Leaky backhoe, normal?

[OrangePower]

You should be able to raise the relief pressure for that function that is getting droopy. Some machines get very conservative settings from factory. Far lower thresholds than the factory pressure spec. Should have a lock ring and an Alan key adjustment. It is very sensitive. I would only move it 1/4 turns at a time until your satisfied. Your boom is more than likely the culprit valve. Which should be the first or second from the right side of the valve stack (standing off the machine).

I heard they have some kind of dampening technology so things dont get jerky. I dont know if that is for the L/R swing cylinders only, or? If that is the case on the bucket control, that may be why, for me.

 

[OrangePower]

I’d just tap the curl every little bit rather then holding it all the time.

I tried just tapping and it takes a LOT of talent when your rocking around going through ruts n stuff. I dont like holding it continuously either, heating up the fluid more, but, beats having to backup and scooping up the log/rock again when it drops.

 

[GreensvilleJay]

re: Because of the displacement difference on each side of the piston, when the valve is not operated, the only way the cylinder can drift is leakage on the ground or past the spool.

Not always true.
With the ram seals leaky, oil will be 'pumped' from one side to the other due to the external pressure (the bucket and /or gravity. While the volume of oil is constant, where it is/goes depends on the external forces on the ram.

Had a professionally rebuilt master cylinder fail. They'd overbored and undersized the seals. Press the pedal ,brake oil went 'round and round' INSIDE the master..never got to the brakes. I found that out going down a small mountain with 1 1/4 of topsoil in the truck. Jus another 'fun' day...back in '78.

 

[TheOldHokie]

re: Because of the displacement difference on each side of the piston, when the valve is not operated, the only way the cylinder can drift is leakage on the ground or past the spool.

Not always true.
With the ram seals leaky, oil will be 'pumped' from one side to the other due to the external pressure (the bucket and /or gravity. While the volume of oil is constant, where it is/goes depends on the external forces on the ram.

Not always true is correct.

Without external leaks the volume of oil in the cylinder is constant.

On extend the piston can bypass because the total volume inside the cylinder is increasing as the rod extends. The oil moves to the base end and the load on the rod pulls a vacuum in that end.

But on retract the overall volume of the cylinder is decreasing. and tere is no place for the excess oil to go. Its incompressible and keeps the rod locked. A single acting displacement type cylinder that does not have a piston or seal is a perfect example.

Now look at the bucket cylinder on the backhoe and you will see it is moving in the retract direction.

Dan

 

[Henro]

Not always true is correct.

Without external leaks the volume of oil in the cylinder is constant.

On extend the piston can bypass because the total volume inside the cylinder is increasing as the rod extends. The oil moves to the base end and the load on the rod pulls a vacuum in that end.

But on retract the overall volume of the cylinder is decreasing. and tere is no place for the excess oil to go. Its incompressible and keeps the rod locked. A single acting displacement type cylinder that does not have a piston or seal is a perfect example.

Now look at the bucket cylinder on the backhoe and you will see it is moving in the retract direction.

Dan

Dan, please explain further.

For example, say a cylinder was put together without any seals. Wouldn't the piston be free to move in whatever direction the force on the cylinder rod wanted it to push it?

So a leaky seal situation should be the same, except for time it takes the piston to move.

Seems like this should be the same for movement in either direction. The OP's question related to why his bucket/thumb would not hold constant pressure when the control valve was centered, and when he was not trying to change anything.

Seems to me that a problem with the seals in the cylinder could cause this result. But I think you are saying that leaking seals would affect one direction, but not the other?

 

[GreensvilleJay]

No, works (fails...) both ways ! All depends on whether the cylinders rod is being pushed or pulled by the 'external force' ( gravity on the bucket in this case).

Shock absorbers are good examples of 'controlled' flow from one side of the cylinder to the other. Internal valving controls the rate of flow between 'top' and 'btm' chambers

In this case there IS a difference in flow as the rod side of the cylinder has less volume, due to the area of the rod. When you look at cylinders on equipment, the NON rod side will always exert more force though go slower. One of them 'physics' things.

 

[TheOldHokie]

No, works (fails...) both ways ! All depends on whether the cylinders rod is being pushed or pulled by the 'external force' ( gravity on the bucket in this case).

Shock absorbers are good examples of 'controlled' flow from one side of the cylinder to the other. Internal valving controls the rate of flow between 'top' and 'btm' chambers

In this case there IS a difference in flow as the rod side of the cylinder has less volume, due to the area of the rod. When you look at cylinders on equipment, the NON rod side will always exert more force though go slower. One of them 'physics' things.

Purely and simply no. When the cylinder is retracting the overall volume inside the cylinder is decreasing. If there is no external leak the rod cannot displace the fluid.

Dan

 

[TheOldHokie]

Dan, please explain further.

For example, say a cylinder was put together without any seals. Wouldn't the piston be free to move in whatever direction the force on the cylinder rod wanted it to push it?

So a leaky seal situation should be the same, except for time it takes the piston to move.

Seems like this should be the same for movement in either direction. The OP's question related to why his bucket/thumb would not hold constant pressure when the control valve was centered, and when he was not trying to change anything.

Seems to me that a problem with the seals in the cylinder could cause this result. But I think you are saying that leaking seals would affect one direction, but not the other?

You are using your noodle. What you are describing is called a displacement type cylinder. They are single acting and have no piston or internal seal. Common applications are fork lifts, hydraulic jacks, and the old in ground automotive lifts. They are one of the simplest types of hydraulic cylinders and you probably studied them in high school physics

When you pump oil into the base end the entire volume inside the cylinder is at the same pressure and the hydraulic pressure on the end of the rod forces it to extend. It will hold extended as long as the oil in the cylinder has no place to go. Also note that if the rod is not really large in diameter it can be pulled out with the application of very little force. The only thing holding it in is air pressure (14.69 lb/sq in) acting on the exposed end. Hence it is single acting.

A double acting cylinder with leaky piston seal is effectively a displacement cylinder.

Dan

 

[Henro]

You are using your noodle. What you are describing is called a displacement type cylinder. They are single acting and have no piston or internal seal. Common applications are fork lifts, hydraulic jacks, and the old in ground automotive lifts. They are one of the simplest types of hydraulic cylinders and you probably studied them in high school physics

View attachment 71964

When you pump oil into the base end the entire volume inside the cylinder is at the same pressure and the hydraulic pressure on the end of the rod forces it to extend. It will hold extended as long as the oil in the cylinder has no place to go. Also note that if the rod is not really large in diameter it can be pulled out with the application of very little force. The only thing holding it in is air pressure (14.69 lb/sq in) acting on the exposed end. Hence it is single acting.

A double acting cylinder with leaky piston seal is effectively a displacement cylinder.

Dan

Thank you Dan for taking the time to explain further.

I forgot about the fact that when the cylinder rod goes into the cylinder, it has to displace a certain volume of oil equal to the volume of the cylinder rod entering the cylinder. So if there is nowhere for the oil to be displaced, the cylinder rod will not enter the cylinder, even if there were no internal seals on the piston/rod assembly.

This is extremely interesting to me because I have a situation with my mini ex, where the cylinder for the blade extends to lift the mini ex for stability when digging. kind of like outriggers on a backhoe.

In my case, the cylinder leaks down. When doing so, the cylinder rod is entering the cylinder. This apparently should not happen if the control valve is good. If the control valve was good, both lines to the double acting cylinder would be closed off, and there would be no place for oil in the cylinder to be displaced to.

You have proven that it is possible to teach an old dog new tricks.

Thanks again!

 

[GreensvilleJay]

re: When the cylinder is retracting the overall volume inside the cylinder is decreasing

the volume inside the cylinder is a constant. say the empty cylinder is 100 cu in, and the rod + seal makeup 30 cu in. That means there's 70 cu in of space for the oil. That can never change. what does change is where the oil is. With 'leaky' seals the oil can flow from one side to the other depending on where the rod gets pushed

 

[TheOldHokie]

re: When the cylinder is retracting the overall volume inside the cylinder is decreasing

the volume inside the cylinder is a constant. say the empty cylinder is 100 cu in, and the rod + seal makeup 30 cu in. That means there's 70 cu in of space for the oil. That can never change. what does change is where the oil is. With 'leaky' seals the oil can flow from one side to the other depending on where the rod gets pushed

You really need to think about that. Perhaps read Hendo's last post and look at the image I posted.

Hint: When the 30cc rod is fully retracted there is 70cc of oil in the 100cc cylinder. When the rod is fully extended there is 100cc of oil in the cylinder.

Dan

 

[GreensvilleJay]

Ok...if you put the cylinder on a bench WHERE does the extra 30cc of oil come from ?
The total volume is 100cc ( 70 cc of oil, 30 cc for the rod), you can't magically get another 30cc of volume into the 100cc cylinder.
Perhaps we're looking at this differently, as I assume the cylinder is not connected to the tractor.
You need to disconnect(isolate) the cylinder from the spool valve. Otherwise you have no way of knowing WHERE the leak is.

My head hurts, off to eat some chili I made at 6 today.......

 

[Henro]

re: When the cylinder is retracting the overall volume inside the cylinder is decreasing

the volume inside the cylinder is a constant. say the empty cylinder is 100 cu in, and the rod + seal makeup 30 cu in. That means there's 70 cu in of space for the oil. That can never change. what does change is where the oil is. With 'leaky' seals the oil can flow from one side to the other depending on where the rod gets pushed

Actually, that volume does change. When the cylinder rod enters the cylinder the volume gets smaller. Cylinder volume is a variable that changes with how much cylinder rod is in the cylinder...OR when the cylinder rod leaves the cylinder the internal volume for fluid increases...NOT a fixed number.

 

[TheOldHokie]

Ok...if you put the cylinder on a bench WHERE does the extra 30cc of oil come from ?
The total volume is 100cc ( 70 cc of oil, 30 cc for the rod), you can't magically get another 30cc of volume into the 100cc cylinder.
Perhaps we're looking at this differently, as I assume the cylinder is not connected to the tractor.
You need to disconnect(isolate) the cylinder from the spool valve. Otherwise you have no way of knowing WHERE the leak is.

My head hurts, off to eat some chili I made at 6 today.......

This is a 7th grade geometry problem.

The volume of oil in the cylinder is equal to the volume of the cylinder minus the volume of the rod that is inside the cylinder. When the rod is pulled out there is room for more oil When the rod is shoved in there is less room for oil. It doesn't matter where the cylinder is or if its on or off the tractor.

1. If the rod is out and the cylinder is full of oil for the rod to move in the volume of oil must decrease to allow for the added volume of the rod.
2. Since the oil is incompressible and there is no outlet for the oil the pressure in the fluid rises to match the force pressing on the rod. The rod is hydraulically lockeed in that direction.

Its a "physics" thing. If you want I will provide you with the algebra.

Dan

 

[RCW]

My head hurts, off to eat some chili I made at 6 today.......

Its a "physics" thing. If you want I will provide you with the algebra.

Jay and Dan - - - I want to understand this too. It frustrates me I'm not getting either side of the equations...I used to be a lot smarter.

I was about to get pen and paper out to help myself figure it out.

I have 46 things I need to do for and before holidays for home and at work....how about you guys take this after January 4, 2022?!?!!?.....

 

[TheOldHokie]

Jay and Dan - - - I want to understand this too. It frustrates me I'm not getting either side of the equations...I used to be a lot smarter.

I was about to get pen and paper out to help myself figure it out.

I have 46 things I need to do for and before holidays for home and at work....how about you guys take this after January 4, 2022?!?!!?.....

Simple example:

If the cylinder is full of oil and the rod is 2" in diameter and you push it 1" into the cylinder it will displace 1/2(2")^^2*pi*1" cubic inches of oil. The portion of the rod that was outside the cylunder is now inside the cylunder taking up space that was previously occupied by oil. Accordingly the volume of oil in the cylinder must decrease by 3.1416 cubic inches.

It does not matter whether it is a single acting cylinder, double scting cylinder with good piston seals, or a double acting cylinder with leaky seals. The volume of oil in the cylinder has to decrease to make room for the additional length of rod.

Dan

 

[kubotafreak]

I heard they have some kind of dampening technology so things dont get jerky. I dont know if that is for the L/R swing cylinders only, or? If that is the case on the bucket control, that may be why, for me.

On these basic designed backhoes, the dampening you describe is done with restrictors/orfice devices at the hose mounts on the valve body. No variable here, just a predetermined hole. The operator is the real dampening with his/her use of the controls and rpm. If kubota stepped up their game, they might add an accumulator for the BH feed line, but that still doesn't affect your issue at hand.

I looked up the control valve body on your model. It looks like the bucket curl does not have its own relief. I know I mentioned boom earlier, I meant bucket. Many times the boom dips as well like your bucket curl letting go. You may very well have a bypass in the valve, or the cylinder. I would swap your lines from the dipper at the valve, to bucket for a bit to make confirm if the problem stays or follows. Problem stays its cylinder, if it moves its the valve. You are lucky in the sense Kubota even offers the individual spools separate from the valve for $128. A seal kit is $305 for the bucket cylinder.

 

[TheOldHokie]

On these basic designed backhoes, the dampening you describe is done with restrictors/orfice devices at the hose mounts on the valve body. No variable here, just a predetermined hole. The operator is the real dampening with his/her use of the controls and rpm. If kubota stepped up their game, they might add an accumulator for the BH feed line, but that still doesn't affect your issue at hand.

I looked up the control valve body on your model. It looks like the bucket curl does not have its own relief. I know I mentioned boom earlier, I meant bucket. Many times the boom dips as well like your bucket curl letting go. You may very well have a bypass in the valve, or the cylinder. I would swap your lines from the dipper at the valve, to bucket for a bit to make confirm if the problem stays or follows. Problem stays its cylinder, if it moves its the valve. You are lucky in the sense Kubota even offers the individual spools separate from the valve for $128. A seal kit is $305 for the bucket cylinder.

Jay and Dan - - - I want to understand this too. It frustrates me I'm not getting either side of the equations...I used to be a lot smarter.

I was about to get pen and paper out to help myself figure it out.

I have 46 things I need to do for and before holidays for home and at work....how about you guys take this after January 4, 2022?!?!!?.....

Perhaps a few pictures with example calculations will clarify things. The shaded area represents the oil inside the cylinder and the dark black is the rod and piston. As the rod and piston move inwards the volume of oil inside the cylinder has to decrease to make room for the retracting rod.

Dan

 

[Henro]

Perhaps a few pictures with example calculations will clarify things. The shaded area represents the oil inside the cylinder and the dark black is the rod and piston. As the rod and piston move inwards the volume of oil inside the cylinder has to decrease to make room for the retracting rod.

Dan

The light turned on in my head when I realized (after reading Dan’s explanations) that if the oil has nowhere to go, the rod will not retract, even if pushed on by an outside force.

Since oil is essentially non compressible, the oil acts the same as if it were concrete. (Unless there is a leakage path for the oil, through a control valve, rod seal, leaky hose, etc. Then the oil acts like oil…)

 

[GreensvilleJay]

TOH, those pictures work for a cylinder with good seals, but in my example, leaky seals will allow the oil to go from the 'left' side of the cylinder to the 'right' side. The ram pushes the oil past the leaky seal to the other side.