Proceedings of SerbiaTrib '13

flexion) that the most loaded rows are those locatedclose to 40°…60° from the equatorial plane (therows located lower have small loads on each balls,and for the rows located higher each ball carries abigger load but the number of balls is low). Thedistribution of rolling balls loading versus thezenith positioning angle of the rolling ball ispresented in Fig. 3.Analyzing the graph, the following conclusionscould be drawn:1. As reported before, there is a slight asymmetryof the distribution even for the initial position. Thisasymmetry evolves with flexion leading tounloading of some balls located peripherallyoutside the hemispherical area characterized bycompressive loading pole.2. The peripheral balls located closer to thecompressive loading pole are generally highlyloaded, but the highest loaded balls remain thosepositioned in intermediate rows (between 40° and60° from the equatorial plane).Seizure burdens to different loadings andcontact surfaces roughness influence over theseizure burden have been determined.Even though the minimum value of the wearmust be the same with the minimum value of thesurfaces roughness, given the experimentalconditions, it came out from the trials results onwear that the lowest level of wear is acquired at acertain value of roughness, not at the lowest levelof roughness.For the tests we used the test rig presented inFig. 4. In this test rig, the friction pair is formedfrom a bush with spherical profile and a flat diskshaped with a diameter of 18 mm and a thickness of5 mm (see Fig. 5). The sphere's radius is r = 11.5mm.Figure 4. Experimental test rigFigure 3. The rolling balls loadings versus zenithpositioning angle of ball.For the extreme maximum loadings of the ballsduring the analyzed flexion, a preliminaryevaluation of tribological parameters of contactbetween femoral head and rolling balls and betweenrolling balls and acetabular cup has been performedby using formulae (1) - (3).3. LABORATORY TRIALSExperiments revealed a certain seizure in somestrain conditions. Laboratory trials for balls/planeHertzian contacts have been restarted in order todetermine seizure behaviour depending on theroughness of the flat area. The trials have beencarried out in BSF (body simulated fluid)lubrication conditions, much closer to the realoperating conditions up against the initial tests withdistilled water.Figure 5. Used friction coupleIn the static contact, compression stresses incontact spot, p max si p med (maximum pressure andmean pressure) are:3pmax 1.5 PE 2 / π r 2 (1 – μ 2 ) 2 (5)Pp med 2a(6)and the radius of the contact surface, a, is:a32 1.5 1PrE(7)334 13 th International Conference on Tribology – Serbiatrib’13