Proceedings of SerbiaTrib '13

The tribometer operates in principle of inclinedplane. Contact pair together with system for heatingand probe for temperature measurement is rotatedfrom horizontal to the desired angle α. The rotatedangle of inclined plane is measured with readingprecision of one minute, which for a wide intervalof possible values of the coefficient of rollingfriction causes the measurement error less than 3%.Figure 1 – The balance of rolling body at inclined plateThe authors’ starting point was from theoreticalassumption that the contact between ball and flatsurface in laboratory conditions will be achieved onthe small number of unevenness in a regard anumber of unevenness at higher temperatures.Further, it is assumed that due to thermal expansionof material in the contact zone, will result asincreasing of value e (figure 1). This means that asa consequence we will have an increase of rollingmoment resistance and a parallel increase of thecoefficient of rolling friction. The authors believethat there is some correlation between staticcoefficient of rolling friction and the value ofthermal dilatation of contact pair. If we have inmind the stochastic nature of real contact area andnonlinear temperature field, it is hard totheoretically quantify the influence of temperatureon coefficient of friction. Hence, in order toquantify the influence of various parameters oncoefficient of friction, authors will provide relativeextensive experimental research.3. EXPERIMENTAL TESTSExperimental tests were performed on a specialdesigned and constructed tribometer. The completemeasuring instrument is showed on figure 2. Also,all positions are marked with numbers anddescribed in following text. The tribometer consistsof three bigger parts, as follows:1 – Thermoregulator. The main aim of this part is tovary a temperature (in our case is 200°C). There aretwo small screens; one is showing desiredtemperature and another current temperature.2 – Block with thermocouple. Inside of this block,beside thermocouple, there is a system for heatingand probe for temperature measurement. Also, inthis part of tribometer the contact between objectand block is made.3 – Counterweight. This part enables to makerotating of the block with thermocouple with verygood precision.Figure 2 – Measurement instrumentation(1-Thermoregulator, 2-Counterweight,3-Block with thermocouple)The tests were performed with rolling balls ofdifferent diameters over channels with differentradius bends. Balls weight and balls diameter werein a range from 0.04 to 0.08N and from 2.32 to13mm respectively. Bend radius of the blockcovered a range from 2.5 to 8 mm. Balls and blockwere heated on selected temperatures, 20, 100, 150and 200°C. Chosen material for balls and block wassteel DIN 17230 (100Cr6) with hardness 62-66HRC. Hardness is achieved by quenching andtempering process. Ball roughness is Ra=0.002µm.The roughness of the block channels surface was ina range between: Ra=0.8-1µm. The figure 3 is adiagrammatic representation coefficient of rollingfriction dependence regarding temperature andnormal load.Figure 3 – Coefficient of rolling friction dependenceregarding temperature and normal load398 13 th International Conference on Tribology – Serbiatrib’13