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Wow! Sometimes, all you got to do is ask. Thanks for the info, guys. Check out this email I got: Good evening from our farm in central California I am unable to post on YT because ISP problems. Kim is aware of the posting problems and working on it. So I will e-mail you with an answer to your question. If you can figure how to post this e-mail in the thread, please feel free to do so.
In agricultural tractors, there are two types of hydraulic systems, open center and closed center. The directional control valves typically have 4 ports: 1) P- pressure from the pump, 2) T- return to the hydraulic reservior (tank), 3) A- working port to one side of a cylinder or hyd. motor, 4) B- working port to other side of the cylinder or motor. In an open center hydraulic system with a typical 3 position directional control valve, the hydraulic oil enters the P port and exits the T port when the valve is in the center position. The hydraulic pump is circulating the oil thru the valve and back to the reservoir. When the handle is pulled in one direction, the pressure oil directed to one of the working ports (imagine port A) and the return oil from the cylinder or motor enters thru port B and exits the hydraulic valve thru port T to the tank. Pulling the handle in the other direction will direct the flow thru port B and the return thru port A and exits the hydraulic valve thru port T.
In a closed center hydraulic system, the directional control valve has a different spool. When the 3 position valve is in the center position, hydraulic oil is blocked from entering port P and exiting thru port T. When the handle is pulled in either direction, the valve acts the same as described ih the previous paragraph. The terms closed center and open center refer to the configuration of the hydraulic directional control valve but other hydraulic components are required: namely THE HYDRAULIC PUMP.
In an open center system, the hydraulic pump is a fixed displacement pump. It could be a gear or vane pump. For every revolution the pump turns, a certain volume of oil is pumped. It is usually measured in cubic inches per revolution at various PSI. When the directional valve is in the center position, the oil has a place to go. It is recirculated to the tank. The system pressure at rest is only line pressure (friction) and is very low, usually less than 100 PSI. In a closed center system, the hydraulic pump is a variable displacement type such as an axial piston pump. The pump has several small piston on a squash plate which is controlled by pressure. Typically these pumps are called pressure compensated. When the hydraulic system is at rest (no flow), system presssure is maintained (1500 to 2800 PSI) and the pressure moves the squash plate so that the pump only lubricates itself and produces no flow. When flow is required, the reduced hydraulic pressure will allow the squash plate to move to pump oil and maintain system pressure.
Closed center systems have been used on John Deere since the 60's, I believe. They allow the tractor to use multiple hydraulic functions with one pump. Power steering with open center systems require a priority valve to maintain a certain flow available for steering the tractor unless the tractor has a dedicated pump for power steering. Hydraulic brakes (JD again) used the hydraulic system for activation of the turning brakes.
I replaced a PC pump on a JD 2640 about two years ago. The pump cost $550 from the local dealer. An equivalent fixed displacement pump would cost one half. On newer tractors, most systems are closed center and for obvious reasons. I hope this long winded response makes sense. Happy Thanksgiving to all. Medfarm
aka Ray Medley
Medley Farms
San Joaquin Valley, CA
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