adding hydraulic thumb kubota B21

[orangetree]

You don't *have* to use a return to tank (eg unpressurized) here

You can use the same valve as a 3rd function, which will have four ports in use: cylinder A, B, power beyond in, power beyound out.

Many of these 3rd function valves are on/off, meaning the valve is kept simple. keep in mind its the pressure *difference* (A to B) that operates the cylinder. if a valve uses a tank port, that pressure is just atmosphere, so the cylinder will see the full line gauge pressure. a 3rd function valve with no tank line is beholden to the rest of the PB circuit being free-flowing, but this is not a problem just a note.

 

[joea99]

You don't *have* to use a return to tank (eg unpressurized) here

You can use the same valve as a 3rd function, which will have four ports in use: cylinder A, B, power beyond in, power beyound out.

Many of these 3rd function valves are on/off, meaning the valve is kept simple. keep in mind its the pressure *difference* (A to B) that operates the cylinder. if a valve uses a tank port, that pressure is just atmosphere, so the cylinder will see the full line gauge pressure. a 3rd function valve with no tank line is beholden to the rest of the PB circuit being free-flowing, but this is not a problem just a note.

That is kind of what I thought early on, but when I did some asking around and other "research", some said, including the vendor, that Tank must go to tank in all cases. Some mentioned some valves are not built to handle full pressure everywhere internally. I just accepted those statements without really thinking about it, but remained a bit puzzled. Only have the schematic (below) to go by, no idea of the valve internals. I would tend to think the PB should block the T ports (goes into "N") otherwise it would "short" the next in line, but who knows, maybe they are concerned about leakage past internal seals?

I suppose I can hook every thing up with a temporary line from the Tank port to the tank fill and see what happens when I fire it up. If nothing comes out at "center" I
can put the implement through a couple of cycles and watch for any fluid out.

The diagram is for a 3 spool, but same for all, I guess.

 

[rScotty]

Thanks. I can see that being the case when the valve is the last one "in line", but these will each have a PB port in play so when not actuated most, hopefully all, of the flow should be directed to the next valve, right?

Regardless, if I just size them the same as existing tank lines it should be fine. I can see it would be foolish or false economy anyway, to size smaller.

joe a.

No. That's not correct. I see the problem. Assume that the PB is not involved in return flow. In fact, begin by considering a spool valve assembly that doesn't have a PB at all.

Instead, think of it this way: Every valve assembly - in fact every spool valve within an assembly - will have its own return path which is opened by the movement of the spool. Moving the valve spool always does two things simultaneously - it provides pressure to one end of the cylinder and also provides a return path for fluid from the other end of that cylinder.

To help visualize this, think on what it means when you are not moving the spool valve. When the spool is centered, the oil that is going to become the return oil is trapped in the cylinder because it cannot get past the spool valve to go anywhere. In fact, that trapped oil is helping to hold the loader arms and bucket from flopping as you drive.

Now take another look at your valve schematic. Follow the arrows inside the schematic as the spool valve is moved back and forth. You will see that the arrows are dead-ended until the spool valve is moved laterally. Unfortunately, hydraulic schematics almost never illustrate how the ports line up when the spool is moved laterally. I wish that they did; but you have to either draw it out or use your imagination.

Back in 2017, "www.FluidPowerWorld.com" published a 10 or 20 part monthly tutorial on reading hydraulic schematics. It is inspired teaching and very readable.
starts with:
Hydraulic symbology 101: Understanding basic fluid

I've heard that some folks have successfully used the PB port as a return port, but that is not its function, & connecting it that way doesn't give dependable results. Some valves will work that way and others won't. Most won't. It depends on their internal construction.
At this point in your understanding I think you should just accept that the PB and the Tank return should be completely different circuits. That is simplest.
rScotty

 

[orangetree]

Tank must go to tank in all cases. Some mentioned some valves are not built to handle full pressure everywhere internally.

There are many different types of valves. If the specific valve requires tank(atmosphere drain) then YES it requires it. But this is not the case for all valves. Take this Summit 3rd function valve with EXACTLY FOUR ports. NO tank here. And note that some valves can do BOTH - in this case the valve will typically have a cavity that is shared by tank & PB, and if you want PB (eg pressurized output/return, you need a power beyond "sleeve" adapter

I suppose I can hook every thing up with a temporary line from the Tank port to the tank fill and see what happens when I fire it up.

I STRONGLY recommend against experimenting here. As tough as it is to figure out the specifics, you simply do NOT want to mess with 3000psi unless you're sure you have it wired correctly!

 

[joea99]

There are many different types of valves. If the specific valve requires tank(atmosphere drain) then YES it requires it. But this is not the case for all valves. Take this Summit 3rd function valve with EXACTLY FOUR ports. NO tank here. And note that some valves can do BOTH - in this case the valve will typically have a cavity that is shared by tank & PB, and if you want PB (eg pressurized output/return, you need a power beyond "sleeve" adapter


I STRONGLY recommend against experimenting here. As tough as it is to figure out the specifics, you simply do NOT want to mess with 3000psi unless you're sure you have it wired correctly!

Point taken.

The vendor has stated I need to hook up the Tank return, so that should have been the end of it, regardless of my own concerns. Not fully understanding "why" always bugs me.

 

[joea99]

No. That's not correct. I see the problem. Assume that the PB is not involved in return flow. In fact, begin by considering a spool valve assemble without any PB.

Instead, think of it this way instead: Every valve assembly - in fact every spool valve within an assembly - will have its own return path which is opened by the movement of the spool. Moving the valve spool does two things. It provides pressure to one end of the cylinder and it provides a return path for fluid from the other end of that cylinder.

To help visualize this, when you are not moving the spool valve, the oil that is going to become the return oil is trapped in the cylinder because it cannot get past the spool valve to go anywhere. In fact, that trapped oil is helping to hold the loader arms and bucket from flopping as you drive.

Now take another look at your valve schematic. Follow the arrows inside the schematic as the spool valve is moved back and forth. You will see that the arrows are dead-ended until the spool valve is moved laterally. Unfortunately, hydraulic schematics almost never illustrate how the ports line up when the spool is moved laterally. I wish that they did; but you have to either draw it out or use your imagination.

Back in 2017, "www.FluidPowerWorld.com" published a 10 or 20 part monthly tutorial on reading hydraulic schematics. It is inspired teaching and very readable.
starts with:
Hydraulic symbology 101: Understanding basic fluid

I've heard that some folks have successfully used the PB port as a return port, but that is not its function & connecting it that way doesn't give dependable results. Some valves will work that way and others won't. Most won't. It depends on their internal construction.
The PB and the Tank return should be completely different circuits.
rScotty

I'll be sure to go over that tutorial and have already bookmarked it. I was guessing at the lineups and meaning of the symbols, so it should help.

EDIT - BTW, It seems reasonable that in order for the over pressure relief on this valve to protect the valve (?) the Tank port MUST go to tank, rather than rely on anything past the PB port, and be able to pass full pump output, which in this case is just under 9 GPM. If that made sense. (Ignoring that the relief MAY only be to protect against operating mishaps that "spike" the pressure due to hitting something during implement motion. . .)

So, back to routing where the lines will go.

joe a.

 

[rScotty]

I'll be sure to go over that tutorial and have already bookmarked it. I was guessing at the lineups and meaning of the symbols, so it should help.

joe a.

When learning hydraulic schematics it helped me to take the picture of the spool valves in the standard "center-off" position - which is how all schematic show them. And then take out my pencil and draw it again with the spool moved one way and then the other.

Interestingly, this whole hydraulic thread shows the difference between how technical subjects used to be taught, and how they are learned now through social media in the digital age.

It used to be that the student had a textbook that was always correct. A basic understanding of science helped, but there was never any doubt about the technical part of the text book being correct.

Today, a lot of learning is by "researching" opinions from sources online. The student trying to learn by comparing has to judge between competing opinions instead of facts. The student ends up comparing opinions that sound correct with those that are self-serving or just plain wrong. It's tough to learn that way.

rScotty

 

[joea99]

There are many different types of valves. If the specific valve requires tank(atmosphere drain) then YES it requires it. But this is not the case for all valves. Take this Summit 3rd function valve with EXACTLY FOUR ports. NO tank here. And note that some valves can do BOTH - in this case the valve will typically have a cavity that is shared by tank & PB, and if you want PB (eg pressurized output/return, you need a power beyond "sleeve" adapter
. . .

Just to drag this out a bit more, by "do both" you mean "both", not "either", correct? That is, even with the PB sleeve, which I have installed, it still needs the tank?

That said, the sleeve looks like it would block the internal passage from the P port (pressure supply) to the T "channel", leaving the T ports still open and connected to each other internally as well as to the relief valve outlet.

Does that seem reasonable?

 

[orangetree]

Just to drag this out a bit more, by "do both" you mean "both", not "either", correct? That is, even with the PB sleeve, which I have installed, it still needs the tank?

That said, the sleeve looks like it would block the internal passage from the P port (pressure supply) to the T "channel", leaving the T ports still open and connected to each other internally as well as to the relief valve outlet.

Does that seem reasonable?

It entirely depends on the valve.

I know (because I have one in hand) that W.R.Long sells a 3rd function valve with ONLY 4 ports - NO return to tank. It's generally mounted up right by your FEL valves. This valve would be perfect for your application; put it "in line" with your backhoe. Also, call them - they are the MOST helpful company I've dealt with. If you are still confused, ask to speak with the founder's son (I did, he was very knowledgable).

Other valves do require the atmospheric port. If it requires it, it requires it. I happened to talk to Summit today, it turns out ALL their valves DO require Tank. Sounds like a call to W.R.Long is in order for you

keep in mind - if the valve has an overpressure, it WILL require an atmospheric pressured port (eg 'Tank') - otherwise, it would be impossible for the OP valve to function!

 

[joea99]

It entirely depends on the valve.

I know (because I have one in hand) that W.R.Long sells a 3rd function valve with ONLY 4 ports - NO return to tank. It's generally mounted up right by your FEL valves. This valve would be perfect for your application; put it "in line" with your backhoe. Also, call them - they are the MOST helpful company I've dealt with. If you are still confused, ask to speak with the founder's son (I did, he was very knowledgable).

Other valves do require the atmospheric port. If it requires it, it requires it. I happened to talk to Summit today, it turns out ALL their valves DO require Tank. Sounds like a call to W.R.Long is in order for you

keep in mind - if the valve has an overpressure, it WILL require an atmospheric pressured port (eg 'Tank') - otherwise, it would be impossible for the OP valve to function!

You last sentence pretty much sums it up.

I do have a WR Long solenoid valve at the FEL for the 4 in 1 bucket. It is an HVAIR D03S-2C-12D-35. Even has a metal tag with the schematic on it.

It does have a (marked) T port that seems to function as the PB port, which goes to the BH/3PTH. I think that is what set me down the path of wondering if I needed a Tank at all. Short answer, yes, I do, for the valves I bought.

BTW, that valve does specify "High Tank Port Pressure: Up to 3000 psi tank port back pressure", so, I guess that explains some of the other comments I've gotten over the course of my roller coaster ride of "learning hydraulics".

Thanks to all for all the comments.