front bucket dropping overnight

MtnViewRanch · Nov 28, 2018

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 · Nov 28, 2018

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

Threepoint · Nov 28, 2018

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 · Nov 28, 2018

Threepoint said:
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 · Nov 28, 2018

MtnViewRanch said:
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 · Nov 28, 2018

K5lwq said:
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 · Nov 28, 2018

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 · Nov 28, 2018

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 · Nov 28, 2018

MtnViewRanch said:
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.