A lot of attention is paid to contamination of hydraulic fluid, usually viewed as dirt, water, and air. But heat is also quite detrimental to hydraulic fluid and may account as many component failures as “regular” contamination.
The inconvenient truth about hydraulic machines is they are heat-generating systems. They are not unique in this respect: Energy conversion and control with 100% efficiency remains elusive. But it’s my contention that unavoidable inefficiency, which manifests as energy contamination of the hydraulic fluid, does not command the attention it deserves.
With the exception of the reservoir, every component in a hydraulic system is a heat-generating device. The process of moving hydraulic fluid through a conductor from A to B results in pressure drop and, therefore, heat generation. Installing depth filters to control particle contamination also creates a pressure drop, which increases heat load. Pumps and motors leak internally, resulting still more heat-generating pressure drops. The charge pump on a hydrostatic transmission is 100% heat load. In open circuits, heat-generating orifices, throttles (in all their various forms), and hydrostats are installed to control direction, flow, and pressure—and loads are counterbalanced by installing hydraulic resistance.