front bucket dropping overnight

[MtnViewRanch]

A simple hypothetical example. A hyd system designed to leak at a rate of 1 drop per minute. Extend the lift cylinders on a loader. If the piston seals are faulty, you now have not 1 port leaking at a rate of 1 drop per minute, but 2 ports leaking at a rate of 1 drop per minute. Thus doubling the 1 drop per minute acceptable leakage rate and causing concern about drift.

That is the exact reason when testing, you isolate the cylinder from the rest of the hydraulic system. Then try to have the rod extend, if it extends, then you are happy to have bad seals. (easy fix) If not, then you have a poor control valve that typically needs to be replaced.

Yes it is possible to have both bad seals and a bad control valve. But if you confirm that the cylinders do or do not leak is the starting point. If you have bad seals and change them and still have excessive cylinder drift, then you know that the control valve is out of spec and should be replaced.

A good rule of thumb IMO, if the drifting actually affects you operating the tractor, then the problems need to be addressed. If not and you find that things move after days or weeks, not really a concern.

Just my :2cents: , others may disagree and that is fine, I know that this works for me.

 

[K5lwq]

Thanks Brian, well stated. Also thanks for doing the video demonstrating faulty seals.

 

[Threepoint]

Ok, I'll jump in now with my opinion. Brian is absolutely right. (I've learned from him in prior threads!). RickB and K5lwq are also, right, but Richard and LD1 are more right than RickB and K5lwq. So everyone is right. :drink:

If the cylinder is full to begin with, and if no oil can escape from either end, there is no way the piston and rod can move. Period. So even if the piston seals are bad, or for that matter missing altogether, there can be no leak-down.

BUT, if there is a leak elsewhere in the system (e.g., internal within a valve), the piston can now move, so there can and will be leak-down. Which may be completely normal and within tolerance, as has been pointed out. AND (and here is where RickB is right), if the piston seals are also bad, the leak-down can be even faster, as the oil within the cylinder on one side can leave that chamber not only via the first leak, but also by moving past the piston seals to the other side of the cylinder.

So in that situation, replacing the defective or worn piston seals can indeed reduce the apparent leak-down, but only if the first leakage continues. Which it will, if it's due to the intended system design in the first place.

 

[MtnViewRanch]

Ok, I'll jump in now with my opinion. Brian is absolutely right. (I've learned from him in prior threads!). RickB and K5lwq are also, right, but Richard and LD1 are more right than RickB and K5lwq. So everyone is right. :drink:

If the cylinder is full to begin with, and if no oil can escape from either end, there is no way the piston and rod can move. Period. So even if the piston seals are bad, or for that matter missing altogether, there can be no leak-down.

BUT, if there is a leak elsewhere in the system (e.g., internal within a valve), the piston can now move, so there can and will be leak-down. Which may be completely normal and within tolerance, as has been pointed out. AND (and here is where RickB is right), if the piston seals are also bad, the leak-down can be even faster, as the oil within the cylinder on one side can leave that chamber not only via the first leak, but also by moving past the piston seals to the other side of the cylinder.

So in that situation, replacing the defective or worn piston seals can indeed reduce the apparent leak-down, but only if the first leakage continues. Which it will, if it's due to the intended system design in the first place.

Full in what rod position? Correct if the rod is fully extended, it is impossible for the rod to retract if the unit is full of oil on both sides of the piston. Wrong if the rod is retracted, the rod extends providing room for the oil to seep by the piston seals and fill the void left on the closed end of the cylinder. (non rod end)

 

[4570Man]

Full in what rod position? Correct if the rod is fully extended, it is impossible for the rod to retract if the unit is full of oil on both sides of the piston. Wrong if the rod is retracted, the rod extends providing room for the oil to seep by the piston seals and fill the void left on the closed end of the cylinder. (non rod end)

The cylinder is always full unless you’ve done something out of normal operation to get air in it.

 

[ovrszd]

True enough. However, that does not indicate that the control valve has failed or is out of spec. It also doesn't mean that someone experiencing cylinder drift will correct their problem with a new valve. A new valve could actually make the drift worse.

Explain how the cylinder can collapse without an external leak or leaking control valve?

 

[K5lwq]

I never said it would collapse without a leaking control valve. If you were to read my post again you will see I actually said if the rod was fully extended and completely capped with no leaks then it could not collapse.

However, the problem with most every spool valve out there is they do leak internally. Some much more than others and can vary somewhat within the same brand and model.

Most valve manufacturers will advertise their valve leakage amount usually in ML or CC's per minute.

As Brian explained, when you have a bad cylinder seal, then that rate is doubled and normally when someone might complain about drift.

So, because of that and the fact that a new valve could leak more than the one being replaced, it makes sense to test cylinders for bad seals. Most of the time, as Rick points out, rebuilding the cylinders will correct the drift problem.

 

[millsan1]

I didn't read the whoel thread, so excuse me if this has been said.

Tractors mostly use open systems. Heavy equipment use closed systems.

The part that matters for your question is open systems allow leak by and do not have check valves that would hold fluid in place. Therefore, open systems leak down, even when they are new and in perfect order. It is the nature of the beast.

A closed system should not leak down, especially if it contains check valves. Without check valves, they too can leak down.

 

[Threepoint]

Full in what rod position? Correct if the rod is fully extended, it is impossible for the rod to retract if the unit is full of oil on both sides of the piston. Wrong if the rod is retracted, the rod extends providing room for the oil to seep by the piston seals and fill the void left on the closed end of the cylinder. (non rod end)

But can't that happen only if air can leak past the rod seals to replace, with air, the volume vacated by the rod as it extends? If air can't leak in, and since by assumption no oil can enter the cylinder-end port, there is a perfect vacuum preventing the rod from exiting, no? So for practical purposes the cylinder is locked in both directions. In fact, as counterintuitive as it seems, the rod would be locked in place even without a piston on it, much less piston seals. This is what I think LD1 is referring to. It was very hard for me to get my head around it for awhile a year or so ago when discussed in another thread.

 

[K5lwq]

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

 

[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