Week 6: Four Bar Linkages

Overview

We will examine some of the finer details of Four Bar Linkages and how to create, animate, and simulate them in Fusion 360.

Homework

Let’s look at our animated joints and discuss our intended uses for our Four Bar Linkages.

Four Bar Linkages

Let’s refer to the links by the proper names: Crank, Coupler, Rocker, and Ground or Frame.

Four bar linkage with named links

Crank: link AB

Coupler: link BC

Rocker: link CD

Ground/Frame: link AD

And E is the End Effector.

Let’s create a quick four bar linkage model. We’ll use the “Center to Center Slot” tool and “Revolute” joint.

Remember, CAD lies to you.

You can easily make some impossible linkages. In this example the bars are “ghosting” through each other (around the 5 minute mark in the video).

Coupler Curve

Images from Coupler Curves Video

We are concerned with the “Coupler Curve”. The coupler curve is the path that our end effector (point E) travels in our mechanism. As far as I can tell, Fusion 360 does not have a way to highlight the path of the coupler curve. So we will use our designs we create with the Mechanical Expressions Tool.

Take note, the End Effector does not travel at a constant velocity and acceleration along the Coupler Curve. There can be fast spots and slow spots.

Coupler curve with a Cusp

Cusp, a sharp point on a curve which has useful properties of instantaneous zero velocity.

Coupler curve with a Crunode

Crunode – is where a path crosses itself

Four Bar Linkage in Fusion 360

Load your values for AB, BC, CD, AD, BF, and FE in the Mechanical Expressions Tool. Take a screen grab of your linkage with coupler curve path.

Insert your screen grab as a Canvas in Fusion 360

Four Bar Linkage screen shot used as a canvas

Calibrate Canvas to the proper scale.

Create a Kinematic Diagram – a sketch with just lines that represent the links. Fix the two Ground points by right clicking on them. Dimension the lengths of each line.

Kinematic Diagram of a four bar linkage

Create a Mechanism Skeleton – another sketch that is basically a 2D projection of everything in your four bar.

Mechanism Skeleton of a four bar linkage

Extrude your links as new components.

Four components making up a four bar linkage

Create Revolute joints between each links and animate model.

Animated Four Bar Linkage

The four bar linkage travels the curve I want, but there are two problems with the above mechanism. One, link BC (the coupler) “ghosts” through link AB (the crank), this is an impossible machine. And two, the end effector E is not exactly right. The center of the curved end of E follows the Coupler Curve, not the tip of E. Some modifications need to be made to have the mechanism work correctly. Remember, CAD lies to you.

What to do with your Mechanism Skeleton

At this point, I would normally create a functional prototype from the Mechanism Skeleton. I would laser cut each linkage and connect them with fasteners (screws, nuts, and washers in this case). I would see what’s working and what needs improving.

The prototype below in a little more involved than a simple laser cut proof of concept, but it does the job.

Take note of the top view of the initial prototype. My links need to be secured to each other and not bang into each other. Screws, nuts, and many washers needed to be used to get the mechanism to operate.

Top view of the four bar linkage

How the linkages are fastened to each other will have to be considered.

Designing Our Mechanism

Knee and hip joints by Jeff Clune

Now that we have our functioning four bar linkage, we can truly design our mechanism.

Here is how I started working on my “Block Pusher”.

Working with my mechanical skeleton, I started adding and editing elements. Linkages with more complex geometry (but keeping the same lengths to my kinematic diagram). I add fasteners (shoulder screws from McMaster 91327A295 and 91327A294), and see where there are collisions. I add the block, and see what needs to change in my mechanism. Now I need to modify links, add a plane for the block to travel, modify the Base, etc, etc.

Assignment

Create a fully functioning, animated Four Bar Linkage system. Have a purpose and/or function in mind for the linkage. Incorporate the purpose/function into your CAD model if it makes sense. The design of the linkages should be something more involved than basic slots. What are the linkages made of? What is the end effector (E) moving, holding, pushing, pulling? Id the four bar is part of a bigger assembly, what does that assembly look like? How does it operate?

You all will be presenting your projects in class next week. You will have 5-7 minutes to present, you will take questions from me and your fellow classmates.

Related Viewing

Mechanical Design Part 5: Four Bar Linkage

Levers and 4 Bar Linkages (great video)