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4 stage double acting telescopic cylinder mis-staging on retract...

speedshop

Plastic
Joined
Nov 20, 2010
Location
toronto,canada
Howdy,
Need help in problem solving: have a 21",4 stage, double acting, telescopic hydraulic cylinder, 56" stroke, 6" dia main body with both ports on main barrel. cylinder hooked up to a 12v DC haldex/barnes power pack with a 2 position, four way electric valve ( 2 coils on one cartridge,push & pull).
system installed on a 2008 toyota tundra dump bed made from scratch.
system extends fine, but mis stages on powered retract by first stage retracting first but for a bit and locks up.
when partially extended and retracting, first stage retracts while the other 3 smaller stages extends at the same time. When fully extended, cylinder locks during first stage retract. video & pics of problematic cylinder in action available upon request. files too large to upload to forum. help!
 
thanks for the reply. you may be right. will undergo more troubleshooting and eliminate any other possibilities before sending it in for testing. by the way, it retracts normally and in correct stages by gravity alone but too slow due to light bed weight (not enough weight to push the stages back down. video coming soon and will keep you posted....thanks.
 
Back pressure may be the reason.
Retraction from partial extension: If there is back pressure on the base the pressure applied to lower the cylinder will cause the stage with the most surface area (power),stage one, to move down first. The remaining stages are free to extend due to the back pressure on the base, adding the displaced oil on the top side to the lowering of the first stage.
From full extension: The pressurized oil lowers the first stage until both sides of the cylinder are equally pressurized. Motion stops or slows to the speed of leakage off the base. Stages 2, 3, and 4 are extended and so cannot accomodate any of the trapped fluid in the base.
Put a pressure gauge on the base side of the cylinder, I think you will find it pressurized on retracting the cylinder, indicating a blockage in the return circuit.
 
Keep in mind there may be a design problem esp. if you are using a surplus cylinder of unknown application, for example one meant only for full-stroke operation. There are a whole lot of varieties of double-acting telescoping cylinders and not all of them have intuitive physics like the stages retracting in the expected order under no-load condition. If it truly locks up and won't move under power that's one thing. If it's just that it retracts in the wrong order, esp. if extended partway and then reversed under no- or low-load condition, that in itself does not mean there's something wrong with the cylinder.
 
You know, I've never seen a double acting telescopic cylinder. Are you sure the second port is not a vent? :D Joking aside, what kind of hoops do you have to go through to bleed the air out of a system like that? I'm wondering if possibly there is a 'bubble' trapped in there, and behaving like a spring?
 
I also think you have a single acting cylinder with a overflow/vent fitting on it. It should use gravity to return and the "top" fitting should go to the reservoir.
 
I posted a link about 2 weeks ago to a site with a cutaway view of a single acting telescopic cylinder. That same site had a cutaway of a double acting as well, along with a description of how they work. I'll see if I can find it again.

Like Hu, I've never seen one that I know of.

http://www.customhoists.com/doublespec.html has a not too great pic but an explanation of how they work that's fairly understandable
 
I know double acting telescoping cylinders exist, but have never seen one nor heard of an application for one.

ASS-U-MEing that you have a flow restrictor on the extend side of the cylinder with free-flow bypass for extend, but restricted flow for retract (so that if the dump bed were loaded with something and the valve opened it doesn't overun the pump on retract and come crashing down to the bottom). Then this restriction would as others describe cause an unloaded bed to possibly retract in backwards order. Not sure why it's locking up on you though.

I have to ask though. On a dump bed why a double acting?? I would just run the rod end to tank and run as single acting. Unless of course you are making something more on the lines of a bale picking bed and need to power down, but I would think the limited amount of force on the retract would make that cylinder an illogical choice. Most of the pickup truck dump beds I've seen use a regular double acting cylinder and a knee joint, would think a 6" base telescope is way overkill on a pickup.
 
Glad I found this forum. At last, I find the right people who knows what they are talking about. y'all have the same conclusions I have and makes sense. just not sure of myself nor this cylinder. Yes, the size is a bit of an overkill, but for $125 bucks and it fits just right under the bed with the desired stroke, I had to get it. plus, the largeness just adds more stability and longevity for such a small application. I got used to the size.
originally, had it single acting (power up, gravity down), but the bed was not heavy enough to push the cylinder back down fast enough on its own.
flow restrictor? hmmmm. let me add that to my troubleshooting and will keep you posted.
I tried to retract from a partial extended stroke on purpose just to show how the stages react. retracting from a full extension, results in the first stage attempting to retract first but stalls the pump or 'locks up', if i termed it right.
I purchased the cylinder thru ebay from a surplus company that had no information on the condition nor the manaufacturer, just the cylinder ID numbers marked on it: 391246-64 (wish me luck in searching for the seal kit for this thing)
As soon as i upload the videos, it will be clearly seen how the cylinder goes out of whack on power retract. In the mean time, I will continue to trouble shoot before bringing it in for testing, if i can't avoid it. tune in for the results....
 
That is one VERY expensive cylinder you have there! Several years ago we had a customer who built a piece of kit that had a serious design flaw in that they specified a 60" stroke double-acting cylinder with a closed length of 22"!!!
As most of the parts had been manufactured a solution to the problem had to be found and a quote was obtained for a telescopic cylinder that could do the job. The quote was nearly £9000 and caused our customer to almost have a heart attack but, with no other easy solution he had to bite the bullet and spend the money.
A few years later the cylinder started playing up with similar symptoms to yours and it was found to be a small bit of rubber from a seal that was moving around and blocking one of the small cross-port holes.
 
The original design of this cylinder may not have been designed for incidental side loading that occurs when tipping a box, as a nice symmetric load would not exhibit this cocking tendency.

I would wonder about binding due to excessive clearance between the components. When a telescoping stage is extending, the distance between the 'piston flange' at the bottom of the moving stage and the 'gland' provided in the top of the previous stage is a very short distance. The shorter this distance, the closer the fit must be to prevent cocking and binding. And frankly, I'm surprised that the big cylinders used on dump trucks retract as well as they do, because they often seem sloppy as hell.

The 'piston flange' is typically just a brass collar affixed to the bottom of each stage. It might be necessary to take the cylinder apart and measure the clearances, and rebuild or modify it. It might even be necessary to lose some stroke in order to get the piston flange farther from each gland.
 
Hydraulic hoses tense up under pressure, if there is a blockage in the return curcuit in the pump when you try to retract the cylinder the line going to the base should move and get stiff. That's an easy field check.
I don't see how, even if this cylinder turned out to be single acting with a vent, this would account for the three stages extending as the first retracts, in the retract from partial extension scenario. I could see oil spewing out of the wipers if there were no seal incorporated below, and perhaps oddball seals/internal wipers locking up with presure oil on the wrong side, but you are still getting some movement so thats out.
How are you lowering the bed now? Cracking the line on the base and bleeding it down, cutting power to the pump and activating the solenoids, let it leak down under power?
More importantly use caution and safety props when working under a raised bed.
 
Given the small ports that are in the retract side of the cylinder. Perhaps a complete tear down and cleaning is in order. As was said above a small piece of something could easily block a port off as they are so small.
 
I have just re-read your first post and I noticed that you say both ports are on the main barrel. This cylinder cannot be double-acting as there cannot be a flow path from one section to the next. The retract port must be at the top of each cylinder section to allow the oil to flow to the correct area of the annulus. Unless around 9 tubes are fitted to provide the correct flow path this must be a single acting cylinder.
 
Actually I think it can. IIRC the double acting telescopes I have seen diagrams of had both ports on the main barrel, the fluid gets to the top of the next stage by small holes drilled the length of the stage rods.
Picture-007.png
 
You are correct! when I saw your picture I remember repairing a ram almost exactly like that one. The port at the bottom of the cylinder led to a chrome rod that went up into the next section and it had a smaller rod that came out of it and continued the scheme.

It was basically a hydraulic ram complete within another ram and there were small holes just above the piston on each of the stages of the "main" ram.

It is very difficult to explain (or comprehend!) but it did work and I think each stage had FOUR pressure seals without counting rod wipers etc.
 
Box and hoist grain trucks usually use telescoping double acting cylinders. The advantage is the cylinders will not run free when an overcenter load wants to extend the cylinders. This becomes an issue when the pivot point for the bed is not at the very rear of the bed. The pivot points are moved under the bed to give a greater dumping angle without a need for as much headroom, and also to dump uniform loads with less cylinder force.
 
Ok guys....here they are...2 video clips of the problematic cylinder and whole setup.

YouTube - cylindernoretract.MOV

YouTube - cylinderpartiallyextendedretractwrong.MOV

Received a response from a retired elevator repair guy with 20 plus years experience. Without seeing the videos yet, he confidently assumed that a 'double sided u-cup seal' needs replacing. Sound familiar to anyone? I wish and hope it is just a blockage. won't find out till I open it up. Can this thing be disassembled using mechanic shop tools? Hydraulic pressing involved here? Fabricating own tools possible to tackle this thing? Would love to see and learn the innards of this cylinder. I'm sure it is expensive when new.
Buchanaan, your right. Researched and found that this type of cylinder is uncommon to mass produce. They are built from scratch for a very specific application and customer, case by case basis.

Grind, troubleshooted by fully extending, safety propped, redirected extend hose straight to reservoir/tank wide open with no slow valve, remove safety props, then power retract. Still the same results. Retract hose stiffens. Gravity retract a bit faster since there are no valve restrictions...just hose straight to tank. No vents/breather on barrel nor end of plunger, just 2 ports on barrel.

Hu, didn't notice any cocking, binding or buckling before till i started locking up the cylinder. it started 'straightening' up after full extension and lock up, especially the large first stage. no oil leaks though. extends and gravity retracts ok. just have to give it a 'tug' to start the descent from a full extension.

Enjoy the videos.....more to come.......feedback is highly appreciated.....

YouTube - cylindernoretract.MOV

YouTube - cylinderpartiallyextendedretractwrong.MOV
 








 
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