Convert rotation movement to a linear movement, in the same axis

Question

I need to convert the rotation movement of a motor to a linear movement (it has to be in the same axis of the motor).

I made a 3D model in Tinkercad consisting of two parts:

• the first part will mount on the motor and has a shape like screw inside

  

  

• the second part is a plunger that should move in linear direction when the motor rotate with the first part

  

  

  

I printed this model but the plunger gets stuck inside the first part and do not move.

So it fails.

I want a simple way to convert the rotary move to linear in the same axis of motor.

I broke part #1 ("the first part") from behind to see what happened inside. I found it sticks as part #2 jumps above the screw shape.

I think this mechanism is hard to be done.

So I switched to scotch yokes mechanism.

Hope it works!!

The vertical gear will pull the horizontal gears to move the articulator in a linear movement.

Answer 1

Simple Machines

We got a circular motion, and need a linear one. The most simple set up for this is known from steam engines:

The downside is, that this is a 90° turn from the motor. But to turn a circular motion by 90° is easy: Gears.

There are many options of gear types that would work to drive the transmission disk in a right angle from the motor:

• Helical Gears engage in a 90° to one another. McMaster Carr describes them as "For smooth, quiet operation at high speeds under heavy loads, helical gears have curved teeth that engage gradually and stay in contact longer than straight teeth. Because the curved teeth create thrust loads (loads that are parallel to the shaft), helical gear systems often require thrust bearings to prevent wear from misalignment."
• Miter Gears have their teeth mounted on a cone. There are 2 types (normal and spiraled), and McMaster Carr describes them as "Miter gears have straight teeth and a conical profile for transmitting motion at a right angle without changing shaft speed or torque. They're more efficient than spiral miter gears, which means they require less power to do the same work; however, they are noisier when run at high speeds and under heavy loads."
• Very similar are Beveled Gears, but the tooth geometry is simpler. McMaster Carr says: "Similar to miter gears, bevel gears have straight teeth and a conical profile for transmitting motion at a right angle. Unlike miter gears, one of the gears (sometimes called a pinion) is smaller than the other."
• The outside of the disk is connected to a worm gear. The linear motion will be slow but VERY powerful, and the machine can't drive the motor. McMaster Carr says: "Worm gears use screw threads to reduce shaft speed by ratios of 18:1 and greater while transmitting motion at a right angle. Unlike bevel gears, worm gears operate in one direction only; they transmit motion from worm to gear and cannot be reversed."

Redesigning the original part: the Angled plane in motion

Your basic idea works but your design had two large problems:

• The alignment feature for the pushrod is way underdesigned
• The Pushrod is missing a secondary alignment feature

That is easy to solve, and reduced to the working areas, this is what might appear in your construction, approximating the actually very complex surface to an angled face:

The rotating angled plane is good to act. To prevent rotation of the green pushrod, I made the retainment lugs much beefier and part of the whole shaft, not an addition onto the shaft. That made it a large rectangle, which prevents any rotation in the grey housing part.

Do note that the rendition as a simple ramped cylinder is just a very rough approximation of the actual body you need, which would be the rotational cut of a rotating and sliding cylinder. The required cam track can be quite tricky to design in CAD.

The shaft also protrudes through the bottom angled surface in the most forward position, which prevents the shaft from misaligning at that critical point and not hitting the bore it has to get back into. This protrusion can be much shorter than shown, but it is very much aiding in reliability and preventing the misalignment you saw.

As an additional factor, the turning disk might need to be made from two halves that are connected, so it can be put around the shaft and not contain an insert-slot that would result in misalignment. You might need interlocking pegs between the parts of the rotating barrel to force alignment.

However, there's one problem with the design in itself: the setup has very large contact areas and will induce lots of friction.

Answer 2

The following might seem obvious, but it shouldn't be overlooked either.

Unless you have one of those Wire EDM cutting machines, with very precise tolerances¹, that can seamlessly cut a shape out of blocks of metal that then, seemingly, glide out of the surrounding block, with little to no stiction², then your printed surfaces of the screw will have imperfections that may cause the binding. The first step would be to shave, sand and smooth the surfaces. Maybe even treat with another chemical, such as acetone (depending upon your printing material) to smooth the surfaces even further.

However, in addition to that, some lubricant may be in order. Every gear box or mechanism contains a little oil or grease.

So, your original design may still work, with a little help from smoothing and lubricating any mating surfaces. As Trish states, your design has a lot of surface friction, so every bit of polishing of the surfaces will help.

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¹ Approaching $\frac{5}{1000}$ (i.e. five thousandths) of a millimetre

² Such as the objects shown at the start of this video: How these impossibly thin cuts are made