In this system, a control-valve spool must be open in the center to allow pump flow to pass through the valve and return to the reservoir. Figure 2-3, shows this system in the neutral position. To operate several functions simultaneously, an open-center system must have the correct connections, which are discussed below. An open-center system is efficient on single functions but is limited with multiple functions.

Series Connection. Figure 2-4, shows an open-center system with a series connection. Oil from a pump is routed to the three control valves in series. The return from the first valve is routed to the inlet of the second, and so on. In neutral, the oil passes through the valves in series and returns to the reservoir, as the arrows indicate. When a control valve is operated, the incoming oil is diverted to the cylinder that the valve serves. Return liquid from the cylinder is directed through the return line and on to the next valve.

This system is satisfactory as long as only one valve is operating at a time. When this happens, the full output of the pump at full system pressure is available to that function. However, if more than one valve is operating, the total of the pressures required for each function cannot exceed the system’s relief setting.

Series/Parallel Connection. Figure 2-5 shows a variation on the series-connected type. Oil from the pump is routed through the control valves in series, as well as in parallel. The valves are sometimes stacked to allow for extra passages. In neutral, a liquid passes through the valves in series, as the arrows indicate. However, when any valve is operating, the return is closed and the oil is available to all the valves through the parallel connection.

When two or more valves are operated at once, the cylinder that needs the least pressure will operate first, then the cylinder with the next least, and so on. This ability to operate two or more valves simultaneously is an advantage over the series connection.

Flow Divider. Figure 2-6, shows an open-center system with a flow divider. A flow divider takes the volume of oil from a pump and divides it between two functions. For example, a flow divider might be designed to open the left side first in case both control valves were actuated simultaneously. Or, it might divide the oil to both sides, equally or by percentage. With this system, a pump must be large enough to operate all the functions simultaneously. It must also supply all the liquid at the maximum pressure of the highest function, meaning large amounts of HP are wasted when operating only one control valve.