There are two types of Hydraulic systems in the ag market: open-center and closed-center hydraulics. Learn more about each below.
Closed-center hydraulics are closed in a continuous loop. It has the advantage of using a single central pump. Open-center hydraulics have more than one pump in stages that supply power to different applications as the needs arise. For example, in an open system, the tractor’s steering and PTO would have separate pumps that supply the oil to make those important systems work. A closed system would use only one to supply power to both.
Open center refers to the open central path of the control valve when the valve is in a neutral position. The hydraulic pump is a continuous flow type. When the valve is neutral, then hydraulic fluid goes back to the reservoir or the tractor housing. This design is a bit simpler and generally uses pumps that are less expensive.
Closed-center circuits supply full pressure to the control valves, whether any valves are actuated or not. The pumps vary their flow rate, pumping very little hydraulic fluid until the operator actuates a valve. The valve’s spool, therefore, doesn’t need an open center return path to the tank. Given the pump’s need to react or sense what the need of the machine is or is not, these systems tend to be a bit more complex and expensive. They are powerful and are used in most heavy equipment and modern high-performance aircraft.
In review, open-center systems always have oil flow. Closed-center systems are always under pressure, but oil does not flow until you activate a lever asking the system to perform. Closed systems build and hold pressure, and, in the past, took some hits because the pressure held at high levels made initial power at start-up more difficult.
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