tailieunhanh - Engineering Tribology Episode 2 Part 11

Tham khảo tài liệu 'engineering tribology episode 2 part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fundamentals of Contact Between Solids 475 Figure Schematic illustration of micro-slip and creeping movement in a rolling contact. When the roller or sphere sustains traction the micro-slip increases in level and extent over the rolling contact area 94 95 . When micro-slip prevails over the entire rolling contact area gross sliding or skidding of the roller or sphere will commence. A fundamental difference between rolling and sliding friction is that other energy dissipation mechanisms which are negligible for sliding friction become significant for rolling because of the very low friction level. Major sources of energy dissipation which are not discussed further here are aerodynamic drag of the rapidly rotating roller and repetitive compression of air inside a pneumatic tyre. Another important source of energy dissipation is hysteresis in the mechanical response of the rolling material. Hysteresis means that the compressive stresses ahead of the centre of the rolling contact are greater than the compressive stresses behind the rolling contact. Ahead is defined as not yet reached by the centre of the rolling contact while behind is defined as already rolled on by the centre of the rolling contact. The resulting asymmetry in compressive stresses generates reaction forces that oppose the rolling motion. For example hysteresis is found to be the principal component of rolling friction in polymers 87 . An effect similar to mechanical hysteresis may also be generated by adhesion between the roller and rolled surface 96 . Adhesion behind the rolling contact causes the compressive forces behind the rolling contact to be less than the compressive forces ahead of the rolling contact. Adhesive effects are significant for rubbers 96 where the adhesion is generated by van der Waals bonding between atoms of the opposing surfaces 97 . Rolling is nearly always associated with high levels of contact stress which can be sufficient at high contact loads to cause plastic .