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Open & Closed Center Systems

Objectives

  • Differentiate between the terms open loop and closed loop.
  • Read a schematic to correctly identify an open or closed center hydraulic system.

Terminology Clarified


Discussions around hydraulic system designs often result in confusion when terms such as open loop, closed loop, open center, and closed center are used.
Open Loop
VS
Closed Loop

When we are talking about the loop, we are referring to the system as a whole. Open loop systems start and end at the reservoir whereas closed loop systems start and end at the pump.

Open Center
A
B
P
T
VS
Closed Center
A
B
P
T

When we are talking about the center we are referring to the center position of a directional control valve. Open center valves connect the P and T ports when centered. Closed center valves block the P port from the T port when centered.

Open Loop Systems

An open loop hydraulic system is any system where all of the fluid starts at the reservoir and is returned to the reservoir.

The pump takes in fluid from the reservoir, then sends the fluid to the valves and actuators, and then ultimately, back to the reservoir via a filter.

Now that we know what an open loop system looks like, let's have a look at open and closed center systems that are common in open loops.

Closed Center Systems

The following systems are all of the open loop design.

In this first example, oil is drawn in from the reservoir by the pump and sent out to a number of directional valves where the P or Pump Port is blocked.

Pump
Reservoir
Reservoir
P
T
A
B
P
T
A
B
Relief
Filter

This design results in a system where the pressure is determined by the setting of the relief valve or some other pressure limiting device when all the directional valves are returned to their center position.

The directional valves are in the center position.

In this case, we have constructed a closed center system using float center valves.

But a closed center system can also be constructed using closed center valves.

When all valves are at their center position, oil from the pump is forced through a spring loaded relief valve.

Using the relief valve as an alternate path for the pump flow should only be done for short periods of time as the oil will become very hot, and energy will be wasted at the prime mover.

An even better solution for closed center systems is the use of a pressure compensated pump!

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