Keeping hydraulic systems clean
Common threats to manufacturing machinery are air, moisture, machine particles, process chemicals, heat, microbes and atmospheric particulates.
Monitoring and controlling contaminants is key to the management of hydraulic fluids. Air, water, particulates and heat threaten hydraulic equipment in the same way they affect oil and components, raising the risk of failure and reducing machine capacity.
In a technical presentation to the Society of Tribologists and Lubrication Engineers (STLE) in Hamilton, Larry Feldcamp, president of Feldcamp Equipment Ltd. in North Bay, Ont., talked about the importance of hydraulic filtration and the need for proper hydraulic tank design.
He says to control entrained air, the tank should be divided into a high flow zone and a quiet zone, then allow for a dwell time of 4:1 or 5:1 ratio. A tank for, say, a 50-gallon-per-minute pump flow should thus have a capacity of 200 gallons. The high flow zone should be changed out every three to 10 minutes. This allows the air to dissipate.
Locations for hydraulic filtration are critical. Remember that so-called “new oil” out of the drum is 10 to 20 times too contaminated for use in a hydraulic system, hence the need for filtration.
Feldcamp says that most hydraulic pump manufacturers don’t like to have suction filters or strainers on the suction lines installed in the system. Strainers have too fine a mesh that can cause clogging and cavitation. Besides, they’re not filters and are typically too fine to act as orifices to the pump. If used at all, they should not be mounted vertically because air will be trapped in the cap. When mounted horizontally, the air is often captured on the underside of the fine mesh. The best way to mount them is at a 45-degree angle to allow trapped air to get out of the cap and to run along the pleats. Strainer mesh should be open enough for a strainer pressure drop of virtually zero.
Using pressure is an effective way to filter hydraulic fluid. A pressure filter protects the valving and actuators in the system against contamination coming from the pump. Unless the valve components are very sensitive, Feldcamp recommends 10- or 20-micron absolute elements with an initial pressure drop at full flow of no more than 25% of the indicator setting. This provides adequate component protection and a reasonable element life. A “no bypass” design filter assures that no contamination gets from the pump to the actuators and valves.