An Experimental Study of the Effect of Load and Moving Speed on Free Rotating Rubber Contact using Fluorescence Microscopy

An experimental study was conducted to evaluate the effect of the applied load and moving speed on the free rotating rubber specimen contact area on a smooth-surfaced glass plate. The contact area of the rubber specimen and flat surface was observed using fluorescence microscopy by utilizing ultraviolet as an excitation light source and pyranine as a dye substance. The apparent contact area between the rubber specimen and the flat surface was measured using image processing software based on the Otsu thresholding method. The result reveals an increasing trend line due to applied normal load dependency, which agreed with the Hertz theory. On moving-speed influence, the trend line of rubber contact increased at a lower speed, reaching the highest value at a moving speed of 8 mm/s, and decreased as the moving speed increased further.

Apparent contact area; Fluorescence; Free rotating; Rubber

Since the past few decades, many scientists have been conducting several experiments related to the contact mechanics of soft materials due to their significant applications in many industries (Nakajima and Takahashi, 2002; Fujii, 2008; Bódai and Goda, 2012; Fowell et al., 2014). One of them is the research about rubber tires of vehicles. The important role of a tire in the vehicle system can be comprehended as its function in transmitting forces on the vehicle through tire rubber to the road surface in a safe and comfortable manner. Therefore, traction force or grip between the tire and road surface becomes one of the most important performance characteristics needed to understand in the automotive industry (Liang et al., 2020). For example, it helps us to understand what range of friction is needed to keep the vehicles safe when braking or cornering or how fast the vehicle can be utilized to maintain the vehicle under control. If the frictional characteristics fail to satisfy the required tractional force, then the entire system of the vehicle will be out of control, leading to a car accident.

        In general, the grip performance of rubber tires is affected by both internal and external factors. One internal factor is the filler inside the tire, which greatly affects its performance. Thus, enhancing the quality of the filler enables the tire to increase its grip performance. This result was reported by Hasan et al. (2020) in their study about the properties of tire rubber by modifying the clay filler of tires.

 Similarly, external factors such as moving speed and loads on vehicles also have a significant impact on vehicle performance. The more weight is loaded on the vehicle, the faster the tire wears down. Faster vehicle runs cause the driver to lose control easily. Therefore, studying these two factors is vital to maintain the tire life and also for driving safety. Although the importance of tire grip performance is applied on both dry and wet surface conditions, its influence is much more reduced on wet surfaces. This phenomenon was shown in a report from Micheline Corporation (Michelin, 2001). Zheng et al. also reported similar results, as the adhesion coefficient reduced when the tire ran on a wet surface, and it reduced further as the water thickness increased (Zheng et al., 2018). Due to its significant impact on the grip performance of tires, we decided to observe and measure the contact condition of tire rubber on wet surfaces.

Tire grip performance or the coefficient of friction is generally calculated based on frictional force, as stated in “laws of friction.” It was reported that the friction force (F_?) of elastomer contact is generally produced by two types of forces: adhesion force (F_a) and hysteresis force (F_h) (Tabor, 1960). Thus, frictional force can be expressed as follows:

                                                                (1)

However, in rough surface contacts such as between the tire and road surface, the most dominant factor on the friction force is hysteresis force; in contrast, the effect of adhesion force is very minimal (Persson, 2001). On the other hand, when the surface of rubber contact is very smooth, the adhesion force makes the biggest contribution to the friction force (Persson and Volokitin, 2006). As the current study utilized a smooth surface of both the rubber wheel and glass plate, the frictional force solely depends on the adhesion force. Thus, the coefficient of friction can be expressed as follows:

                                                                         (2)

Maegawa et al. (2015) reported that the correlation between kinetic friction force and the contact area of the elastomer was linear. Therefore, the frictional force of the elastomer can also be measured based on the contact area between the rubber tire and the mating surface. The coefficient of friction of adhesion force can be expressed as follows:

                                                               (3)

where ? is the shear strength and A is the contact area between the elastomer and the mating surface. As the shear strength does not change, the only factor affecting the adhesion force is the elastomer contact area. This means that the contact area of tire rubber determines the amount of frictional force of the vehicles in a certain manner.

The only way to directly observe the contact condition in situ is by using visualization methods such as fluorescence microscopy. Fluorescence microscopy is the best method for observing and measuring the contact area of elastomer material. Fluorescence microscopy provides detailed information based on the amount of fluorescence dye that exists within the contacting parts. Therefore, the intensity produced in fluorescence microscopy represents the gap within mating contact. Moreover, differentiating the contact region and noncontact region is becoming easier and more accurate. There are several samples of fluorescence microscopy applications in the field of tribology. For example, Fowell et al. (2014) used fluorescence microscopy on elastomeric seal material to measure the lubricant film thickness of contact area at several entrainment speeds. Petrova et al. (2019) utilized fluorescence microscopy to visualize solid-to-solid contact regimes. As it is a very sensitive observation method, the fluorescence technique is also used more frequently in other fields of studies, such as chemistry (Rozaini et al., 2012), biology (Hötzer et al., 2012), and medicine (Marcu, 2012).

As the contact condition of tires is closely related to the coefficient of friction and the grid performance of tires, a lot of data is required to design and produce better performing tires. The information about the contact condition of tires can also be used to analyze the condition of tire wear, to predict the lifetime of tires, and for some other purposes. However, the currently existing reports and data are still far from enough to satisfy the needs of data on many circumstances of tire contact conditions. One of them is the contact condition of rubber tires due to the influence of moving speeds and applied loads on wet surfaces.

Therefore, to contribute to providing data for these purposes, this study conducted an experiment to investigate how the applied loads and moving speeds affect the contact condition of rubber tires. As a fundamental study, the experiment was carried out as a rubber tire wheel running on the top of the flat surface of a glass plate under several applied loads and moving speeds. The contact condition of the tire and mating surface was observed using the ultraviolet-induced fluorescence technique. The moving speed in this study indicates free rotation, which is a mixed movement of rolling and sliding. The rubber wheel had the same material as the tire rubber of regular vehicles, and the mating surface was a smooth glass plate (BK7). During the experiment, the fluorescence liquid made of the pyranine-dyed solution was covered on the top of the mating surface, simulating the condition of wet surfaces.

    The effect of the applied load and moving speed on the free rotating rubber specimen contact area on a smooth-surfaced glass plate was studied. The results indicate different tendencies between applied-load and moving-speed dependencies. Regarding applied-load dependency, the plot distribution of the rubber specimen apparent contact area was within the 20 mm² range for each applied load from three tests. Overall, the apparent contact area increased linearly as the applied load increased in the graph with the log–log axis. This result fits the function of the Hertz theory, as the half-width of the contact area is proportional to the applied load in the power of ½. Regarding moving-speed influences, some information can be obtained from the results. First, the applied load also increased the apparent contact area of the rubber specimen. Second, the moving speed of the rubber specimen increased the rubber apparent contact area at a lower speed up to 8 mm/s, and then it decreased as the moving speed increased further.

    This work was supported by the Indonesia Endowment Fund for Education (LPDP), the Ministry of Finance of Republic Indonesia (KEMENKEU), the Ministry of Research and Technology of Republic Indonesia (KEMENRISTEK), and the Ministry of Education and Culture of Republic Indonesia (KEMENDIKBUD) in the form of a scholarship for a doctoral degree at Kanazawa University with the grand contract number PJR-1570/LPDP.4/2019. Also, thanks to TOYO TIRE for the financial support and being a collaboration partner with the Tribology Laboratory of Kanazawa University.

Bódai, G., Goda, T.J., 2012. Friction Force Measurement at Windscreen Wiper/Glass Contact. Tribology Letters, Volume 45(3), pp. 515–523

Fowell, M.T., Myant, C., Spikes, H.A., Kadiric, A., 2014. A Study of Lubricant Film Thickness in Compliant Contacts of Elastomeric Seal Materials using a Laser Induced Fluorescence Technique. Tribology International, Volume 80, pp. 76–89

Fujii, Y., 2008. Method for Measuring Transient Friction Coefficients for Rubber Wiper Blades on Glass Surface. Tribology International, Volume 41(1), pp. 17–23

Hasan, A., Aznury, M., Purnamasari, I., Manawan, M., Liza, C., 2020. Curing Characteristics and Physical Properties of Natural Rubber Composites using Modified Clay Filler. International Journal of Technology, Volume 11(4), pp. 830–841

Hötzer, B., Medintz, I.L., Hildebrandt, N., 2012. Fluorescence in Nanobiotechnology: Sophisticated Fluorophores for Novel Applications. Small, Volume 8(15), pp. 2297–2326

Johnson, K.L., 1987. Contact Mechanics. Cambridge, United Kingdom: Cambridge University Press

Klueppel, M., Muller, A., Le Gal, A., Heinrich, G., 2003. Dynamic Contact of Tires with Road Tracks. In: 2003 Technical Meeting of the American Chemical Society, Rubber Division, San Francisco, USA, Paper No. 49

Liang, C., Li, H., Mousavi, H., Wang, G., Yu, K., 2020. Evaluation and Improvement of Tire Rolling Resistance and Grip Performance based on Test and Simulation. Advances in Mechanical Engineering, Volume 12(12), pp. 1–14

Maegawa, S., Itoigawa, F., Nakamura, T., 2015. Optical Measurements of Real Contact Area and Tangential Contact Stiffness in Rough Contact Interface Between an Adhesive Soft Elastomer and a Glass Plate. Journal of Advanced Mechanical Design, Systems, and Manufacturing, Volume 9(5), pp. 1–14

Marcu, L., 2012. Fluorescence Lifetime Techniques in Medical Applications. Annals of Biomedical Engineering, Volume 40(2), pp. 304–331

Michelin, 2001. The Tyre: Grip. Clermont-Ferrand, France: Société de Technologie Michelin

Nakajima, Y., Takahashi, F., 2002. Increase of Frictional Force of Rubber Block by Uniform Contact Pressure Distribution and Its Application to Tire. Rubber Chemistry and Technology, Volume 75(4), pp. 589–604

Otsu, N., 1979. A Threshold Selection Method from Gray-Level Histograms. IEEE Transactions on Systems, Man, and Cybernetics, Volume 9(1), pp. 62–66

Persson, P., 2001. Theory of Rubber Friction and Contact Mechanics. The Journal of Chemical Physics, Volume 115(8), pp. 3840–3861

Persson, P., Volokitin, A., 2006. Rubber Friction on Smooth Surfaces. The European Physical Journal E, Volume 21(1), pp. 69–80

Petrova, D., Weber, B., Allain, C., Audebert, P., Venner, C.H., Brouwer, A.M., Bonn, D., 2019. Fluorescence Microscopy Visualization of the Roughness-Induced Transition Between Lubrication Regimes. Science Advances, Volume 5(12), pp. 1–8

Rahman, J., Iwai, T., Koshiba, K., Shoukaku, Y., 2018. Observation of Contact Area of Rubber Wheel using an Ultraviolet-Induced Fluorescence Method. In: Proceedings of Asia International Conference on Tribology 2018, Serawak, 17–20 September, Malaysia, pp. 291–292

Rahman, J., Shoukaku, Y., Iwai, T., 2020. In Situ Rubber-Wheel Contact on Road Surface using Ultraviolet-Induced Fluorescence Method. Applied Sciences, Volume 10(24), pp. 1–13

Rozaini, M., Ali, R., Ros, L., 2012. Normal Micellar Value Determination in Singular and Mixed Surfactant System Employing Fluorescence Technique. International Journal of Technology, Volume 3(4), pp. 103–109

Tabor, D., 1960. Hysteresis Losses in the Friction of Lubricated Rubber. Rubber Chemistry and Technology, Volume 33(1), pp. 142–150

Zheng, B., Huang, X., Zhang, W., Zhao, R., Zhu, S., 2018. Adhesion Characteristics of Tire-Asphalt Pavement Interface based on a Proposed Tire Hydroplaning Model. Advances in Material Science and Engineering, Volume 2018, pp. 1–12