Understanding hydraulics and cylinder drift

[willie59]

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.

 

[norite]

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.

Not sure what your asking about the tipper rams.

I think he is talking about the hoist cylinders on a dump truck, I believe that is what he means by a tipper.

 

[oldseabee]

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.

 

[Yepblaze]

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.

 

[oldseabee]

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 A$$ is NEVER CAN HAPPEN.

 

[khp-mech]

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.

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.

 

[Lee-online]

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

 

[motrack]

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.

 

[willie59]

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.

 

[stumpjumper83]

A+ for atco

 

[Hendrik]

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.

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.

 

[willie59]

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.

 

[oldseabee]

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

 

[Hendrik]

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.

 

[Hendrik]

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.

 

[pete13]

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

 

[oldseabee]

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.

 

[oldseabee]

You should probably move this thread to the Skid Steer part of the forum for more input.

 

[mrtutz]

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

 

[stinkycat]

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