Proceedings of SerbiaTrib '13

etween metal and rubber will not occur shockabsorber will have better absorbing properties.Figure 8. Effects of surface conditions on thestress/deflection curve for rubber under compression [8]Significant sliding compromises the assemblystability and has a great effect on lowering ofnormal resulting force. Furthermore, the increase ofthe friction coefficient on the contact surfaces ofthe rubber element increases the shear stress and itsshare of the total stress also. The increasing shearstress further increases the total stress in theelement and the force which resists the deformationof the element. The increase of the shear stressshare of the total stress leads to the enhancedamortization capacity of rubber elements. As thehigh values of normal resulting force are the designrequirement, it is necessary to find the balancebetween the sliding allowance and resulting normalforce. It can be achieved by influencing thetribological contact parameters (lubrication, surfaceroughness of the metal part, contact pressure, …)and thus the friction coefficient value.Based on above it can be concluded that it is notpossible to actually perform the virtualdevelopment process of the shock absorber withrubber metal spring without the knowledge of thefriction coefficient value in contact between therubber and metal parts.The coefficient of friction of rubber is highlydependent on contact pressure. As the contactpressure between the rubber and the free metalplates in shock absorbers is approximately 20 MPa,it is extremely difficult or even impossible toexperimentally determine the actual value offriction coefficient at noted operating contactpressure.The compound friction coefficient can bepredicted based on experiments with rubberspecimens or based on existing data on normalreaction force in similar operating conditions. Bysimulation of experiments on rubber specimens orpreviously performed experiments, the frictioncoefficient can be determined by goal drivenoptimisation procedure. The value of frictioncoefficient will be approximately determined whenthe normal reaction force obtained by simulation isequal to experimentally obtained one.As an example, it is necessary to determine thefriction coefficient in contact between rubber withtrade name TG-B-712 (manufactured by companyTIGAR, Pirot) and metal plate at contact pressureof 3 MPa. The rubber specimen (with dimensions(35.7 x 17.8 mm) was compressed between steelplates at specific tribological conditions for which itwas necessary to determine the value of frictioncoefficient. The force-displacement data wasrecorded during the experiment (Figure 9).Figure 9. Experimental force – displacement dataFigure 10. Functional dependence between frictioncoefficient and force determined by virtual experimentThe friction coefficient was determined byvirtual experiment from which the functionaldependence between friction coefficient and normalresulting force was obtained (Figure 10). Based onrealistic experimental data (Figure 9) it is clear thatthe maximal resulting normal force correspond tofriction coefficient value of μ = 1.5.290 13 th International Conference on Tribology – Serbiatrib’13