Understanding Open and Closed Loop Systems in Fluid Power

Blog | February 26th, 2019

Let’s cut straight to the meat of the article. Open loop fluid systems start by sending a fluid stream through the system inlet, where it’s then picked up and boosted by a hydraulic pump. The hydraulic oil leaves its reservoir, passes through a pump outlet port, and it’s channelled towards an actuator or another active system device. From here, the loop takes the oil back to the reservoir.

Closed Loop Differences

A relief valve or a directionally controlled device has done its job on the open loop circuit. Congratulations are in order, for the first system type has been described. Now, what about closed loop hydraulic circuits? Well, there’s a fundamental difference built into this system type. To start with, the reservoir and pump are still in the mix. This time, though, the return line hooks up to the hydraulic pump’s drive inlet. Instead of the reservoir providing loop power, the output stream is drawn directly from the return flow. The loop is short-circuited, so it becomes a closed loop system.

Closed Loop VS. Open Loop Advantages

Generally speaking, open loop circuits are cheaper to run. The equipment is also easier to maintain and repair. That latter feature is made possible by a simpler, easier to troubleshoot system architecture. In the cons column, though, open loop equipment is designed to operate at lower pressures. And, despite that minor drawback, they still require larger than average fluid reservoirs. Worse still, open loop circuits generate excess quantities of thermal energy when their working pressures rise too high. On the other hand, closed loop hydraulics is more expensive, but the gear features a more adaptable layout. Closed loops can easily reverse direction without the aid of numerous valves, and they deliver more control options, along with builds that carry higher pressures.

Selectability Guidelines

It’s not always that easy, though. A designer doesn’t just look at a list of closed and open circuit design advantages and make a choice. Just as a single example of this observation, flow-sensitive systems don’t work well with closed circuit hydraulics, not when the liquid volumes on either side of a single rod cylinder are out of balance. Cavitation incidents multiply when such conditions remain unresolved.

Still, closed loop hydraulics is a desirable configuration. On mobile equipment, systems that don’t allow much space, a reverse-forward feature that doesn’t require a mass of pressure compensating valves is clearly advantageous. That same architecture allows for fewer fittings and hoses, too. Incidentally, although open loop equipment is indeed cheaper to fit and easier to troubleshoot, the smaller reservoirs used in closed loop systems makes this circuit layout a more fitting match for mobile equipment.

Mobile Hydraulic Specialties Pty Ltd

Factory 89, 38-40 Popes Road
Keysborough, Victoria, 3173

Phone: (03) 9798-6511

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