Robotics Education & Competition Foundation
Inspiring students, one robot at a time.

Retractable Sprockets


Entry ID #: 2593
Created: Wed, Jan 13, 2016 6:48 AM

Introduction In past years, our team had often considered using a transmission as a part of our robot’s drive train but always concluded that it wouldn’t be worth it. In order to change speeds, we would need to dedicate an already scarce motor to shift the gears, which limited the maximum power output we could achieve, with the shifter being unable to contribute whatsoever. Furthermore, the only other alternative would have been to install a heavy (and expensive) pneumatics system, needlessly increasing the cost and weight of the robot. To remedy this, we sought to create a transmission that would not need an usually-idle shifting motor or pneumatics system, and be able to extract power from all of the motors it used. Design The part that we have designed is a retractable sprocket, composed of 2 grooved plates sandwiching several teeth that slide inwards or outwards in the grooves. By spinning the plates in opposite directions, the teeth could be made to extend outwards or retract into the plates, changing the sprocket between a cylindrical and sprocket form. Thus, a short axle would be inserted into each plate, and connected to the drive trains of 2 motors, one for each side, and a chain looped around the retractable sprocket. Spinning the motors in opposite directions would retract or extend the teeth and engage or disengage the chain, while spinning in the same direction would result in the normal combination of torque of two motors. By connecting two separate retractable sprockets to the same motors, arranged so that as one expands, only one sprocket will ever be engaged to a chain at a time. When the motors turn in opposite directions, the engaged sprocket will retract while the other expands, switching which retractable sprocket is engaged. In this way, the retractable sprockets may be used to switch motor output, by using the different chains connecting to the sprockets to drive different outputs, change gear ratio or speed, by using the chains to drive different sets of gears, and disengage motors from outputs entirely, by using only one sprocket that can engage or disengage a single chain and output, all while utilizing the full power of both input motors. CAD Modeling To create our new part, our team first downloaded part files of the standard Vex gears in order to create tooth profiles for the gears we used. This was essential to ensure that the teeth in our part would successfully mesh with the standard Vex parts. First, we projected the shapes of the teeth of the standard gears to transfer the angles and shapes onto our parts. In order to create a three-dimensional part, we extruded the teeth to a desirable height, and then added an extrusion of a circular peg to each side of the tooth to hold it in place in the future. Then, we projected the circular shape of the Vex gear onto a new sketch and extruded a suitable width to create sprocket plates to hold the sliding teeth. To ensure that the gear teeth were regularly spaced, we used the circular pattern tool to evenly distribute extruded grooves of the same width as the tooth pegs along the circumference of the gear. As a result, the teeth would be sandwiched between two grooved plates, with pegs holding them securely in place. Once the necessary sprocket parts had been created, the final portion of modeling our part entailed assembling the gears, plates, and axles in one assembly model. First we placed a sprocket plate, and then we inserted sliding teeth into each of the grooves. We finished it off symmetrically with another sprocket plate on top of the teeth. In order to ensure that everything fit together properly, we used multiple constraints. To keep teeth flush with plates, we used a mate face to ensure there was contact between them. We also used the insert constraint to ensure that an axles passed through the gears and plates, and tangent constraints to ensure that the gear teeth were properly inserted in their corresponding grooves. Finally, once the assembly was completed, we added animations to demonstrate how the part functions: as the plates spun in opposition, the teeth were able to extend and contract, effectively changing the gear ratio. Conclusion Autodesk Inventor Professional is an extremely vital component of every engineer's' arsenal as it allows for comprehensive modeling and analysis of three-dimensional structures, a key component of engineering. Our team will certainly utilize Inventor in the future in order to model future robots for VEX as well as engineering projects through the rest of high school, in college and for the rest of our lives. During the course of this project, we learned that 3-dimensional models do not always translate well into real-world devices. Even though the models worked well in a virtual environment, applying the proper tolerances, friction reducing techniques, and other slight adjustments required us to go through over a dozen cycles, refining the part at every step along the way. Our team learned how important organization truly is for an engineering project, such as being able to properly organize the increasing number of part files for use in the subassemblies and final assembly. Furthermore, organization within individual components is essential while constraining all the dimensioned elements, as well as in version control to track iterations of our designs. Autodesk Inventor will also be useful for communication of ideas, as we can create visual models and technical sketches to allow others to better grasp our otherwise abstract concepts. Such visualization prior to making physical models is key to producing high-quality projects, as we can tweak our idea before committing to costly physical prototypes. In our current scope as a Vex Robotics team, Inventor greatly helps during documentation processes. Additionally, being able to create 3-dimensional assemblies helps to understand internal mechanisms, while also provides testing benefits for new mechanisms. Learning 3D design software such as Autodesk Inventor Professional will aid us aspiring engineers, as we will be able to develop, refine, and produce products in professional environments more quickly through analyzing virtual models prior to real-world development.


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Links / Videos

An animation of the CAD mode of the retractable sprocket, showing how it may expand and retract.

This video shows how the teeth in the 3D printed sprocket may retract and extend when the outer plates are twisted.