding. Jan 15 '20 at 10:18

You mentioned availability, so it sounds like you want to get away with the components you have. While there may not be a solution to that, I want to share my idea, how to overcome the "contact surface" problems in B and C.

Yould you mill or cut a notch along the lead screw rod, such that at three angles 120° between each noth, there is a smooth surface along the rod. This would in no way prevent the nut from performing as required. The Roller bearings in C would roll in these notches. You may want the shape of the notch to match the roller bearings, such that there is a line contact, rather than point contact. But, this really depends on the lateral load, that you have.

I am a little confused about the description of your setup and the goal.

Taking a cut through the center, according to the description - the pulley spins, the rod will move up and down freely. Since the nut and the rod are threaded together, then the nut must follow the motion of the rod, if so the rod will not spin, as it is independent of the pulley. Thus, the only thing required to maintain the rod move in the vertical direction, without shaking laterally, is a ring/cylinder with a borehole size slightly less than the diameter of the rod to allow for free motion.

If this is not your intention, you need to better describe how the setup functions.

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• "then the nut must follow the motion of the rod," the nut is fixed to the pulley which is fixed to the structure, when the pulley spins, the nut spins which causes the threaded rod to move up and down, exactly like a spline shaft. Your suggestion to have a ring /cylinder as support is valid (like a bushing if I'm following correctly), except the rod is threaded not smooth so I'm not having a threaded rod move through such a bushing is going to 'scrape'. Thanks for the input May 24 at 10:44