tractor hydraulics

[zkted]

I am sort of familar with hydraulics on aircraft but do not know much about tractor hydraulics. I have 2 questions please.
How does the power beyond valve or port work on a tractor
On an Iseiki tractor I saw 3 hyd hoses coming from rear to a non standard valve bank with hand levers. They told me the original valve bank at rear of the tractor was broken and 2 expensive so new valve was plumbed in and the oringinal valve body had ports taken out of it.
Why 3 lines?

 

[Bird]

zkted, we've got a lot of guys on the forum who know a lot more about hydraulics than I, but I'll try a short version.

<font color=blue>How does the power beyond valve or port work on a tractor</font color=blue>

The pump is pushing that oil to the inlet port on the valve. Most of us have "open centered" valves so the oil flows through the valve and onward to power other things - such as the 3-point hitch. If there is no power beyond port, or if it is plugged and not in use, then the oil goes in the inlet and out the outlet, but when you use that valve - move the lever - you are interrupting the flow temporarily and whatever is downstream from there cannot be used simultaneously. However, if you have the power beyond, then even when you are using that valve, oil is still flowing through the power beyond port on to the other valves downstream.

<font color=blue>Why 3 lines?</font color=blue>

When you have the power beyond port in use, so oil is continuously flowing through it downstream, as mentioned above, then you need a line from the outlet port on the valve to dump the oil back to the reservoir or tank when the valve is used.

Example: My front end loader valve has power beyond (3 lines - inlet from the pump, power beyond, and outlet to the tank) so even when I'm raising or lowering the front end loader, I can simultaneously raise or lower the 3-point. But then I added another two spool valve on the fender for hydraulic top 'n tilt. The power beyond port is plugged, so it simply has the inlet and outlet ports is use. When I am using one of those levers, I cannot raise the 3-point hitch until I return the top 'n tilt lever(s) to the center (spring loaded so they return to center when I release them).

Hope this helps; maybe someone else can expand on it if you have other questions.

Bird

 

[allmobile]

The other thing that happens if you don't have PB, is that when you are using the 3-pt and there is a load on it, whatever pressure is in the line counteracts against the relief valve in the aux. valve, i.e. 3-pt is making 1000 psi., the aux. valve relief is 2000 psi. you have now increased it to 3000 psi. which can start to cause damage especially as these pressures increase.

Brian

 

[Bird]

Brian, you lost me on this one (don't know enough about the hydraulics, I know). Can you explain a little more?

Bird

 

[allmobile]

I'm not sure I can explain without a picture of a relief valve. It would be easy for me to show you. I'll have to look around for a picture to help me out. Hopefully somebodyelse has the ability to explain in words.

Brian

 

[kubmech]

Brian, in this case 1+2 does not equal 3, actually 1+2=1.
If the 3ph relief (usually tractor main relief) is set at 1000 psi
and an aux valve is tied into the circuit let's say prior to the main tractor relief valve (in this case it would have to have a relief in the valve itself) and the aux valve relief is set at 2000 psi. While operating an aux valve function the relief for that circuit will operate at 2000 psi and no flow will go to the 3ph, except for the fluid bypassing relief and trying to find its way back to tank through the 3ph sytem, which if you were a very patient person, would eventually operate the hitch and blow by main relief at 1000 psi. (wow, how hot is that fluid at this point) When the aux valve is in neutral, fluid flows through the "open center" to the 3ph, with the 3ph operating against relief excess pressure above 1000 psi is sent back to tank and never builds enough to back up against aux relief which pops at 2000 psi.

 

[kubmech]

zkted, on most aircraft the hydraulic system is closed center, a very expensive variable displacement hyd pump with an internal pump compensator set at designed system pressure that will stroke or destroke depending on system demands, with a relief valve in the system to back up the pump compensator. Open center systems rely soley on the relief valve to set system pressure.

 

[Bird]

Brian, and Kubmech, I guess I'm too dense to understand. I'm using an open centered, two spool valve for my hydraulic tip 'n tilt (power beyond port plugged as it came from the manufacturer). I guess what I'm asking is whether you're saying that this might cause problems because I'm not using the power beyond, whether you're saying it'll cause problems only if I try to use the auxilliary valve and the 3-point simultaneously, or whether you're saying it's going to cause a problem anyway, even though I don't try to use them simultaneously.

Bird

 

[kubmech]

Bird, from your first post it sounds like you're set up correctly,
I'm assuming you tapped off of the PB port of your loader control valve and I don't see where you will run into a problem.

 

[Bird]

No, kubmech, it seems that tapping off the loader valve power beyond is a popular way to do it, but I left the loader and its valve alone, and used the "optional" hydraulic outlet block back toward the back of the tractor on the right side. It's something the B2710/2910s have, although none of the three dealers I talked to knew anything about it.

Bird