I've been considering posting this for quite some time, but I've been reluctant to do so because it's going to disagree with what many people think about hydraulic systems, more specifically, hydraulic cylinders and diagnosing problems with them. Nevertheless, I decided it's necessary to discuss this subject because it's just wrong for me to allow a misconception to remain and ignore the facts of mechanical physics. My intent is not to expose any individual as being wrong, rather, to increase the understanding of all persons, whether they are a novice or a professional. It's my opinion that understanding how something works is one of the most important factors in troubleshooting and repairing mechanical devices. Hydraulics has many qualities that makes it an excellent method of producing mobile power to do work using a number of actuators, such as motors or cylinders, making machines that allow you to not only have speed and efficiency, but still allow very slow and fine precise movements with the same machine. One attribute of hydraulic principle that allows you to do these things is because fluid/oil under pressure (via a pump) does not compress, instead, it displaces. The fact that this physical property exists is an important fact in understanding how a hydraulic cylinder works and how it relates to a failure condition that is frequently experienced, that is, hydraulic cylinder creep, drift, or movement when the cylinder is not in use. In many cases, someone will diagnose the problem as failed piston seals in the cylinder. In some situations this may very well be the case. In others, it's all but impossible. This is not theory, but indisputable fact. Which is why I felt it was important for us to discuss this, because when someone encounters a cylinder drift problem, it's not always accurate to simply diagnose the problem as failed cylinder piston seals. So I would like to point out a few characteristics of hydraulic cylinders, what's going on inside them, and how it relates to cylinder drift. Keep in mind, a cylinder is just a single part of a hydraulic system or circuit, and there are various methods and componets that control the operation of any given cylinder depending on the application, so the things I'm disscusing about them are in general, they do not apply to every situation, one must consider all components of a cylinders hydraulic circuit to properly diagnose problems with a cylinder. I'm going to start this discussion talking about the scenario where it's all but impossible for piston seal failure to cause a double acting cylinder to drift. To understand how this is possible, the very first characteristic about a cylinder we must discuss is probably the most simple, but at the same time the most important. Below is two diagrams, one shows the cylinder fully extended, the other shows the cylinder fully retracted. When the cylinder is fully extended is when there is the most area inside the cylinder because the rod is outside of the cylinder, it's simply a full open area inside. When it's fully retracted, the rod is now taking up space inside the cylinder, so the area inside is less compared to fully extended. The result is the cylinder holds more oil inside when fully extended, and less oil when fully retracted. The numbers (quantity) in this illustration are for comparative purposes only, have no math or method behind them, but to make all this easy, let's just say it's a 2 to 1 ratio difference of oil quantity inside the cylinder between fully extended and fully retracted. With that fact in mind, let's discuss the situation where it's all but impossible for failed piston seal to cause drift. And there are applications where this attribute is not only desired, but rather a necessity. Applications such as crane booms, telehandler booms and fork carriages, and aerial manlifts. If sudden failure of piston seals could cause drift on these types of machines, the results could/would be bad to say the least. When you put a crane boom in the air...it has to stay there. This is done by taking advantage of the inherent properties discussed above. 1) fluid/oil does not compress, it instead displaces, and 2) there is more oil inside an extended cylinder than a retracted one, difference in area. Now, how do we take advantage of these properties and make a double acting cylinder to where it's not possible for seal failure to cause drift. To do so requires blocking the ports of the cylinder. This is typically done by using pilot operated holding valves, but that's a topic for a later discussion. For now, let's keep it simple. Just imagine the cylinder has shut off valves at the ports like in this illustration showing the valves closed. Now, let's make the cylinder operate. We open the blocking valves on the cylinder, and operate the control valve to extend the cylinder. This forces blue oil into the cylinder via oil being delivered by the pump. Now oil is being pumped in to fill the piston end of cylinder, it encounters the piston seal and can't pass by it. Pressure builds and the oil displaces the piston since it's the only movable part of the cylinder and it forces the piston/rod to extend. At the same time, the green oil on the rod end of cylinder is being displaced from the movement of the piston. The blocking valve on the rod end is open, and the displaced oil in the rod end simply gets sent back to tank through the control valve. So, at this point, we have the cylinder extended half way out. Working with the 2 - 1 ratio mentioned previously, then let's say there's 2 gallons of blue oil in the piston end of cylinder, and that there's 1 gal of green oil in the rod end. Let's close off the blocking valves at both ends, and remove the piston seal, simulating piston seal failure. Now imagine this is a crane boom cylinder with the rod holding the boom up. At first glance, one would think the weight of the boom would force the cylinder to retract since there is no piston seal now. The fact is, it won't. With the cylinder ports blocked by the locking valves, the cylinder can't retract. With the ports blocked, the oil is trapped in the cylinder, it cannot escape and go back to tank, or anywhere else for that matter. Remember, fluid/oil does not compress, and it displaces. Since there's more oil (2 gal) on the piston side of cylinder, and the rod side can only take 1 gallon of oil in this position and not a drop more, there's no where for the oil in the piston side to go to, there is no where for the oil to displace. The rod side of the cylinder already has all the oil it can handle. Look at it this way. Let's say the piston moved just one inch and retracted the cylinder. And in doing so, the rod side would take in 1 ounce of oil by the piston moving one inch. Well...2 to 1 ratio, remember? This would mean the piston side would displace 2 ounces of oil when the piston moved one inch. Therefore, if the cylinder, with no piston seal, were able to move one inch, the piston side would have to displace 2 ounces of oil into the rod side, which could only take 1 ounce of oil within that one inch of movement. See the problem...there's no where for the additional ounce of oil from the piston side to go.
Still trying to get your head around displacement? Let's try this; a single acting cylinder from a typical warehouse type straight mast forklift. Many of these cylinder designs don't even have a piston, or a piston seal. They only have a flange on the inside end of the rod to bottom out on cylinder gland so rod can't spit out of cylinder. The seal in the gland of the cylinder is the only thing that prevents oil from escaping. Now, let's operate the control valve and pump some oil into the retracted cylinder, and it's going to go in there. Since more green oil is going into the inside of the cylinder, something has to displace. Well, the body of the cylinder is solid, it won't move. Likewise, the gland won't move. The only thing that will move (displace) is the rod. So adding oil to the cylinder displaces the rod and makes it extend. Then, when you stop extending the cylinder by closing the control valve, the oil is now trapped in the cylinder, and is now being held in that extended position. No piston, no piston seals, basically just a rod in the cylinder, load can not make cylinder go in. For the rod to be able to go in, oil has to be displaced and there is nowhere for oil to go since it's trapped. This is the same effect as would happen in a double acting cylinder that has blocking valves (holding valves) on the cylinder ports if the piston seal was to fail, cylinder will not drift because there's nowhere to send the displaced oil. This principle applies to cylinders that don't have holding valves as well, such as the boom lift cylinders on many excavators that raise the boom by extending cylinders. The control valve acts as a holding valve when the cylinder is not in use because it blocks the internal tank ports when the valve is in the center position. This is how you can raise the bucket off the ground by raising the boom and shut the engine off. If the control valve didn't block the ports, the boom would simply drop and the bucket would be back on the ground. There is some internal leakage by the spool in the control valve, but it is minimal, not enough to cause a noticable drift. So if you had a failed piston seal on the boom lift cylinder of an excavator with no holding valves, the cylinders would still hold the boom up because the control valve blocks the ports. If you had a boom drifting down noticably, you'd more likely have a problem with a circuit relief valve or anti-cavitation valve in the boom section of the control valve. On the other hand, there are applications where cylinder drift can be caused by failed piston seals, such as the bucket roll cylinders on a Bobcat skid loader. In this application, the load of the bucket is pulling the cylinder out, extending it. In that case, a failed piston seal allows oil to transfer from the rod side of the cylinder to the piston side, which will hold more oil than the rod side, the excact opposite of a cylinder under retracting load. Well, that covers a few basics of hydraulic cylinders and their operations, there's a whole lot more that's not even covered here concerning the various valves and cofigurations of controlling a hydraulic cylinder. The point is, one must properly test circuits and components when diagnosing cylinder drift as it's not always simply failed piston seals.
good job Atco you put a lot of work into that posting-I fully agree with you in princple except for the "fluids not being compressible"-fluids do compress, some worse than others-we do a lot of fine position hydraulic control applications-the resistance to hydraulic oil compressing is referred to as "bulk modulus" and it and entrained air in the oil have to be taken into consideration. Take a hydraulic cylinder and line that may hold 25-50 gallons and the entrained air alone can be 3-5%-this and the fluid compression can cause a lot of grief, cylinders drifting enough to lose a limit switch and shutting down a process. Tom
You're absolutely right maytag. Like I stated previously, the discussion was "generally speaking" and using terms such as "does not compress" to keep the conversation in terms the are easy to understand. There are so many technical variables to account for in detailed design and engineering that including all those details tend to make the simple discussion much more complicated. It may be of value to continue on with technical details within this thread. We all may glean some good info from it.
carry on you got the ball I do quite a bit of fluid power classes, where you "removed" the piston seals, I'll remove the piston. People are amazed when you do the math for pressure intensification and rod gland failures. Occasionally I'll get an engineer that can't believe that everything that was drawn up by Rexroth or Vickers or whoever doesn't work in the real world-usually entertaining. Hopefully others will join in the discussion. Tom
LoL, yeah, like, eyes pop out like a stomped on bullfrog! I really considered including the compressive variable, but the more I thought about it, the subject is already complex enough while attempting to keep it simple. More and more details only add layers of detail that tend to twist the brain in knots. I do realise, however, that it would have probably been better if I had prefaced the "does not compress" factor by saying something like the compressive properties of oil are mitigated during pumping oil into the cylinder causing movement at which point max compression threshold is achieved. Naaaaa, too much gobblety goop! :tong
mis information You see a lot of confusion/bad info in heavy industry too-seen several expensive valve stacks replaced when they had a bad cylinder. Tom
Excellent tutorial ATCO:thumbsup Yes there is quite a pressure spike on the rod gland of the cylinder when the piston seals start leaking past on a stationary cylinder like an outrigger or crane boom.My outrigger cylinder on the crane is 6" in diameter and the rod is 3".Let's say the outrigger has 50,000 LBS on it.Pressure =Force"lbs" divided by area"inches squared" .That puts pressure around 1769 psi with the 6" piston.Now lets say the piston seals are leaking past and do the math on the 3" rod .Wow..That put's it at 7,077 psi .Sometimes we get an outrigger rod seal leaking abit and I always attributed it to the piston seal leaking past & spiking out the pressure.Who knows.
Nicely done Atco. Q.E.D. quod erat demonstrandum
Years ago when going through those hydraulic classes we learned about those principles and being told that it was theoretically impossible for drift when the weight was being put on the piston side of the cylinders. However after these many years in the industry and having actually experienced the problem more times than I can remember, I can relate a few little points to the discussion. First item is that all cylinders are designed with a small percentage of leakage by the packings. If that leakage didn't exist the packing would burn up from the friction of sliding against the barrel. I have seen that in the form of dieseling and charred packing rings from freshly rebuilt cylinders. That same principle of leakage and lubrication applies to the control valve spools. The second point I can make is that being that some leakage is inherent, the cylinder will drift down until the pressures on each side of the piston will equalize. Now if you were to open the valve on the rod side of the cylinder, the piston would shoot out for a second. That is because there is more surface area on the piston side of the packing than on the rod side. On old systems with manual control valves you could check this by picking up the weight, shutting down the engine and waiting some minutes until creep is seen, and then moving the control to the down position. The boom would jump up slightly and then go down. My third point has to do with the equalization of pressures on both sides of the piston and I've never worked out the math but here is the premise. Once the pressures are equal the problem becomes one of displacement. So since as oil leaks from the piston side of the cylinder to the rod side, the piston has to move down. As that piston moves down the displaced oil fills rod side and the pressure is kept equal. Intuitively then the piston must reach an equilibrium where it will no longer drop which I believe is true. However the cylinder has a specified total amount of fluid it can hold. My thought is that now the piston can go down as long as no new fluid is added to the cylinder. You see the amount of leakage will equal the amount of drift in the cylinder as long as the total amount of fluid remains the same. My last point we have to consider is the other components in the circuit which would include the control valve. Remember it also has to have a specified amount of leakage. When all the pressure is on one side of the cylinder you have the valve leaking a small amount. When you equalize the pressure on both sides of the cylinder and thus both ends of the control valve spool you essentially double your leakage rate. Multiply that times the number of hours of use and the natural wear of hydraulic components and add in any bad maintenance and you will see there are lots of places for oil to go when associated with blown packing in a cylinder. Now days anything used to suspend a load is required to have load locks. When I first saw them installed on machines they were always installed on the cylinders. What we were told is that should a hydraulic hose break the load would not crash down on someone. Now days they are installed on the machine behind the cylinders with a hydraulic hose between it and the cylinder. Basically they took the leakage factor out of the control valve spools. It took down pressure from the pilot system to open the load lock in order to let the oil out of the piston side of the cylinder. There are other problems that came from that but I'll leave that for another discussion. Nice job AtcoEquip
TD25C, The total surface area of the piston is 28.26 inches squared. The total surface area of the rod side of the piston is 7.07 inches squared. 28.26 minus 7.07 equals 21.19 inches squared. If you equalize pressure on both sides of the piston you get 37,491 pounds of down force on the rod side of the piston. That gives you 12,509 pounds to spare in holding the piston in place. But now you have 1769 psi of pressure against the rod side of the control valve leaking off. I haven't seen an outrigger in years though that didn't have a load lock. Great thought.
Nice job Atco
Hee hee...see how complex this stuff can get? Great posts guys about the technical details guys. My original intent to making a thread about cylinder drift was on occasion someone would make a post asking about a cylinder drift problem they're having. It would not be unusual to get a response that the cylinder piston seal is bad. Knowing that's not always the case, it's kinda hard just to respond back by saying "that's not your problem" without being able to explain why. And it's just a wee bit hard to fit all this stuff in a simple reply. So I wanted to post a thread about the subject (figured I'd step out on the limb), keep it as basic and simple as possible, so there would be an information resource available to show why someone needs to sometimes look at other possiblities and not just simply say it's blown piston seals.
info As some of you know I deal with hydraulics in heavy industry I'm not well versed as to the "norm" in the mobile applications. What type piston seals are the norm for most of your cylinders? Tom
It really depends on the application maytag. Machines like excavators that have to deal with shock loads commonly use a square cut urethane ring with a square cut (lathe cut) rubber loader in the ID of the urethane ring. These cylinders will typically use a buffer ring in the gland to protect the u-cup seal from shock load and have even seen white iron rings on each side of the piston seal to protect piston seal from shock load. Other machines, like manlifts, that don't experience the shock loads as much will use other types of 2 and 3 piece seals, like crown seals, TP seals, and such. Occasionally, you run across a configuration that uses opposing U-cup or Polypack seals on the piston, but not often, as you have to deal with trapped oil between the two seals.
more cylinder The opposing u-cups is what is the norm for most of the industrial applications I have dealt with, occasionally we'll have a steel ring assembly where high temp is a factor-I was curious if the steel rings were used in your world-apparently not. Had a little entertainment the other with a OEM rep from a German manufacturer, this is a 2 cylinder circuit, cylinders in parallel, mounted rod down with an overhanging load, the cylinders were drifting and losing the prox switch. When the line wasn't running we checked and sure enough a new cylinder was bypassing-the rep couldn't believe it and was trying to convince me that 1 cylinder would hold the load-when I sketched out the circuit and drew the bypassing cylinder as an open valve between the cap and rod end a light bulb light up and they had the bad cylinder repaired. Takes all kinds, funny that if you want steel mill equipment you go to the old Axis powers from WW2. Germany has SMS, and Voest Alpine, Italy has Daneili and Japan has Kawasaki, nobody else can build a complete mill. Tom
Nice article Atco- well written. An interesting displacement case to think about is a closed system like the damper cylinder on a tilt deck or a steering stabilizer. The oil is trapped inside the cylinder and transfers through a hole in the piston from one side to the other as the deck tilts up or down. When you re-assemble one, understanding displacement let's you figure out whether to fill the cylinder with the rod fully extended, retracted, or somewhere in between.
Now that we have discussed some of the workings of a cylinder, now would be a good time to talk about another component of the cylinder and how it works, the piston seal. Contrary to what one might think, the piston seal is not fitted in such a way that it's excessively tight when installed in the barrel of the cylinder. If it were, there would be excessive friction against the seal as the cylinder moved in and out. Here's a diagram that shows a typical square cut piston ring with an o-ring loader under the ring. When the piston is installed in the cylinder barrel, the seal ring just contacts the inner bore of the cylinder, it's not fitted extremely tight against innner bore. In order for the seal ring to make a tight seal it needs a little help from pressure. When oil enters the cylinder from the pump, it encounters the load that's bearing on the cylinder and builds pressure to move the cylinder. This oil pressure goes between the piston and the cylinder bore and encounters the piston seal and the o-ring loader. The piston seal is fairly rigid, doesn't distort much. On the other hand, the o-ring loader is the more pliable component in this area. The oil pressure bears down on the o-ring loader and distorts it's shape, sort of squashes it. It's very much like if you were to place a balloon on the ground, then place your hand on top of it and start pushing down on it. As you pushed down, the sides of the balloon would start expanding outward equally. Just another example of displacement. So as the o-ring is being displaced, by the pressure changing it's shape, the o-ring's shape can't go inward because it encouters the piston groove it's fitted in, likewise it encouters the wall of the piston groove on the backside. So the only direction the material of the pliable o-ring can be displaced is outward against piston seal. This displacement of the o-ring pushes against the piston seal ring and forces it outward against the bore of the cylinder. The more pressure is increased inside the cylinder, the more the o-ring loader bears against seal ring forcing it against cylinder wall. This priciple of displacement happens to nearly every seal throughout the hydraulic system. O-rings in nearly every fitting seal using this principle. That's the reason why an o-ring will begin to leak when it gets hard from long term exposure to heat, it's no longer pliable and loses it's ability to displace when pressure bears against it.
I am extremely impressed with this thread. It shows superb knowledge, and considerable work.
OK if I understand this correctly a hyd cylinder can drift one way (rod going out) but not the other (rod going in) if the cylinder ports are blocked? Which means that in order to establish if a piston seal is bad, you need to pull on the rod and block off the cylinder ports? The reason I ask (apart from curiosity) is that the backfill blade on my Kubota KX91-3 drops at a fairly quick rate (it takes 20-30minutes to fully drop), now if I where to raise the machine with the blade then it should not drop unless there is a problem with the control/relief valve? BTW do tipper hydraulic rams work by filling the sections with oil?
Your backfill blade dropping is pulling the cylinder out. Fairly easy to test if it's a piston seal problem Hendrik. Raise your backfill blade fully, then stick your digging bucket under it to hold it up. Disconnect the hose connections to the blade cylinder and block the ports with appropriate plugs/caps. Pull your digging bucket out from under the blade and monitor the blades movement. If it drifts down, got bad piston seal. Not sure what your asking about the tipper rams.
Great post ATCO, best explanation I have ever read. Should keep lots of guys from rebuilding cylinders for no reason when the problem is in the control valve/ relief setup. Hopefully someone will make this thread a sticky. I think he is talking about the hoist cylinders on a dump truck, I believe that is what he means by a tipper.
One thing I have noticed on front end loaders is that the boom cylinders tend toget air trapped on the rod end. This is because the rod wiper is only to keep the dust and dirt out and gland packing is to keep oil in the cylinder. typically when the boom is lowered quickly, in spite of recirculation/anti void circuits in the valve when you drop a boom you have weight pushing down on the cylinder so it will retract faster then the pump can fill the rod end, drawing a vacuum on the rod side and some air can get pulled in through the gland. Gradually you will get a pocket of air in the rod end that can't get out, it will get compressed and pushed down the rod side hose or tube but not far enough to get through the valve and dispersed into the tank. To prove this drop your boom to the ground then try to lift the front end of the loader off the ground. If the cylinder rod end and supply line was full of oil and no air, the machine would start to rise almost instanly. The same effect is on a tractor/loader /backhoe when you put down pressure on the boom to raise the back of the machine. If the cyllinder packing is bad the boom will drift down untill the bypassing oil can compress the air in the rod end. The typical complaint will be that when you raise the boom with a load and let go of the lever, the boom will drift down if the packing is bad, but the operator will raise the boom back up and it will stay in place because the air in the rod end is already compressed, however if the operator just lets the boom continue dropping, it will stop when the air in the rod end is compressed enough that there is no more room for more oil.
But according to all the pictures and diagrams one would be LED TO BELIEVE that it would not fall because of inability to make up the total internal volume differential. Yea I'm either a wise arss or just don't understand. Oh, and what about the overpressure relief's built in to some of the pistons? I am soooo confused some days.
It will only fall enough to bypass enough oil to compress the air trapped in the cylinder, maybe 6 inches or a foot or could be more or less. It all depends how much air is in the cylinder. The pictures do not allow for air trapped. ATCO has done a great job explaining the process in theory as he explained at the beginning, There are always circumstances that modify the theory. All that I am doing is injecting a circumstance that you might run into in the real world. The one statment that can bite you in the *** is NEVER CAN HAPPEN.
Great discussion started by ATCO and great job with the visuals. One of the things I have found was when dealing with cylinder drift issues. Damaged circuit reliefs ( line reliefs) and main spool valve damage and this was causing the cylinder drift but the line reliefs were getting overcome by the pressure intensification in the cylinder by the piston leakage or the reliefs damaged by foreign material. The foreign material was coming from the cylinder barrel and piston do to a loosened piston nut or damaged barrel wall. I agree with you totally that cylinder drift issues can be wrongly troubleshot as piston seals leaking but pressure intensification in a cylinder can overcome the limitations of the circuit reliefs and control valve spools also if there is enough weight pushing down on the rods of the cylinders.
Most earthmoving equipment does not use cylinder lock valve so drift will happen. Cat has specs on how much is allowed and it is normally something like a maximum 5mm of drift on the rod in 5 minutes. With a good machine there is usually no noticeable drift within that 5 minutes. Now when it does drift but within spec, it is the oil that is supporting the load leaking off back to tank through the machining tolerances of the valve, spool and seats of the load check. Drift that is out of spec and is too much can be caused by a worn out cylinder piston seal or even the barrel. This allows oil to return to the valve via both ports and now doubles the allowable calculated leakage. There is alway a possibility of it being other things like, line reliefs, load checks, ride control valves, makeup valves etc. Here is how i check a cylinder to see if it is causing the drift. Move the cylinder to roughly the middle of its travel, support the load/boom/loader arms with a crane. Now disconnect the hose on the piston/barrel end and cap the cylinder and hose immediately. Now remove the rod end hose and cap the hose but leave the cylinder un capped. Lower the load with the crane. If the cylinder is good it will support the load after it settles a little bit. If it is bad it will retract and oil will flow out of the rod end port because oil is bypassing the piston. If there are two cylinders like boom cylinder on an excavator, i do one at a time with the other left open or unpinned so not to interfere with the test. BTW, oil does compress, 0.5% at 1000psi if i remember correctly
Graders will use cylinder lock valves on the moldboard and wheel tilt functions while most modern day excavators use a lock valve for the boom circuit thats incorporated into the main valve body.
I have worked on cranes and manlifts since the early 80's, aerial type machines have been using holding valves for years. Like I stated earlier, when you put a hydraulic crane boom in the air...it has to stay there. :yup Worked on a Bucyrus-Erie 45C 30 ton hydro once that had a hose come from the base of each boom lift cylinder to a holding valve mounted on the structure base behind boom cylinder mount pivots. B-E installed a velocity fuse in the fitting of the boom lift cylinders to prevent rapid boom drop in the case that the hose failed. Oh yeah, for Hendrik, If you are in fact referring to a dump bed cylinder, maybe even a multi-stage cylinder, all the ones I've been around work by filling them with oil.
A+ for atco
So the potential for them to come down in a hurry, if a hose or seal on the multi stage blows, is there. Obviously I only get under a raised tipper/dump bin when it is empty and blocked, with the weight being supported by the block.
Oh yes, the potential is there for sure on those units. Not sure how they build them in Oz, but here in the states, most dump beds I've worked on are gravity down cylinders. Simply move the spool on the dump control valve and it comes down, even with engine not running. Hose or seal failure would do the same thing.
Most ADT's have to have down pressure on the dump cylinders for two reasons. 1. At full tip the bed will go over center and require rod end pressure to pull the bed past the over center point. 2. The situation is aggravated if you get a pile of sticky material stuck on the tail end of the bed at full tip
Well I knew this already but I wanted the pros to clarify that if you get under a tipper/dump bed you are dicing with death. No safety or second chances. Work hard and work safe.
And I just want to say that working on hydraulics requires a fair degree of knowledge, equipment and skill. Hydraulic fluid under pressure can puncture your skin and kill you. If you are not sure..........don't touch it.
Bucket Tilt Leaks Down! Please Help I have a bobcat s220 and the bucket tilt falls on its on and it does it on or off. both tilt cylinders are rebilt and still have same problem.. I have did everything possible looking for a leak and cant find one.. i dont have a schematic or service manual.. so im looking for some input on what to check next. thanks for any help
Most likely a circuit relief bypassing. Your tilt circuit should have a circuit relief in the control valve at each end of the tilt spool. Other possibilities are a broken spool, or crack in the valve body. Don't know how your machine is set up but the above should be valid unless you have a remote relief somewhere.
You should probably move this thread to the Skid Steer part of the forum for more input.
Hi Gents. 1st post on here, I found this forum while searching for an answer to a technical question that has been bothering me all day. I am hoping one of you kind gents can shed some light on an issue I am having with a truck mounted boom lift. The machine has twin lift cylinders and the main boom is dropping slightly over a period of time. Measured drift is approx 13mm (1/2 inch) over a period of 3 hours. The drift starts off faster and slows down as time goes on. You would think that this level of drift is permissible however when this machine is used for media work with a camera man in the cage, the cage levelling can go out which can cause problems, also, at maximum outreach the machine could possibly creep out of it's working envelope. The cylinders are double acting and have pilot operated holding valves. These were replaced yesterday and we still have cylinder drift. The port on the rod side of the cylinders has been removed and ball valves and sight gauges fitted to the machine (filled with oil) to try and determine if the cylinder seals are passing from the piston side to the rod side. The oil in the sight gauges was sucked into the cylinder, which to me suggests that the oil in the piston side has compressed. My question relates to aerated oil. Have any of you ever come across an instance of cylinder drift happening because of aerated oil? I have never had a problem before however I am wary of the fact that there's always a first time. We have had the pipe off one of the cylinders and operated the machine to gauge how much air is in the oil going into the cylinders, we have noticed bubbles however if they are a factor I don't know. The twin pumps for the machine are mounted lower than the tanks so I suppose that there is a possibility that one or both pumps could have an air leak on the suction line side that is aerating the oil, however I can hear no pump cavitation noises so I have ruled them out up to now. The manufacturer of the machine reckons that one cylinder alone is enough to hold the machine up and static, and during tests today I did notice drift in 2 other cylinders which lends more weight to the theory of air in the oil however as I said before I have never had that causing drift. The question I have is have any of you ever had aerated oil cause this to happen? I have had pumps on machines knock as they are priming however they have never caused a machine to drift before. Regards
Yes aerated oil will cause cylinder drift. The air in the oil compresses at different rate than the oil and will cause drift as well as damage the pump(s) it can be a slow process to bled the air from the system
And I would be curious as to what is causing aerated oil in the first place, that's not normal.
Check out the shaft seals on the pumps-I've had them go bad and suck air in to aerate the oil. The pump does not have to leak oil to suck air in but have seen them leak oil when not running. Good luck in your endeavor, Maytag
Thanks for the replies gents. Apologies for the delay in responding. I was convinced that the only way for air to be entering the system would be though somewhere on the suction lines to one of the 2 pumps. I made a point of checking the hose fittings and clamp bolts on the suction lines first thing Thursday and got a nip on most of them (they weren't too loose however I reckon I got a turn or 2 on most of them). Worked the machine for 20 mins or so then set it back up in the same position and started testing for drift. Test results were much better (4.5mm in 3 hours) so we are thinking we may have sussed it. We then operated the machine for a good 2 hours to get it warm and retested. The drift was back and worse than before! I'm still thinking it's an air leak on the suction line (maybe one that opens up when the oil gets warm). Each suction line has an SAE fitting with an O ring on it at either end held on with 2 half moon clamps, and a ball valve by the tank end. I have known air to get in the suction line through a leak in the ball valve spindle before now. Maybe some sort of smoke test near the suction lines will show a result? I'll keep you posted
The return to tank situation might well be worth checking out, it needs to be away from the oil feed tap and if possible piped low into the tank to limit frothing, HV 46 oil is a very stable bit of fluid to use Winter-Summer, the only thing that will compress in the oil is air, oil will not compress it acts as a solid media in the Law of Hydros (Greek Stuff) it is usual to find on Boom cylinders an amount of lift in hight when left in 1 position due to Sunlight heating up the cylinders, are your pumps load sensing V.P.S.P or just a direct pump set up like a bent axis piston pumps or gear pumps??? do both pumps draw from the same tank or is there 2 tanks with a ballance line?
Hi TC Thanks for responding. Tank is massive for the application (450 ish L). Machine is using standard 32 grade hydraulic oil. 2 Rexroth piston pumps (pressure compensated). as soon as Hi rpm is selected the compensators move the swash plates one way which brings system pressure up to 200 bar. Idle pressure is approx. 20 bar. (adjusted on the pumps). Tank has a baffle plate in the centre with the suction lines on the opposite diagonal to the return line. We have had the access plate off the top of the tank and sometimes at full flow you can see small bubbles coming back into the tank. However it's on the opposite diagonal to the suction ports! One tank for the machine, 2 main pumps and one small electric pump in case of main engine failure (main pumps PTO driven, back to back) Can't see any froth in the tank, just what I'd call small bubbles in the oil (like what you'd see in flat lager or cider). Regards Phil
i am glad this just came to the top of "shop talk" i love this stuff thanks atco for this great post even though i am 2 years late..............theironoracle
You're quite welcome theironoracle, but one is never too late to noodle out how things work. In most cases, understanding theory of operation plays a big role troubleshooting problems.
Great explanation Willie! Phil I if its just bubbles like a flat beer I doubt its enough aeration to cause the type of drift your describing.
Phil, if the oil return to tank is good the only logical thought is that air is entering via the line from Tank to pump inlet, in the U.K. we tend to only use the Pressure Compensated set up to adjust the Delta P of the pump/s with the Load Sensing system being the prefered method to lift (activate) the swash of the pumps, on the Cranes that I install I also set into the ECU a warm up speed , 4 mins at 1200 rpm, then low idle speed around 700-750 rpm and 2 other speeds upto max rpm oil flow checked not max engine speed asI find using Load Sensing does not need top engine design speed, the boom hoist cylinders are usualy way up to the task of holding things up, I have more trouble with the extension side (Recycling Vave) on the stuff I work on, as your crane has a 2 pump through drive it would possibly have 2 oil feeds from the tank?? are both pumps making Froth? is it possible to swap around pipe work on pumps? tctractors
What an excellent discussion to discover. One question I have, and I qualify this by saying that I haven't done the sums, but, in the circumstance where we assume the piston seals are cactus and we're assuming that the load is being supported by the difference in displacement between the above piston space, with rod, and the below piston space, without rod; surely there's potential for fairly severe amplification of pressure? It seems to me to be a form of leverage and I can imagine that in many circumstances this could be enough to open a pressure relief valve and allow the load to drop. The point I'm making is that whilst theory says that dodgy piston seals can't allow the load to drop (without squirting oil out the rod seals!), surely they can cause excessive pressure in other areas which will result in the same thing?
Good point nutwood. And as you stated, one would actually have to work out the math, but pretty much anything is possible. The reasoning for me posting this info was not to imply "this is the way it is and no way else". On the contrary, it was a response to so often when an individual is experiencing cylinder drift that they automatically consider it to be failed piston seals. That was my main point, that this is not always the case, and I attempted to present an argument that supported such. Yes, things can get way more complex than the situations that I described, which is all the more reason that one should do a little troubleshooting before they pull a cylinder apart because it's leaking down.
Nutwood, you are exactly right, when a cylinder changes from a piston type cylinder to a displacement type cylinder due to the piston seals leaking pressures can increase by orders of magnitude. If a cylinder has a 4" piston, the area of the piston is roughly 12 square inches. A 12,000 pound load would create a 1,000 psi pressure on the piston side of the cylinder. If the same cylinder has a 2" rod, then the displacement area of the rod is roughly 3 square inches, if the piston seals are bad then the same 12,000 pound load would induce about 4,000 psi on BOTH cylinder ports, piston and rod end. If there are no relief valves opening to tank, then damage can occur to the cylinder tube, rod packing, hoses, steel lines, or even the control valve. Note that a crossport relief block won't help at all because the pressures are equal on both lines due to the piston seal leakage. Sorry, just felt like preachin' for a minute I guess, lol
Not preaching, good stuff! Thinking about this, and considering that few seals are absolutely 100% tight, I'd suggest that this potential pressure amplification is a really good reason not to leave a load on a hydraulic cylinder that's not been designed for continuous load holding. I've always considered it bad practice but I've never considered that, combined with worn piston seals, you could be seriously stressing other parts of the system. Fascinating stuff!
When I tried to explain some of this stuff the other day, the guy politely told me 'all you need to do is remove and re-install the cylinder the other direction and that will show you it doesnt matter'.... No need to comment, stuck this in here for the pure amusement.
A customer the other day was telling me about a cracked boom on his 3000hr 324d cat excavator. I have seen plenty of cracked booms but this was the most premature I had ever heard about. I know the conditions this machine has worked in since new and it has only been standard use. Because of this thread I got to thinking what could have caused this? My conclusion that perhaps one of the cylinders had bad piston seals. So I explained to them over the phone to lift the bucket off the ground and see if it settles then stops (equalizing pressures in all four fluid zones of cylinders). Sure enough they said it dropped slowly then stopped! Next I instructed them to safely isolate each cylinder and determine which one allowed the drift and which one had no drift. They identified the bad cylinder and repaired. Thanks to all who post high quality info on HEF, I love the theory of mechanics.........TIO
Right on Don. :yup Which is why crane and aerial lift cylinder designs have to account for this and be designed strong enough. With cranes (that support loads over persons/property) and aerial lifts (that support the persons themselves over people and property) you simply can't have failures like this. Result would be major damage and/or people dead. Exactly, never argue with an idiot...they will defeat you with superior ammunition. It is some awesome info that folks have posted here TIO.
Started the thread thinking, "huh?" Then, reading more of the posts, and thinking about it all it made sense: the rod changing the volume of the barrel. I think the single-acting forklift example brought it together for me. Thanks for the post!
Great thread willie59. I have one piece of equipment that uses the displacement cylinder princible. The old Shaver post driver,mine has about a 2 inch cylinder bore using a 3/4 inch dia rod. Instead of a piston & seals it has a finned guide at the bottom of the rod that lets the oil pass by it quickly allowing the H beam hammer to drop fast.So it is using the 3/4 inch displacement to lift the 300 lb hammer. www.shavermfg.com/standard-post-drivers
Iron Oracle: Great bit of diagnostics there, the twin boom cylinder situation never occurred to me, the racking must have been intense with one cylinder carrying the load!
I found this presentation of the very thing you are discussing on this thread. http://www.hydraulicsupermarket.com/flash/cyl_drift_video.html it might help with the more "visual" learners.
Good find, pajibson... :thumbsup Here's some more from HydraulicSupermarket.com ... I posted some of this years ago; I suppose it's in the Archives by now... lol Anyway, there's some good reading... :cool2 Hydraulic technical library: http://www.hydraulicsupermarket.com/technical.html Hydraulic product library: http://www.hydraulicsupermarket.com/products.html Brendan Casey's blog: http://www.hydraulicsupermarket.com/blog/ Really, bookmark it, and reference it every now and then... :yup OCR
Its a really a great post. learned many things about hydraulic cylinder and drifts and learned about structures of their. Operations also help me out in better ay to understand more deeply about them.
I don't understand why a bobcat roll cylinder is possible to drift but a crane or excavator doesn't. is it because of weight ??? because a excavator bucket has weight also as well as the boom
Read this article and there is alot of great information, being that I am really green when it comes to hydraulics. hopefully someone can give me some insight being that I'm a rookie:/ I have an aerial forklift with parallel hydraulic cylinders (jlg skytrak) I had the cylinder taken in due to the cylinder leak, no testing was done just took it in (rookie move I know) so I get the cylinder back put it in and I am able to lift the cylinder but it doesn't want to go down. I can hear the machine trying to pull it down but won't go, at times it will retract but really slow. I have checked my lines, all seems well. Sometimes I will get a slight whistle pull i'll then pull lever after the whistle it goes down just fine. Air In Cylinder? Relief valve? Cylinder by passing? Tia
Tia, those cylinders are equiped with a cylinder check valve, this is a safety valve that keeps the boom from dropping if there is a failure in the boom up curcuit like a blown hose. The valve only opens when the pressure on the down hose opens it and allows return oil to flow. It is screwed into the block at the base of the cylinder replace it and see what happens. TIO
Thank you theironoracle, I will give it a shot, thanks for your input
Be very careful removing that holding valve on the boom hoist cylinders. Make sure the boom is fully at rest, not in the air, and even then, when you remove that valve cartridge there may be some residual pressure, just be very slow and careful removing it.
Thank you willie I will.
Update, I replaced the valve and all is ok! Thanks again fellas, really appreciate it! What might of caused this valve to fail? Wear and tear?
If it was fine before you took the cylinder in, and not working correctly after, it's most likely got some contaminant stuck in it, part of a seal or a metal shaving from inside the cylinder, or possibly some dirt that got into the lines when you removed or replaced the hoses. Just guessing.
Yep, it doesn't take much of a fragment to foul up a holding valve cartridge.
Makes sense it would be garbage. Missed quite a bit of garbage when I pressure washed it, doesn't help that it's spends most it's life on a dairy. Next time ill make sure i get all that crud off before disconnecting lines. But now the boss is happy happy happy
I can relate to this thread. Yes I know this is not a tractor, but. Some of the aerial knuckle booms where I was working at a few years ago used in the matching elbow cylinders, a fixed holding valve on one cylinder and an adjustable holding valve on the other cylinder. We would sometimes have to adjust the adjustable side so the boom tip did not move sideways when movement started in the up or down direction. When the adjustable valve closely match the fixed the boom tip would move straight up and down when the function was moved. We used a holding valve that allowed 5 drips per minute on most of our cylinders. there is a german manufactured holding valve that has a positive close and is made with "O" rings to stop even that drift rate. Back on the drifting of cylinders. Heat is one thing that was not mentioned. While it is more pronounced on a cylinder that is 288" long, during operation with normal temp hydraulic oil introduced into that cylinder, I have seen them cool after being raised and move down as much as 3" inches as well as that same cylinder in cool weather after raised during operation. Enter the warm sun out from behind the clouds warming the cyl and it expands (raising) the boom extension. Sorry just another perspective on that. Another Tom V. IFPA MHM AJPP
So I've got a bobcat 442 with significant boom drift. Haven't had much time to pin point the issue, but if I understand correctly it shouldn't drift because it is putting down-pressure on the piston side and also the holding valve on the cylinder should "hold" the load? The ram seal seems good so I'm guessing my holding valve is shot? Also if the holding valve isint functioning correctly, the control valve should still hold the load with minimal drift, so I probably have boom control valve issues also? Great thread, very informative! Thanks for your insight