AKKAMAAN
Veteran Member
I think there is some different opinions about what causes cylinder drift/drop.....
It is usually gravity loads that makes cylinder drift....some times even inertial loads can make cylinders drift....while loads from friction never do....
A double action cylinder can hold a load on each side of the piston.....the capped side have a larger piston area than the rod side of the piston....the rod cross area make the net piston area that smaller on the rod side....
Let's make an example...we put a load on a double action cylinder capped side, and lets also say the hoses are disconnected and plugged with QD's.
If a certain load, lets say 1000 lbs, is held up by the pressure on the capped side, that pressure will sooner or later equalize both sides of the piston. If the cross area of the bore is 20 square inch, the pressure will initially be 1000/20=50psi. Due to inevitable piston natural seal leakage, pressure will equalize, and we will have the same pressure on both sides of piston. But now it is not 20 sqin that hold the load, it will be the net area difference between the capped side and the rod side, which is the area of the piston rod, lets say 5 sqin. 1000/5=200psi.
OK, piston seal have leaked a little but the cylinder WILL NOT DRIFT, unless there is an external leakage. Cylinder will actually not drift until something bursts.
If this cylinder would be connected to a control valve, it will be the control valve only that can make cylinder drift. PERIOD!! Either through work port valves or through the spool "tolerances".....
Spool leakage would be either natural from design, a few c/c per minute, if more it it would be due to MANY years of wear on the spools. It is unheard of, that spools can wear so much so a bucket can just starting dropping down fast during "a lunch break".
So, if the cylinder is suddenly dropping down "fast" it can only be the work port valve, or another valve connected to the hydraulic lines.
Next example will be when the load is held up by the rod side of the double action cylinder....
If we use the same cylinder and load as the above example, the initial pressure will be 1000/(20-5)=1000/15=67psi. At this setup the piston rod is being pulled out of the cylinder and will create a void on the capped side. Then this pressure will never equalize the sides of the piston, and the result will be that cylinder will drift if piston seal is leaking. Still, leakage through the control valve is possible. This can be tested by disconnecting and plugging the rod side hose. If we still are drifting it will be from a bad piston seal.
Now I go camping over the weekend and will be back on Sunday!
It is usually gravity loads that makes cylinder drift....some times even inertial loads can make cylinders drift....while loads from friction never do....
A double action cylinder can hold a load on each side of the piston.....the capped side have a larger piston area than the rod side of the piston....the rod cross area make the net piston area that smaller on the rod side....
Let's make an example...we put a load on a double action cylinder capped side, and lets also say the hoses are disconnected and plugged with QD's.
If a certain load, lets say 1000 lbs, is held up by the pressure on the capped side, that pressure will sooner or later equalize both sides of the piston. If the cross area of the bore is 20 square inch, the pressure will initially be 1000/20=50psi. Due to inevitable piston natural seal leakage, pressure will equalize, and we will have the same pressure on both sides of piston. But now it is not 20 sqin that hold the load, it will be the net area difference between the capped side and the rod side, which is the area of the piston rod, lets say 5 sqin. 1000/5=200psi.
OK, piston seal have leaked a little but the cylinder WILL NOT DRIFT, unless there is an external leakage. Cylinder will actually not drift until something bursts.
If this cylinder would be connected to a control valve, it will be the control valve only that can make cylinder drift. PERIOD!! Either through work port valves or through the spool "tolerances".....
Spool leakage would be either natural from design, a few c/c per minute, if more it it would be due to MANY years of wear on the spools. It is unheard of, that spools can wear so much so a bucket can just starting dropping down fast during "a lunch break".
So, if the cylinder is suddenly dropping down "fast" it can only be the work port valve, or another valve connected to the hydraulic lines.
Next example will be when the load is held up by the rod side of the double action cylinder....
If we use the same cylinder and load as the above example, the initial pressure will be 1000/(20-5)=1000/15=67psi. At this setup the piston rod is being pulled out of the cylinder and will create a void on the capped side. Then this pressure will never equalize the sides of the piston, and the result will be that cylinder will drift if piston seal is leaking. Still, leakage through the control valve is possible. This can be tested by disconnecting and plugging the rod side hose. If we still are drifting it will be from a bad piston seal.
Now I go camping over the weekend and will be back on Sunday!
