How do you adjust a hydraulic valve?

[MadReferee]

</font><font color="blue" class="small">( I wish I could suck this in, but if you put a pressure gauge at the nozzle, how much pressure would you get? My guess would be only a few psi. which is created by the restriction of the hose diameter, not the pressure that you would see if the white hose was shut off. )</font>

Not sure but probably true, but in a tractor we are not talking 30-60 psi which is what your water pressure is. We are talking upwards of 1900 psi. The water hose analogy was used to simply describe the open/closed center systems.

In a tractor, the pump and pressure it generates is directly related to the engine speed. Even at idle you can operated the hydraulics, albeit very, very slowly.

 

[Steve_Miller]

<font color="orange"> Open center valves pass fluid thru the valve from inlet to power beyond when the valve's spools are in neutral. The return to tank outlet is only used for work port exhaust fluid and depending on valve design, also can be used to return fluid when the relief valve pops.
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Mad,
Are there three ports to the valve? If so, this is all starting to make sense. Where Junkman teed in was between the pressure port and the PB port?

Steve

 

[MadReferee]

</font><font color="blue" class="small">( Are there three ports to the valve? If so, this is all starting to make sense. Where Junkman teed in was between the pressure port and the PB port?)</font>

Nope, the picture only shows an inlet, an outlet, and the work ports.

Based on his postings, he teed into a pressure line, either before or after the loader valve, and the return to tank line.

Take a look HERE for a nice picture description of the open/closed/PB circuits.

 

[Henro]

<font color="blue">Pressure is always there because the pump is always pumping fluid thru the system. Almost, if not all, CUT's have a fixed displacement pump that is constantly pumping fluid. This pumping action generates pressure. </font>

At the risk of sounding like this is a debate... /forums/images/graemlins/blush.gif

The fixed displacement pump develops flow...but no pressure to speak of. It is resistance to flow that causes pressure to be developed, not the pumping action. The electrical analog would be current flowing through a resistance causing a voltage to be developed.

Now if you are saying that the pumping action causing flow to be pushed through the resistance of the open circuit loop, causes pressure to be developed it is time to move on. This is certainly true.

Madref, have you ever measured or heard anyone reporting what the pressure at the inlet to a loader valve is, or might be? For example, in our tractors (your B2710 or my B2910) what might the pressure developed at the inlet to the loader valve likely be when all controls in the loop are centered? 1.000 PSI? More?

In Junkman's case, is it likely that it could be as high as 1,500 psi? If so this would answer the question of where the pressure is coming from when the normal circuit is open.

Your hose example is, I think, a good illustration of what is probably happening in Junkman's setup, if one starts looking at it from the point the main valve is opened. /forums/images/graemlins/smile.gif

The key has to be flow resistance in the open circuit loop causing pressure to develop at across the loop, and since the loader is the first point in the loop, the inlet side of the loader valve would feel the highest pressure. Perhaps high enough to work Junkman's graple just fine...even though that would not be possible in a perfect world. /forums/images/graemlins/grin.gif

 

[Steve_Miller]

<font color="blue"> ( Are there three ports to the valve? If so, this is all starting to make sense. Where Junkman teed in was between the pressure port and the PB port?) </font>

Let me rephrase this statement. According to the third picture down on the site you posted the link to there is a pressure port, PB port and a return port, correct? These are what I meant by three ports. Sorry for the confusing description, I'm good at that. Where Junkman teed in was pressure line or PB and the return? Or is this not the type of valve he has, if so I'm still confused.

Steve

 

[MadReferee]

</font><font color="blue" class="small">( The fixed displacement pump develops flow...but no pressure to speak of. It is resistance to flow that causes pressure to be developed, not the pumping action. The electrical analog would be current flowing through a resistance causing a voltage to be developed.
)</font>

Ok, so my terminology sometimes sucks, but you get my point. His circuit is correct. He will get flow into the CC valve once it's spool is opened no mattter which pressure side of the loader valve he tees into. Fluid is still always flowing, it just backs up to the CC valve and stops.

Gotta go, Later...

 

[MadReferee]

</font><font color="blue" class="small">( Let me rephrase this statement. According to the third picture down on the site you posted the link to there is a pressure port, PB port and a return port, correct? These are what I meant by three ports. Sorry for the confusing description, I'm good at that. Where Junkman teed in was pressure line or PB and the return? Or is this not the type of valve he has, if so I'm still confused.)</font>

Ok, one last one then I'm gone.

A loader valve is the third picture down (open center circuit with PB). The add on valve Junkman has is the second picture down (closed center circuit). Actually his connection is more like the fourth picture only with a fixed displacement pump where he tees into the main line just after the pump.

later...

 

[Steve_Miller]

<font color="blue"> The key has to be flow resistance in the open circuit loop causing pressure to develop at across the loop, and since the loader is the first point in the loop, the inlet side of the loader valve would feel the highest pressure. Perhaps high enough to work Junkman's graple just fine...even though that would not be possible in a perfect world.
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Henro,
If this is truly what is happening, then you are absolutely right. The only draw back is that he would have very little clamping pressure on the grapple unless he happened to use a loader function at the same instant increasing the grapple pressure to the loader valves relief pressure. I'm going to bed. Check back in the morning.

Steve

 

[Henro]

Steve,

Looks like MadRef beat you to bed! /forums/images/graemlins/grin.gif /forums/images/graemlins/blush.gif
<font color="blue">
Where Junkman teed in was pressure line or PB and the return? Or is this not the type of valve he has, if so I'm still confused. </font>

I'm pretty sure you are not confused...now you can go to bed! /forums/images/graemlins/grin.gif

I would guess that Junkman is connected between the line coming into the loader valve (pressure port) and the return to tank. This would provide maximum pressure availability in the system, and would also keep pressure available when the loader was operated. /forums/images/graemlins/smile.gif

This is an interesting application of a closed center valve in an open center system, something we normally hear as being a no no.

It works because of the pressure that exists at the output of the pump, due to flow resistance in the circuit (pressure drop) between the pump and hydraulic tank.

 

[Henro]

<font color="blue">The only draw back is that he would have very little clamping pressure on the grapple unless he happened to use a loader function at the same instant increasing the grapple pressure to the loader valves relief pressure. </font>

This is what I was thinking except I am now thinking that the 1,500 psi that Junkman measured at his grapple valve is probably the pressure his tractor's pump is developing across his open loop circuit when all the control valves are centered. So that should provide a fair amount of clamping force.

The final chapter in the discussion would be what happens to the pressure at the grapple control valve when the loader goes into a stall situation, like full up or down, or end of movement of the bucket, when the system pressure goes up to the pressure relief valve trip point?

My bet? I would bet the pressure on the grapple can vary from the 1,500 PSI Junkman measured all the way up to max system pressure, depending on what is being done by other cylinders controlled by the open center control valves at any given point in time.

In fact, I would even suspect that the system pressure drops a little when that closed center grapple valve is operated, since it would provide a parallel path for hydraulic flow, and lower resistance, so for the given pump flow the pressure develeoped would be less. /forums/images/graemlins/shocked.gif

What happens when the grapple and loader are operated at the same time is harder to estimate.