Twin Screw Supercharger Model
The Twin Screw Supercharger Model was a group project that I completed for the Additive Manufacturing course I took my last semester at CSU. Initially, this project consisted of students modeling and 3D printing an object using a variety of techniques we learned over the course of the class, however the project scope was changed in the middle of the semester due to COVID-19. After this shift, we were no longer required to 3D print our object and only had to produce a CAD model. As team lead, it was challenging to lead such a large group of fellow classmates remotely, however these difficulties were overcome and the end result was a success.
The only main requirements for this project consisted of including an articulating component and an area for metal inserts, as well as to describe which additive process would have been used to print each part. Although we had a lot of freedom to choose what we wanted to create, the group was made up of 8 students in total, therefore we needed to come up with an idea that would provide enough work for everybody. For these reasons, the group landed on modeling a twin screw supercharger.
BACKGROUND
Twin screw superchargers are located on top of a car engine and are most commonly found in drag racing vehicles and muscle cars. They are designed to increase the engine’s efficiency by forcing compressed air into the intake manifold, thus producing more horsepower. The key components that make up a twin screw supercharger are 2 rotors, the housing, and the driving mechanism. First, air is pulled into the intake of the housing due to the rotation of the meshed male and female screw rotors, which are driven by the crankshaft. The air is trapped in the pockets bounded by the contours of the male and female screws. The rotation of the screws direct the air axially along the length of the supercharger, squeezing the air into a smaller and smaller space and causing to compress.
THE MODEL
Shown below are various images showing the final model. It is made up of 3 main parts – the twin screw supercharger, the base, and the crankshaft. The dimensions of our model fully assembled is approximately 170mm x 125mm x 250mm, meaning that this is a scaled down representation of a real twin screw supercharger. The main use for this model would be for demonstrational or marketing purposes. Due to its compact and portable design, as well as its ease of assembly and disassembly, this model gives the user a tangible means of understanding how a twin screw supercharger functions, without the hassle of obtaining a real, full size twin screw supercharger or the ambiguity of learning how one works using online resources.
The exploded view of the model below shows how all the components come together. The supercharger, which is made up of the housing, ball bearings, bearing enclosure, front cover sits on top of the base and features the twin screw rotors inside. The housing lid attaches to the top face of the supercharger and gives the user direct access and easy visibility to the twin screws without having to disassemble the entire supercharger. The crankshaft sits inside the base, attaching directly to the handle and a sprocket. The decision to include the crankshaft in the model was an important one, as it provides a more realistic representation of how a twin screw supercharger is driven.
This image below shows where the group incorporated the articulating component and embedded metal thread requirements. The chain, shown in pink, links the sprocket from the crankshaft to the sprocket of the supercharger, causing the twin screw rotors in the supercharger to rotate as the handle of the crankshaft is turned. The full chain would be printed as one part, meaning that it would be ready to use right off the printer, without the need to be assembled. The embedded metal threads, shown in blue, would have allowed the main housing to be easily attached to the housing front cover, which protects the enclosure for the ball bearings.
Embedded metal thread
Articulating component
ADDITIVE PROCESSES
To take advantage of the resources available to us as CSU students, it was our goal to utilize a variety of 3D printers to print all the parts that made up the twin screw supercharger. The group decided on a fused deposition modeling (FDM) printer, a material-ink jetting printer, and a liquid polymer printer, all detailed in the figure below. We wanted to utilize photopolymer printers in our project for their high accuracy capabilities as well as a melt extrusion printer for the parts which have lower tolerance requirements. Since the tolerances are so incredibly important at the interfaces between the screws themselves and the screws to the surrounding housing, we wanted to invest in high quality additive manufacturing processes for those parts.
The first printer listed in the table and shown in the photo on the left is the Prusa MK3 printer, which has decent precision and a good sized print volume. The Stratasys Objet30 Pro, which is shown in the middle photo, is a material jetting printer has very high accuracy while being able to accommodate larger parts. Seen in the photo on the right is the Autodesk Ember, which is a liquid polymer DLP printer. It provides the highest precision but comes at a tradeoff of a small print volume. These factors were all considerations that needed to be taken into account when determining which specific printer would be used to print certain parts.
Melt Extrusion - Prusa MK3
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It was decided that the the parts that would be melt extruded are the crankshaft, handle, chain, base, gears/sprocket, and bearing enclosure because none of those parts needed a high degree of precision. The advantages of FDM printing is it is the most cost efficient, however the disadvantage is that it is the least precise. The parts would have been printed in PLA, due to the material's good aesthetic properties, relatively high strength, low cost, and easiness to print compared to ABS or PETG filament. The costs to print these parts in PLA at 30% infill are broken down in the table below.
Material-Ink Jetting - Objet30 Pro
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The housing body, front cover, and housing lid would have been material-ink jetted. The Objet30 Pro Stratasys printer has very high printing accuracies, and is capable of printing larger parts with nice surface finishes. However, the main disadvantage of this machine is the cost. Because of this, we limited the use of this printer to the parts which require high precision but are too large to print with the liquid polymer Autodesk Ember printer. The costs to print these parts using RGD850 primary material and RGD705 secondary material are broken down in the table below.
Liquid Polymer - Autodesk Ember
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The Autodesk Ember would have been used to print the male and female rotors screws. This printer has the highest dimensional accuracy of all three printers, however it is limited to a very small print volume and the resin is very expensive. Due to these constraints, we would have only printed the male and female screws because these parts have very tight tolerances and it is important for them to fit together properly. The costs to print these parts with PR48 Resin are broken down in the table below. It can be seen that the cost of these components were relatively low considering their tight tolerance requirements.
DESIGN FOR ADDITIVE
The intent of this project was to create a scaled down model of a twin screw supercharger, so that it could be used for marketing or education purposes. Using additive processes was the best way to manufacture the supercharger, because it was much cheaper than creating a tool and injection molding the parts or creating a mini version of a real twin screw supercharger out of metal. Using additive manufacturing processes allowed for some of the parts to be combined and this reduced the total number or parts. Also, because this would be used for educational and marketing purposes it needed to be portable and to fit on a table so printing the parts will allow for the whole assembly to be much more lightweight. This model’s intent was to gives the user a good understanding of twin screw supercharger mechanism without requiring all internal components of a car while staying as true to the real system as possible.​​