Health and Handling Basic Hydraulic Systems  Open Center Hydraulic Systems 12-6 Figure 12-3.  Open center hydraulic system. Figure 12-2.  Basic hydraulic system. number of subsystems, with a selector valve for each  subsystem. Unlike the closed center system, the selector  valves of the open center system are always connected  in series with each other. In this arrangement, the system  pressure line goes through each selector valve. Fluid is always  allowed free passage through each selector valve and back  to the reservoir until one of the selector valves is positioned  to operate a mechanism. When one of the selector valves is positioned to operate an  actuating device, fluid is directed from the pump through one  of the working lines to the actuator.  [Figure 12-3B]  With the  selector valve in this position, the flow of fluid through the  valve to the reservoir is blocked. The pressure builds up in  the system to overcome the resistance and moves the piston  of the actuating cylinder; fluid from the opposite end of the  actuator returns to the selector valve and flows back to the  reservoir. Operation of the system following actuation of the  component depends on the type of selector valve being used.  Several types of selector valves are used in conjunction with  the open center system. One type is both manually engaged  and manually disengaged. First, the valve is manually moved  to an operating position. Then, the actuating mechanism  reaches the end of its operating cycle, and the pump output  continues until the system relief valve relieves the pressure.  Regardless of its function and design, every hydraulic system  has a minimum number of basic components in addition to a  means through which the fluid is transmitted. A basic system  consists of a pump, reservoir, directional valve, check valve,  pressure relieve valve, selector valve, actuator, and filter.  [Figure 12-2] Some phosphate ester-based fluids are blended with  performance additives. Phosphate esters are good solvents  and dissolve away some of the fatty materials of the skin.  Repeated or prolonged exposure may cause drying of the  skin, which if unattended, could result in complications,  such as dermatitis or even secondary infection from bacteria.  Phosphate ester-based fluids could cause itching of the  skin but have not been known to cause allergic-type skin  rashes. Always use the proper gloves and eye protection  when handling any type of hydraulic fluid. When phosphate  ester-based mist or vapor exposure is possible, a respirator  capable of removing organic vapors and mists must be worn.  Ingestion of any hydraulic fluid should be avoided. Although  small amounts do not appear to be highly hazardous, any  significant amount should be tested in accordance with  manufacturer’s direction, followed with hospital supervised  stomach treatment. An open center system is one having fluid flow, but no  pressure in the system when the actuating mechanisms  are idle. The pump circulates the fluid from the reservoir,  through the selector valves, and back to the reservoir.  [Figure 12-3]  The open center system may employ any  Hand  pump Filter Motor-driven pump Reservoir Actuating cylinder Pressure relief valve One-way check valve Suction Pressure Return Reservoir Relief valve Pump Directional control valves Suction Pressure Return A B