Hydraulic power is something that is easy to use but not always easy to understand. At its root, hydraulics is the science of how fluids are harnessed to perform mechanical tasks. That can be anything from folding a disc at the end of a field to running a generator at the bottom of a huge dam.
Hydraulics – Open vs. closed take me back to my days as an undergrad at Virginia Tech. Even though I was not an engineering student, many of my friends were and they would engage me in their discussion on the topics of “fluids”… namely “was air a fluid?”
A fluid can be defined as having no shape, yield easily to external forces and have the ability to flow. The sticker to the fluid question dealt with the subject of compressibility. Air is compressible; most fluids are not.
I think about this sometimes when I am using one of my floor jacks. The oil inside the jack is not compressible so as I increase the pressure on the oil by pumping the jack, the jack raises the cylinder and, in turn, the vehicle as well.
The hydraulic oil inside your tractor acts in much the same manner. As the hydraulic pump creates pressure within the system, the oil inside the tractor or the oil within the hoses connected to the tractor transfers that pressure to the point of mechanical action, which might be the cylinders on your round baler or the brakes that stop you from rolling where you really don’t want to go.
A properly working hydraulic system is a must in today’s world of high-horsepower, heavy equipment. Let the power steering go out of your pickup truck and you understand quickly how important good hydraulics are to the completion of our tasks.
In tractors, two main types of systems are used: open-center and closed-center hydraulics. “Center” is a bit of a misnomer. It would probably be more accurate to replace center with circuit, but it is the term that many of us have grown up with so there is no sense in debating the point here.
Closed-center hydraulics are just that, 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 neutral position. The hydraulic pump is a continuous flow type. When the valve is neutral, then hydraulic fluid goes back to reservoir or the tractor housing. This design is a bit more simple 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 opencenter return path to 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.
So in a nutshell, 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.
Read more: Extension Answers: Hydraulics – Open vs. closed