As the weather gets cooler, you probably won’t worry too much about rising oil temperatures, but the truth is that any industrial hydraulic system running above 140 degrees is too hot. Note that oil life is halved for every 18 degrees above 140 degrees. Systems operating at high temperatures can form sludge and varnish, which can cause valve plugs to stick.
Pumps and hydraulic motors bypass more oil at high temperatures, causing the machine to run at a slower speed. In some cases, high oil temperatures result in a loss of power, causing the pump drive motor to draw more current to run the system. O-rings also harden at higher temperatures, causing more leaks in the system. So, what checks and tests should be carried out at an oil temperature above 140 degrees?
Every hydraulic system generates a certain amount of heat. About 25% of the electrical power input will be used to overcome heat losses in the system. Whenever oil is transported back into the reservoir and does no useful work, heat is released.
Tolerances in pumps and valves are typically within ten thousandths of an inch. These tolerances allow small amounts of oil to continually bypass internal components, causing fluid temperatures to rise. As oil flows through the lines, it encounters a series of resistances. For example, flow regulators, proportional valves, and servo valves control the flow rate of oil by restricting flow. As oil passes through the valve, a “pressure drop” occurs. This means that the valve inlet pressure is higher than the outlet pressure. Whenever oil flows from higher pressure to lower pressure, heat is released and absorbed by the oil.
During the initial design of the system, the dimensions of the tank and heat exchanger were designed to remove the generated heat. The reservoir allows some heat to escape through the walls to the atmosphere. When properly sized, the heat exchanger should eliminate heat balance, allowing the system to operate at temperatures of approximately 120 degrees Fahrenheit.
Figure 1. The tolerance between the piston and cylinder of a pressure compensated displacement pump is approximately 0.0004 in.
The most common type of pump is the pressure compensated piston pump. The tolerance between piston and cylinder is approximately 0.0004 inches (Figure 1). A small amount of oil leaving the pump overcomes these tolerances and flows into the pump casing. The oil then flows back into the tank through the crankcase drain line. The drain stream in this case does not do any useful work, so it is converted into heat.
Normal flow from the crankcase drain line is 1% to 3% of the maximum pump volume. For example, a 30 GPM (gpm) pump should have 0.3 to 0.9 GPM of oil returning to the tank through the crankcase drain. A sharp increase in this flow will result in a significant increase in oil temperature.
To test the flow, a line can be grafted onto a vessel of known size and time (Figure 2). Do not hold the line during this test unless you have verified that the pressure in the hose is close to 0 pounds per square inch (PSI). Instead, secure it in a container.
A flow meter can also be permanently installed in the crankcase drain line to monitor flow. This visual inspection can be done periodically to determine the amount of bypass. The pump should be replaced when the oil consumption reaches 10% of the pump volume.
A typical pressure compensated variable displacement pump is shown in Figure 3. During normal operation, when the system pressure is below the compensator setting (1200 psi), the springs hold the internal swashplate at its maximum angle. This allows the piston to move fully in and out, allowing the pump to deliver maximum volume. The flow at the pump outlet is blocked by the compensator spool.
As soon as the pressure increases to 1200 psi (fig. 4), the compensator spool moves, directing oil into the inner cylinder. When the cylinder is extended, the angle of the washer approaches the vertical position. The pump will supply as much oil as needed to maintain the 1200 psi spring setting. The only heat generated by the pump at this point is the oil flowing through the piston and crankcase pressure line.
To determine how much heat a pump will generate when compensated, use the following formula: Horsepower (hp) = GPM x psi x 0.000583. Assuming the pump is delivering 0.9 gpm and the expansion joint is set to 1200 psi, the heat generated is: HP = 0.9 x 1200 x 0.000583 or 0.6296.
As long as the system cooler and reservoir can draw at least 0.6296 hp. heat, the oil temperature will not rise. If the bypass rate is increased to 5 GPM, the heat load increases to 3.5 horsepower (hp = 5 x 1200 x 0.000583 or 3.5). If the cooler and reservoir cannot remove at least 3.5 horsepower of heat, the oil temperature will rise.
Rice. 2. Check the oil flow by connecting the crankcase drain line to a container of known size and measuring the flow.
Many pressure compensated pumps use a pressure relief valve as a backup in case the compensator spool gets stuck in the closed position. The relief valve setting should be 250 PSI above the pressure compensator setting. If the relief valve is set higher than the compensator setting, no oil should flow through the relief valve spool. Therefore, the tank line to the valve must be at ambient temperature.
If the compensator is fixed in the position shown in fig. 3, the pump will always deliver the maximum volume. Excess oil not used by the system will return to the tank through the relief valve. In this case, a lot of heat will be released.
Often the pressure in the system is randomly adjusted to make the machine perform better. If the local regulator with a knob sets the compensator pressure above the relief valve setting, excess oil returns through the relief valve to the tank, causing the oil temperature to rise by 30 or 40 degrees. If the compensator does not move or is set above the relief valve setting, a lot of heat can be generated.
Assuming the pump has a maximum capacity of 30 gpm and the relief valve is set to 1450 psi, the amount of heat generated can be determined. If a 30 horsepower electric motor (hp = 30 x 1450 x 0.000583 or 25) were used to drive the system, 25 horsepower would be converted to heat at idle. Since 746 watts equals 1 horsepower, 18,650 watts (746 x 25) or 18.65 kilowatts of electricity will be wasted.
Other valves used in the system, such as battery drain valves and bleed valves, may also not open and allow oil to bypass the high pressure tank. The tank line for these valves must be at ambient temperature. Another common cause of heat generation is bypassing the cylinder piston seals.
Rice. 3. This figure shows a pressure compensated variable displacement pump during normal operation.
Rice. 4. Pay attention to what happens to the pump compensator spool, inner cylinder, and swash plate as pressure increases to 1200 psi.
The heat exchanger or cooler must be supported to ensure that excess heat is removed. If an air-to-air heat exchanger is used, the cooler fins should be cleaned periodically. A degreaser may be required to clean the fins. The temperature switch that turns on the cooler fan should be set to 115 degrees Fahrenheit. If a water cooler is used, a water control valve must be installed in the water pipe to control the flow through the cooler pipe to 25% of the oil flow.
The water tank should be cleaned at least once a year. Otherwise, silt and other contaminants will cover not only the bottom of the tank, but also its walls. This will allow the tank to act as an incubator rather than dissipate heat to the atmosphere.
Recently I was at the factory and the oil temperature on the stacker was 350 degrees. It turned out that the pressure was unbalanced, the hydraulic accumulator manual relief valve was partially open, and oil was constantly supplied through the flow regulator, which actuated the hydraulic motor. The engine-driven unloading chain operates only 5 to 10 times during an 8-hour shift.
The pump compensator and relief valve are set correctly, the manual valve is closed, and the electrician de-energizes the motor way valve, shutting off flow through the flow regulator. When the equipment was checked 24 hours later, the oil temperature had dropped to 132 degrees Fahrenheit. Of course, the oil has failed and the system needs to be flushed to remove sludge and varnish. The unit also needs to be filled with new oil.
All these problems are created artificially. Local crank handlers installed a compensator above the relief valve to allow the pump volume to return to the high pressure reservoir when nothing is running on the paver. There are also people who cannot fully close the manual valve, allowing the oil to flow back into the high pressure tank. In addition, the system was poorly programmed, causing the chain to operate continuously when it only needed to be activated when the load was to be removed from the stacker.
The next time you have a thermal problem in one of your systems, look for oil that is flowing from a higher pressure system to a lower one. Here you can find problems.
Since 2001, DONGXU HYDRAULIC has provided hydraulics training, consulting and reliability assessments to companies in the industry.
Foshan Nanhai Dongxu Hydraulic Machinery Co., Ltd. has three subsidiaries: Jiangsu Helike Fluid Technology Co., Ltd., Guangdong Kaidun Fluid Transmission Co., Ltd., and Guangdong Bokade Radiator Material Co., Ltd.
The holding company of Foshan Nanhai Dongxu Hydraulic Machinery Co., Ltd.: Ningbo Fenghua No. 3 Hydraulic Parts Factory, etc.
Foshan Nanhai Dongxu Hydraulic Machinery Co., Ltd.
&Jiangsu Helike Fluid Technology Co., Ltd.
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Post time: May-26-2023