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Tractor Talk Discussion Forum

hydralic systems

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PhilCaseinPA

04-19-2004 09:06:22




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What are the differences between open center and closed center systems? What are the advantages / disadvantages of each?




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JMS/MN

04-19-2004 22:03:51




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 Re: hydralic systems in reply to PhilCaseinPA, 04-19-2004 09:06:22  
kyhayman has it right. Open center systems, as in older tractors, were simpler, but with today's mondern tractors and increased usage, excessive heat and power loss would develop. Closed center systems deliver power on demand, and only when needed. With hydraulics, you do not have pressure without flow. You cannot have one without the other. A closed center system develops pressure upon demand when you activate the valve. An open center system is always pressurized, but if not being used by a cylinder, hydraulic motor, etc., just goes through the bypass, back to the resevoir- so the pump is always pushing oil through the system.

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Greg

04-19-2004 18:12:05




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 Re: hydralic systems in reply to PhilCaseinPA, 04-19-2004 09:06:22  
A few things to clarify on hydraulic systems. No matter how fast the pump/engine is turning it is not being limited on output by the main relief valve. HYDRAULIC PUMPS PRODUCE FLOW, RESTRICTIONS IN THE SYSTEM CREATE PRESSURE. This is true no matter what system is used, closed or open center. A closed center system must also use some type of a load sense signal to either the pump regulator to stroke it as in a piston pump, or shift a valve to allow flow as in a power steering priority valve.

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kyhayman

04-19-2004 14:12:11




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 Re: hydralic systems in reply to PhilCaseinPA, 04-19-2004 09:06:22  
In a nutshell the o.c. system pump output increases with pump (engine) rpm up to relief valve setting. At full speed of engine pump is operating at rated capacity the entire time (with oil being dumped back into the resorvoir if no valves are open). Cheap, reliable, limited capacity, and wastes energy by pumping all that fluid all the time. Closed center only delivers fluid as needed, increasing at an increasing rate up. Allows for higher overall capacity b/c you are not using that capacity all the time (would creat a huge power loss and waste heat probs if it was running wide open all the time). Much more expensive to service and more complicated but much more capacity.

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txblu

04-19-2004 12:17:01




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 Re: hydralic systems in reply to PhilCaseinPA, 04-19-2004 09:06:22  
blue and green.

blue uses open which is a series system. Fluid has to flow thru 1 to get to 2. Use 2500 psig with ~5 gpm.

green uses closed in a parallel system where there is a static pressure of 2500 psig and a 25 gpm rated pump just daring something to attempt to lower the pressure (i.e. someone opened a valve somewhere).

Don't know about other colors and I am referring to old iron, not current technology.

txblu

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