Proceedings of SerbiaTrib '13

experimentally determined that for specific externalload, coefficient of static friction will decrease ifplastic characteristic of material increases andsurface energy decreases. D.-H. Hwang et al [11]concluded that the coefficient of static friction ishigher if contact pair is made of different material -steel/alumina, while the lower value is determinedfor similar materials (steel/steel). This result isconsequence of “stick-slip” effect. Also, one ofconclusions was that the influence of surfaceroughness has less influence for similar materials incontact pair, as well as the increasing value ofnormal load affects on increasing coefficient ofstatic friction in contact pair of different materials,while there is no significant influence for contactpairs of the identical materials.Etsion and Amit [12] experimentally researchedthe influence of normal load on coefficient of staticfriction with very smooth metal surfaces in acontrolled laboratory conditions. Dramaticallyincreasing coefficient of static friction was noticedwhen the normal load is on the lowest level.Behaviour like this is assigned to adhesion forceswhich have more important function regardingsmall normal loads and surface smoothness.A small number of papers deal with coefficientof static friction under influence of temperature[13-16]. The most important conclusion thatauthors made in this papers is that the coefficient ofstatic friction will increase if temperature isincreased, which is resulted of increasing plasticcharacteristics of the most contact material atincreased temperature. Reviewing the literature isnoticed that experimental tests of coefficient ofstatic friction were performed on experimentalequipment with different design, construction anddifferent contact geometry. Also, very interestingare measurement instruments for static coefficientof rolling friction [17-19]. Friction characteristicsof rolling bearing elements depend on contact pairmaterial, design, tolerance, topography of contactsurfaces and lubricants. Authors in this papernoticed, during literature review, that there are noany papers which based their research attempts onstatic coefficient of rolling friction, while inconditions at higher temperature referring to issueabove, there was no paper found (when this paper iswritten).The aim of this paper is to determine influenceof temperature on static coefficient of rollingfriction on contact elements made of steel.Experimental measurements were performed oninstrumentation that authors designed, developedand constructed regarding very precisedetermination of static coefficient of rolling frictionat higher temperatures and relatively small valuesof contact pressure with changing radius bends ofcontact elements.2. THEORETICAL CONSIDERATIONAccording to literature, the static coefficient offriction increases with increasing temperature. It isfound that temperatures above 200°C lead toincreasing of coefficient of friction which can beinterpreted as a result of increasing plasticcharacteristics of material at increased temperature.The static coefficient of rolling friction tested incondition of increased temperature has not beensubject of either theoretical or experimentalresearch. The authors will determine the influenceof temperature, normal load and radius bend ofcontact elements on coefficient of rolling frictionby experimental methods. According to that,measure instrumentation is designed andconstructed, based on inclined plane principle.In the case of rolling friction (contiguous case –figure 1), coefficient of rolling friction isdetermined from formula 1 and 2:where are:M N e(1)M PM moment of resistance andMP rolling moment.From equations 3 and 4: F R(2)eF N f N(3)Ref tanα(4)Rand from the body balance at inclined plate (figure2), we get the following equation:N G cosα(5)esinα R N e G cosα e tanα (6)Rwhere are:f static coefficient of rolling friction;N normal force;e coordinate that defines resultant reactionposition N;R radius of rolling body;G the force of gravity;α angle of inclined plane.13 th International Conference on Tribology – Serbiatrib’13 397