earth moving equipment

It's important to differentiate between hydraulic circuits and their electrical analogues:

• An open valve is equivalent to a closed electrical circuit.
• A closed valve is equivalent to an open electrical circuit.

Does this help?

It can also mean the flow path. A closed circuit recirculates the fluid back to the beginning. An open circuit dumps the fluid when the energy is utilised and it is not recirculated. A simple open hydraulic circuit example is a pressure washer. A closed circuit is a hydraulic jack for lifting.

My brain is an open circuit. Things go in and spill right out. Nothing is retained and it does a bit of cleaning in the meantime. My wife's brain is a closed circuit where nothing new can make it inside and the same old story keeps going around in a circle "I should have married the dentist . . . I should have married the dentist"

Damn....I think we're married to the same woman !!!

You must both be Mormon?

You may be thinking about the difference between open center valving and closed center valving. Check this article.

Early in my career I was taught the terminology was "Closed Loop" and "Open Loop" circuits.

Closed Loop referred to circuits like Hdra-Static Transmissions where all pump flow went to a hydraulic motor and all the hydraulic motors return flow went to the opposite pump port of a Bi-Directional pump. This circuit can also be used with cylinders but gets complicated when the cylinder is Single Rod due to unequal flow in both directions.

Open Loop was for any circuit that used a pump that had its Inlet connected to Tank, its Outlet connected to Directiona Control valves that directed flow to an actuator and returned spent oil back to Tank.

If an Open Loop circuit used Three Position Directional Control valves the center conditon could be "All Ports Open,""All Ports Closed (Blocked)," Tandem Center, "P" to "T," "A" and "B" Closed, "Float Center," "P" Blocked "A" and "B" to "T" and Regeneration Center, "T" Blocked, "P" to "A" and "B."

There are six other center conditions that are cataloged by most manufactures. Four of these I have never used or come across in my travels in the Fluid Power world. Some are merely another center condition installed the opposite end first so it was a no brainer to offer it as an option.

Another thing about Directional Control spool valves that is not understood by many and overlooked by some designers of hydraulic circuits is the conditions of the flow paths as the spool transitions from one position to another. Using the wrong Cross Over condition can cause a lot of problems in an otherwise great circuit.

In a basic training book I use to teach clases in Fluid Power I show the Standard and Non-Standard Spool types and the Cross Over conditions of each one. You can see the book at www.hydraulicspneumatics.com at the Ebook link. There has been some troubles with getting the links to work but I understand it is being worked on.

If you would like to see a brochure of the book and a second one I use for an advanced class go to http://www.ifps.org/Links/Company_Links.html and scroll down to HYDRA-PNEU CONSULTING and click on the book links to get a .pdf file.