Assuming there is load on the cylinder and the piston seal is known to be bad. Reason I ask is I was just reading on insider secrets to hydraulics and the guy is adamant that the rod will not drift, referring to a simulation that shows pressure equalization side to side. Key here is VOLUME of oil. Even with no piston seals at all, if you try to move the rod from one end to the other with both ends blocked, there would be a required change in volume in the cylinder. If that volume is not made up, a vacuum or pressure will occur. If the cylinder is otherwise allowed to "make up" this volume through the valving, drift would occur. I should further expand this with the question of "if the piston seals leak in a system which would cause an increase in pressure on the rod side with compression, will the valving happily accept the change in fluid volume?
I'm going with yes it will drift. The cylinder on our ditching bucket would drift off to one side all the time. Even with the machine turned off and the pressure relieved. Based on past troubles we checked the piston seal and found it was bad.
Volumetrically, that makes no sense, but I am wondering about the reflex effects of system valving, specifically on double acting cylinder systems. I am not sure if the make up valves and such would automatically allow fluid to pass in these situations or not. So the real question is, you experienced drift, fixed the cylinder, did it drift after repair ???
cylinder drift On a double acting single rod end cylinder with a resistive load the cylinder will not drift-you will have pressure intensification as the load will be supported solely by the area of the rod. This same cylinder with a tractive load will drift/extend Tom
If the lines were removed and the cylinder was capped it would not drift, but in all realty on a machine it will drift. The load check should stop the cylinder from drifting on the head end but if the piston seal is leaking then oil will leak past and out the rod end and back to the valve where there is always leakage.
Lee, I hear what you are saying and tend to agree. I should probably get to the point of my question - our CAT 225 excavator had drift in the boom cylinders. Right one would heat up quickly in the center, was not lifting right, etc. We KNOW it was leaking. However, looking at the valving, when the spool is centered, there "should" not be any leakage past the valves so even if the piston seals were leaking, it "should" not drift. The question really is since we KNOW with load on the cylinders, there was bypassing from one cylinder (they operate in parallel) to the rod side, how to heck would this cause drift? I mean what would really change once the cylinders are rebuilt? Load on the cylinder will be pushing on the load check regardless of cylinder condition. Also, would the load check be the most common suspect regarding drift if valving was to blame? I guess that would create the least mess to check but I am still trying to pin down how leaky vs tight cylinders would change that.
Fastline, a double acting cylinder that does not have a holding valve fitted to the cylinder will drift, albeit slowly, if the piston seal is bad, regardless if the control valve spool is centered blocking ports. There is a small amount of leakage that goes around spool in control valve. If the spool was tight enough to have no leakage, it would be so tight you wouldn't be able to move the spool to operate machine.
Let me try to quantify, boom all the way up, stick all the way out, shut engine down, boom will be on the ground in 10-15min. On newer machines, we have left the boom in the air overnight without noticeable drift. We have already rebuilt the boom cylinders but the question now is if we should dig deeper to try and correct or minimize the drift. We need to use the machine for standing some structures where drift can be a real PITA. I should not that drift seems to about the same in the comp/tension loads. It would be great if the cylinder rebuild fixes the issue but would nice to know now if we need to dig deeper. Willie, you mention a cylinder will drift due to leakage past the spool if the cylinder seals leak but why would that change if the piston seals did not leak? There would still be considerable pressure back to the load check and line relief. Curious
Fastline, I'm only saying a cylinder that does not have a holding valve on the cylinder ports will experience a certain amount of drift because control valve components, such as spool, cannot be made so tight to be leakage proof. Most manufacturers allow for a certain amount of drift. With that said, for it to drift so quickly to be on the ground in 10 to 15 minutes would be far outside of expected normal drift. As to which components would be causing your problem, that I don't know as I'm certainly no expert on excavator valve components. I hope you have some luck finding info.
Yes even though your cylinders work parallel to each other, when one has a bad piston seal it doesn't matter it will leak down. If you have rebuilt the cylinders I have seen quite a few people rip a piston seal putting one back together. Twice this year actually.
With an imposed load on the cylinder (the equipment up in the air will do) then with all the controls in neutral the cylinder will drift downwards due to oil passing from one side of the piston to the other via the leaky piston seal. The pressure is not the same on both sides of the piston. A positive pressure will be created on one (bottom) side by the weight of the equipment, with the control valve spool in neutral the pressure on the other side will be as near as damnit zero. With the hydraulic oil at operating temperature and everything in good condition the boom lift cylinder should not drift more than 25mm (1") in 5 minutes. If the cylinder is getting hot and you have already re-sealed it then there is a good chance the barrel is out of tolerance. Did you check the inside diameter when you replaced the seals ..? Maybe it's been honed 030" oversize which is an approved repair technique but oversize seals need to go in it for it to seal properly after repair. It may be more than just a cylinder, but a hot cylinder barrel is sure a good indication that something's wrong.
The system has not been tested with the new cylinder rebuild. I am just trying to avoid hanging the cylinders only to find that I need to drain the main lines again to service the valves. Nige - if the ports are blocked on a cylinder, there will be pressure equalization but there is no volumetric way for it to compress. I guess that is why I am trying to understand where the fluid is going and if the rebuild will actually fix the drift.
You are dealing with an old 225 excavator with dinosaur hydraulics. Just the age of the machine tells me internal leakage was pretty normal when they were new and almost impossible to fix being this old. At this point your cheapest check on the cylinders is to measure the outside diameter of the barrel getting hot. If it is swelled inside, it is also swelled outside. You can also listen for the leakage with a screwdriver or if you are better tooled a stethoscope. I wouldn't be afraid to hold up the boom with a fork lift or something else so you could cap off the piston ends of the cylinders and take loose the rod side. Let the weight rest on the piston ends and see if oil comes gushing out the rod ports. I used to keep a hose handy for running the oil into a drain pan. I don't know of any 225 that had load lock valves on the piston side of the boom cylinders so I would suspect that wear on the spools, load checks and circuit reliefs in the main valve have a lot to do with your drifting problem. Good Luck!
I've sat in many hydralic schools in the last 30 years and every one of them has he same volume displacement theory. But in the real world if that cyl has blown packings that cyl is going to drift. I've rebuilt to many to beleive anything diffrent.
Amen to that. It's a totally different thing teaching the theory to what you find out there on the equipment ........... the theory says that hydraulic oil is a incompressible fluid, the practicality is that all hydraulic fluids have a small amount of dissolved air in them, thus making them compressible to a slight extent. It's difficult to do without a drawing, but I'll have a bash at explaining it. Imagine your machine with the equipment up in the air and the control valves in neutral. Engine can be stopped or running, makes no difference. The weight of the equipment is imposed through the pin at the rod end of the cylinder via the rod to the piston, because they are all connected rigidly together. Therefore the weight of the equipment is felt by the oil in the cylinder that is directly under the piston and so the weight of the equipment is trying to compress that oil. It can't go anywhere because the spools in the control valve are in the neutral position. Meanwhile the oil on the top side (rod side) of the piston feels almost no pressure because the piston is trying to move downwards under the imposed weight of the equipment. Therefore the pressure in the oil in the chamber under the piston is positive whereas the pressure in the chamber on top of the piston (rod end) is at best zero and could even be slightly negative. Remember what I said above about the oil being slightly compressible due to the amount of air dissolved in it. Nature abohrs a vacuum, and the only thing keeping the "high" pressure chamber separate from the "low" pressure chamber is the piston, or more correctly the seals on the piston. Therefore if the seals are not in top condition or if the internal bore in the barrel is damaged or oversize then oil will leak from the chamber underneath the piston to the chamber above the piston via the seals. Ergo, your equipment will come down. Hope that makes sense.
Nige, I understand what you are saying but what you have to remember is the "volumes". As the rod moves from full extension to full retraction, the overall volume of the cylinder (both sides together) decreases thus the oil cannot just "go to the other side" because there is not enough room. I understand what is being said about machines tend to drift regardless of volumetric principles. I know we are not cheating physics here so just trying to understand where the oil is going. Willie mentioned that the oil would sneak by the load check and possibly the spool but if that is the case, the cylinders should drift if leaky or not. I am not trying to be difficult, I am just trying to understand why leaky cylinders would cause drift but tight cylinders would not. Some of the data I have run indicates that if the piston seals are bad, any load on the cylinders is very much compounded as psi in the cylinders. IE, it is very easy to exceed the pressure limit of the relief valve if the piston seals are not working. As load acts on the rod, it acts directly with the rod, NOT the piston. This decrease in surface area with the same load condition will increase the psi (not overall force) in the cylinder.
more cylinder drift Hopefully you have a drawing to use for reference of the the same cylinder set up three different ways. Cylinder A will not drift with a bad seal, you can remove the piston and it still will remain in position- a bad seal will cause a rise in pressure as the rod will support the load. Cylinder B, vertical rod down and the same 20000 lb load, will extend if the psiton seals are bad. Both of the scenarios are supported by the "volume" difference that you have mentioned. Circuit/cylinder C-you can experience drift(cylinder retracting), with even a good cylinder as your pressure relief valves are not "leak-free". In addition to this leak path, with bad piston seals the pressure will rise as the load is only supported by the area of the rod. The pilot operated check valve will open and you will get fluid migrating thru your "blocked center" DCV like Slick Willie already mentioned. HTH, Tom
On a hex the boom head end will use a load check. This will all but stop any oil passing back until the oil pressure is equalized and thus eliminate drift. Now on the rod end there is no need for a load check and only the spool stops the oil, now the spools are not going to stop a drift issue because they are machined to a tolerance to allow it to move. Drift on the rod end is never a problem because of gravity only works one way. Now if the piston seal is bad then it will drift because the head oil will pass by the piston and into the rod end and back to the valve where it will leak past the spool and the boom will drift.
Assuming that your piston seal is leaking there are several reasons why the boom can now drift down. most have been addressed above. 1 Assuming your machine does not have a load holding valve you will always get some leakage past the spool lands. You have to take into account that your static load pressure will now be about 33% higher because of the decreased load holding area of the faulty cylinder. 2 Bearing in mind that now you have the same pressure on both sides of the cylinder so you will have hydraulic pressure on both the upper and lower port relief valve and also both the upper and lower sides of the spool valve, Which probably leak. 3 As mentioned earlier this is an old machine with probably its fair share of wear, So if you add up all the extra places seeing load pressure and the fact that your pressure is in fact higher that normal I think this can explain why you see a big increase in boom drift. I would reseal the cylinder and see how it goes.
My back gruond is ag equipment so forgive me while I butt in with my 2 cents after inflation. Except for special aplications relief valves are on the pump side of the valve, when you close the valve the cylinder is normaly blocked from the relief valve. Some tractors have a cushion relief built into the cylinder ( and possibly a few other aplications), other wise cylinders are not relieved when the valve is closed. You can blame volume cant transfer without leaking past the valve all you want, I have rebuilt leaking down cylinders many times(not as many as most of you) and by fixing the cylinder it held like nobodys business. So much for the oil has to go somewhere else. Again just my 2 cents after inflation.
Ag systems are usually low pressure and have little chance of a large shock load being input into the cylinders by outside forces. In construction and mining equipment which now days operate in the 4,000 to 5,000 PSI range, there is always a circuit relief between the control valve spool and the cylinder. Most now even run a combination anti cavitation and safety valve. The circuit reliefs generally are set about 5 to 10 percent higher than the main reliefs.
Youre right, ag stuff runs about 3000 psi, but mounted implements can go 5 - 10 tons and create high shock loads on ruff terrain, especialy when they can extend 15' and more behind the tractor. That is the reason to put cushion reliefs in the lift cylinders. Lift computers will also shut down in some cases.
What we have is basically a difference in terminology between industries and manufacturers. That is why I try to describe the function of the component rather than being stuck on specific names of the item. My post was only to rebut your statement: "Except for special aplications relief valves are on the pump side of the valve, when you close the valve the cylinder is normaly blocked from the relief valve." What you are describing is the system relief which protects the pump. Circuit relief valves are used in almost all construction and mining equipment in almost all circuits with cylinders. They provide safety for the components in the circuits; main control valve, piping and cylinders. Cushion valves are generally thought of as being used in motors and prevent harsh starts and stops for items that carry a great deal of inertia. Stroke delimber booms come to mind. The cross over reliefs in swing and travel motors in excavators also could be considered cushion valves. I don't want you to think I'm criticizing you on your statement. I just want to make sure that all understand the ideas and concepts being discussed.
John, no criticisium taken. You definetly have a wealth of wisdom, thanks for sharing. Seems like some questions are not answered? Have any of you repaired a drifting cylinder and find that there is virtualy no drift afterwards? If so does it mean that volumetric displacement or lack of displacement exist as, theory, fact, which might exist only on a chaulk board or something else? Can a piston move appreciabaly thru a cylinder with the fluid ends capped?
Refer to the crude drawing I made-I've repaired plenty of cylinders in over three decades. The bulk of my experience is in heavy industry/steel plants-most of the hydraulic circuits have the lockable isolation valves I penciled in (for safety)-with these installed in a circuit and test ports its pretty easy to troubleshoot a bad cylinder-also easy to see the pressure intensification when the load is supported by only the area of the rod. Tom
I think your 3 example sketch pretty much explains it all. :thumbsup Other than slight movement from air bubbles in example A the rod simply can not move.
Yes you will have some entrained air and the oil is slightly compressible (1/2% per 1000psi I think is the rule of thumb). Tom
Most definitely yes, remember you can have pressure equalisation but still have a difference in pressure. nut isn't it but if you take a cyl it has two different surface areas for the fluid to act on as the rod side of the piston is smaller so 6k over 6"dia ............. a surface area of 28.27"= 169,688lbs/sq inch. on the other side it will be 28.27 sq inch -dia of the rod so (1.5x1.5)x3.142x6k= 42,417in/sq..............
If you have leaking cylinder seals I don't believe a difference of pressure would apply. You could remove the piston all together and just have the rod pushing into the cylinder and this would give the same result as a leaking piston seal. Maytag's example in sketch C is more "real world conditions" with pressure boost from the smaller diameter of the rod causing oil to leak past the other components at a faster rate. Changing the leaking piston seal lowers the pressure to normal and the seepage past the control components becomes much more managable.
To throw hard data in the mix for consideration, we got the 2 rebuilt boom cylinders reinstalled in the machine and she does not seem to drift at ALL anymore. Before with the stick extended, it would drift about 2-3in/min at the bucket. So, we know physics laws apply so blocked but leaky cylinders would not drift in compression, also noted above, the load check might totally prevent back flow, BUT where was that oil going? Cannot help but go with my gut here in that the line pressure is actually much higher with a leaky cyl and leaking by the relief. Also - on a tangent, though the cylinders do not drift now, there is an apparent issue with the boom dropped on occasion when lifting the boom. IE, go to lift boom, boom drops a ft before going up. Does NOT happen every time. There is a load check in the main valve and a "boom check". Really not sure where to go on this one but something is not quite right.
The oil that was bypassing the piston would be returning to the tank via the line from the rod end of the cylinder (that does not have a load check valve) and passing through the spools in the control valve to the return line, which obviously only has atmospheric (or tank if the tank is pressurized) pressure in it. Much less than the pressure below the piston caused by the wieght of the implements. Regarding your boom "dropping" phenomenon I would first look at the pilot pressure to the boom lift circuit.
check out http://www.machinerylubrication.com/ it has some good reading
Assuming is where you went wrong. the simulation is correct the rod will not drift, and the adamant guy proves this on the simulation but what he does not tell you and what you assumed is there is no load on the cylinder in his simulation. I emailed Mr Casey with this comment: I understand this concept, and have seen it work, but the cylinder in question does this and continues to creep until the digger bucket is on the ground. The condition of the bore is good, seals and seal grooves are good, and no extrenal leaks. The cylinder does the same thing whethter the hoses are connected or if the ports are capped.. Any insight would be greatly appreciated Richard, If it is rod down with a load hanging on it, it is likely it will continue to creep. Regards, Brendan Casey
Welcome to HEF HeavyMan. I'm not sure who you are addressing but there are more scenarios in this thread than the one you are asking about. Rod Down are the key words here. Capping the cylinder off to take the valves out of the picture, the rod will creep out. It will also create a vacuum and air pocket in the cylinder. Rod Up , totally different outcome. You could remove the entire piston and the rod will not drift because the rod is trying to decrease the area inside of the cylinder.