Can someone please explain to me the difference between a valve with power beyond port and a valve with a tank port. I may be wrong but from my findings but power beyond ports are used when another hydraulic control valve is to be used down stream of another valve to supply hydraulic power to it. And a valve with a tank port sends the hydraulic fluid back to the tank to be pumped through the system again. What I dont understand is why cant the tank port on a hydraulic valve be used to feed the next valve in a system.
The reason I ask is, I had my local hydraulic shop source out an auto cycle log splitter valve for the fire wood processor I am building. It didnt come with power beyond but he told me his supplier told him there was another piece I could get to screw into the tank port on the valve that would give me power beyond capability with the valve. I asked him to explain but he was unsure also. Said he would find out but I havent heard from him yet
You are correct in that PB port feeds downstream hydraulics, but the T port is only to return fluid to the tank (or reservoir).
A convertible control will have two T ports when unconverted. If there are no downstream hydraulics, the PB adapter is not installed, either port can return fluid to the tank and the unused T port is simply plugged.
If there are downstream hydraulics, then the adapter is installed in one of them (and it will be specified which one must be used for PB). BOTH ports are then used. IE: you require 3 hoses instead of 2: one from the pump, one to the tank and one from PB to the downstream hydraulics.
When the spool valve(s) are in the neutral position, fluid flows from the pump and out the PB port. When the spool valve(s) are operated, fluid flows from the pump to one side of the cylinder(s) and the fluid from the other side of the cylinder(s) flows back to the tank through the T port.
If the load on the cylinder(s) is too great, the relief valve setting is exceeded and the relief valve bleeds the excess pressure from the pump port to the T port.
You cannot (well, should not) try to get away with only using 2 lines by connecting downstream hydraulics to the T port because of the risk of compounding relief valve settings. For example, let us say the downstream hydraulics are the 3ph cylinder. That relief valve is set for the maximum permissible pressure on the system -- say, 2,000 psi. And your log splitter relief valve is also set to 2,000psi. If both controls are inadvertently operated simultaneously, the pressure on the system could theoretically hit 4,000psi. Of course, something would break before the pressure got that high.
So it's just the routing of the fluid inside the valve body that creates low pressure return to the tank or keeps the high pressure for other hydaulic functions. So by adding this screw in adapter it blocks the "low" pressure ports so the exiting fliud stays at high pressure for further use.
No. This is an "open center" system. The fluid is always at low pressure until and unless a spool valve is operated because the center (neutral position) of the spool valve is connected to the tank line. There is no resistance to build pressure against. Pressure only builds when a cylinder is being operated.
"Closed center" systems are always at high pressure -- the pressure may even drop somewhat when a cylinder is operated. The center (neutral position) of the spool valve is solid, not connected to anything. When controls are in neutral, fluid returns to the tank through the relief valve so the pressure is always at the relief valve setting. Closed center systems do not require PB ports. Instead of being in series, closed center valves are connected in parallel to each other.
So: Open center = low pressure neutral, hydraulic functions in series, only one can be operated at a time.
Closed center = high pressure at all times, hydraulic functions in parallel, can operate all functions at once (although, each function will be slower).
Clear as mud?
