4 stage double acting telescopic cylinder mis-staging on retract...
Forgot to include this pic of the cylinder with its identificaation markings. can anyone decipher this code?
I would say that it is single acting. The top fitting is located too far from the top to be double.
Unless you plan to haul feathers I think that if you put a load on that bed the cylinder is going to push up in the middle. It dosen't look strong enough where the cylinder mounts.
I have worked on 18 yard dump trucks and up to 38' dump trailers years ago, 1970-75. Up to 5 sections. We use to replace packing on the top of the sections. We would remove a spanner nut on top of each section, remove old packing and install new. WITH WEIGHT OFF OF CYLINDER.
From the experienced hands above, it sounds like a rebuild, with new seals, and new oil without the bits of old seal might solve the problem.
Or, from the site metlmuncher listed:
"Because of the area differential between the retract side and extend side the flow into retract port (B) must be multiplied by this differential to determine the flow out of port. It may be necessary to install a dump-to-tank valve to speed up the retracting cycle." But it sounds like it hangs, and is not just slow to close, right?
Or maybe the cylinders you have are under-sized. Consider these, from Hyco Canada:
I'm assuming also that this is just a single acting cylinder because it looks like one and acts like one.
Observation: the operating angle of the cylinder is pretty low, IMO to have a nice straight push down on retract, and the cocking/binding effect is going to be right there waiting to bite. A typical dump truck has the cylinder almost vertical at the front of the box, and although it does tip quite a few degrees as the box is lifted, it would never get anywhere near the angle your cylinder is working at.
Your cylinder orientation is more typical of a scissor lift type hoist, with a double acting, non-telescoping cylinder. The knuckle joint in the sissor lift hoist redirects a good portion of the gravity load into the natural direction that the cylinder wants to retract. So the columnar load stays in alignment with the rod axis, otherwise it could banana bend it, which I think your telescoping cylinder is prone to do.
In the single acting cylinder, powering it down is not really going to work as expected, because the only 'piston' you've got to push on is the annulus at the bottom end of each stage that keeps the stage from pooping right out the end of the previous stage. So the largest stage has the biggest annulus, which means its got the most square inches of piston area, so it moves first. It doesn't move far before the cocking action of too much extension, at too low of an angle, causes it to bind up in the gland and/or bore of the base stage.
I don't know how much testing you want to do, but I would suggest that you remove the box or block it up, undo the top end of the cylinder and swing it upright against the back of the cab. Then run the cylinder all the way up and power it back down. I bet it will come down without locking up, biggest stage first, smallest stage last......or maybe the last stage simply will never power downwards if there is no oilspace above its annulus and the top seal. You might just have to open the port and hope it will come down by reason of its own weight.
Edit: it seems like you have lots of extension angle on the box and probably do not need that much cylinder travel. If you take the cylinder apart, you might think about adding about a 2" spacer on each stage. The spacer needs to be captive between the stage 'piston annulus' and the gland. This will shorten the extension of each stage by 2" but will decrease the cocking/binding tendency of each stage because you getting a longer bearing area, keeping the pistons farther away from the glands.
Ok, downloaded the videos, and had a think on it. I've got an old Surplus Center catalogue and flipping through it several telescopic cylinders have "first stage double acting for pulling over center load". As you say its an ebay special, this would explain why only the first stage retracts under power. Still the problem of why the cylinder extends the last stages as the first powers down from partial extension. There must be oil trapped in the base, I don't see any other way. There wouldn't happen to be a cartridge valve screwed into the base port that would restrict outflow to control the lowering of the load in it's original application? Or how about a piece of hose lining loose and blocking outflow? Thats happened before.
Ok, first stage only double acting would make sense I think. The oil flow into the retract would cause something like 3x the flow going out the bottom of the cylinder. So lets say you pump 10 gpm into the retract, that would mean the cylinder is trying to push out 30 gpm. If the hoses are too small (like under 3/4" or so, and the valve on the power pack looks quite restrictive for higher gpm) then that itself would cause a restriction to that flow and if there is no load on the bed the flow would simply stay in the cylinder and extend the rest of the stages if they are not yet extended.
And if only the first stage is double acting then once the first stage was fully retracted it would appear to lock up if there isn't enough weight on the bed to push the rest of the stages together on its own or is binding at all.
I don't think you have a double acting telescopic cylinder. Go to this link: http://www.parker.com/literature/Mob...%20service.pdf and go through it to pg 18 and there is a cylinder that resembles yours in cutaway. The upper port on the main body is an optional port that allows the cylinder to act slower because you're moving the oil through the transfer holes in the stages. It also shows a double acting telescopic in a cutaway view. Generally on a double acting the ports are located in the rod end and have a tube extending through the last stage and sealed off at that point so that it has two sides of a cylinder to work on
Yes, there are double/acting Multi-stage cyls but they are for very specific cases....
The two applications that come to mind is the last (smallest stage) is D/A only to bring back an over-center load (in dump truck setup), then gravity kicks in right at the center of balance....
The other is using the cyl in the horizontal position (with no induced load from weight making it mistage ) for retract of cylinders used in Refuse Trucks to unload the garbage from in the body of the truck....
I've also worked on the same type of M/S D/A cyl setup used in an incinerator for pushing the cinder ash out of the combustion chamber.....mounted in horizontal position again and needs most of its power to "Push Out", the only pull power needed was for retracting back into it's resting place...This cyl was 6-stage and each tube was approx 8 ft long so it was a bugger to rebuild.....
My suggestion would be to test the cyl in a horizontal/flat position and see if it stages properly.....if not and only one stage moves then binds hydraulicly then it has to have the right balance for retract with an overcenter load....
My thoughts are that this cyl was manufactured for a very specific use only (as others have mentioned), and not that it needs a rebuild......sometimes the only way to determine that would be to open it up and investigate internally.......this cyl either way could be modified to be straight forward single acting but would require a complete teardown.....
Another diagnostic check.
Run the cylinder out fully. Disconnect power to pump and block the pump pressure side of the valve. Run your valve like you are extending it more (thus opening up the top port to tank). Does the cylinder retract by gravity? If it does then it is just a multi port single acting cylinder. If not then it is a double acting cylinder and you might need to tear it down to figure out just how it's set up.
I just thought of something else......a truly double acting multi-stage hyd cyl has a live end rod for the connections......meaning the hyd oil is plumbed through the hyd rod......it has a hollow rod with a piece of tubing welded internally to isolate the extend and retract porting......thus having both ports on the smallest stage and none on the barrel (with the exception od bleed ports)....
The pic from Bosleyjr shows a live end rod D/A Hyd Cyl with the ports on the small stage....
Which leads me to believe this 4 stage cyl from the OP is just D/A on one stage.....leading to binding as the intent was probably gravity down for the remainder of the stages......Yes /No ?
Thanks for the feedback guys. Decided to disassemble this 200 pounder after I conduct some tests some of you mentioned. alot of the local hydraulic cylinder repair shops near me are also scratching their heads and would love to see the innards of this beast. Rest assured, i will return with images for you all to see and de-myth this thing once and for all. stay tuned........
Ok. didnt have to crack open the cylinder as evident by the test videos below.
I am suspecting something is amiss in the plumbing and/or the valve itself. Here is an image of the 3pos/4way tandem spool valve im using below. I ruled out the cylinder after having it bench tested at a local hydraulic shop. the first stage can actually power retract and extend freely now from a full extension unlike my previous test/video. Here is the video of the cylinder being bench tested at a local shop: YouTube - hydraulicshop1.MOV
Though the first stage is working as it should, the other three stages still seem to 'creepingly' extend while on continous power retract mode as evident by this video: YouTube - hydraulicshop2.MOV
Is this normal? I think I can live with this as long as the first stage is working right. I will install a secondary switch to gravity retract after the first stage is done power retracting. Gravity retract thru the 3/4way valve is fine, but awfully slow on the first stage originally.
My plumbing could also be an issue due to having 3 seperate reservoirs supplying hydraulic oil to the pump by vacuum thru clear hosing. The pump does whine loudly than normal, indicating a presence of a vortex in the pump, introducing air. Plan to add an inline booster pump to eliminate this delimma. Forced to use seperate reservoirs due to space constraints in enlarging the pump tank. Back to the drawing board with this project...will keep you all posted....bob
Here's a nice diagram from Prince:
showing both single and double acting telescopic