Designing Four Bar Linkage in CAD

For my first project, I wanted to start off with something simple, yet challenging enough for me to be able to have the opportunity to learn. With the resources I currently have, I decided to start off with a CAD project. I have been seeing people designing four bar linkages in CAD all over my feed (TikTok and Instagram), so I thought that this would be a fun start.

There are three main types of four bar linkages. To understand the basics of what they are and how they move, watch the video below.

Before starting the designing process, I decided to do research on four bar linkages, attempting to answer the overall question: what the purpose of them? Through my research, I discovered that although they appear to be a simple idea, they are a key concept worth understanding. The four bar linkage essentially transforms one type of motion into another. For example, the motion that the human knee puts in to pedal a bike is transformed into motion that moves the bike forward. The knee is an example of a natural four bar linkage, and it is drawn out below. There are many machines and tools that use the four bar mechanism, even if it doesn’t seem obvious.

Ex. 1: Scissor linkages (used in scissor lift extenders, maybe seen commonly in your local Home Depot/Lowes?)

Ex. 2: Bicycle movement (explained more in depth in the video above)

 

Before getting into designing, I first mapped out my initial dimensions for the linkages. I did not have a plan when originally creating the measurements, but I had a rough idea of the ratio between the different links (for instance, the rocker should be longer than the crank).

Dimensions of Trial One

Once the linkages were made, it was time to assemble. I had the basic understanding beforehand of how connectors (or mates) worked, but I was not well versed in all of them. In order to understand, I watched a few videos of how mates worked in order to get an idea of which ones I would use for this project. I already figured that I would need the cylindrical mates since they allow for rotation, but what I didn’t know was that I’d also need planar mates. The planar mates ensure that the links are actually connected to each other while the cylindrical mate only allows for rotation around the holes that are connected. Below is the product of trial one.

As you can see, trial one ended up appearing to work, but there were flaws in the actual mechanism due to overlapping of the crank and rocker from the crank attempting to do a full rotation. After seeing the links continuously overlap when trying to test the linkage, it was clear the measurements needed to be tweaked, meaning that dimensions actually do matter. With a little more research, I learned about the Grashof's Law which is a principle that essentially states that the sum of the longest and shortest link must be less than or equal to the sum of the other two links in order to work correctly. With this in mind, I was able to begin trial two after adjusting the dimensions so that they comply with Grashof’s Law.

Dimensions of Trial One and Two

With this, trial two of the basic crank-rocker motion worked. After this, it was easy to create the double crank and double rocker linkages since the process is repetitive. All that was needed to be done was to replace the linkages with the other. Though this project was simple, I enjoyed improving my skills in CAD while simultaneously enhancing my understanding for simple mechanisms, like the four bar linkage. Below are the final linkages in motion.