Hydraulic Temperature Control

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Operating temperatures consistently above 160°F promote chemical reactions that change the properties of the oil. Effects of high temperature are listed below:

1. Oxidation of the oil
2. Formation of insoluble gums, varnishes, and acids
3. Deterioration of seals (they harden and leakage begins)
4. Loss of lubricity
5. Changes in viscosity

The gums and varnishes clog orifices and cause valves, particularly valves that shift infrequently, to stick. The acids attack the metal surfaces of the components and cause corrosion.

The most significant effect of high temperature is the reduction in viscosity and subsequent reduction in lubricity. Loosely defined, lubricity is the ability of the fluid to maintain a film between moving parts. As viscosity decreases, this film thins. At some point, metal-to-metal contact occurs, and damage results. Metal particles from the damaged surface circulate with the fluid and erode other surfaces as they impact these surfaces. This damage cycle begins with high temperature. It is now readily apparent how the two subjects of temperature and contamination control are interconnected.

It is recommended that a hydraulic system be designed to operate at less than 140°F under worst-case ambient conditions. Over time, components wear, leakage increases, and more heat is generated. It is typical for the maximum operating temperature to slowly increase over the life of the system. Oil temperature at the return to the reservoir (or inlet to the heat exchanger if a heat exchanger is used in the return line) is easy to measure, and it is a valuable indicator of the overall condition of the system. If this temperature reaches 160°F, corrective action is needed.

If the oil has been heat damaged, it will have a darker color and an odor of scorched oil. Both of these characteristics indicate a problem that should have been solved earlier and would have been solved if oil temperature was being monitored.

Another indicator of high oil temperature is heat-peeled paint on the surface of the components or reservoir. If this is observed, it indicates a poorly designed and/or poorly maintained system. Considerable damage has been done by the time heat peeling of paint occurs.

Hydraulic energy is converted to heat energy whenever there is a pressure drop, and no mechanical work is done. Heat generation is unavoidable in fluid power circuits. As discussed in all previous chapters, pressure drops can (and should) be minimized.

Another source of heat generation is the compression of air bubbles in the oil. These bubbles are compressed as pressure is developed by the pump. When gas is compressed, the temperature increases; thus, compression of air bubbles introduces heat into the fluid. Solving a pump cavitation problem not only reduces the damage caused by shock waves as the bubbles burst, it also reduces the problems caused by heat generation.

When heat is generated within a component, say a DCV, part of it flows into the oil, and part of it is conducted through the housing to an outer surface where it is exchanged into the surroundings by convection and radiation. Some of the heat energy in the oil is exchanged into the area around the lines (hoses and tubing) through which the oil flows. Again, this heat is exchanged by convection and radiation. On mobile machines, the length of the lines and the amount of air flowing around the components as the machine moves will often provide enough heat exchange that a separate heat exchanger, referred to as an oil cooler, is not needed. On the other hand, a mobile machine in direct sunlight is subjected to a high radiant energy input from the sun. Temperature of an exposed surface can reach 140° when the machine is sitting still on a clear day. Obviously, the potential for heat exchange from the oil is a function of the ambient conditions. A good design will maintain fluid temperature in the desired range for worst-case ambient conditions.

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