Confirm My Hydraulic Analsis Please

[Henro]

The old mini ex I bought has an issue. The Blade cylinder does not hold. It leaks down.

Now the cylinder extends to cause the blade to lift the front of the excavator for stability.

So after thinking about things, I have concluded the following. I could be missing something. Thus my request for assessment.

First, the cylinder extends to hold the front of the excavator up for stability. BUT it leaks down rather rapidly. Requiring constant extending of the cylinder.

Now the theory would be if the control valve were not leaking down, the cylinder would no retract, because the cylinder rod that is fully extended takes up volume when it enters the cylinder, so if there would be nowhere for this oil to go, the rod will not go far into the cylinder. But mine does....full stroke really.

So without doing testing, like removing hoses and applying pressure and looking for external leakage, which there is none of actually, just seems pretty obvious that the issue must be within the control valve, and not the cylinder.

Am I missing something? I don't think so but can't help but ask.

I think the solution is to put a pilot operated check valve between the control valve and the cylinder.

Granted the true solution would be to repair the control valve, but not worth the time and effort on an old excavator. For me anyway.

What do you guys think?

 

[PHPaul]

Possibly, but not necessarily. The piston seal in the cylinder could be bad and allowing oil to bypass from the "down" side to the "up" side of the piston.

Of the two, I'd be more likely to suspect the piston than the valve.

It's (usually) not horribly difficult to disassemble and re-pack a cylinder. Seal kits may be available, if not a decent hydraulic shop can measure and source individual seals.

 

[motionclone]

My Bobcat 337 has load checks in the valve body to prevent this. My grade blade was occasionally not holding and i found that my load check valve was loose in the valve body and allowing fluid to leak by. I simply tightened it and all was good. Get a schematic of your hydro system and see if and where your load checks are.
In my system, you remove the hose and adapter from the main valve body and up inside is a check valve.

 

[Henro]

Possibly, but not necessarily. The piston seal in the cylinder could be bad and allowing oil to bypass from the "down" side to the "up" side of the piston.

Of the two, I'd be more likely to suspect the piston than the valve.

It's (usually) not horribly difficult to disassemble and re-pack a cylinder. Seal kits may be available, if not a decent hydraulic shop can measure and source individual seals.

One thing that is not obvious at first is that due to the cylinder rod, if it is to go into the cylinder, the oil it displaces must do somewhere. That is back to tank through a leaking control valve. Without an external leak, no place else to go.

 

[Henro]

My Bobcat 337 has load checks in the valve body to prevent this. My grade blade was occasionally not holding and i found that my load check valve was loose in the valve body and allowing fluid to leak by. I simply tightened it and all was good. Get a schematic of your hydro system and see if and where your load checks are.
In my system, you remove the hose and adapter from the main valve body and up inside is a check valve.

I have the hydraulic diagram. No check valves in this case.

 

[TheOldHokie]

One thing that is not obvious at first is that due to the cylinder rod, if it is to go into the cylinder, the oil it displaces must do somewhere. That is back to tank through a leaking control valve. Without an external leak, no place else to go.

You got it. If there are no external leaks the valve is leaking down. A POCV on the lines should get it to hold.

Dan

 

[JerryMT]

One thing that is not obvious at first is that due to the cylinder rod, if it is to go into the cylinder, the oil it displaces must do somewhere. That is back to tank through a leaking control valve. Without an external leak, no place else to go.

Why doen't it just go to the other side of the piston?

 

[TheOldHokie]

Why doen't it just go to the other side of the piston?

Because there is not enough room for all of the oil that is being displaced by the movement of the rod.

When a cylinder retracts the volume of the base end decreases faster than the rod end increases. Its hydraulically locked and cylinder cannot retract seal or no seal.

When extending the opposite is true. Volume of the base end increases faster than rod end decreases and oil can bypass around the seal allowing the rod to drift outward.

Dan

 

[motionclone]

I have the hydraulic diagram. No check valves in this case.

What do you have for a machine? Pressure relief valve on that circuit? Maybe your swivel joint is leaking by internally.
What about the linkage between the handle and the actual valve, is the valve returning to center when you let off the handle?

 

[Henro]

What do you have for a machine? Pressure relief valve on that circuit? Maybe your swivel joint is leaking by internally.
What about the linkage between the handle and the actual valve, is the valve returning to center when you let off the handle?

The machine is a Kubota KX41-2v. I parked it on a large sheet of cardboard and saw no leakage.

But actually it doesn’t matter where the leakage is if external to the cylinder. A pilot operated check valve at the cylinder will/should solve the issue for me. Simple enough. Old mini ex used for personal purposes. Not for profit, aside from me profiting from having it and saving my back a bit.

 

[motionclone]

The machine is a Kubota KX41-2v. I parked it on a large sheet of cardboard and saw no leakage.

But actually it doesn’t matter where the leakage is if external to the cylinder. A pilot operated check valve at the cylinder will/should solve the issue for me. Simple enough. Old mini ex used for personal purposes. Not for profit, aside from me profiting from having it and saving my back a bit.

Your swivel joint could leak internally. lift the machine with the boom and blade so the tracks are off the ground. operate the tracks both ways, does the blade go down at all with track input? Or leave tracks on the ground and lift with just the blade, swivel the house, does the blade drop with the house swung in a specific direction? All indicators of swivel seals leaking by. I rebuilt mine, its fairly cheap to do but its like heart surgery.

 

[dvcochran]

The old mini ex I bought has an issue. The Blade cylinder does not hold. It leaks down.

Now the cylinder extends to cause the blade to lift the front of the excavator for stability.

So after thinking about things, I have concluded the following. I could be missing something. Thus my request for assessment.

First, the cylinder extends to hold the front of the excavator up for stability. BUT it leaks down rather rapidly. Requiring constant extending of the cylinder.

Now the theory would be if the control valve were not leaking down, the cylinder would no retract, because the cylinder rod that is fully extended takes up volume when it enters the cylinder, so if there would be nowhere for this oil to go, the rod will not go far into the cylinder. But mine does....full stroke really.

So without doing testing, like removing hoses and applying pressure and looking for external leakage, which there is none of actually, just seems pretty obvious that the issue must be within the control valve, and not the cylinder.

Am I missing something? I don't think so but can't help but ask.

I think the solution is to put a pilot operated check valve between the control valve and the cylinder.

Granted the true solution would be to repair the control valve, but not worth the time and effort on an old excavator. For me anyway.

What do you guys think?

The quick test is to remove one hose and stroke the cylinder until it deadheads on the same end. If the piston seal is leaking, you will see the oil bypassing. It may not be much of a stream (depending on how fast the cylinder is leaking down) but it should be measurable in a cup. Just know there will be some residual oil to take into consideration. Repeat the process on the other side of the cylinder by reattaching the first hose and removing the other hose.
It is harder to do with the valve because the oil volume out is much less, but you can test it the same way. If easy to get at, I think I would disconnect the hoses at the valve and place a cup underneath it for a while looking for oil accumulation. Internal valve leakage is not nearly as common.
If you are not confident in doing these test, and it is not a PITA to remove the cylinder, take it to a local hydraulic shop and have it tested.

 

[PHPaul]

While I agree that ALL the oil can't bypass to the rod side due to the volume taken up by the rod, it's certainly possible that enough will bypass to allow the blade to move until the tracks are back on the ground.

With the blade and tracks all sitting on the ground, there's still travel in the blade cylinder to pick it UP for travel.

 

[TheOldHokie]

While I agree that ALL the oil can't bypass to the rod side due to the volume taken up by the rod, it's certainly possible that enough will bypass to allow the blade to move until the tracks are back on the ground.

With the blade and tracks all sitting on the ground, there's still travel in the blade cylinder to pick it UP for travel.

No oil will bypass internally because the cylinder is 100% full of oil. For the rod to move inward in any amount the volume of oil inside the cylinder has to go down - period. If you completely remove the piston seals on a double acting cylinder it effectively becomes a single acting cylinder as shown below. Think about it.

Either the valve is leaking internally (see post #15) or there is an external leak in the circuit

Dan

 

[TheOldHokie]

It is harder to do with the valve because the oil volume out is much less, but you can test it the same way. If easy to get at, I think I would disconnect the hoses at the valve and place a cup underneath it for a while looking for oil accumulation. Internal valve leakage is not nearly as common.

All spool valves leak internally.

New out of the box 5-6 cc/min @ 100 bar would be a superior number. 10-12 cc/min @ 100 bar would be typical. Over time the spools and bores in the valve body wear, the oil clearance opens up, and spool leakage increases.

Dan

 

[motionclone]

All spool valves leak internally.

New out of the box 5-6 cc/min @ 100 bar would be a superior number. 10-12 cc/min @ 100 bar would be typical. Over time the spools and bores in the valve body wear, the oil clearance opens up, and spool leakage increases.

Dan

There are several other sources of internal leaks besides the valve and the cylinder seals that are specific to an excavator like the swivel joint. Im betting that individual circuits on his excavators valve body have pressure relief valves too that could be hung open slightly and leaking by.

 

[jyoutz]

Possibly, but not necessarily. The piston seal in the cylinder could be bad and allowing oil to bypass from the "down" side to the "up" side of the piston.

Of the two, I'd be more likely to suspect the piston than the valve.

It's (usually) not horribly difficult to disassemble and re-pack a cylinder. Seal kits may be available, if not a decent hydraulic shop can measure and source individual seals.

If a seal in the cylinder was bad, wouldn’t it leak hydraulic fluid?

 

[TheOldHokie]

There are several other sources of internal leaks besides the valve and the cylinder seals that are specific to an excavator like the swivel joint. Im betting that individual circuits on his excavators valve body have pressure relief valves too that could be hung open slightly and leaking by.

Sure - there are multiple possibilities for internal leakage. But what is NOT a possibility is the piston seal on the cylinder(s).

Dan

 

[Henro]

There are several other sources of internal leaks besides the valve and the cylinder seals that are specific to an excavator like the swivel joint. Im betting that individual circuits on his excavators valve body have pressure relief valves too that could be hung open slightly and leaking by.

BUT, where theleakage is occurring is not the issue. If external to the cylinder.

The question related to whether my analysis was correct, without doing anything physically to confirm it.

Seems so, and adding a pilot operated check valve block should be the solution. Easiest way for me to accomplish what I need.

 

[motionclone]

BUT, where theleakage is occurring is not the issue. If external to the cylinder.

The question related to whether my analysis was correct, without doing anything physically to confirm it.

Seems so, and adding a pilot operated check valve block should be the solution. Easiest way for me to accomplish what I need.

Well yeah it is the issue. You got fluid going somewhere and you want it to stay in your cylinder to hold your blade down.
Whats a pilot operated check valve, the plumbing to make it all work cost? Best to figure out what your actual problem is so you dont end up wasting money on a bandaid. Hydraulic hoses, fittings and this pilot operated check valve costs big money nowadays. A check valve wont give you pressure relief protection either.

You could save yourself a bunch of money just get a couple pairs of vice grips and crimp both hydro lines going to your blade cylinder every time you want your blade to stay down.