Isothermal EHL problem for chemically degrading lubricant

A new formulation for an elastohydrodynamic problem with degrading lubricant is proposed. The formulation takes into account stress-induced degradation of polymer additive to lubricant on the basis of the kinetic equation for polymer degradation. Degradation of polymer additive results in an irreversible viscosity loss, which, in turn, leads to a reduced lubrication film thickness and change in other contact parameters. The problem is solved numerically. A solution of the kinetics equation is compared with some experimental polymer degradation data. The calculated value of the viscosity loss in a lubricated contact is similar to an earlier obtained one in experiments

Lubricants With Non-Newtonian Rheology and Their Degradation in Line Contacts

A plane isothermal elastohydrodynamic problem for a line contact lubricated by a degrading fluid with non-Newtonian rheology is studied. The lubricant is represented by a base stock with a polymer additive which undergoes stress-induced degradation caused by scission of polymer molecules. The polymer molecules are considered to be of linear structure. The effective lubricant viscosity experiences reversible and irreversible losses. The reversible loss of the effective lubricant viscosity (shear thinning) is due to the non-Newtonian rheology of the fluid and variations in the fluid shear rate. The irreversible loss of the effective lubricant viscosity is caused by the degradation process of the polymer additive dissolved in the lubricant. The degradation process of the polymer additive while it passes through the contact is described by a kinetic equation. The kinetic equation is solved along the lubricant flow streamlines. The solution of the kinetic equation predicts the density of the probabilistic distribution of the polymer molecular weight versus polymer molecule chain length. The changes in the distribution of polymer molecular weight affect local lubricant properties. In particular, the lubricant viscosity experiences reversible and irreversible losses and, in general, is a discontinuous function. The changes in the lubricant viscosity alter virtually all parameters of the lubricated contact such as film thickness, friction stresses, pressure, and gap. The considered non-Newtonian rheology of the lubricant causes a small reversible loss of its viscosity. As a result of the polymer additive degradation the lubricant may experience a significant irreversible loss of its viscosity which, in turn, leads to a noticeable reduction in the lubrication film thickness in comparison with the case of a non-degrading lubricant with similar rheology. Some comparisons between the cases of lubricants with Newtonian and non-Newtonian rheologies with and without lubricant degradation are considered

Modeling of Line Contacts With Degrading Lubricant

A plane isothermal elastohydrodynamic problem for a lubricated line contact is studied. The lubricant represented by a base stock with some polymer additive undergoes stress-induced degradation due to scission of polymer additive molecules. The polymer molecules have linear structure. The degradation process of a polymer additive dissolved in a lubricant while the lubricant passes through the contact is described by a kinetic equation. The kinetic equation is solved along the lubricant flow streamlines. The solution of the kinetic equation predicts the density of the probabilistic distribution of the polymer molecular weight versus polymer molecule chain length. The changes in the distribution of polymer molecules affect local lubricant properties. In particular, the lubricant viscosity changes as polymer molecules undergo scission. These irreversible changes in the lubricant viscosity alter virtually all parameters of the lubricated contact such as film thickness, frictional stresses and pressure. As a result of the polymer additive degradation the lubricant experiences a significant viscosity loss. The viscosity loss (up to 60 percent), in turn, leads to a noticeable reduction in the lubrication film thickness (up to 12 percent) and frictional stresses applied to contact surfaces in comparison with the case of a nondegrading lubricant. Moreover, the pressure distribution in degrading lubricants exhibits extremely sharp spikes of about 2.15 to 2.82 (depending on the slide-to-roll ratio) times greater than the maximum Hertzian pressure. That may lead to noticeable variations in fatigue life of the contact surfaces

A General Case of a Line Contact Lubricated by a Non-Newtonian Giesekus Fluid

A steady plane hydrodynamic problem of lubrication of a lightly loaded contact of two parallel cylinders lubricated by a non-Newtonian fluid with Giesekus rheology is considered. The advantage of this non-Newtonian rheology is its ability to properly describe the real behavior of formulated lubricants at high and low shear stresses. The problem is solved by using a modification of the regular perturbation method with respect to the small parameter α, characterizing the degree to which the polymeric molecules of the additive to the lubricant follow the streamlines of the lubricant flow. It is assumed that the lubricant relaxation time and the value of α are of the order of the magnitude of the ratio of the characteristic gap between the contact surfaces and the contact length. The obtained analytical solution of the problem is analyzed numerically for the dependencies of the problem characteristics such as contact pressure, fluid flux, lubrication film thickness, friction force, energy loss in the lubricated contact, etc., on the problem input parameters