OK, alrighty then. Lets do a thought experiment shall we?
Lets say we have a cylinder with a very very leaky seal. It struggles to lift the FEL arms and bucket so that the cylinder is halfway extended. Now we place the joystick/valve in neutral so that the rod side port on the cylinder no longer has a flow path to the tank and of course the cylinder side no longer has connection with the pump flow. We turn off the tractor engine. So far so good? And in our system lets assume that the control valve is machined to some really really tight tolerance so that it can never never leak. The hoses and fittings are perfect and never never leak.
Now remember the internal piston seals leak like a sieve. We now throw some weight into the bucket. Lets say 1000 lbs.
The stage is set.. very leaky piston seals, perfect hoses and connections, an a totally non leaky control valve.
Now some facts. The space behind those leaky pistons seals is full of oil isn't it? AND the rod side of those leaky piston seals is full of oil and the rod itself isn't it? The rod is made of good steel, and the external rod seals are perfect and don't allow any oil to get past the rod and drip onto the ground.
You tell me how even with 1000 pounds in the bucket exerting inward force on the rod, how that rod is going to enter into the cylinder?
Think about it, the hydraulic oil is basically incompressible. Oh, its molecules might compress just a tad, but very very very little.
It don't matter one little bit that the internal seal leaks like a sieve in this scenario because all the fluid thats behind the seal and all the force of that 1000 lbs trying to press that rod into that cylinder cannot press it in there even if the internal piston seal was totally removed in some manner.
You cannot stuff the rod into the cylinder no matter what. Even if the piston body was totally gone, you still cannot stuff that rod into that cylinder full of oil. Can't be done.
Why do we need piston seals at all then you ask? Because we need them to raise the loaded bucket when we put the flow of hydraulic fluid from the control valve when it moves to open the circuit from the pump, then pressure builds behind the piston, AND, this is the important part, when we move that control valve we are not only opening up that circuit, but we are also opening up another circuit from the other side (the rod side) of the cylinder to the control valve on back to the tank.
NOW the oil on the rod side has a nice low pressure place to go. If our internal piston seals were very very leaky, guess where our flow from the valve/pump would go? Yep that is right it would go right past our leaky seals and head right back to the tank, and we would not be able to raise our load.
Now someone please tell me how I am FOS.
