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swing barrel fix?

Started by krusty, July 02, 2018, 05:53:21 PM

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mike_belben

Quote from: Grandpa on July 07, 2018, 07:25:45 PM
Quote from: mike_belben on July 06, 2018, 10:32:34 PM
Quote from: Don P on July 06, 2018, 07:12:50 PM

So as a basic order of troubleshooting, if a double acting cylinder is drifting, unless there is fluid coming out of the cylinder or other external leak, check the valves first?
Yes sir.
Mike, I would disagree with your answer, if a double acting cylinder is drifting the oil should return to the tank, as per the link you posted above.

If cylinder oil is reaching the tank when the valve is closed, we have a valve leaking.   
Praise The Lord

Jack S

 from my experience with double acting cylinders if the seal on the piston is bad  drift will happen until the internal pressures equalize on each side of the piston. I have rebuilt many and the problem is cured. In fact I just did the crowd cylinder on my woods backhoe this year. Jack

Grandpa

Mike
     In reply 4 you posted a link that says the oil returns to the tank via the port relief. Now you are saying it is a leaky valve. Can you clarify, keeping in mind the original question by DonP was troubleshooting a drifting double acting cylinder?

It has been my experience also that when a double acting cylinder drifts, it is most often the cylinder packing. Valves can and do leak, but it is much less common.

Of course all of this assumes no external leaks.

mike_belben

If the cylinder is not leaking externally, drifting all the way down is probably in the valve, but it isnt in the piston.  Even with piston seal removed, a cylinder without an external leak can only drift until pressures on both sides of the piston face equalize.  If no leak from rod seal or hose, that will not likely be a full cylinder stroke worth of drift before equalization occurs. 


Here is a quick crummy sketch to illustrate only the cylinder, lines and spool.  



Once the pressures above and below eqialize the drift stops even with no piston seal at all.  If drift continues its because something allowed one side to continue losing pressure.  Either the rod seal or a hose or a crack or the valve spool.  It cannot go from the cylinder to the exhaust port, or back through the pump when shutoff, without first going by the valve. Granted there are plenty of parts in a multisection valve to consider so thats a blanket answer but it does start narrowing down.  It could be a spool, a relief, a section O ring, a cracked section, a cracked fitting, a bad O ring on and SAE or ORFS connection etc etc etc.  But if it behaves like this under these constraints ...it aint the piston seal.  Thats the whole point.  One less part to needlessly rebuild.
Praise The Lord

Skeans1

I've had cylinders with no external leaks drift off or fully let off issue was the piston packing.

kiko

What don't want to confuse things and I may repeat previous comments. The rack swing cylinders I have dealt with did not have a gland, just a piston on each side of the rack , so if the drift issue was caused a piston leak oil would be exiting around the rack.  So a rack swing cylinder is two single acting cylinders more or less.  As far as theory goes gravity will not let the pressures equalise on both sides of a piston untill it no longer a factor as with a boom cylinder where the boom had made to a stop or cylinder had bottomed out, or the swing has reached the low spot.   The spool seal leaking would not likely cause drift.  With the spool in neutral,  the oil must travel through the port relief before returning to tank and that back flow would not be applied the spool.  Also I have never seen a gresen valve with out spool seals, but that does not mean they don't exist. 

kiko

Kolstrand.com has spool seals for 25p valves.

snowstorm

i think he thought there would be seals sealing the spool to the valve body internally . i have had quite a few valves apart over the years never seen one built that way. spool to body machined fit only seal is for what oil may seep past that machine fit 

mike_belben

Same here.  All i can recall are ground finish spools and honed to size valve bores.  Dust seals on the end.  

I have seen a few crossleaks from bad sectional or end cap Orings. 
Praise The Lord

krusty

I did see the seals  that Kiko has mentioned and they are cheap enough to order and try for phase #2 of the diagnosis. My fear is that if I cannot get this fixed I am at risk of the boom swinging around and either blowing the cylinder or knocking me off the platform.

Plus it has be in that "I must finally fix this problem out of principle" mode. I bought two ball valves to insert in the lines to the cylinders. They are 3/8" lines and due to back packaging of the valves, one was 3/8" one was 1/2" so will get the right one on the way home today. Even with just one valve it did still move a bit. Also noticed that when I cracked the line at the valve body, the fluid was highly aerated. Will also replace the suction line on the pump this week. If there is mucho air in fluid as I have seen, I would assume it would cause spongyness as well. There was enough air in the fluid to cause fluid to be pushed out of the hose at an alarming rate. The boom was grounded so there was no movement. Also the presser relieved by the valve in opening it in either direction.

Stay tuned........

mike_belben

Air throws my drift info out the window..  It will facilitate drift by its compressible nature.

 Its tough to know if its trapped air thats been in for years or if you have a rod seal thats letting air in every time you retract the cylinder.  


Praise The Lord

kiko

Again, I was afraid I might confuse the issue.  I  Only call them spool seal because they go over the spool.  However ,the spool looks good and it seems the issue has been identified, or at least has to be addressed before moving foward .A cavitating pump will make a distinct sound.  Also check for a suction strainer when addressing the suction line.

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