Why Your Hydraulic Machine Probably Needs an Oil Cooler – and a Big One!

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.

The point is that energy wasting-pressure drops are a fact of life in hydraulic systems. They can (and should) be minimized, but they can’t be completely eliminated. So let’s stop ignoring the elephant in the room. Because if left unchecked, energy contamination is just as problematic as particle contamination, and arguably more so.

Energy Contamination Affects Lubrication

Adequate lubrication of hydraulic components and efficient power transmission both depend on appropriate oil viscosity. If hydraulic fluid temperature is allowed to exceed that required to maintain viscosity at around 20 centiStokes (cSt), the likelihood of boundary lubrication—resulting in friction and wear—increases dramatically.

he temperature at which this point is reached depends on the fluid’s viscosity grade and its viscosity index (VI). The VI is a measure of an oil’s resistance to change in viscosity with a change in temperature. An oil with a high VI is often called a multi-grade oil. Multi-grade oils are often specified for equipment that must operate in cold. The high VI helps prevent the oil’s viscosity from increasing (thickening) at low temperatures. However, a high VI also helps prevent its viscosity from decreasing (thinning) at high temperatures.

In other words, the critical temperature as far as viscosity is concerned can be relatively low or high, depending on the oil being used…

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