Web Exclusive - Is your hydraulic fluid ready for summer?

Figure 1. Rendering shows the typical reservoir components, including breather caps, oil level switches, thermometers, and coolers.
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Other than oil storage, the two main purposes of the reservoir are to allow contaminants to settle and to dissipate heat. If the reservoir is not cleaned, it will act as a heat sink and can cause system temperatures to rise above 140°F. Oil will then start breaking down, causing sludge and varnish in the system. If the contaminants are not removed from the reservoir they will be drawn into the pump, which can cause premature failure of the system components. Be sure to use a lint free cloth when cleaning out the reservoir.

System cleaning and flushing
When the oil is removed from the reservoir it should be filtered to 1-∝m into a storage tank with a flushing and filtering unit that will remove solid contaminants and water. Unless the oil is severely degraded it is not necessary to change it. Run the oil through the 1-∝m filters when re-filling the unit. The entire system should then be flushed to clean the oil in the lines to the valves and actuators. System flushing is done by connecting the inlet and outlet lines of the cylinders and motors together. Figure 2 shows the flushing unit that is used by our consultants during this process.

Figure 2. GPM120 System flushing machine attached to the reservoir as the system operates.
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If possible, electrically or manually actuate the directional valves to allow the fluid to re-circulate through the piping. If this is not possible, then bypass the directional valves by connecting the pressure and tank lines to the outlet lines to the actuators. Use the existing pump on the machine to re-circulate the oil through the lines. Connect the flushing unit so that it re-circulates the oil in the reservoir through the 1-∝m filters. Allow the system to run for as long as possible. In Figure 3, the purity of the oil is shown for a system before it was flushed then after one, four, and 16 hours.

Check all accessories
Reservoir heater setting - Check the heater thermostat (no. 2 in Figure 1) on the reservoir to verify that it will turn on at a minimum of 70°F. If the pump is mounted on top of the reservoir and the oil temperature drops below approximately 60°F then cavitation will occur.

Oil level switches - Most reservoirs use two switch settings (no. 7 in Figure 1), a warning and a shutdown. The problem with this is that the difference between the two levels may be several hundred gallons of oil. By eliminating the warning switch and setting the shutdown at a higher level, oil loss will be minimal when a hose ruptures.

Figure 3. These jars of oil show the visible results from hydraulic system flushing.
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Breather cap - Verify that the breather cap (no. 8 in Figure 1) has a rating of approximately 10 ∝m. This is the first line of defense for contaminants entering the tank. Depending on the location, the breather cap may need to be changed a couple of times a year. Other options include pressurizing the reservoir with an internal bladder or using a moisture removal type breather.

High-temperature switch - Oil will start breaking down at 140°F and many systems will not shut the unit down until the oil temperature reaches 160°F. Hydraulic systems are designed to operate below 140°F. If the oil temperature rises above that level then a problem in the system exists. This could be caused by a cooler malfunction, excessive bypassing at the pump, valves, cylinders, and hydraulic motors. Set the high temperature switch (no. 10 in Figure 1) at 140°F to shut the pump off, preventing oil breakdown.

Heat exchanger flushing and cleaning - The tubes in a water type cooler (no. 12 in Figure 1) should be flushed periodically to remove deposits. A mild alkaline solution such as Oakite or a 1.5% solution of sodium hydroxide or nitric acid can be used. If an air cooler is used, verify that the cooler fan is turned on at approximately 120°F and turned off at about 110°F. A metal comb should be used to straighten the fins on the unit if necessary.

Leaks - Don't be surprised if new leaks develop and old leaks get worse once the temperature is up. Viscosity drops as temperature increases. Thinner oil gets through orifices more easily so it would be wise to fix current leaks and inspect seals, junctions and hoses before summer arrives.

Once the system is thoroughly inspected and upgraded a preventive maintenance schedule should developed to perform these checks on a regular basis. Many times no thought is given to the hydraulic system as long as it is running and the machine is operating. By performing the checks in this article, your systems will operate at maximum efficiency, operate more safely, and reduce downtime.