front bucket dropping overnight

[Threepoint]

Threepoint watch the video that is linked in post #11. Maybe that will help.

No, I'm making a different point not addressed in the video: The cylinder is completely full of oil at the start of the test, so the only way the oil in the rod end can even begin to move past the piston seals into the cylinder end is if either (1) oil or (2) air simultaneously enters the cylinder from outside it in order to compensate for the volume of rod exiting the cylinder. Otherwise, vacuum pressure will prevent the rod from even beginning to extend under normal conditions, e.g. the weight of an implement or, in the case of the video, Brian's body weight and strength. That's what I mean by the cylinder being "locked" by vacuum pressure.

In the video test, disconnecting the QDs prevented any additional oil from entering the cylinder, so the fact that Brian is able to pull the rod out with his arms and weight says to me that (1) the piston seals are leaking, as he says, and (2) air is simultaneously entering the cylinder past the rod seals (gland seals). This doesn't mean that the rod seals are also bad, since air molecules can pass when the larger hydraulic oil molecules cannot.

Anyway, that's the point I was making. I'm pretty sure this is right, but I'm not a fluid dynamics engineer, and I didn't stay at a Holiday Inn Express recently. If we have one on the board, maybe he or she can weigh in.

 

[MtnViewRanch]

No, I'm making a different point not addressed in the video: The cylinder is completely full of oil at the start of the test, so the only way the oil in the rod end can even begin to move past the piston seals into the cylinder end is if either (1) oil or (2) air simultaneously enters the cylinder from outside it in order to compensate for the volume of rod exiting the cylinder. Otherwise, vacuum pressure will prevent the rod from even beginning to extend under normal conditions, e.g. the weight of an implement or, in the case of the video, Brian's body weight and strength. That's what I mean by the cylinder being "locked" by vacuum pressure.

In the video test, disconnecting the QDs prevented any additional oil from entering the cylinder, so the fact that Brian is able to pull the rod out with his arms and weight says to me that (1) the piston seals are leaking, as he says, and (2) air is simultaneously entering the cylinder past the rod seals (gland seals). This doesn't mean that the rod seals are also bad, since air molecules can pass when the larger hydraulic oil molecules cannot.

Anyway, that's the point I was making. I'm pretty sure this is right, but I'm not a fluid dynamics engineer, and I didn't stay at a Holiday Inn Express recently. If we have one on the board, maybe he or she can weigh in.

I took the piston seals off the piston. The cylinder was 100% full of fluid. I could only pull X amount of vacuum on the cylinder. The rod then sprung back into the cylinder when I let go.

I pressurize every single cylinder I build with 140PSI. I have yet to have any seals leak air. I think that it's funny that some people actually think that 14.7 PSI of air gets by a hydraulic seal that holds back over 10,000PSI of fluid. Maybe it will over a very long period of time, but no way in a short time frame.

 

[K5lwq]

The point I was hoping to get across is that just because there is cylinder drift, extending or collapsing, does not necessarily mean a directional control valve is faulty. :drink:

 

[Threepoint]

I took the piston seals off the piston. The cylinder was 100% full of fluid. I could only pull X amount of vacuum on the cylinder. The rod then sprung back into the cylinder when I let go.

I pressurize every single cylinder I build with 140PSI. I have yet to have any seals leak air. I think that it's funny that some people actually think that 14.7 PSI of air gets by a hydraulic seal that holds back over 10,000PSI of fluid. Maybe it will over a very long period of time, but no way in a short time frame.

Brian, I think I just had a Eureka moment. If the following reasoning is wrong, please let me know. Grasshopper wants to learn, Master! Also, Richard, K5lwq, LD1, anyone else still following this thread, pls. jump in with any thoughts!

Watching the video demonstration again, I realize now why Brian was able to pull the retracted rod out with only arm strength, even though he had just completely filled the cylinder and hoses with oil. It was not because air was entering through the rod's gland seal as I first suspected. Rather, it was because the male plug of the Quick Connect on the cylinder-end hose was disconnected from the QC socket on the tractor, and thus was not actually sealed against external, atmospheric pressure. The spring valve in the QC's plug is designed to seal against internal pressure pushing outward, not against the vacuum that was induced by pulling on the rod. That vacuum reduced the internal pressure on the spring enough to allow air (under atmospheric pressure) to enter the hose. Now, with air in the cylinder, and with a condition of bad piston seals, or no seals at all, the oil at the rod end of the cylinder now has a place to go. As the rod extends, oil can move to compress the air until a condition of equilibrium of pressure is restored. Once Brian stopped pulling on the rod, the induced vacuum was released and the compressed air expanded, forcing some of the oil back past the piston seals, causing the rod to spring partially back into the cylinder.

Here is another interesting video demonstrating a bypass test of a hydraulic cylinder. This method eliminates the possibility that air can be introduced through the Quick Connects, as only internal pressure is applied to the fittings.
Bypass testing a hydraulic cylinder - YouTube

 

[LD1]

Possibly could have pulled air in.

Air is smaller on the molecular level than oil, and harder to seal out.

IE: something that seals oil out might not seal air.

But even if those quick disconnects were replaced with caps....results would likely be the same. The cylinder would extend, drawing vacuum...and creating a void in the oil. But that void don't necessarily mean air has to enter. It's just a void. Vacuum is the ABSENCE of air.

And final thought.....alot of times gland seals are a vee packing or u-cups. They are designed to hold oil in....not really designed to seal the other way....only against dirt and contaminats.

 

[K5lwq]

Threepoint,

I truly do not believe air was pulled into the cylinder. As LD1 said, even if it was capped with no posable way for air to get in, the results would be the same.

The pressure in the cylinder is always wanting to be equal. When Brian pulls on the rod he created an area of high pressure and low pressure. The low pressure side was pulling against Brian but was no match for his brute strength! So the only thing it could move is the oil from the high pressure side which allowed the rod to extend.

Once Brian stopped pulling, the low pressure side can now move the rod to equalize pressure easier than pulling the oil. Therefore, the rod retracted till pressure was equal.

Is that a better way of looking at this?

 

[ishiboo]

As has been beaten to death, it's normal.

Hydraulic control valves are typically designed for operation and not for static support of a load. That is why hydraulic load holding check valves exist - to keep a load at a certain position. Cylinders and valves can also be built to higher standards for critical applications where positioning is important. For example, my 50' scissor lift even if a hose blew it would not cause the thing to fall uncontrolled, and once shut off it will sit in the exact same spot with essentially zero drift.

This occurs on brand new tractors, not because the seals are bad but because the control valves "leak". It can also happen internally on a cylinder once the cylinder itself is worn, as K5 said, because the fluid bypasses the piston seal. With an imperfect seal you could fill a cylinder with oil and plug both ports and still be able to move it with no external leakage.

 

[LD1]

As has been beaten to death, it's normal.

Hydraulic control valves are typically designed for operation and not for static support of a load. That is why hydraulic load holding check valves exist - to keep a load at a certain position. Cylinders and valves can also be built to higher standards for critical applications where positioning is important. For example, my 50' scissor lift even if a hose blew it would not cause the thing to fall uncontrolled, and once shut off it will sit in the exact same spot with essentially zero drift.

This occurs on brand new tractors, not because the seals are bad but because the control valves "leak". It can also happen internally on a cylinder once the cylinder itself is worn, as K5 said, because the fluid bypasses the piston seal. With an imperfect seal you could fill a cylinder with oil and plug both ports and still be able to move it with no external leakage.

Just so we are clear....

Moving a cylinder full of oil.....creating drift by allowing fluid to bypass piston seals....ONLY happens if the cylinder is trying to extend.

Mobile lifts utilize a DPCV to prevent sudden falling in the event of a failure

 

[MtnViewRanch]

Just so we are clear....

Moving a cylinder full of oil.....creating drift by allowing fluid to bypass piston seals....ONLY happens if the cylinder is trying to extend.

Mobile lifts utilize a DPCV to prevent sudden falling in the event of a failure

Can we please emphasize the ONLY? Just so others won't miss it.

 

[LD1]

Can we please emphasize the ONLY? Just so others won't miss it.

Still won't make a difference