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Mechanics Corner: Hydraulics: Open vs. closed

Published on 19 October 2017

Hydraulic power is something 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 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?”

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A fluid can be defined as having no shape, yielding easily to external forces and having 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 transfer 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 many of us have grown up with, so there is no sense in debating the point here.

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Closed-center hydraulics are just that … closed in a continuous loop. They have 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 power take-off 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 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 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 open-center 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 very 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.

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Many open-center systems rely on a power priority valve to maintain the critical systems that require pressure. As the name suggests, the valve prioritizes the oil flow and pressure going to each requirement.

It maintains oil flow to the brakes first, followed by steering and so on, with the common sense approach that if all else fails, you want to be able to stop and steer – and if you cannot steer, you definitely want to be able to stop.

Open-center pumps are gear-driven, giving them a set output. These systems tend to be more efficient with power delivery and less susceptible to oil contamination. That said, your choice of hydraulic fluids is very important regardless of the system your tractor and equipment uses.

Like motor oils, hydraulic fluids have grades and specifications that help you evaluate the viability of the fluid in your equipment. Even with the ability to compare specs, it is also wise to consider not all of the important information is included.

Additives that help maintain the integrity of the seals and O-rings of the hydraulic system are very important. The loss of any of these seals will result in at least a partial loss of function, and total failure is a possibility.

Some of these “leaks” can be controlled by running a tractor “overfull” on hydraulic fluid. In fact, some aftermarket kits allow tractors to “gain” fluid capacity by adding filter extensions or other means. These remedies can work well but, again, they are not a substitute for a system with seal integrity.

Power steering and other functionality can be lost when the tractor turns downhill and air gaps in the system cause the system to lose pressure.

In the end, the decision between an open or closed system is up to the individual needs and desires of the equipment owner. Just like the tractor they are contained in, choose what works best for you and maintain it. In the end, it is a lot easier to pay for what you want than to settle for less.  end mark

Andy Overbay holds a Ph.D. in ag education and has more than 40 years of hands-on dairy and farming experience.

Andy Overbay
  • Andy Overbay

  • Extension Agent
  • Virginia Cooperative Extension
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