tailieunhanh - Handbook of Lubrication Episode 1 Part 8

Tham khảo tài liệu 'handbook of lubrication episode 1 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Volume II 169 If M is taken as 800 kg the frequency will be 216 Hz. Nayak32 has shown that random surface characteristics can excite oscillation at preferred frequencies. Johnson and Gray20 demonstrated that such vibration can cause the interacting surfaces to develop corrugations . Some evidence of plastic flow has been found in the crests of corrugations but none in the troughs. The position is so serious that a number of railways periodically reprofile rail surface by grinding. Adhesion When perfectly clean flat metal surfaces are brought into close proximity less than 1 5 nm they unite chemically. When separation is wider they are attracted to each other by van der Waals forces. At small separations less than 10-8 m these are governed by a square law and at greater separations greater than 10 -7 m by a cube law. Practical surfaces are usually covered by oxide films and are so rough on the atomic scale that when bodies are brought together only a tiny fraction of the contacting area about 1 1000 at the peaks of asperities is subjected to powerful adhesive forces. Experimental measurements of adhesive forces are available with soft materials which conform when pressed together 19 with mica which has been cleaved to produce an exceptionally smooth surface 17 and with hard spheres of very small 36 Adhesive forces were sometimes two to three orders of magnitude higher than those applied initially to force the spheres Buckley8 measured the force to rupture junctions made within a vacuum system evacuated to 10-11 torr. Crystals of copper gold silver nickel platinum lead tantalum aluminum and cobalt were cleaned by argon ion bombardment before being forced against a clean iron 01 1 surface by a force of 20 dyn. When iron was pressed against iron a separating force greater than the 400 dyn was required. In the case of other metals this force varied from 50 to 250 dyn. In every case the strength of the junction was greater than the force used to