Many of the projects with your STEM students will involve mechanical motion of some kind. And whether its a project ranging from a mousetrap car to wind powered generator, from a Rube Goldberg machine to a giant 120 pound FIRST Robotics Challenge robot, engineering a mechanism will require some combination of connections between motors and driveshafts, wheels or axles.
One obstacle commonly encountered by teachers and students is once they have amassed a collection of dc motors from various sources, is how to connect these devices to transfer motion of the motor to the object that needs to be moved. It might be a wheel, pulley, gear or shaft. When prompted, students who have little experience with mechanical devices will often be stumped. They may even suggest things like rubber bands, tape or glue. It’s not their fault of course, these sorts of challenges are not in the realm of their experience. Yet.
The solution to the connection problem is a common device called a coupler, and there are many different types. Each has a specific purpose, and can be purchased or manufactured. In an earlier STEM activity, we looked at manufacturing an axle-to-wheel coupler (or hub) with the help of SprutCAM machining software. In this lesson, we will again use SprutCAM to create a type of connector specifically designed to compensate for misalignment of parts, called the oldham coupler. Misalignment is a very common condition with any project, as it is very difficult to align shafts in exactly the same plane when taking into account spacing issues, bolt hole alignment errors and so on. Connecting together parts that are out of alignment and rotating them at speed can create serious vibration issues that effect performance and reliability of projects. See the clip below to see this compensation in action:
Oldham couplers are made of three pieces. We typically use a combination of the CNC mill and 3D printer to make them in my classes. The center section is typically a nonmetal material such as hard rubber or plastic. My students make this part with the 3D printer for even heavier duty applications, and this is recommended as ABS plastic has proven itself a quite robust. If you are printing in PLA, you will have to try it and let us know how it works out. If you do not have access to a CNC mill, you can manufacture the entire coupler in plastic on the 3D printer for very light duty situations, but take extra care and print with very high infill.
Design software options are many, but chances are you already use one if you have the machinery required to do this activity. Design packages can range from free open source examples such as FreeCAD and OpenSCAD to reasonably priced higher end products like Alibre and IronCAD.
Students can design couplers to fit any type of shaft diameter. We generally use set screws to keep the couplers in place. If the shaft you are mating with the coupler does not have a flat edge for the set screw, you should grind it down a bit so the set screw can get a good bite on it to prevent rotation. Oldham style couplers can be made in a variety of ways, but we try to keep things simple and use designs such as this:
Two of these will form the outer sections of the coupler. Note that the diameters of the shafts can be different, so you can connect a 6mm shaft to a .375 inch shaft without any problem by extrude cutting the appropriately sized hole. Also note that there is a hole for a set screw that will retain the coupler on its respective shaft. If you haven’t already, it is recommended that you purchase variety pack of set screws so your students can have greatest possible flexibility in their project.
The center section in this particular design looks like this. It will be manufactured out of ABS plastic on the 3D printer. Note that the sliders are positioned perpendicular to one another.
Remember that plastic does have a tendency to ooze a bit, and you will need to compensate for that in your slider dimensions. You will want to print with external support for best results.
The completed part looks like this.
The set screws are clearly visible here, don’t forget them!
Now that the designs are complete, it is time to manufacture them. Exporting a .stl file to your 3D printer will be enough, since they use an integrated machine Gcode generator and machine controller such as Replicat or Marlin for example.
The metal outer portion of the Oldham coupler will be made on the CNC mill, and we will use SprutCAM software to generate the machine code. We have several tutorials on using SprutCAM on the site, including STEM lessons and videos. Take advantage of them, if this is your first encounter with SprutCAM. Tutorial links are at the bottom of this page.
We import the part as an .igs file, and create some stock around it. Next we orient the axes in the proper orientation for our operations.
Next we will drill the hole for our shaft. We will use a standard drilling operation here. Be sure and compensate for the height of the stock above the part. If you are coupling two different shaft sizes, don’t forget the import both models and run them separately.
Our next operation will be a pocket for the center slider. Select the edges that need to be milled out, then choose the correct depth in your parameters.
A 2D contour finishing operation will cut the part out of the stock. To prevent the part from shifting during this operation, students can use tabs or leave a thousandths then knock it out. Trim the excess with snips or a deburring tool.
At this point my students will flip the part over and put the part in the drill press or drill on the CNC for the set screw. Tapping is manual as well, again since this is just a single threaded hole.
That’s it. The oldham coupler is one of many versatile devices that students can create in your STEM classroom that provides not only skills in design and manufacturing, but a better understanding of the different options they have when challenged to make complex devices with what is available to them at minimal cost.