check valve? bypass loop? I have been using dr Batson's book to build the lines and I do not see anything about that, thank you for the suggestionA high pitched whine in hydraulics is often fluid running over a small orifice, or a dead-headed pump. Do you have a check valve in the system? How is your cylinder bypass loop controlled?
check valve issue: The fluid can't be allowed dead-head when the the cylinder isn't being operated, so however your bypass loop is set up could be forcing the fluid to backflow to the back side of the check valve. You can usually feel chatter on the check valve with your hand. Look for a valve or T or some fitting that is getting way too hot. That is your choke point causing backpressure on the check valve.
dead head: If you are dead-heading the pump against a closed valve when the cylinder isn't operating, that's a real no-no. The fluid has to always flow back to the return sump if the pump is running.
thanks John, I am using a prince 4-way control valve with the high pressure side of the pump hooked up as the control valve schematic showsA hydraulic system can be as simple or as complex as you want to make it. Simple is inexpensive but it will usually do some undesirable things.
Can you describe your system? It sounds like you have a pump which then is plumbed to a valve block. (Assuming you have a cylinder that is driven in both directions by the pump: is it a two-position reversing valve?) then the valve block goes to the cylinder. The head end of the cylinder (where the rod comes out) is your retract, and the cap end (the blanked off butt end) is plumbed to Extend. And whenever the cylinder is neither extending or retracting the pump begins whining loudly. Does this sound about right?
If that's what is going on, then I would say your problem is that you are using a two-position reversing valve. The only time any fluid can move is if the cylinder is being actuated to extend or retract. Sure, that will work. But your pump is hating life whenever the cylinder is not moving. **If your cylinder is a spring retract, that will change the valve you need. There are a zillion valves to choose from so that you can do whatever you want to do, very easily.
The pump whine:
A hydraulic pump cannot dead-head. Dead-head means the pump is pumping fluid, but there's nowhere for it to go because the valve it feeds is closed. This means the pump is pushing all kinds of fluid to nowhere, and being incompressible, the fluid is pushing back on the pump. This causes cavitation and will destroy the pump.
To protect the pump, many manufacturers will build in a bypass loop inside the body of the pump. (this is not the bypass I was referring to earlier.) This internal bypass port allows *some* fluid from the discharge side of the pump to circulate back to the inlet side through a small machined channel. Some fluid movement is better than none, and this will keep a little bit of fluid circulating to cool the pump bearings and reduce cavitation. But that tiny little channel is a small orifice, and it's going to sing like blue blazes when that high pressure fluid squeezes through there. The pump is also going to get real hot after a bit.
I realize this is just a lot of assumption on my part because I can't see your system. If I'm on the right track with your setup, let me know. If I'm right about the valve you are using, this is a simple fix. You would just have to change the valve with one that has a center position. A center position that has pass-through will allow you to run a line from the center position outlet to the sump, then the pump can run freely and the fluid will flow from the pump and back to the sump. That's the "bypass" loop I was referring to.