Since the beginning of the 20th century, hydraulic sealings and journal bearings are designed employing Reynolds lubrication theory /1–3/. The Reynolds lubrication theory presumes the no slip boundary condition at the liquid-solid interface. Recent studies conducted by the authors show, that the assumed no slip boundary condition at the liquid-solid interface is not valid for most fluid power applications; cf. /4/. This effects the prediction of leakage flow and frictional behaviour of sealing systems as well as bearing capacity of journal bearings. Thus, considering slip at the liquid-solid interface is important for the design of hydraulic components. The concept of wall slip was already discussed by Navier /5/ and Stokes /6/, when deriving the momentum equation for Newtonian fluids in the 19th century. Stokes favours the no slip boundary condition and justifies his hypothesis by a good agreement of the theory with experimental investigations of Poiseuille /7/. In contrast, Navier /5/ formulates the slip boundary condition, with the slip velocity being the product of the shear rate and the slip length. Regardless of this discussion, the no slip boundary condition is established over the centuries, based on the insufficient measurement techniques. However, in many technically important applications of fluid power technologies wall slip is not negligible. This is the case if the quotient of slip length and typical flow geometry is less than 10E-3. Thus, for typical hydraulic systems is reasonable to consider slip, if the gap geometries are of the order of magnitude of 10 μm
