tailieunhanh - Mechanical Engineering-Tribology In Machine Design Episode 8

Tham khảo tài liệu 'mechanical engineering-tribology in machine design episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 162 Tribology in machine design Figure where y is the surface tension and R and R2 are the radii of the meniscus in mutually perpendicular planes. In the case of parallel plane surfaces R can be taken as infinity and Ri as approximately h 2 where h is the separation of the surfaces. Assuming a surface tension of N m the thickness of the fluid film is about 5 X 10 7 m and the pressure difference resisted by the seal can amount to 8 X 104 Pa. Thus in the situation depicted in Fig. where the fluid wets the surface a pressure of 8 X 104 Pa acting from right to left can be resisted. However in the absence of this pressure the fluid would continue to be drawn into the cavity with the interface advancing to the right. It has been shown experimentally that when the meniscus reaches the end of the constricted passage it begins to turn itself inside out as indicated in Fig. . Owing to the contamination of engineering surfaces the contact angles of oil against synthetic rubber and steel under industrial conditions are found to be high so that the sealed oil does not spread along the steel shaft. In addition to the equilibrium meniscus effect any local variation of the surface tension of a liquid induces a driving force to a fluid. This is known as the Marangoni effect and its implications for the action of seals have been investigated. When a temperature gradient exists on a solid surface a droplet of liquid laid on that surface will spread out more rapidly towards the lower-temperature side. Even when conditions are generally isothermal differential evaporation of the constituents of a multicomponent liquid may produce local variations in surface tension which markedly affect spreading behaviour. The constituents of mineral oils having higher molecular weights will tend to spread more rapidly by reason of their greater surface tensions. This process promotes segregation of the constituents of blended oils thus depriving the high-temperature side where .