FRICTION LIFTING MECHANISM:
During the static analysis course in my second year of my undergraduate program, we were assigned a project. This project required us to lift a 5 lbf bar measuring 1.5 inches using only forces of friction. This assignment was interesting because it was the first time we had designed a simple machine and analyzed it ourselves. Most teams on this project developed unsuccessful scissor type mechanisms and one team even used a modified bear trap to create friction in the bar. Unlike most teams we chose to use cams to create large compression forces and thus, large friction forces. Our design is shown performing the task with nearly 40 lbs, eight times the requirement of this project.
We started out with three concepts. First, a scissor lift mechanism that closed tighter as the arms pulled up. Second, a spring-loaded activation that created the normal force through spring compression. Lastly was the cam idea seen above. Due to simplicity and low part count, the third idea was moved forward. We had to make this design. Using scrap metal we made the frame and the cams. Originally, the cams were meant to be a spiral form, but making those proved to be a very difficult machining task as this was before the department had a CNC machine. We spent some time approximating a spiral from a circle, turned the outside on the lathe and sliced the rod with a bandsaw. This provided a functional approximation of our spiral cam shape. when used, the cams are initially rolled apart to provide clearance for the bar. When the bar contacts the string, the cams close around the bar and provide the necessary friction.
Next, we had to find a material with a high coefficient of friction between the device and the steel bar. The aluminum-steel interface coefficient was much too low for this device as we expected to find from our preliminary calculations. After giving it some thought, we identified a high coefficient of friction material: the fluorosilicone rubber mat advertised to hold your phone on the dashboard of a moving vehicle. These anti-slip mats were tested to determine the coefficient of friction. This material was showed to have a slightly non-linear coefficient of friction, but we estimated for a linear coefficient for the purpose of this project. The material was difficult to bond to the aluminum. Fluorosilicone is notoriously difficult to use with adhesives. A professor who's research emphasized in polymers advised us to flame treat the surface to promote adhesion. After a series of test samples, we identified that flame treating the surface and using a polyurethane-based glue provided for the best adhesion. Other glues tested were cyanoacrylates, two-part epoxies, and E6000. Once these were bonded, we created one more part that held a strain gauge bonded with a two-part cyanoacrylate so that we could measure stress in this bar. This project enabled the student to design and test their own solution to an open-ended problem while also exercising their knowledge of free-body diagrams and force analysis.
