ABSTRACT
There are two basic forms of hydraulic power transmission — hydrostatic and hydrodynamic (or hydrokinetic). The hydrostatic drive employs a positive displacement pump and delivers oil to a hydraulic motor, power being transmitted by fluid pressure without great changes in fluid velocity. In the hydro-dynamic system a centrifugal-type pump circulates large amounts of oil driving a turbine-type hydraulic motor. Power is transmitted by changes in the fluid velocity. Modern refining methods produce oils with good natural resistance to oxidation. Generally, the less viscous oils also tend to have greater natural resistance to oxidation. The oil’s natural resistance to oxidation can be extended by the use of oxidation inhibitors. In general the ideal temperature range for most hydraulic systems is 50–65 °C. All hydraulic oils contain air in solution which has little effect on the operation of the system, although when a hydraulic oil contains air in the form of bubbles in the body of the fluid it can cause problems.
