Controlling Hydraulic Oil Temperatures

With cooler weather on the way, you may not be too worried about rising oil temperatures, but the fact is, any industrial hydraulic system running higher than 140 degrees is too hot. Consider that for every 18-degree increase in temperature above 140 degrees, the life of the oil is cut in half. Systems that operate at high temperatures can produce sludge and varnish, which result in the sticking of valve spools.

Pumps and hydraulic motors bypass more oil at high temperatures, causing the machine to operate at a slower speed. In some cases, high oil temperatures can waste electrical energy by making the pump drive motor pull more current to operate the system. O-rings also harden at higher temperatures, leading to more leaks in the system. So what checks and tests should you perform if the oil temperature is higher than 140 degrees?

Causes of Heat Generation

Every hydraulic system generates a certain amount of heat. Approximately 25 percent of the input electrical horsepower will be used to overcome heat losses in the system. Whenever oil is ported back to the reservoir and no useful work is done, heat will be generated.

The tolerances inside pumps and valves are normally in the ten-thousandths of an inch. These tolerances permit a small amount of oil to continuously bypass the internal components, causing the fluid temperature to rise. When oil is flowing through the lines, a series of resistances will be encountered. For example, flow controls, proportional valves and servo valves control the oil’s flow rate by restricting flow. When oil flows through the valves, a “pressure drop” occurs. This means that a higher pressure will exist at the valve’s inlet port than the outlet port. Anytime oil flows from a higher pressure to a lower pressure, heat is generated and absorbed in the oil.

When a system is initially designed, the reservoir and heat exchangers are sized to remove the generated heat. The reservoir allows some of the heat to dissipate through the walls to the atmosphere. If properly sized, heat exchangers should remove the balance of the heat, enabling the system to operate at approximately 120 degrees F…

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