Confused about air in hydraulic lines

MarcusCarr · Mar 21, 2006

I am in need of an explanation on how the air gets out of the hydraulic lines of the grapple I am building.

I understand that oil flows into, then out of, the valve. It makes sense to me that the air would be pushed back into the oil tank.

The part I don't understand is the lines going from the valve to the cylinder. Each line goes to a closed chamber. Without gravity to move the oil to the bottom of the cyclinder and the air to the top...how will I get the air out of the lines and cylinder?

Soundguy · Mar 21, 2006

How about colapse the sid eof the da cyl you ar working on.. then pressurize the disconnected line till oil comes up tot he end of the line.. then connect loosely, and pressurize.. should bubble for a second then be straight oil.. tighten up.. then set valve for float, and colapse da cyl so that other side is empty, and do the same?

Soundguy

EdKing · Mar 21, 2006

I think this is how it works, someone more knowledgeable about hydraulics can correct me if I am wrong.

The control valves used on tractor hydraulics are open center, meaning oil flows through them all the time. When you first hook the cylinder up and use the control, you are pushing oil into the cylinder closed side, pushing the air out of the open side. The air is then pushed back to the oil tank. When you release the control, oil continues to flow through the control valve pushing any air that came from the cylinder to the control valve back to the tank. When you operate the control in the other direction, it pushes oil into the now closed side (formerly open side) pushing the oil and remaining air out of the open side (formerly closed side). A few cycles of this will push all the air out of the cylinders and lines and back to the tank, where it is released from the vent hole.

MadReferee · Mar 21, 2006

All you have to do is cycle the cylinder from one extreme to the other extreme a couple of times. The pressure of the moving cylinder (not gravity) will force all the air from the one end of the cylinder and its hose out thru the spool and back to the tank. Cycling the cylinder will do the same for the opposite side of the cylinder and its hose.

It doesn't matter if the system is open or closed center (your tracter is open center). When the spool is in the neutral position, the fluid is held in the cylinder and hose.

rambler · Mar 21, 2006

2000# of pressure does a real number on an air pocket - cycle the cylinder a few times, it will be real jerky as the air uncompresses & compresses. The air disappears into the oil tank - pretty violently, but it's all closed in the lines & tank & all.

--->Paul