Friction Tester Components

Executive Summary

The objective of this project was to design and develop a friction tester in order to measure the dynamic friction between various materials and ice in a temperature enclosed environment. Professor Sungho Jin works with super-hydrophobic materials made of PEEK, or polyether ether ketone, with a nanostructure on the surface to give the material strong water repellant properties. This project was sprung from the idea that these materials could be used in the application of downhill snow skis or ice skates in order to reduce capillary drag, which can be the primary source of friction at the high speeds that these applications undergo.

The functional requirements were designed based on the high speeds, weight distribution, and temperature conditions that snow skis experience. The friction tester was expected to maintain a constant velocity of two meters per second and placed inside of a temperature enclosure ranging from   -20 °C to 5 °C. Based on the four square centimeter size of samples that would be used, it was determined that a mass of 50 to 120 grams would have to be used depending on the angle of the slope that would want to be replicated.

Using these requirements and looking at other friction tester designs, the decision was made to create a pin on disk friction tester. This tester would be comprised of a stationary, loaded pin that would sit on a rotating disk of ice and undergo the effects of dynamic friction. This frictional force would be found by creating a bending beam load cell out of strain gauges to measure the strain in the beam. This strain measurement would be used with the known physical properties of the bending beam and bending beam equations to find the applied frictional force at the end of the beam.

The first step in the design of this friction was to create the rotating turn table that would be used to spin the dish of ice. Many motors and set-ups were researched to build this turntable, but were out-shined by an old hard disk spin table that was provided by one of the project sponsors, Professor Frank Talke. This spin table was able to spin at very high speeds with very little vibration, which was one of the main concerns with building one from scratch. Due to the very low frictional forces that would be measured, the bending beam was designed to be made out of shim stock, which would increase the strain measured by the strain gauges due to the extremely thin geometry. To measure the strain, a full Wheatstone bridge was used in order to gain an accurate reading of the small strain that would be measured.

The results of the test were graphed in MATLAB so that the coefficient of fiction over time could be analyzed. Figure 1 shows a comparison of the different samples tested. After several  test were run, the result were tabulated and summarized into Table 1. This shows that out of all the samples tested, the ski resulted in the lowest average coefficient of friction. This was expected to show the lowest value between the ski, Teflon, and waxed wood, and hydrophilic sample since the base of skis are made of a Teflon compound that is designed for minimal friction on snow and ice. We had hoped that the hydrophobic material would outperform all other materials but this did not turn out to be the case. Some of the reasons for this were that the samples provided to us had a much sharper edge than all other sampled that tended to catch on the ice which increased the measured force. Also the hydrophobic coating wore off of the samples during testing, being completely eliminated after sixty seconds of testing.  However, the main reason for building this tester was to provide Professor Sungho Jin’s lab with a way to quickly test their hydrophobic samples so that improvements could be made to their material. This friction tester succeeded in accomplishing that goal as it was able to accurately measure the friction of various materials on ice.

Table 1 - Measured coefficients of friction for materials tested

Figure 1- Coefficient of Friction vs. Time For Materials Tested