What are the important characteristics for a hydraulic lubricant in service?

Shear stability

Addition of a viscosity index improver (VII) enables a widening of the functioning ranges of hydraulic fluids. However, there is a large number of VIIs which have quite different shear stabilities.
Indeed, VIIs are polymers (long carbon chains) which may be cut into smaller chains when subjected to shear. In this case, the viscosity index is reduced and the operating viscosity may no longer be suitable for the operation.
In order to guarantee optimum functioning of the installation over time, it is necessary to resort to shear stable lubricants.

Fluid behaviour when subjected to air and water:

Air and water are pollutants for a hydraulic fluid. Their presence makes the fluid compressible, which leads to reduced precision in the transmission of the force and/or movement. In addition, they may cause breakdowns in the oil film and reduce the duty life of the operating oil. The properties enabling separation of the oil with these pollutants are therefore very important: foaming and air release properties regarding the air, and demulsibility regarding water.