As part of the locomotion sub-team of New York University’s Tandon School of Engineering Robotic Design Team, the goal every season is to design and construct the manner in which the robot will traverse the environment. One of ideas proposed for the 2021-2022 season was a screw tank, and I wanted to understand the feasibility of this design approach.
Given the nature of the design and the manufacturing constraints of a school engineering team, I assumed certain materials and manufacturing methods that I incorporated into my analysis. Since this design would be operating in a mixture of sand and rocks, I decided to assume a material of hardened steel since it has the least abrasion to sand (according to this resource: https://nvlpubs.nist.gov/nistpubs/jres/5/jresv5n3p553_A2b.pdf). Additionally, I concluded that the most cost-effective and simplest solution for this design, was to use sheets of hardened steel to construct the screw cylinders and its threads by wielding them together.
After graphing the equation for necessary torque with respect to the angle of the thread, and with conservative constraints (I set the maximum diameter of each screw to total 0.5 meters, the largest allowable width of the robot), the lowest torque required to overcome the weight of the robot was found to be at a thread angle of approximately 73.6 degrees, to be 0.94 ft-lbs.
Overall, there was an increasing trend of smaller required torques for higher thread angles. This was expected because the smaller the thread angle, the lower the pitch (the distance between thread tips), which means that the number of threads on the screw will increase, putting less force and stress on each individual thread.
However, because the number of threads on the screw has increased, the screw must rotate more, to go travel the same distance as a screw with less threads. As a result, a motor with a relatively high RPM would be required for this configuration. More interesting however, was if the diameter of the screw decreases, then the required torque will also decrease, but the output torque will be larger than a screw with a larger diameter because of its higher mechanical advantage.
Unfortunately, at an all-team meeting this design proposal was discontinued in favor of redesigning the traditional locomotion wheels, given the familiarity of its cost-effectiveness and manufacturability, and its ease of integration with the other subsystems.