Kubota Hydraulic Info and Q/A

[TheOldHokie]

To the admins - if this is out of line or inappropriate please delete it.

I am starting this thread to give folks a chance to learn the basics of Kubota tractor hydraulics. Its the number one topic here and multiple people have asked for something of this sort. Its an open discussion. Feel free to ask questions and provide answers. If you are providing answers make sure you are speaking fact not fiction. Public flogging of purveyors of missinformation is encouraged and that includes me!!

Please keep questions and answers short​

I am going to start things off with something simple. Here is a diagram of an open center circuit similar to the loader and third function on a Kubota tractor. Its the source of much confusion. Ask a question and I or someone ekse will try to answer it. One question at a time and keep the question short and specific.



I will start the discussion with a few fundamental concepts. The pump produces a constant flow of oil which must always have a way to circulate. If it does not the pump is deadheaded, pressure spikes way above the limits if the components and catastrophic damage is immediate.

Because deadheading means instant death my diagram has a pressure relief valve right at the pump outlet. This valve acts as an over pressure safety device. It is normally closed but in the event something in the circuit deadheads the pump the relief opens at a predetermined max safe pressure setting and oil is diverted to the tsnk circuit and from there back to sump to prevent pressure from going any higher. The relief remains open until the deadhead condition is removed and pressure drops. At that point the relief returns to the closed state.

Keeping all that in mind, with ALL valves in neutral ALL oil flows continuously around the outer circle. Red is called the neutral circuit and blue is called the tank circuit. Please be precise when using those terms.

In this state pressure in both the neutral and tank circuits is "zero". This state is often called "standby" and the only work the pump is doing is circulating free oil and pressure in the neutral circuit is essentially zero. Now two very important concepts.

PRESSURE IN THE NEUTRAL CIRCUIT ONLY GOES HIGH WHEN A VALVE IS SHIFTED AND THE PUMP IS USED TO DO WORK

Similarly

PRESSURE IN THE TANK CIRCUIT IS ALWAYS ZERO.

Burn those two very important concepts into your brain and let the discussion begin!!

Dan

 

[Grandad4]

Great idea! Keep going, please.

 

[xrocketengineer]

Fantastic! I am assuming that the 3rd Function valve in the diagram is not a "bypass valve" and the Position Valve Control refers to the three point hitch control. Is that right?

 

[Joisey]

Fantastic idea. If one is unfamiliar with hydraulics or has limited knowledge, an illustration can make a world of difference. Even for those that are familiar with hydraulics, the diagram will make a good way to double check how things work.

 

[TheOldHokie]

Fantastic! I am assuming that the 3rd Function valve in the diagram is not a "bypass valve" and the Position Valve Control refers to the three point hitch control. Is that right?

Good kickoff question

Lets assume the 3rd function valve is a 4 way solenoid operated directional control valve with tandem center spool. That is what Kubota and third party suppliers use in their kits.

The loader valve is Kubota OEM with all the factory's "special features".

The position control valve is synonymous with 3pt control valve.

Dan

 

[Russell King]

Please explain what T port does on the loader (FEL) valve.

 

[Russell King]

Please explain what the PB (power beyond) port does and why it is (usually) installed in a T (Tank Return) port.

 

[Russell King]

Please provide a description of the valve port names and any additional names (ie HPCO v PB).

You may want to expand the loader valve diagram to show the flow through the valve when passive and when diverting to a cylinder?

 

[TheOldHokie]

Please explain what T port does on the loader (FEL) valve.

Nice softball and I am not going get sucked into the full complicated answer. We will save that for graduate classes

When the spool in the loader valve is shifted:

1. The neutral circuit is blocked at PB,
2. The oil coming in on the neutral circuit is rerouted to the loader cylinder(s).
3. Oil coming back from the cylinder(s) is sent out the tank port and goes directly back to the sump.

TEASER: that is "standard" behavior but on current generation Kubota valves that is only true for the bucket roll back function

 

[TheOldHokie]

Please provide a description of the valve port names and any additional names (ie HPCO v PB).

You may want to expand the loader valve diagram to show the flow through the valve when passive and when diverting to a cylinder?

Thats exactly what I am attempting to avoid. My intent here is to focus on one specific thing at a time. Hopefully that will develop logically.

Broad complicated questions and answers overwhelm and confuse people. Lets stick to softballs and no curves like you tossed on that first one That question and answer covered a basic behavior in simple terms that anyone should be able to understand. Lets try to keep it that way.

Dan

 

[TheOldHokie]

Please explain what the PB (power beyond) port does and why it is (usually) installed in a T (Tank Return) port.

The PB (power beyond) port is used to supply high pressure oil to a down stream valve. Power beyond separates low pressure tank return oil from high pressure carry over oil. The mechanics of how that is done and why you want to do it is an advanced topic for later discussion. In general the PB port is just the continuation of the neutral circuit.

Dan

 

[MapleLeafFarmer]

I have a mx5100 that is gear driven. Pulls like a monster and has never let me down. Engine runs like new.
Problem is with the loader. Let's say it's lift capacity is just underwhelming.
I think i'm getting a leakage in the control valve area. My brother disagrees and says it's probably the hydraulic pump or maybe leaky cylinders. He also just never wants to agree with me.
Any suggestions to help isolate always appreciated. We are very handy growing up farming. Everything we have though his self taught trial and error. Our supply of tools and spare parts pretty extensive. Hopefully, putting in our own bendpak lift later this year. Just short on book learning.

Thanks in advance

 

[Bee-Positive]

I have a mx5100 that is gear driven. Pulls like a monster and has never let me down. Engine runs like new.
Problem is with the loader. Let's say it's lift capacity is just underwhelming.
I think i'm getting a leakage in the control valve area. My brother disagrees and says it's probably the hydraulic pump or maybe leaky cylinders. He also just never wants to agree with me.
Any suggestions to help isolate always appreciated. We are very handy growing up farming. Everything we have though his self taught trial and error. Our supply of tools and spare parts pretty extensive. Hopefully, putting in our own bendpak lift later this year. Just short on book learning.

Thanks in advance

I think you're missing the intent of this thread as being a learning tool not a problem solving thread.
You should maybe post your question in a separate post.

 

[TheOldHokie]

I have a mx5100 that is gear driven. Pulls like a monster and has never let me down. Engine runs like new.
Problem is with the loader. Let's say it's lift capacity is just underwhelming.
I think i'm getting a leakage in the control valve area. My brother disagrees and says it's probably the hydraulic pump or maybe leaky cylinders. He also just never wants to agree with me.
Any suggestions to help isolate always appreciated. We are very handy growing up farming. Everything we have though his self taught trial and error. Our supply of tools and spare parts pretty extensive. Hopefully, putting in our own bendpak lift later this year. Just short on book learning.

Thanks in advance

Your brother sounds much like mine. Your problem (hydraulics not brother) is pretty common.

My approach would be isolate and test the suspect components.

1. If the pump was weak you would see it elsewhere but its easy to test on that tractor.
   1. Disconnect the pump supply hose from the loader valve and connect it to a 3000 PSI hydraulic gauge. Now the only thing in the circuit is the pump and main system relief.
   2. Start the tractor and read delivery pressure. If its low the pump and/or relief valve is defective
2. Cylinders are equally easy to test.
   1. Retract the cylinders and remove the pins in the rod end.
   2. Disconnect the coupler on the base end hoses, start the tractor, move loader control to pressure down and hold it there. If the cylinders extend one or both are leaking internally

 

[TheOldHokie]

I think you're missing the intent of this thread as being a learning tool not a problem solving thread.
You should maybe post your question in a separate post.

Agreed but I answered anyway.

Dan

 

[MapleLeafFarmer]

Your brother sounds much like mine.

1. Start the tractor and read delivery pressure. If its low the pump and/or relief valve is defective.

Thanks!!! And appreciated..... and there in lay my dilemma.
I guess I should just tear into the valve. And if problems still exist,
then I guess it's time to look at a pump replacement as it's not leaking probably just old and worn.

Definitely gonna wait to the fall. Don't wanna do anything extra in this hot weather that I definitely just can avoid.

 

[Russell King]

Is there a pressure relief valve near the pump in ALL Kubota tractors or could you destroy the pump by deadheading the pump (as you directed the test in post #14)?

 

[TheOldHokie]

Thanks!!! And appreciated..... and there in lay my dilemma.
I guess I should just tear into the valve. And if problems still exist,
then I guess it's time to look at a pump replacement as it's not leaking probably just old and worn.

Definitely gonna wait to the fall. Don't wanna do anything extra in this hot weather that I definitely just can avoid.

Where did I say anything about taking anything apart or replacing components? I said isolate and test.

 

[Henro]

Are there standard colors used for explanation of hydraulic diagrams for the purpose of instruction? For example, I might have chosen blue for the general, flowing circuit, perhaps green for the return tank, and maybe be saved red to illustrate working parts of the circuit when they’re activated.

Just curious and nothing more.

 

[TheOldHokie]

Is there a pressure relief valve near the pump in ALL Kubota tractors or could you destroy the pump by deadheading the pump (as you directed the test in post #14)?

Not all models have a relief between the pump and loader valve. My test suggestion was specific to that model.