Diagram for installing replacement pool valve on Wagner Load

I think my spool valve on my Wagner loader went out completly today. When I raise the bucket it quickly dops unless I hold the spool lever in the up position. I braced the buket up and removed the upper hose and then lifted the arm and the bucket fell, no fluid came out of the top hole on the ram. Could someone tell the least expensive spool valve I cn use? Also all of the valves I've seen are set up for double acting cylindrs on each lever. The lift arms are signle acting cylinders. Would someone have a diagram of how I could plum in the replacement valve.
Thanks,
Bill

most valve setups can be set for fast drop / single acting.

if this is a standalone valve with nothing down line like the 3pt... then you could even get away with a cehap logsplitter dual spool. set one up for sa.. just make sure you get open center.. or a convertable one then set it up as OC. if your old spool was the relief source.. make sure your new valve has relief too.

(quoted from post at 17:50:52 01/10/14) I think my spool valve on my Wagner loader went out completly today. When I raise the bucket it quickly dops unless I hold the spool lever in the up position. I braced the buket up and removed the upper hose and then lifted the arm and the bucket fell, no fluid came out of the top hole on the ram. Could someone tell the least expensive spool valve I cn use? Also all of the valves I've seen are set up for double acting cylindrs on each lever. The lift arms are signle acting cylinders. Would someone have a diagram of how I could plum in the replacement valve.
Thanks,
Bill

Click to expand...

Haven't we been down this road before? You will be hard pressed to beat this price:

[u:51e02669fb]Prince MB21BB5C1 Two spool DA Valve[/u:51e02669fb]

If you need a picture of how to hook it up send me email.

TOH
[email protected]

(quoted from post at 17:50:52 01/10/14) I think my spool valve on my Wagner loader went out completly today. When I raise the bucket it quickly dops unless I hold the spool lever in the up position. I braced the buket up and removed the upper hose and then lifted the arm and the bucket fell, no fluid came out of the top hole on the ram. Could someone tell the least expensive spool valve I cn use? Also all of the valves I've seen are set up for double acting cylindrs on each lever. The lift arms are signle acting cylinders. Would someone have a diagram of how I could plum in the replacement valve.
Thanks,
Bill

Click to expand...

Looking back at your test I don't see how you have determined the valve is at fault. You will get that result if the bucket cylinder seals are blown. The oil inside the cylinder simply moves from one side of the piston to the other as the rod extends and nothing comes out the open port.

TOH

I removed the hose from the top of the bucket cylinder where it attached to the frame, with the bucket liifted and supported. I then raised the lift arm and the bucket leaked down. No oil came out of the opening in the top of the cylinder. If the seal was leaking inside of the cylined, I would think the oil would come out of the top of the cylinder once the bucket had totaly dropped.

(quoted from post at 18:44:19 01/10/14) I removed the hose from the top of the bucket cylinder where it attached to the frame, with the bucket liifted and supported. I then raised the lift arm and the bucket leaked down. No oil came out of the opening in the top of the cylinder. If the seal was leaking inside of the cylined, I would think the oil would come out of the top of the cylinder once the bucket had totaly dropped.

Click to expand...

Why would it come out? The piston moved from one end of the cylinder to the other and the oil went right past it. No oil has entered or exited the cylinder. What is going to push oil out the open port?

I sent you the valve diagram but you really need to make sure it is the problem first.

TOH

good price!

"Why would it come out? The piston moved from one end of the cylinder to the other and the oil went right past it. No oil has entered or exited the cylinder. What is going to push oil out the open port?"

Volume? The side of the piston with the rod has a lot less area in the cylinder bore than the side with no rod. Fill a cylinder with the rod extended, then lower it with the oil pushing past the piston and the top side won't be able to hold all the oil because of the space taken up by the rod. The excess oil will come out the top port.

(quoted from post at 00:27:20 01/11/14)

"Why would it come out? The piston moved from one end of the cylinder to the other and the oil went right past it. No oil has entered or exited the cylinder. What is going to push oil out the open port?"

Volume? The side of the piston with the rod has a lot less area in the cylinder bore than the side with no rod. Fill a cylinder with the rod extended, then lower it with the oil pushing past the piston and the top side won't be able to hold all the oil because of the space taken up by the rod. The excess oil will come out the top port.

Click to expand...

That is true and a point to consider. But this is a bucket cylinder and your scenario would be a bucket roll operation. In his test the bucket is dumping and the rod is moving out of the cylinder increasing the internal volume of the cylinder. There is plenty of room in the cylinder for all of the oil.

The test I gave him the first time he asked is the one I use and is "volume neutral". It isolates the cylinder from the rest of the hydraulic system and is simple and definitive:[list:eaec4008a7][*:eaec4008a7]Roll the bucket back and block or tie it up.[*:eaec4008a7]Remove both hoses and plug/cap both ports on the cylinder.[*:eaec4008a7]Remove the bucket support[/list:u:eaec4008a7] If that bucket drops the seals in the cylinder are leaking.

TOH

"Roll the bucket back and block or tie it up.
Remove both hoses and plug/cap both ports on the cylinder.
Remove the bucket support
If that bucket drops the seals in the cylinder are leaking. "

That won't work as a piston seal test. There's a volume difference because of the rod. If both ports are closed off the rod and piston can't move, even if there is a 1/2" hole in the piston. You would be pulling the big volume end towards a "vacuum". The rod can only move if there is a leak at the rod seal end to let air in. If you leave the rod end port open, then it would test the piston seals.

(quoted from post at 10:58:50 01/11/14)
"Roll the bucket back and block or tie it up.
Remove both hoses and plug/cap both ports on the cylinder.
Remove the bucket support
If that bucket drops the seals in the cylinder are leaking. "

That won't work as a piston seal test. There's a volume difference because of the rod. If both ports are closed off the rod and piston can't move, even if there is a 1/2" hole in the piston. You would be pulling the big volume end towards a "vacuum". The rod can only move if there is a leak at the rod seal end to let air in. If you leave the rod end port open, then it would test the piston seals.

Click to expand...

With the spool on a properly functioning control valve positioned in neutral you have EXACTLY the same situation as a plugged/capped cylinder. Are you suggesting that if the control valve is working properly a piston seal leak can't cause leak down because "vacuum" will hold the cylinder in place? The only difference in my test is that both sides of the control valve spool have been eliminated as a potential source of oil/pressure leakage.

It's been a very long time since freshman physics class but here is my analysis. When we initially remove the support under the bucket the weight of the bucket is applied to the rod trying to pull it out of the cylinder. Oil is incompressible so if the seals are tight the oil pressure on the rod end side of the piston simply goes up offsetting the added force and the rod is held in place by the increase in oil pressure. In fact that trapped oil pressure will hold much much more than the weight of the bucket as long as the seals are tight. But when the seals are leaking that elevated pressure is transmitted through the leaking seal to the oil on the lower pressure side of the piston. That causes an increase in the pressure on the base end, the piston moves and the rod extends. Once the oil pressure on both sides is once again equal the piston and rod stop moving. The pressure will never be equal on both sides until the bucket has dropped to the mechanical limit of it's travel and something other than oil pressure is supporting its weight.

TOH

"When we initially remove the support under the bucket the weight of the bucket is applied to the rod trying to pull it out of the cylinder. Oil is incompressible so if the seals are tight the oil pressure on the rod end side of the piston simply goes up offsetting the added force and the rod is held in place by the increase in oil pressure. In fact that trapped oil pressure will hold much much more than the weight of the bucket as long as the seals are tight."

True.

"But when the seals are leaking that elevated pressure is transmitted through the leaking seal to the oil on the lower pressure side of the piston. That causes an increase in the pressure on the base end, the piston moves and the rod extends."

No, that can't happen. If the rod extends and the piston tries to move toward that end, The volume of oil displaced by the movement of the piston that goes past the piston seal is less than the volume of oil needed to fill the space created behind the piston. There's no rod on the back side of the piston so that space is larger. Air would have to be introduced to allow the piston to move.

"Once the oil pressure on both sides is once again equal the piston and rod stop moving. The pressure will never be equal on both sides until the bucket has dropped to the mechanical limit of it's travel and something other than oil pressure is supporting its weight."

It has nothing to do with pressure. It's volume. Take a double acting cylinder. Take the piston out and throw it away. Insert the rod halfway into the cylinder. Fill the cylinder completely with oil and cap both ports. There's no piston at all, so that's a big seal leak. Try to move the rod. You can't. You can't move it in or out. You cannot cap both ports to test for a piston seal leak.

(quoted from post at 13:05:10 01/11/14)

"When we initially remove the support under the bucket the weight of the bucket is applied to the rod trying to pull it out of the cylinder. Oil is incompressible so if the seals are tight the oil pressure on the rod end side of the piston simply goes up offsetting the added force and the rod is held in place by the increase in oil pressure. In fact that trapped oil pressure will hold much much more than the weight of the bucket as long as the seals are tight."

True.


No, that can't happen. If the rod extends and the piston tries to move toward that end, The volume of oil displaced by the movement of the piston that goes past the piston seal is less than the volume of oil needed to fill the space created behind the piston. There's no rod on the back side of the piston so that space is larger. Air would have to be introduced to allow the piston to move.

It has nothing to do with pressure. It's volume. Take a double acting cylinder. Take the piston out and throw it away. Insert the rod halfway into the cylinder. Fill the cylinder completely with oil and cap both ports. There's no piston at all, so that's a big seal leak. Try to move the rod. You can't. You can't move it in or out. You cannot cap both ports to test for a piston seal leak.

Click to expand...

OK. Just so happens the bucket cylinders on my Kubby are leaking internally and high on my list of things to fix. Here is my leakdown test.[list:367e1adb88][*:367e1adb88]Bucket rolled back and tied in place with that blue strap[*:367e1adb88]Double acing cylinder full of oil and both ports tightly plugged.[*:367e1adb88]Strap removed from bucket. [*:367e1adb88]Weight of bucket immediately begins to pull rod from cylinder and stops once bucket hits the mechanical stops.[/list:u:367e1adb88]What happened? Keep in mind - pressure and volume are related.

TOH

(quoted from post at 19:23:49 01/11/14)

(quoted from post at 13:05:10 01/11/14)

"When we initially remove the support under the bucket the weight of the bucket is applied to the rod trying to pull it out of the cylinder. Oil is incompressible so if the seals are tight the oil pressure on the rod end side of the piston simply goes up offsetting the added force and the rod is held in place by the increase in oil pressure. In fact that trapped oil pressure will hold much much more than the weight of the bucket as long as the seals are tight."

True.


No, that can't happen. If the rod extends and the piston tries to move toward that end, The volume of oil displaced by the movement of the piston that goes past the piston seal is less than the volume of oil needed to fill the space created behind the piston. There's no rod on the back side of the piston so that space is larger. Air would have to be introduced to allow the piston to move.

It has nothing to do with pressure. It's volume. Take a double acting cylinder. Take the piston out and throw it away. Insert the rod halfway into the cylinder. Fill the cylinder completely with oil and cap both ports. There's no piston at all, so that's a big seal leak. Try to move the rod. You can't. You can't move it in or out. You cannot cap both ports to test for a piston seal leak.

Click to expand...

OK. Just so happens the bucket cylinders on my Kubby are leaking internally and high on my list of things to fix. Here is my leakdown test.[list:69cd0c3afd][*:69cd0c3afd]Bucket rolled back and tied in place with that blue strap[*:69cd0c3afd]Double acing cylinder full of oil and both ports tightly plugged.[*:69cd0c3afd]Strap removed from bucket. [*:69cd0c3afd]Weight of bucket immediately begins to pull rod from cylinder and stops once bucket hits the mechanical stops.[/list:u:69cd0c3afd]What happened?

TOH

Click to expand...

Oil does not compress nor expand (maybe a little when heated). Since the back end of the cylinder has more area than is displaced by the front side of the piston, it's physically impossible for the rods to extend if the cylinders are full of oil without introducing more oil or air to the cylinder. The rod seals allowed air to be drawn in. The air then migrated to the rear side of the piston so the bucket could drop.

(quoted from post at 14:35:06 01/11/14)

(quoted from post at 19:23:49 01/11/14)

(quoted from post at 13:05:10 01/11/14)

"When we initially remove the support under the bucket the weight of the bucket is applied to the rod trying to pull it out of the cylinder. Oil is incompressible so if the seals are tight the oil pressure on the rod end side of the piston simply goes up offsetting the added force and the rod is held in place by the increase in oil pressure. In fact that trapped oil pressure will hold much much more than the weight of the bucket as long as the seals are tight."

True.


No, that can't happen. If the rod extends and the piston tries to move toward that end, The volume of oil displaced by the movement of the piston that goes past the piston seal is less than the volume of oil needed to fill the space created behind the piston. There's no rod on the back side of the piston so that space is larger. Air would have to be introduced to allow the piston to move.

It has nothing to do with pressure. It's volume. Take a double acting cylinder. Take the piston out and throw it away. Insert the rod halfway into the cylinder. Fill the cylinder completely with oil and cap both ports. There's no piston at all, so that's a big seal leak. Try to move the rod. You can't. You can't move it in or out. You cannot cap both ports to test for a piston seal leak.

Click to expand...

OK. Just so happens the bucket cylinders on my Kubby are leaking internally and high on my list of things to fix. Here is my leakdown test.[list:adbd3b773e][*:adbd3b773e]Bucket rolled back and tied in place with that blue strap[*:adbd3b773e]Double acing cylinder full of oil and both ports tightly plugged.[*:adbd3b773e]Strap removed from bucket. [*:adbd3b773e]Weight of bucket immediately begins to pull rod from cylinder and stops once bucket hits the mechanical stops.[/list:u:adbd3b773e]What happened?

TOH

Click to expand...

Oil does not compress nor expand (maybe a little when heated). Since the back end of the cylinder has more area than is displaced by the front side of the piston, it's physically impossible for the rods to extend if the cylinders are full of oil without introducing more oil or air to the cylinder. The rod seals allowed air to be drawn in. The air then migrated to the rear side of the piston so the bucket could drop.

Click to expand...

We can discuss the physics at greater length but there are no external oil leaks and we have detected a leaking piston seal with both ports plugged. :idea:

More experimental physics results from the Garaj Mahal lab. When the rod is disconnected from the bucket it partially retracts. What is that all about? Freshmen physics leads me to say that is the result of a positive air pressure differential acting on the end of the rod that is outside the cylinder - it is not in pressure equilbrium with the portion of the rod that is inside the cylinder and it moves to equalize the two pressures. If we had sucked a bunch of air into the cylinder there would be no pressure differential and the rod would be in equilibrium. Hmmm....

More experimenting - I can easily pull the rod out increasing the volume of the base end because I am simply overcoming air pressure and creating a [u:adbd3b773e]vacuum[/u:adbd3b773e] in the base proportional to the change in volume. That "vacuum" is simply a pressure differential between the hermetically sealed interior of the cylinder and the atmospheric air pressure OUTSIDE the cylinder.

More experimenting - I cannot push the rod in even the slightest - hard as a rock. Further indication that there is no gas (e.g. air) on the back side of the cylinder to be compressed. The backside appears to be 100% full of incompressible oil and it would seem my seals are only leaking in one direction. That is imformation we would not get if we only plugged one port.

Agree or disagree on the physics?

TOH

(quoted from post at 21:02:38 01/11/14)

(quoted from post at 14:35:06 01/11/14)

(quoted from post at 19:23:49 01/11/14)

(quoted from post at 13:05:10 01/11/14)

"When we initially remove the support under the bucket the weight of the bucket is applied to the rod trying to pull it out of the cylinder. Oil is incompressible so if the seals are tight the oil pressure on the rod end side of the piston simply goes up offsetting the added force and the rod is held in place by the increase in oil pressure. In fact that trapped oil pressure will hold much much more than the weight of the bucket as long as the seals are tight."

True.


No, that can't happen. If the rod extends and the piston tries to move toward that end, The volume of oil displaced by the movement of the piston that goes past the piston seal is less than the volume of oil needed to fill the space created behind the piston. There's no rod on the back side of the piston so that space is larger. Air would have to be introduced to allow the piston to move.

It has nothing to do with pressure. It's volume. Take a double acting cylinder. Take the piston out and throw it away. Insert the rod halfway into the cylinder. Fill the cylinder completely with oil and cap both ports. There's no piston at all, so that's a big seal leak. Try to move the rod. You can't. You can't move it in or out. You cannot cap both ports to test for a piston seal leak.

Click to expand...

OK. Just so happens the bucket cylinders on my Kubby are leaking internally and high on my list of things to fix. Here is my leakdown test.[list:2e1f32d508][*:2e1f32d508]Bucket rolled back and tied in place with that blue strap[*:2e1f32d508]Double acing cylinder full of oil and both ports tightly plugged.[*:2e1f32d508]Strap removed from bucket. [*:2e1f32d508]Weight of bucket immediately begins to pull rod from cylinder and stops once bucket hits the mechanical stops.[/list:u:2e1f32d508]What happened?

TOH

Click to expand...

Oil does not compress nor expand (maybe a little when heated). Since the back end of the cylinder has more area than is displaced by the front side of the piston, it's physically impossible for the rods to extend if the cylinders are full of oil without introducing more oil or air to the cylinder. The rod seals allowed air to be drawn in. The air then migrated to the rear side of the piston so the bucket could drop.

Click to expand...

We can discuss the physics at greater length but there are no external oil leaks and we have detected a leaking piston seal with both ports plugged. :idea:

More experimental physics results from the Garaj Mahal lab. When the rod is disconnected from the bucket it partially retracts. What is that all about? Freshmen physics leads me to say that is the result of a positive air pressure differential acting on the end of the rod that is outside the cylinder - it is not in pressure equilbrium with the portion of the rod that is inside the cylinder and it moves to equalize the two pressures. If we had sucked a bunch of air into the cylinder there would be no pressure differential and the rod would be in equilibrium. Hmmm....

More experimenting - I can easily pull the rod out increasing the volume of the base end because I am simply overcoming air pressure and creating a [u:2e1f32d508]vacuum[/u:2e1f32d508] in the base proportional to the change in volume. That "vacuum" is simply a pressure differential between the hermetically sealed interior of the cylinder and the atmospheric air pressure OUTSIDE the cylinder.

More experimenting - I cannot push the rod in even the slightest - hard as a rock. Further indication that there is no gas (e.g. air) on the back side of the cylinder to be compressed. The backside appears to be 100% full of incompressible oil and it would seem my seals are only leaking in one direction. That is imformation we would not get if we only plugged one port.

Agree or disagree on the physics?

TOH

Click to expand...

The backside cannot be 100 percent full of oil with the cylinder extended. There was not enough oil in the front of the cylinder to begin with to fill that void. The rod end holds less oil because of the space the rod takes up in the bore. You're ignoring my simple example. Take a double acting cylinder. Take the piston out and throw it away. Insert the rod halfway into the cylinder. Fill the cylinder completely with oil and cap both ports. There's no piston at all, so that's a big seal leak. You can't move the rod in or out. You can't push it in because there's no place for the displaced oil to go. You can't pull it out because there's no oil to fill the void it leaves when coming out. It's hydraulically locked. Having a piston on the rod makes no difference. The only way a capped cylinder can move is to introduce more air oir oil into it as it extends. It's simple, basic physics. I don't make the laws.

Take a cylinder with the piston centered and fill both ends with oil. Run an oil filled hose between the 2 ports. It still cannot move because of the displacement difference on each side of the piston.

(quoted from post at 18:15:23
The backside cannot be 100 percent full of oil with the cylinder extended. There was not enough oil in the front of the cylinder to begin with to fill that void. The rod end holds less oil because of the space the rod takes up in the bore. You're ignoring my simple example. Take a double acting cylinder. Take the piston out and throw it away. Insert the rod halfway into the cylinder. Fill the cylinder completely with oil and cap both ports. There's no piston at all, so that's a big seal leak. You can't move the rod in or out. You can't push it in because there's no place for the displaced oil to go. You can't pull it out because there's no oil to fill the void it leaves when coming out. It's hydraulically locked. Having a piston on the rod makes no difference. The only way a capped cylinder can move is to introduce more air or oil into it as it extends. It's simple, basic physics. I don't make the laws.

Take a cylinder with the piston centered and fill both ends with oil. Run an oil filled hose between the 2 ports. It still cannot move because of the displacement difference on each side of the piston.

Click to expand...

Well I thought I understood basic physics but maybe I don't. Physics can be unintuitive at times and it wouldn't be the first time I was fooled by a force problem. Just for clarifiication when I said the backside was 100% full of oil I was talking about the partially extended state after the bucket has been disconnected. At that point the backside volume is equal to the sum of the front and backside volumes at the start of the test. We are both in agreement - it grows faster then a the front side decreases as the rod extends.

I am not trying to be argumentative just for the sake of being argumentative. Your volume explanation is quite compelling and you just about had me convinced which is why I went and did the test. But the cylinder did move with the ports plugged so what I am looking for is an explanation of that observed behavior. So far you haven't provided one that matches all of my observations. If I have introduced air into the backside of the piston why can I not compress that air by pushing the rod in?

TOH

(quoted from post at 23:54:35 01/11/14)

(quoted from post at 18:15:23
The backside cannot be 100 percent full of oil with the cylinder extended. There was not enough oil in the front of the cylinder to begin with to fill that void. The rod end holds less oil because of the space the rod takes up in the bore. You're ignoring my simple example. Take a double acting cylinder. Take the piston out and throw it away. Insert the rod halfway into the cylinder. Fill the cylinder completely with oil and cap both ports. There's no piston at all, so that's a big seal leak. You can't move the rod in or out. You can't push it in because there's no place for the displaced oil to go. You can't pull it out because there's no oil to fill the void it leaves when coming out. It's hydraulically locked. Having a piston on the rod makes no difference. The only way a capped cylinder can move is to introduce more air or oil into it as it extends. It's simple, basic physics. I don't make the laws.

Take a cylinder with the piston centered and fill both ends with oil. Run an oil filled hose between the 2 ports. It still cannot move because of the displacement difference on each side of the piston.

Click to expand...

Well I thought I understood basic physics but maybe I don't. Physics can be unintuitive at times and it wouldn't be the first time I was fooled by a force problem. Just for clarifiication when I said the backside was 100% full of oil I was talking about the partially extended state after the bucket has been disconnected. At that point the backside volume is equal to the sum of the front and backside volumes at the start of the test. We are both in agreement - it grows faster then a the front side decreases as the rod extends.

I am not trying to be argumentative just for the sake of being argumentative. Your volume explanation is quite compelling and you just about had me convinced which is why I went and did the test. But the cylinder did move with the ports plugged so what I am looking for is an explanation of that observed behavior. So far you haven't provided one that matches all of my observations. If I have introduced air into the backside of the piston why can I not compress that air by pushing the rod in?

TOH

Click to expand...

Never mind. I've presented the basic science of hydraulic behavior with the simplest example I can think of. I have no doubt you are fully capable of understanding it. But you prefer to ignore it and argue rather than admit that your your testing method is flawed. Have at it. I'm done.

(quoted from post at 20:21:19 01/11/14)

(quoted from post at 23:54:35 01/11/14)

(quoted from post at 18:15:23
The backside cannot be 100 percent full of oil with the cylinder extended. There was not enough oil in the front of the cylinder to begin with to fill that void. The rod end holds less oil because of the space the rod takes up in the bore. You're ignoring my simple example. Take a double acting cylinder. Take the piston out and throw it away. Insert the rod halfway into the cylinder. Fill the cylinder completely with oil and cap both ports. There's no piston at all, so that's a big seal leak. You can't move the rod in or out. You can't push it in because there's no place for the displaced oil to go. You can't pull it out because there's no oil to fill the void it leaves when coming out. It's hydraulically locked. Having a piston on the rod makes no difference. The only way a capped cylinder can move is to introduce more air or oil into it as it extends. It's simple, basic physics. I don't make the laws.

Take a cylinder with the piston centered and fill both ends with oil. Run an oil filled hose between the 2 ports. It still cannot move because of the displacement difference on each side of the piston.

Click to expand...

Well I thought I understood basic physics but maybe I don't. Physics can be unintuitive at times and it wouldn't be the first time I was fooled by a force problem. Just for clarifiication when I said the backside was 100% full of oil I was talking about the partially extended state after the bucket has been disconnected. At that point the backside volume is equal to the sum of the front and backside volumes at the start of the test. We are both in agreement - it grows faster then a the front side decreases as the rod extends.

I am not trying to be argumentative just for the sake of being argumentative. Your volume explanation is quite compelling and you just about had me convinced which is why I went and did the test. But the cylinder did move with the ports plugged so what I am looking for is an explanation of that observed behavior. So far you haven't provided one that matches all of my observations. If I have introduced air into the backside of the piston why can I not compress that air by pushing the rod in?

TOH

Click to expand...

Never mind. I've presented the basic science of hydraulic behavior with the simplest example I can think of. I have no doubt you are fully capable of understanding it. But you prefer to ignore it and argue rather than admit that your your testing method is flawed. Have at it. I'm done.

Click to expand...

I'd be happy to acknowledge my testing error and your explanation if I understood it. But I don't. I'll think about it some more.

Thanks for trying,

TOH

(reply to post at 20:43:00 01/11/14)

I'll think about it

Click to expand...

Thinking wasn't helping so I decided to do some independent research. I almost immediately found an exhaustive discussion of the physics of hydraulic cylinder drift on the Machinery Lubrication website,:

Link to Full Article

As I said in another thread - intuition and common sense can sometimes fool you. Here is the salient portion I extracted from the article - exception two is the scenario we are dealing with in the bucket experiment:

[color=blue:e7b90b4a5a][i:e7b90b4a5a]A popular misconception involving hydraulic cylinders is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal can be the root cause of cylinder drift, the physics involved are often misunderstood.[/color:e7b90b4a5a]

[color=red:e7b90b4a5a]Fact is, if the piston seal is completely removed from a double-acting cylinder, the cylinder is filled with oil and the ports are plugged, the cylinder will hold its load indefinitely, unless the rod-seal leaks.[/color:e7b90b4a5a]

In this condition, due to the unequal volume on either side of the piston, fluid pressure equalizes and the cylinder becomes hydraulically locked. Once this occurs, the cylinder can move only if fluid escapes from the cylinder via the rod seal or its ports.


[u:e7b90b4a5a][b:e7b90b4a5a][color=red:e7b90b4a5a]Exceptions to the Rule[/color:e7b90b4a5a][/b:e7b90b4a5a][/u:e7b90b4a5a]

There are two exceptions to this theory. The first is a double-rod cylinder (Figure 1) where volume is equal on both sides of the piston.



Figure 1. Double-rod cylinder - oil volume is equal on both sides of the piston

The [b:e7b90b4a5a][color=red:e7b90b4a5a]second exception[/color:e7b90b4a5a][/b:e7b90b4a5a] involves a load hanging on a double-acting cylinder (Figure 2). In this arrangement, the volume of pressurized fluid on the rod side can easily be accommodated on the piston side. [b:e7b90b4a5a]But as the cylinder drifts, a vacuum will develop on the piston side due to unequal volumes, and [u:e7b90b4a5a]depending on the weight of the load[/u:e7b90b4a5a], this vacuum may eventually result in equilibrium that arrests further drift.[/b:e7b90b4a5a]



Figure 2. Load hanging on double-acting cylinder[/i:e7b90b4a5a][/color]

You can do some cool things with double rod cylinders. I've used them to evenly raise a big load with a shifting center of gravity using a double rod cylinder on all 4 corners. Hydraulic pressure is applied to one end of one cylinder, the oil from the other side goes to the pressure side of the next, and so on until the last output goes back to tank. It works because all of the volumes are the same. It's a bugger to bleed but once it's full the results are great, and you get the added benefit of using the rod glands as linear bearings.

(quoted from post at 00:06:53 01/13/14) You can do some cool things with double rod cylinders. I've used them to evenly raise a big load with a shifting center of gravity using a double rod cylinder on all 4 corners. Hydraulic pressure is applied to one end of one cylinder, the oil from the other side goes to the pressure side of the next, and so on until the last output goes back to tank. It works because all of the volumes are the same. It's a bugger to bleed but once it's full the results are great, and you get the added benefit of using the rod glands as linear bearings.

Click to expand...

At the risk of starting another mind-numbing physics discussion :twisted: Here is some food for thought:

You could connect four equal volume single acting cylinders in parallel and reduce the hydraulic pressure needed to lift the load by 4X. But it won't do what you want. There is more to the physics of your double rod cylinder rigging than just having cylinders with equal displacements and I had to think about it for awhile to reason it out.

TOH

(quoted from post at 16:02:06 01/13/14)

(quoted from post at 00:06:53 01/13/14) You can do some cool things with double rod cylinders. I've used them to evenly raise a big load with a shifting center of gravity using a double rod cylinder on all 4 corners. Hydraulic pressure is applied to one end of one cylinder, the oil from the other side goes to the pressure side of the next, and so on until the last output goes back to tank. It works because all of the volumes are the same. It's a bugger to bleed but once it's full the results are great, and you get the added benefit of using the rod glands as linear bearings.

Click to expand...

At the risk of starting another mind-numbing physics discussion :twisted: Here is some food for thought:

You could connect four equal volume single acting cylinders in parallel and reduce the hydraulic pressure needed to lift the load by 4X. But it won't do what you want. There is more to the physics of your double rod cylinder rigging than just having cylinders with equal displacements and I had to think about it for awhile to reason it out.

TOH

Click to expand...

I happen to like mind-numbing physics discussions. Four single acting cylinders will advance at different rates if they are loaded unevenly. Flow dividers help but the cylinders still get out of time due to internal leakage. The double rod cylinder setup is great because the oil out of one cylinder is the oil in to the next so they advance evenly no matter what.

(quoted from post at 22:50:19 01/13/14)

(quoted from post at 16:02:06 01/13/14)

(quoted from post at 00:06:53 01/13/14) You can do some cool things with double rod cylinders. I've used them to evenly raise a big load with a shifting center of gravity using a double rod cylinder on all 4 corners. Hydraulic pressure is applied to one end of one cylinder, the oil from the other side goes to the pressure side of the next, and so on until the last output goes back to tank. It works because all of the volumes are the same. It's a bugger to bleed but once it's full the results are great, and you get the added benefit of using the rod glands as linear bearings.

Click to expand...

At the risk of starting another mind-numbing physics discussion :twisted: Here is some food for thought:

You could connect four equal volume single acting cylinders in parallel and reduce the hydraulic pressure needed to lift the load by 4X. But it won't do what you want. There is more to the physics of your double rod cylinder rigging than just having cylinders with equal displacements and I had to think about it for awhile to reason it out.

TOH

Click to expand...

I happen to like mind-numbing physics discussions. Four single acting cylinders will advance at different rates if they are loaded unevenly. Flow dividers help but the cylinders still get out of time due to internal leakage. The double rod cylinder setup is great because the oil out of one cylinder is the oil in to the next so they advance evenly no matter what.

Click to expand...

Yes - that is the description of the practical result. What is the physics that produces that difference in behavior?

The key is understanding static pressure. Different cylinder loads produce different static pressures inside the cylinder. The pressure inside a cylinder supporting a 1000# load is double the pressure of that same cylinder supporting a 500# load.

Four SA cylinders connected in parallel are all operating at the same static pressure. So a heavier load on any one causes it to retract and displace oil into the cylinders at a lower pressure causing them to extend.

As we learned earlier a double acting cylinder is "hydraulically locked" in both directions at all times. So even though they are interconnected on a single circuit the static pressure in each one can vary depending on the load they are supporting without displacing any oil into the others.

TOH

Duplicate post deleted

(quoted from post at 23:33:54 01/13/14)
As we learned earlier a [color=red:9b72322b91]double acting[/color:9b72322b91] cylinder is "hydraulically locked" in both directions at all times. So even though they are interconnected on a single circuit the static pressure in each one can vary depending on the load they are supporting without displacing any oil into the others.

TOH

Click to expand...

Aha, I think I found our disconnect. I'm talking double [i:9b72322b91]rod[/i:9b72322b91] cylinders and you're thinking double [i:9b72322b91]acting[/i:9b72322b91] cylinders. It's the double rod that make the volume the same on both sides of the piston and prevents them from hydraulically locking in the circuit I described. I should make a schematic.

(quoted from post at 00:50:57 01/14/14)

(quoted from post at 23:33:54 01/13/14)
As we learned earlier a [color=red:0e27fde279]double acting[/color:0e27fde279] cylinder is "hydraulically locked" in both directions at all times. So even though they are interconnected on a single circuit the static pressure in each one can vary depending on the load they are supporting without displacing any oil into the others.

TOH

Click to expand...

Aha, I think I found our disconnect. I'm talking double [i:0e27fde279]rod[/i:0e27fde279] cylinders and you're thinking double [i:0e27fde279]acting[/i:0e27fde279] cylinders. It's the double rod that make the volume the same on both sides of the piston and prevents them from hydraulically locking in the circuit I described. I should make a schematic.

Click to expand...

I'm thinking double rod cylinders as shown below. They have a sealed piston and are double acting. If I understand your system properly you are only using them to hydraulically move the load up and hold it. Gravity is sufficient to lower the load on it's own

TOH

single rod, double acting cylinder. Power up and down.

A picture is worth a thousand words. The cylinders can be powered up or down.

(quoted from post at 21:46:57 01/14/14) A picture is worth a thousand words. The cylinders can be powered up or down.

Click to expand...

Oh - I understood the setup. The point I was trying to make was that the reason it works as you want is that the cylinders are hydraulically locked against each other. And because teh displacements in each end of the cylinder are equal the rods all move the same amount.

But if i have got the physics right (always a big if) the downside is that means the cylinder loads are additive and the hydraulic pressure requirement is the same as if you only had one cylinder lifting the load. You could use 100 cylinders and it would not change the pressure requirement. One calculation is worth a thousand words:



Total load being raised:6000 pounds
Effective piston area of each cylinder: 2 sq in.

Assuming the load to be lifted is evenly distributed across all four rod ends (e.g. 1500#) the hydraulic loading on the individual cylinders is:

C4 = 1500# piston load @ 750 PSI
C3 = 3000# piston load @ 1500 PSI
C2 = 4500# piston load @ 2250 PSI
C1 = 6000# piston load @ 3000 PSI

Agree or disagree?

TOH

(quoted from post at 09:34:36 01/15/14)

(quoted from post at 21:46:57 01/14/14) A picture is worth a thousand words. The cylinders can be powered up or down.

Click to expand...

Oh - I understood the setup. The point I was trying to make was that the reason it works as you want is that the cylinders are hydraulically locked against each other. And because teh displacements in each end of the cylinder are equal the rods all move the same amount.

But if i have got the physics right (always a big if) the downside is that means the cylinder loads are additive and the hydraulic pressure requirement is the same as if you only had one cylinder lifting the load. You could use 100 cylinders and it would not change the pressure requirement. One calculation is worth a thousand words:

Total load being raised:6000 pounds
Effective piston area of each cylinder: 2 sq in.

Assuming the load to be lifted is evenly distributed across all four rod ends (e.g. 1500#) the hydraulic loading on the individual cylinders is:

C4 = 1500# piston load @ 750 PSI
C3 = 3000# piston load @ 1500 PSI
C2 = 4500# piston load @ 2250 PSI
C1 = 6000# piston load @ 3000 PSI

Agree or disagree?

TOH

Click to expand...

Exactly right. That's the downside, you have to buy 4 "full sized" cylinders. But usually the cost is offset by all of the linear bearings and other guidance you don't have to buy. The other downside is that the circuit baffles maintenance guys that are used to a more traditional setup. I've used this setup more than once with good results though.