Leaky backhoe, normal?

[baronetm]

I'm a little late to this thread, my L3901 with a BH77 hoe had the same issue with about 2-3 hours on the hoe and less than 40 hrs. on the tractor. I initially noticed a bit of slack in the bucket curl cylinder while digging. But it became very apparent quickly there was a serious issue when the bucket would drop any load held in it or by the thumb within a minute at most.

My guess was defective rod cylinder seals, (there were no external leaks) the dealer doubted my diagnosis. It was confirmed, with their finding a scored cylinder bore on the bucket curl cylinder. The score had cut the cylinder rod seals, thusly leaking hydraulic fluid from the pressure side to the tank. A new Cylinder was installed, and all is well.

In my many years of industrial maintenance it's not the first time I have seen new cylinders fail, and its usually in a very short period of time after putting them into service. The usual issue would be damaged piston seals, probably while assembling the cylinder. I have never seen a new cylinder that was scored so that was a surprise. I also do not recall ever having valves mechanically fail on a new clean system.

 

[Henro]

I'm a little late to this thread, my L3901 with a BH77 hoe had the same issue with about 2-3 hours on the hoe and less than 40 hrs. on the tractor. I initially noticed a bit of slack in the bucket curl cylinder while digging. But it became very apparent quickly there was a serious issue when the bucket would drop any load held in it or by the thumb within a minute at most.

My guess was defective rod cylinder seals, (there were no external leaks) the dealer doubted my diagnosis. It was confirmed, with their finding a scored cylinder bore on the bucket curl cylinder. The score had cut the cylinder rod seals, thusly leaking hydraulic fluid from the pressure side to the tank. A new Cylinder was installed, and all is well.

In my many years of industrial maintenance it's not the first time I have seen new cylinders fail, and its usually in a very short period of time after putting them into service. The usual issue would be damaged piston seals, probably while assembling the cylinder. I have never seen a new cylinder that was scored so that was a surprise. I also do not recall ever having valves mechanically fail on a new clean system.

Not trying to dispute your experience. Just wondering how the cylinder fluid could leak out if the cylinder control valve was holding correctly.

The theory seems to be that the cylinder rod would extend when pulled, but should not retract (much) even when forced, if there is no leakage path for fluid To leave the cylinder. Granted, hoses can expand and so on.

BUT you experience seems contrary to the theory.

I am finding this thread a worthwhile learning experience.

 

[baronetm]

Not trying to dispute your experience. Just wondering how the cylinder fluid could leak out if the cylinder control valve was holding correctly.

The theory seems to be that the cylinder rod would extend when pulled, but should not retract (much) even when forced, if there is no leakage path for fluid To leave the cylinder. Granted, hoses can expand and so on.

BUT you experience seems contrary to the theory.

I am finding this thread a worthwhile learning experience.

Thanks for your question, it made me think a little deeper into my theory.

I won't pretend to be a tractor hydraulic specialist by any means, I continually try to relate my industrial experience to the tractor world and believe me I sometimes find the way some systems are plumbed and especially the terminology confusing, but many components are the same.

From my experience and comments from many on this site, it appears most Kubota backhoe cylinders leak down over as little as a few hours just sitting while the tractor is shut down. The boom cylinder will drop onto the mechanical safety latch in a few minutes or less, even if the tractor is running

In the case of holding a load with the thumb, you have clamped the load and now you have let the control valve lever go back to a neutral position. You are not continually applying system pressure to the cylinder overcoming the leak.

So, my thought is with one side of the cylinder under pressure (clamping or holding the load) the other has a leakage path to tank thru the valve bank as they are not made to the close tolerances of an industrial valve, thus the leaking seal and valve bank gave a place for the oil to flow to tank, the best explanation I have.

In my industrial experience this would not be the case, I believe the industrial components are made to a much closer tolerance so leakage is much less of or not an issue, also in any industrial system I have worked on if the cylinder was in a vertical or any position and holding a critical or heavy load there would be a counterbalance valve to restrict any flow, not allowing the cylinder rod and any connected load to drift over time.

If there is a tractor hydraulic expert available, I am also willing to learn or have my theory corrected.

 

[OrangePower]

Thanks for your question, it made me think a little deeper into my theory.

I won't pretend to be a tractor hydraulic specialist by any means, I continually try to relate my industrial experience to the tractor world and believe me I sometimes find the way some systems are plumbed and especially the terminology confusing, but many components are the same.

From my experience and comments from many on this site, it appears most Kubota backhoe cylinders leak down over as little as a few hours just sitting while the tractor is shut down. The boom cylinder will drop onto the mechanical safety latch in a few minutes or less, even if the tractor is running

In the case of holding a load with the thumb, you have clamped the load and now you have let the control valve lever go back to a neutral position. You are not continually applying system pressure to the cylinder overcoming the leak.

So, my thought is with one side of the cylinder under pressure (clamping or holding the load) the other has a leakage path to tank thru the valve bank as they are not made to the close tolerances of an industrial valve, thus the leaking seal and valve bank gave a place for the oil to flow to tank, the best explanation I have.

In my industrial experience this would not be the case, I believe the industrial components are made to a much closer tolerance so leakage is much less of or not an issue, also in any industrial system I have worked on if the cylinder was in a vertical or any position and holding a critical or heavy load there would be a counterbalance valve to restrict any flow, not allowing the cylinder rod and any connected load to drift over time.

If there is a tractor hydraulic expert available, I am also willing to learn or have my theory corrected.

I would agree if these are dual feed cylinders. Not going out now in the cold dark to check, but, how many and which cylinders are dual feed on our tractors? My thread, so, I will point to the BX23s first, lol .

 

[TheOldHokie]

Thanks for your question, it made me think a little deeper into my theory.

I won't pretend to be a tractor hydraulic specialist by any means, I continually try to relate my industrial experience to the tractor world and believe me I sometimes find the way some systems are plumbed and especially the terminology confusing, but many components are the same.

From my experience and comments from many on this site, it appears most Kubota backhoe cylinders leak down over as little as a few hours just sitting while the tractor is shut down. The boom cylinder will drop onto the mechanical safety latch in a few minutes or less, even if the tractor is running

In the case of holding a load with the thumb, you have clamped the load and now you have let the control valve lever go back to a neutral position. You are not continually applying system pressure to the cylinder overcoming the leak.

So, my thought is with one side of the cylinder under pressure (clamping or holding the load) the other has a leakage path to tank thru the valve bank as they are not made to the close tolerances of an industrial valve, thus the leaking seal and valve bank gave a place for the oil to flow to tank, the best explanation I have.

In my industrial experience this would not be the case, I believe the industrial components are made to a much closer tolerance so leakage is much less of or not an issue, also in any industrial system I have worked on if the cylinder was in a vertical or any position and holding a critical or heavy load there would be a counterbalance valve to restrict any flow, not allowing the cylinder rod and any connected load to drift over time.

If there is a tractor hydraulic expert available, I am also willing to learn or have my theory corrected.

Not an expert but your analysis is at least partially faulty.

The only path to tank for the cylinder on the back hoe is through the control valve and it doesn't need a leaking cylinder seal to get there. When the thumb is clamped the control valve spool is in neutral and holding the built up pressure on that side of the piston. Any leakage through the spool will bleed off that pressure.

Mobile equipment uses spool valves and all spools leak to some degree. A typical leakage number for a mobile equipment valve of this type is in the vicinity of 5 cc/min min to 12 cc/min max. at 100 bar (1450 PSI). Using those numbers it is pretty easy to calculate a cylinder drift rate. Pressure drops will happen a lot faster and are harder to calculate. If you have an application that needs to hold pressure a check valve of some sort is your best bet. For cylinders pilot operated check valves are one solution.

Dan

 

[OrangePower]

Any leakage through the spool will bleed off that pressure.

Dan

..........if it gets past the initial lever/valve and THEN into the control valve spool, correct?

 

[TheOldHokie]

..........if it gets past the initial lever/valve and THEN into the control valve spool, correct?

I don't follow that. Here is the operation as I see it:

1. When shifted to curl the bucket spool diverts pump flow to the cylinder and establishes the thumb pressure.
2. When allowed to return to center (neutral) it holds that pressure in the base end of the cylinder and the pump flow reverts to the center core where it continues through the backhoe valve and to tank through the 3 pt valve which is also in neutral
3. If the bucket spool is leaking (and it always does to some degree) oil from the curl side of the cylinder bleeds back into the open center where it joins the flow going to tank through the 3pt valve. Clamping pressure on the thumb is lessened/lost.

Is that how you see it and if not how do you see it?

Dan

 

[OrangePower]

I know how the cylinder itself works, I know where the line goes. After that, I would have to double check the actual valving connected to the lever that controls the bucket before commenting.

In a nutshell, yes, it is still confined to a 1 or 2 point leak ( depending on how many seals are on the shaft that controls the bucket) at 1 spot no matter how many levers are shared in the block. Its not leaking in the cylinder, which I think a lot of the posters are confusing, but, at the valve in the block.

 

[TheOldHokie]

I know how the cylinder itself works, I know where the line goes. After that, I would have to double check the actual valving connected to the lever that controls the bucket before commenting.

In a nutshell, yes, it is still confined to a 1 or 2 point leak ( depending on how many seals are on the shaft that controls the bucket) at 1 spot no matter how many levers are shared in the block. Its not leaking in the cylinder, which I think a lot of the posters are confusing, but, at the valve in the block.

The "shaft (spool) that controls the bucket does not have "seals". Spools rely on a very close radial fit in the body of the valve for that function and since they have to slide freely in that body there is always a small amount of leakage through the clearance. The standard fix for that is to install a pilot operated check valve on the cylinder ports. POCV's use poppets which are generally considered fluid tight to seal the reverse flow. Here is an example of an add-on valve that would work for you :

Prince RD1600 Pilot Operated Check Valve RD-1637 - Dalton Hydraulic

The PRINCE valve model RD-1600 is a pilot operated check valve. This valve blocks oil from flowing from the cylinder port to the valve port until sufficient pressure is applied to the pilot port. Oil can free flow from the valve port to the cylinder port.

www.daltonhydraulic.com

Dan

 

[Henro]

Dan's answer brings up a similar solution to my situation, with the blade cylinder on my mini ex.

This discussion has me convinced that the leak down in the blade cylinder that I experience is caused by the control valve leakage. The control valve assembly has a large number of valve sections connected together. To get the one out and repair it looks to be a major undertaking for a person like me. A great alternative solution would likely be installing a pilot operated check valve at the cylinder.

Have to study this more and look at the links Dan provided above.

 

[TheOldHokie]

Dan's answer brings up a similar solution to my situation, with the blade cylinder on my mini ex.

This discussion has me convinced that the leak down in the blade cylinder that I experience is caused by the control valve leakage. The control valve assembly has a large number of valve sections connected together. To get the one out and repair it looks to be a major undertaking for a person like me. A great alternative solution would likely be installing a pilot operated check valve at the cylinder.

Have to study this more and look at the links Dan provided above.

I think a POCV is a lot easier, cheaper, and more likely to fix your issue than trying to replace a spool or entire valve section.

Dan

 

[Henro]

I think a POCV is a lot easier, cheaper, and more likely to fix your issue than trying to replace a spool or entire valve section.

Dan

My thought too. Glad this thread added some clarity to my situation and others as well.

 

[OrangePower]

Well

The "shaft (spool) that controls the bucket does not have "seals". Spools rely on a very close radial fit in the body of the valve for that function and since they have to slide freely in that body there is always a small amount of leakage through the clearance. The standard fix for that is to install a pilot operated check valve on the cylinder ports. POCV's use poppets which are generally considered fluid tight to seal the reverse flow. Here is an example of an add-on valve that would work for you :

Prince RD1600 Pilot Operated Check Valve RD-1637 - Dalton Hydraulic

The PRINCE valve model RD-1600 is a pilot operated check valve. This valve blocks oil from flowing from the cylinder port to the valve port until sufficient pressure is applied to the pilot port. Oil can free flow from the valve port to the cylinder port.

www.daltonhydraulic.com

Dan

Well, if thats the case, then yea, totally makes sense now. I was gonna say thats a bunch of BS that they didnt have seals, but, I dont think they engineered it to be a holding circuit to begin with, so............

Gonna have to look into that POCV. Thanks.

 

[TheOldHokie]

Well


Well, if thats the case, then yea, totally makes sense now. I was gonna say thats a bunch of BS that they didnt have seals, but, I dont think they engineered it to be a holding circuit to begin with, so............

Gonna have to look into that POCV. Thanks.

No BS - the only seals in a spool valve are on the ends where they enter and exit the body and are for external sealing. The glands are precision ground and plated for a sub-thousandths fit in the honed bores. When the spool and body wear the valve is trash and has to be replaced. That's one of many reasons why oil cleanliness is so important.

Dan

 

[lynnmor]

Well


Well, if thats the case, then yea, totally makes sense now. I was gonna say thats a bunch of BS that they didnt have seals, but, I dont think they engineered it to be a holding circuit to begin with, so............

Gonna have to look into that POCV. Thanks.

I am interested in what you come up with. It appears that it might be a bit of a challenge in fitting a POCV since it is almost 6" long and there would be a fair amount of plumbing to do. My new backhoe has been worked on by two dealers with some improvement the last time, but I don't know how long that will last. Pinning the boom every time I need to move isn't acceptable.

 

[OrangePower]

No BS - the only seals in a spool valve are on the ends where they enter and exit the body and are for external sealing. The glands are precision ground and plated for a sub-thousandths fit in the honed bores. When the spool and body wear the valve is trash and has to be replaced. That's one of many reasons why oil cleanliness is so important.

Dan

My point about BS was that to have the bucket hold, the circuit should have had seals. Since it is a slip fit, it is obvious it was designed that way. Still, would have been nice to have it engineered to use seals so it would hold.

 

[TheOldHokie]

I am interested in what you come up with. It appears that it might be a bit of a challenge in fitting a POCV since it is almost 6" long and there would be a fair amount of plumbing to do. My new backhoe has been worked on by two dealers with some improvement the last time, but I don't know how long that will last. Pinning the boom every time I need to move isn't acceptable.

The POCV does not have to go on the cylinder, It just needs to be somewhere between the DCV and the cylinder. For example you could mount it on the boom and run the hoses fromthe valve to it. Then run a couple short hoses over to the ports on the cylinder. If I were doing it that is probably the approach I would use.

Dan

 

[TheOldHokie]

My point about BS was that to have the bucket hold, the circuit should have had seals. Since it is a slip fit, it is obvious it was designed that way. Still, would have been nice to have it engineered to use seals so it would hold.

The industry has had remarkable success doing it this way for almost a hundred years.
The industry solution to spool leakage is not putting seals on the spools. It is ditching spools for other control elements - notably poppet directional control valves. Almost zero leakage but they can't handle nearly as high flow rates as a spool valve and operator control is lessened. You better believe there has been a LOT of thought and energy put into these designs and they exist for good reasons. High flow/high pressure/low leakage/good response is an expensive combination.

Dan

 

[OrangePower]

High flow? I am gonna have to have my tractor sit in with you for some counseling on high flow.


Never the less, POCV it will have to be when I finally settle on a life long tractor.

 

[DustyRusty]

Is this why Kubota doesn't offer a thumb that is hydraulically operated? I was thinking about installing a hydraulic thumb cylinder to my BX23S and using the 3rd function to power it. Is this a good or bad idea?