“Your valve might be bypassing in the fully open position. If the wedge will split at 3/4 handle extension, the problem is not the engine.”
-Not sure he did the test in 3/4 position, sounded like he meant when fully stroked it starts to bog, and when fully released back to center the engine recovers. This would be normal if the engine is not capable of turning the load. Basically the mass of the engine acts as a flywheel and adds some torque to the pump briefly, then the engine goes down.
-the 3/4 position operation still does not rule the engine out or in. All we know is that engine hp is less than pump demand load. The engine can be down, or the load up, we haven’t determined that.
-Probably not leaking valve. If the engine and RV are correct, the engine sees no difference in its load whether the oil is pushing a log, going across leaking valve, going across relief valve, or going across leaking cylinder seals. The engine only sees the torque required to turn the pump at fixed displacement against say 2500 psi. Where it goes beyond the pipe, engine doesn’t care. The key is what is the load pressure.
-An open center spool valve does not suddenly connect pump flow to cylinder. That is physically impossible, so there is a crossover position. If the spool overlapped (closed off the pump to tank connection BEFORE it opened the pump to cylinder connection) there would be a blocked spot in the crossover position, which would send oil across relief each time the spool shifted. There is usually a ‘load check’ which is a check valve on the inlet port from pump to each spool section. The spool is cut to open a path to the load first, before it starts closing off the connection from inlet (pump) to tank. If it just opened the load path from pump to cylinder, while still having the pump to tank path open, the cylinder load (say a farm loader) would push oil backwards through the valve out to tank and the load would drop. The pump inlet check stops that from happening, the oil can’t go backwards.
OK, so what, we don’t have a suspended load to push oil backwards. The load check is not just to prevent load drop, it is to allow moving the load at less than the relief valve setting. A control, efficiency and heat issue I won’t go into. Even if it does not have a load check, the same principle applies: the path to cylinder is opened before the path to tank is closed off.
Here is the key point: Since the oil does not suddenly all go to cylinder, the pressure builds up slowly with handle position, even when cylinder is stalled against a load. The cylinder is first connected to pump, the backcheck prevents the cylinder from going backwards, but the pump oil still has an easier path to tank. The pressure/flow to the cylinder is only the same pressure that it takes to push the rest of the oil across the still open path from pump to tank. Maybe that is 100 psi.
As the handle moves further through its stroke, it is closing off the path to tank, so it takes more pressure to push the oil through that path to tank. Maybe 500 or 1000 psi builds up. This pressure is also on the cylinder, even if cylinder is not moving. Moving spool further builds pressure even further, say to 1500 psi, then 2000, then 2500. Without a relief the pressure would get higher and higher trying to push the (10?) gpm across a tiny orifice. When the spool totally closed to tank, oil could go no where, and pressure would go infinite.
With a relief valve, the pressure only rises to RV setting, then some goes that direction and some still goes across the spool to tank.
Run it at ¼ stroke and read the pressure, then half stroke, then ¾ stroke. If you see the engine stalling at less than the maximum rated pressure that the engine hp could turn that pump, then the engine is weak. If it reaches rated pressure, but keeps going higher and stalls the engine, then the RV is too high.
Both scenarios mean you need to know the pump size, pressure, and engine hp. Then it is easy to calculate the hp load ( flow gpm x pressure psi / 1714 = theoretical hp) and know if it is too much load for rated engine, or too little tired engine for rated load. Then you know whether or not to do valves, rings, etc., to adjust the RV, or to have a smaller pump.
BTW, I assume this thing worked fine before, that the engine and pump were at one time sized and operated together properly. If this is a new machine design and startup, that’s another story.
I suggest again, get pump info, and operating pressures. I think the answer will be very clear then.
k
