6 - Rheology

Abstract

This chapter discusses the concepts and principles of rheology. It begins by defining viscosity. The inner friction or resistance to flow of a fluid is of prime importance when selecting a hydraulic medium for a specific application. This is known as the viscosity of a fluid. The degree of shear stress in a moving liquid is directly proportional to the viscosity. A high viscosity gives rise to high shear stresses and consequently high resistance to relative motion of bodies separated by the fluid film. The chapter mentions the low temperature flow properties. The actual viscosity at the lowest temperature of operation constitutes the real criterion when selecting fluids for low temperature operation. It also describes the temperature and pressure dependence of viscosity. The temperature dependency of viscosity can be expressed by the Walther equation. And the pressure dependence of viscosity is quite pronounced at large pressure changes. Moreover, the shear stability of hydraulic fluids is discussed here. It emphasizes the importance of interpreting shear stability test data with caution when selecting fluids for exacting hydraulic systems.