Tham khảo tài liệu 'friction, lubrication, and wear technology (1997) part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fig. 2 Elastohydrodynamic viscosity-pressure coefficient. Source Ref 7 Viscosity at atmospheric pressure and the EHD viscosity-pressure coefficient both strongly depend on lubricant temperature. Hence for engineering purposes that use simplified EHD equations it is convenient to express almost all of the lubricant contribution to film thickness in dimensionless terms as follows Lubricant speed parameter TỊ 1 11 u K. Eq 10 where u is the mean surface velocity in the direction of motion and jp is the effective radius in the direction of motion. When the lubricant-speed parameter is used in place of the usual speed parameter used by theoreticians the exponent of their materials parameter ehdE is greatly reduced A1 is the effective elastic modulus of the bearing materials . Hence effects on the materials parameter by common lubricants only slightly affect calculated film thicknesses. The lubricant portion of the lubricant-speed parameter EHD is sometimes called the lubricant parameter. At the high pressures encountered within the load-carrying zone in hard EHD lubricants can act as plastic solids with a shear strength. The shear strength increases linearly with pressure according to Ạp Tẹo P Eq 11 where - t is the pressure coefficient of shear strength at temperature t TLH is the shear strength constant at temperature t and 0 gage pressure and Ttp is the shear strength at temperature t and gage pressure p. Low-Temperature Flow Properties. At low temperatures oils become too viscous to pour from a container. Mineral oils also may not pour because they precipitate crystals of wax at low temperature. Pour point is defined by the ASTMD97test. In addition the viscosity that is the shear stress per unit of shear rate at low temperatures is commonly not independent of shear rate or stress nor of temperature and shear history as it is for a Newtonian fluid. Therefore kinematic viscosity or dynamic viscosity derived from kinematic determined at low shear rate is no longer a
