Kubota BX25 FEL falls under own weight

   / Kubota BX25 FEL falls under own weight #21  
Yes, with lift disconnected and trying to lift anyway.....how fast the thing actually lifts (when trying to lower) is an indication of how much oil is actually leaking past the piston seals.

But still wouldn't cause loader drift. You could completely remove the seals....and the loader would still not drift. Oil actually has to EXIT the cylinder for it to drift down. There is no if's, and's, or but's about that.

The issue oldnslo eluded to is that with bad piston seals.....oil can now seep past the lift..AND lower ports in the valve body itself. Giving the oil double the escape paths.

Fixing the cylinders may help the symptom....but the issue is still oil leaking past the valve.

Manufactures usually rate valve leakage. Since valves are metal on metal, and have to move....they cannot be a 100% seal. 3cc/min of oil at 1000psi is a pretty standard number.

Just how much yours is leaking can be calculated
 
   / Kubota BX25 FEL falls under own weight #22  
To isolate weather the lift or lower port in the spool is leaking worse....or if they are about the same....

Raise loader and disconnect lift.....time how long it takes to drop a given distance.
Do the same for the other hose.
 
   / Kubota BX25 FEL falls under own weight #23  
Yes, with lift disconnected and trying to lift anyway.....how fast the thing actually lifts (when trying to lower) is an indication of how much oil is actually leaking past the piston seals.

But still wouldn't cause loader drift. You could completely remove the seals....and the loader would still not drift. Oil actually has to EXIT the cylinder for it to drift down. There is no if's, and's, or but's about that.

I would have to see that and still not sure I would believe it. You should be able to cap both ends and still have movement if the load on the cylinder is greater than what the seals can hold. The oil just moves from one side of the seal to the other as the cylinder rod moves.

Now, I will admit that I am in no way a expert, and most folks in this old world are a heap lot smarter than me. But, I have had many cylinders on the farm that we rebuilt because of drift, and problem would go away. Every implement we had used multiple cylinders. On occasion, one of those would give some trouble.
 
   / Kubota BX25 FEL falls under own weight #24  
No. You can cap both ends of the cylinder and there should be NO compression at all of the cylinder. Even if seals were removed. Heck even if the Piston itself were removed.

Oil HAS to EXIT the cylinder.

Oil will NOT compress.

By compressing cylinder....you are shoving a steel chrome plated rod down into it. Where do you suppose the oil goes that the cylinder rod displaces?
 
   / Kubota BX25 FEL falls under own weight #25  
This is why there is so much mis-information regarding cylinders and drift.

It is physically impossible to compress a cylinder, provided it is full of oil and not air, unless the oil has an exit out of the cylinder.

This has been beat to death here on tbn. Because like you, many people think the oil can simply bypass the seals....and don't understand the concept.

The seals are ONLY there to provide a separation between rod end and base end so you can have a cylinder that will travel BOTH directions.

Without seals....without a Piston....or with badly damaged seals.....you have nothing more than a single a ring cylinder.

Pump the barrel full of oil.....and the oil pushes the rod out of the barrel. (Displacement). Snowplow angle cylinders (single acting) are exactly like this. No Piston or seals at all. Just a rod in a tube.

The reverse is true also. You cannot push the rod back down into the cylinder without expelling oil OUT of said cylinder
 
   / Kubota BX25 FEL falls under own weight
  • Thread Starter
#26  
Well obviously I don't want to throw parts at it if they are not needed. Even though I'm not totally familiar with how each system component works, I do try to troubleshoot using just logical thinking. Here's where I'm going with that..

I noticed yesterday that with the FEL sitting on the ground and you slowly lift it, the lift arms seem to rock on the pivot pins at different times. Once the load is suspended then it's harder to see if the arms are going up at different rates. Once the load is at a given height and you release the lever the load drops to given height and appears to stop or at least drop very minimally. Would it be a fair evaluation that these actions make it seem like one cylinder is doing more work than the other not only during the initial lift but also the support of the load once raised?

The machine has about 200 hrs on it and probably less than 40 hours of that is likely use of the FEL. I know anything is possible but with no previous issues, would a valve problem come on that quickly with no warning?
 
   / Kubota BX25 FEL falls under own weight
  • Thread Starter
#27  
This is why there is so much mis-information regarding cylinders and drift.

It is physically impossible to compress a cylinder, provided it is full of oil and not air, unless the oil has an exit out of the cylinder.

This has been beat to death here on tbn. Because like you, many people think the oil can simply bypass the seals....and don't understand the concept.

The seals are ONLY there to provide a separation between rod end and base end so you can have a cylinder that will travel BOTH directions.

Without seals....without a Piston....or with badly damaged seals.....you have nothing more than a single a ring cylinder.

Pump the barrel full of oil.....and the oil pushes the rod out of the barrel. (Displacement). Snowplow angle cylinders (single acting) are exactly like this. No Piston or seals at all. Just a rod in a tube.

The reverse is true also. You cannot push the rod back down into the cylinder without expelling oil OUT of said cylinder

I see where you're coming from here on the science of how the cylinder works. Maybe I'd better off taking it to someone that knows more about the system and how it functions.
 
   / Kubota BX25 FEL falls under own weight
  • Thread Starter
#28  
This is why there is so much mis-information regarding cylinders and drift.

It is physically impossible to compress a cylinder, provided it is full of oil and not air, unless the oil has an exit out of the cylinder.

This has been beat to death here on tbn. Because like you, many people think the oil can simply bypass the seals....and don't understand the concept.

The seals are ONLY there to provide a separation between rod end and base end so you can have a cylinder that will travel BOTH directions.

Without seals....without a Piston....or with badly damaged seals.....you have nothing more than a single a ring cylinder.

Pump the barrel full of oil.....and the oil pushes the rod out of the barrel. (Displacement). Snowplow angle cylinders (single acting) are exactly like this. No Piston or seals at all. Just a rod in a tube.

The reverse is true also. You cannot push the rod back down into the cylinder without expelling oil OUT of said cylinder

Here is a video that explains clearly exactly what you are talking about here.

Cylinder Drift - YouTube
 
   / Kubota BX25 FEL falls under own weight
  • Thread Starter
#29  
However here's another that shows how to diagnose cylinders that have internal seal failures. I believe this to be the issue I am having. The cylinder in the video appears to be the same as the one on my compact Kubota as well.

Thoughts?

Bypass testing a hydraulic cylinder - YouTube
 
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   / Kubota BX25 FEL falls under own weight
  • Thread Starter
#30  
The recommended local shop won't give me an accurate price to rebuild them over the phone. Something about being metric and such. Said price could be between $120 and $220 each. That pricing makes me want to consider doing this job myself. I've done most of my auto, cycle, watercraft, and small engine repairs since high school. I should be capable of the seal replacements right? lol
 

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