"The Spinning Arrow" GearboxDesigning a fully functional and fun gearbox
Role: Design & Analysis, Solo Project |
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- The Goal -
At Olin College, the course Mechanical Design teaches students to design, analyze, and virtually assemble mechanical parts, machines and mechanisms at a professional level. As a project for the course, I created a rotating arrow gearbox in order to master transmission and FEA in SolidWorks.
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Sketching it Out
The first step to any successful CAD project is to fully understand what you want to design. Sketching, for me, is the most effective way to bring my ideas to life and communicate them to others. The image to the right is a scan of my sketches for this project, including the goals, major components, and potential failures. 
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Solidworks CAD
After sketching the gearbox, I began creating the parts in SolidWorks. The design begins with two identical purple plates to encompass the clock-cage. A stock motor with a mated motor shaft is attached to the back plate using a sub-assembly of a #10-52 Socket Head Cap Screw, lock-washer and washer. A spacer and 30T spur gear is attached to the motor shaft with a 3/16" spring pin to hold in place. Two jack shafts are also placed in the clock cage with a distance of 2.8125 from each other in order to maintain the integrity of the spur gears. Both jack-shafts are placed in ball bearings mounted to the plates. The longer jack-shaft is placed at the end to act as the output. A 60T gear, spacer, and spring pin is placed on the first shaft. A second 30T gear is added to the top face of the 60T gear with a spring pin. A final 60T gear, spacer, and spring pin is placed on the output shaft. The clock-cage is then completed with the second face plate and 4 clock-cage posts attached with mortise and tendon joints and screws, lock washers, and washers. |
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FEA & FOS Analysis
Before applying FEA, I estimated the loads using Free Body Diagrams. In my design, I recognized the two main points of failure as the final gear and the final gear drive pin. Given the torque of the motor and loads of other gears, I calculated the final gear will feel a force of 37.5 inIb and the spring pin will feel 150 Ib. |
Holding the final gearbore fixed and under full 37.5 inlb motor torque, FEA generates a maximum stress of 1,021 lb/in^2 and a factor of safety of 8,94. The maximum stress is well below yeild for a Delrin gear.
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Analyzing the drive gear spring pin under the full 37.5 inlb motor torque, the spring may yield. Therefore, if manufactured, I would user a slightly bigger pin or a solid pin.
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