I apologize in advance as this question might seem very introductory/simple, or it might even sound a bit vague. I am very new to design in any field other than UI/UX design, so I'm definitely very far behind, but I'll home in on exactly what I mean and hopefully this question can be answered effectively.

Explaining the title, I think the most common example that I tend to use is Legos, as with every sorta kit or design, there's always a way to attach items (say, gears) to mounting points through joints (if you want rotational motion to be translated, you could use a plus axle; if you want something to swing freely, you use a beige pin; if you want something to be more or less fixed, you use a black pin, etc). In practical situations that aren't Lego though, this doesn't seem to be a thing. I don't see a lot of plus axles used in machinery diagrams and similar things, and I'm assuming this is probably because it's unoptimal for force distribution or some other impractical reason.

So my main question basically is as follows:

What is the standard way to design machinery that involves motion, either rotational or positional? I've heard of and seen a lot of mechanical designs that allow you to convert motion, such as maybe a rack and pinion mechanism or a four-bar linkage mechanism, but how are any of these anchored to anything else? Say I wanted to pass an axle through a block like this (forgive my terrible drawing) and rotate it on one side:

enter image description here

This already opens up a large amount of questions, like how do I pin the axle in place, how do I deal w/ friction (as I assume that's probably an issue), and a bunch of others. What should the design of this axle be? Should it be keyed so that I can rotate it with decent force? I assume it can't be perfectly round because it would then slip. How would I, say, place a gear on the axle (in Legos, you could just slide it on)?

I was sorta hoping for a general design principle or something that would allow me to create a design that is effective and practical. If any clarification is needed I am happy to provide in comments or edits or otherwise.

Again, apologies for the question being a bit vague or really simple but I've been wanting to start designing machines that use this similar mechanism for the purpose of 3d printing, and I've wanted to start in a CAD software I know, like Fusion 360 or Solidworks, but I'm stuck trying to figure out what I should even be designing. Help is greatly appreciated. Thanks!

EDIT: Looking into bearings, I'm seeing how parts can be coupled together, but this still doesn't seem to cover a lot of other mounting methods, this still somewhat is a problem

  • $\begingroup$ Start by looking at bearings - lots of types to deal with many situations. $\endgroup$
    – Solar Mike
    Oct 1, 2022 at 19:23
  • $\begingroup$ so I actually have looked into bearings as one of my older projects was creating a balisong, and from what I understand it just seems to be a method of reducing friction, but this doesn't seem to help with identifying how anything is held in place or otherwise? $\endgroup$
    – Daneolog
    Oct 1, 2022 at 19:27
  • $\begingroup$ You need to look at types of bearings and how they work. $\endgroup$
    – Solar Mike
    Oct 1, 2022 at 19:30
  • $\begingroup$ I see, will do, okay $\endgroup$
    – Daneolog
    Oct 1, 2022 at 19:31

1 Answer 1


Regarding the design of mechanisms (things containing moving parts which are connected to one another) there is an engineering field dealing exclusively with this called kinematics of mechanisms, which is taught at the graduate level in mechanical engineering schools. It forms a rational basis for the design of things like piston cranks, robotic arms, car window raising mechanisms, aircraft landing gear systems and the like.

Kinematics is based on the mathematics which describe translations in polar coordinates as rotating vectors. Each vector represents a moving part and in the model, you connect the moving parts by adding the tail end of one vector to the head end of another. Those vectors are in the complex number plane which makes it a complicated business to solve kinematics problems by hand, requiring the use of things called dual numbers.

Today, CAD software makes solving kinematics problems easy. The computer does all the math while you sit there and drink coffee from a cardboard cup in your cubicle.


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