I have to deal with a situation where I have two parallel and completely independent rods whose motion is dependent only on external inputs, and I want to add up the lengths traveled by each rod.
My first thought was to use two rack-and-pinion mechanisms by which I move two gears, one by each rod, and then couple them to a single rack. But this is not going to work since the rack when moved by one gear, will cause the other to move as well.

This seems to be an easy thing to do on a macro scale but I wish to 3D print this mechanism and they are going to be very small, so I expect to build a very simple mechanism.

Since the description is not so clear, here is a diagram:
The Basic Idea

  • When Rod A and Rod B are at A0 and B0 respectively, I want Rod Y to be at Y0
  • When either Rod A or Rod B is at state 1 and the other at state 0, I want Rod Y to be at Y1
  • When Rod A and Rod B are at A1 and B1 respectively, I want Rod Y to be at Y1

Rods A and B should be absolutely independent of each other. And the gap between the lower face of Rod A and the upper face of Rod B should be around 35 mm, with all rods being about 3 mm wide. How can this be solved?

  • $\begingroup$ If each rod is only ever rotating in one direction you could use a ratcheting mechanism. $\endgroup$
    – jko
    Aug 4, 2020 at 14:33
  • $\begingroup$ No, the rods move forward and backward along parallel axes $\endgroup$ Aug 4, 2020 at 14:36
  • $\begingroup$ I thought of a ratcheting mechanism but that's why I added the last statement. It's too complex to be 3D printed as a very small piece $\endgroup$ Aug 4, 2020 at 14:37
  • $\begingroup$ If a ratchet mechanism is too complex for you to 3D print, I'm not sure you'll be able to produce any sort of solution. I would suggest a series of differential gears, but again this doesn't seem ideal for your manufacturing methods unless you think the components can be made. What is the scale exactly you're thinking of? $\endgroup$
    – jko
    Aug 5, 2020 at 1:58
  • 1
    $\begingroup$ Since both rods rotate both directions, how does this effect your output? What if both rods move at the same time? And you should move answers of this and other comments into question! $\endgroup$ Aug 5, 2020 at 18:19

1 Answer 1


If I hadn't messed up understanding your idea then you can connect all rods to achieve the desired result.

I've considered two bars connecting to maintain all rods paralel to each other.

The starting position will be with Rod A and Rod B are inactive (position 0-0) and rod Y will remain in $Y_0$. Initial position

The intermediary position Rod A or Rod B will be active, and the other is inactive. Rod Y will stay in position $Y_1$.
Intermediary position

The final position occur when both Rods A and B are active. Rod Y will stop in the position $Y_2$.
Final position

  • $\begingroup$ Perfect. That's exactly what came to my mind a few minutes before you answered this $\endgroup$ Aug 5, 2020 at 23:07
  • $\begingroup$ What software have you used to draw these diagrams? $\endgroup$ Aug 6, 2020 at 13:56
  • $\begingroup$ MS Powerpoint. The easiest option at the moment. $\endgroup$
    – Leafk
    Aug 6, 2020 at 14:05
  • $\begingroup$ Is there a solution where I don't have to use flexible connecting rods and all horizontal rods remain at the same vertical distance wrt each other? $\endgroup$ Aug 6, 2020 at 14:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.