Hyd experts familiar with Kubota grand L

[brocas]

Please help me understand the BH hydraulics if you can.

I have a service manual for my L3940. It does a pretty good job of describing the hydraulics. With my limited understanding of hydraulics, I feel pretty confortable I understand.

Problem is, there is nothing that describes how the add on hydraulics are plumbed.

For the BH, there is the "Loop" hose in the back of the tractor. When you install the BH, you disconnect this loop and connect to the BH and complete the loop from the BH to the tractor. Somewhere in the manual and when you get your demo, you are told that when you remove the BH if you forget to hook the loop back, you will damage the tractor. Of course they dont tell you why. I'm guessing if you dont reconnect the loop hose, the QD blocks flow to the sump. But all this should do is force the releif valve to dump fluid, an undesirable condition no doubt, but they make it sound like the tractor will instantaneously combust and self destruct, so maybe my understaning is incorrect.

Here is how the hydraulics in the tractor are plumbed. From the sump to the pump through filter. From the pump to the FEL block. FEL has PB connection an the releif valve. From the FEL PB to the auxiliary valve block. From aux block to the 3PH. The aux valves do not have PB. When you open an aux valve, there is no flow to the 3PH, meaning you cant operate aux valve and 3PH at the same time. I'm guessing the 3PH valves also have no PB (hard for me to tell as the diagram looks complicated due to the position control). From the 3PH i'm gussing the fluid goes to the sump (it is no clear to me from the diagram but I reason that when no valves are open and sending fluid back to the sump, the fluid has to go somewhere at the end of the circuit and I think the 3PH is the end of the circuit).

So the first question is, where is the BH "loop" in the circuit? Is it in the PB from the FEL or after the 3PH?

There is only one hose going to the BH and one coming back. I'm guessing the hose going to the BH suplies the fluid and the hose going from the BH to tractor just goes to the sump, is this correct?

Why do I need to know? I want to operate a trencher with hyd motor on the 3PH. I speculate the best way to plump the motor is using the "loop". In my bliss ignorant world, all I need is to add is a solenoid valve to turn the motor on/off. I'm reasoning this would have to be a so called float center valve. My undertanding of a float valve is that in the center position (idle) the P ant T ports are connected and the A and B ports are connected. The P and T ports being connected is necessary to allow fluid to flow at all times in the "loop" to allow fluid to return to the sump. Tha A and B (work ports) are connected to allow the motor to freewheel (prevents shocks from motor inertia).

This valve description looks like what I need:

Northman Fluid Power Hydraulic Directional Control Valve – 16.8 GPM, 4500 PSI, 3-Position, Double Solenoid, Float Center Spool, 12 Volt DC Solenoids, Model# SWH-G02-C4-D12-10 | Power Solenoid | Northern Tool + Equipment

but if you look at the symbol, you see that the P port is blocked in center position, and A, B, T ports are connected. This is not exactly what I understand a center float valve to be.

I look at the Northman solenoid valve symbols and dont see any valve which has P connected to T and A connected to B in center position.

Does such solenoid valve even exist?

So, what valve do you think I need?

 

[kennyd]

You need a "open center" valve, like this: Northman Fluid Power Hydraulic Directional Control Valve – 26.4 GPM, 4500 PSI, 3-Position, Double Solenoid, Open Center Spool, 12 Volt DC Solenoids, Model# SWH-G03-C3-D12-10 | Power Solenoid | Northern Tool + Equipment

So the first question is, where is the BH "loop" in the circuit? Is it in the PB from the FEL or after the 3PH?

There is only one hose going to the BH and one coming back. I'm guessing the hose going to the BH suplies the fluid and the hose going from the BH to tractor just goes to the sump, is this correct?

The loop for the hoe is usually after the FEL valve, and before the 3PH. So it taps in between the hose connected to the PB/BYD port on the FEL valve and the manifold.

You are correct, one feed hose and one return hose.

 

[J_J]

Your thoughts and logic about the circuit flow are correct.


You can use the valve that kennyd suggested, but it is an on /off valve with no speed control. You can have some speed control by regulating the tractor engine speed.


Is there not another hose from the tank port to the sump. Did you install the BH.

If your BH dose not have a PB port, then there is only two hoses, and the hyd flow and cyl's expended fluid is passed to the sump through the aux block with no back pressure.

The logic about damage to the tractor with a blocked QD, is that if the relief valve dose not not relieve enough, something could burst, and surely would heat up the fluid quickly.

If you should need speed control, add this valve to the circuit. It is an 0-8 GPM speed control.


Surplus Center - 3/8 NPT HYD FLOW CONTROL VALVE W/RELIEF RDRS137-08

0-16 GPM's

http://www.surpluscenter.com/item.asp?item=9-064-50&catname=hydraulic

On my Kubota, the hose for my 3pt log splitter comes from the hyd switch block to the back of the tractor QD's and loops back to the aux block, and then to the 3pt.

If the switch is to the left, the fluid flows from the aux block directly to the 3pt.

 

[slowzuki]

Adding speed control heats the fluid more, learning to use multiple functions or adjusting the rpm down is better.

The 3 ph has power beyond or the circuit wouldn't be completed back to sump. I believe the power beyond is short circuited directly to sump so you can't use it though.

Blocking the discharge of the pump by not reconnecting the loop will cause rapid heating and degradation of the hydrualic fluid and can damage the pump as its not designed to operate at peak load. The tractor will also operate poorly as a large % of engine output will be diverted to heating hydraulic fluid via the relief circuit. You can simulate this by curling your bucket and holding the lever there.

 

[J_J]

3pt does not have PB, it is the last valve in the circuit, and dumps to tank.

The loop is before the 3pt.

Where does your hyd's flow after going back to the block, to the 3pt or sump?

 

[brocas]

You need a "open center" valve, like this: Northman Fluid Power Hydraulic Directional Control Valve 26.4 GPM, 4500 PSI, 3-Position, Double Solenoid, Open Center Spool, 12 Volt DC Solenoids, Model# SWH-G03-C3-D12-10 | Power Solenoid | Northern Tool + Equipment



The loop for the hoe is usually after the FEL valve, and before the 3PH. So it taps in between the hose connected to the PB/BYD port on the FEL valve and the manifold.

You are correct, one feed hose and one return hose.

Thanks Kenny. After thinking about it, I realized that open center valve would work. Open center connects all ports, so the P flows to T and A/B can flow with each other and T. The motor does not see any pressure because both A and B are at the same pressure so it can free wheel :thumbsup:.

 

[brocas]

Your thoughts and logic about the circuit flow are correct.


You can use the valve that kennyd suggested, but it is an on /off valve with no speed control. You can have some speed control by regulating the tractor engine speed.


Is there not another hose from the tank port to the sump. Did you install the BH.

If your BH dose not have a PB port, then there is only two hoses, and the hyd flow and cyl's expended fluid is passed to the sump through the aux block with no back pressure.

The logic about damage to the tractor with a blocked QD, is that if the relief valve dose not not relieve enough, something could burst, and surely would heat up the fluid quickly.

If you should need speed control, add this valve to the circuit. It is an 0-8 GPM speed control.


Surplus Center - 3/8 NPT HYD FLOW CONTROL VALVE W/RELIEF RDRS137-08

0-16 GPM's

Surplus Center - 1/2 NPT HYD FLOW CONTROL VALVE W/RELIEF RDRS150-16

On my Kubota, the hose for my 3pt log splitter comes from the hyd switch block to the back of the tractor QD's and loops back to the aux block, and then to the 3pt.

If the switch is to the left, the fluid flows from the aux block directly to the 3pt.

Thanks for your sugestion JJ. But this is the part I find amazing, that there are only two hoses connecting to the BH.

If I used the speed valve you suggest, the valve itself needs to dump excess fluid to the sump and the motor would also dump fluid to the sump. I would guess that you could connect both to the loop hose that goes to the sump. There may be issues with this, but I dont know, intuitively I think that the pressure of the fluid the speed valve is dumping is high but its just going to the sump which would give very little back pressuse.

Another issue with the speed valve would be how to reverse motor direction? (or are sugesting adding the speed valve in addition to the solenoid valve?) Actually I dont even know at the moment if reversing is necessary or even possible, I have to check the hyd motor to see if its reversable.

I think also that using engine RPM to control motor speed may be acceptable in this case because the tractor speed has to crawl, and since my tractor is hydro with no crawl, even in lowest gear I'm guessing its going to be difficult to keep the speed down at full RPM.

 

[brocas]

Adding speed control heats the fluid more, learning to use multiple functions or adjusting the rpm down is better.

The 3 ph has power beyond or the circuit wouldn't be completed back to sump. I believe the power beyond is short circuited directly to sump so you can't use it though.

Blocking the discharge of the pump by not reconnecting the loop will cause rapid heating and degradation of the hydrualic fluid and can damage the pump as its not designed to operate at peak load. The tractor will also operate poorly as a large % of engine output will be diverted to heating hydraulic fluid via the relief circuit. You can simulate this by curling your bucket and holding the lever there.

Hi slowzuki, do you mean that the pump is not designed to operate at peak pressure? There should be no issues with the pump operatind the hyd motor continuously at full flow and normal pressure, right?

 

[brocas]

3pt does not have PB, it is the last valve in the circuit, and dumps to tank.

The loop is before the 3pt.

Where does your hyd's flow after going back to the block, to the 3pt or sump?

I dont know at this time as I am separated from the tractor by 100 miles. I guess I should just take the time, pull off the rear wheel and fender and just follow the plumbing.

 

[brocas]

OK so here is an open center solenoid:

https://www.surpluscenter.com/item.asp?item=9-6137&catname=hydraulic

They calling this a motor spool. What specifically makes this OC valve a motor spool rather than just ordinary OC?

And here is one they call reversible.

https://www.surpluscenter.com/item.asp?item=9-7312&catname=hydraulic

What makes it reversible? Isnt the other valve also reversible because it is a 3 position directional? I guess I should read the spec on this valve because it doesnt seem to be a normal OC valve, but something special.