Start up and shut down conditions in elastohydrodynamic lubrication
Abstract
Under a constant applied load, the operation of a typical elastohydrodynamic lubrication involves three stages, namely the start up stage, the normal steady state operation stage and the shut down stage. Previous works focused on the second normal steady state operation stage, in which a continuous lubricant film is separating the contacting surfaces. This dissertation numerically and analytically studies the lubrication behavior in the start up and shut down stages of elastohydrodynamic lubrication operations. A mixed phase lubricated-solid contact model is developed to study the start up stage and demonstrates the transition from the initial solid to solid contact, to the mixed lubricated-solid contact, and finally to the fully lubricated contact. A surface temperature model is developed to calculate the surface temperature rise in the start up process of circular contact in sliding conditions. Because of the solid contact occurring during this process, significant frictional heat is generated and the surface temperature rise is very high. The shut down process of elastohydrodynamic lubrication operation is studied and the numerical results are compared with the experimental results. Because of the characteristics of EHL flow, analytical expressions are derived to predict the pressure and film thickness changes during the shut down process and these predictions agree well with the numerical results. Surface pocket effects in line contact start up condition are then examined in an effort to achieve beneficial tribological performance through surface modification. In the initial static contact situation, lubricant trapped inside the pocket generates hydrostatic pressure and reduces the pressure spikes and subsurface stresses, compared with the dry contact situation. When the surfaces start to move, this hydrostatic pressure enables a pocket lubricant film build up in addition to the inlet lubricant film build up. Essentially the start up process is divided into two parallel micro-EHL start ups. The start up time is reduced and the solid contact vanishes faster than the smooth surface start up. As a result, the frictional heat in the start up process would be much smaller and subsequently the surface temperature rise would be much reduced, compared with the smooth surface start up condition.
Degree
Ph.D.
Advisors
Sadeghi, Purdue University.
Subject Area
Mechanical engineering
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