HYDRO PRESSURE HIGH BX23S

[Kubit]

My BX23S is a 2019 with 135 hours. I had ordered it with what Kubota calls a 3rd function valve and the mechanical thumb options. I converted the thumb to hydraulic & use that valve to power the thumb cylinder. I began reading forum posts regarding PSI & ordered the BXpanded gauge kit. Tested today & I’m getting a 2700 PSI reading at high idle connected to the output from the 3rd function valve—it was a lot simpler to connect there rather than the recommended output to the loader.

I would understand that overall hydro pressure should be constant through-out the system & should never get a “boost” regardless of where it was connected--or am I understanding wrong?

Machine has been running flawless—no leaks or operating issues.
Thanks for any input here.

PS: Other than my mod to the thumb all hydraulics are Kubota original issue.

 

[foobert]

As you are probably aware, that’s well outside of the 1850psi spec.

Supposedly those kit gauges are calibrated. You are the second reporter in a week(ish) to mention very high pressure— I wonder if their gauges are faulty.

 

[DustyRusty]

It could be an erroneous reading being that it was taken at the 3rd function valve. I would take readings at more than one point if it were excessive.

 

[Henro]

It could be an erroneous reading being that it was taken at the 3rd function valve. I would take readings at more than one point if it were excessive.

Agree that taking a reading at a different point in the system is not a bad idea when questions like this arise.

But I am having trouble seeing how the point where one takes the reading could show pressure higher than the system pressure relief valve would hold the max system pressure at.

Granted if the system PRV failed to open, or somehow started to open at a pressure much higher than spec, for whatever reason, the system pressure would be higher than spec. But that would be everywhere in the system, on the pressure side of the pump.

Guess what I am thinking is that taking a pressure measurement improperly should show a lower reading, but not a higher than actual system pressure reading.

Of course, pressure can be higher than system pressure in lines connected to cylinders. Like on a loader when carrying a heavy load and if the tractor bounces over ruts for example. But the OP seems to be taking readings when the tractor is sitting still and not working.

 

[Dave_eng]

By converting a mechanical thumb to a hydraulic one you may have created a situation wherein, one hydraulic cylinder can be pushing against a smaller hydraulic cylinder and in doing so amplifying the pressure in a circuit.

This is similar to the situation where owners destroy bucket cylinders by back dragging especially with forks installed.

The pressure relief valve cannot control or protect a circuit once the circuit's control lever is in the neutral position.

Imaging picking up a bucket of gravel with a FEL. You can barely get the bucket off the ground as the relief valve is limiting the pressure in the cylinders. Now you take a shovel and, with the bucket slightly elevated above the ground, you add more material to the bucket. The pressure in the cylinders and hoses has to increase to support the added weight. The relief valve can do nothing. To the extreme, the cylinders or hoses will fail from excess pressure.

Find somewhere else on your machine to measure the pressure before starting to make adjustments.

Dave

 

[Kubit]

Turns out:
--Okay to take readings from the bypass valve
--Gauge is correct
--PSI ratings here are incorrect

Shout out: THANKS DAVE!

These from the BX23S Workshop Manual WSM showing 3060 psi maximum:

 

[Henro]

Turns out:
--Okay to take readings from the bypass valve
--Gauge is correct
--PSI ratings here are incorrect

Shout out: THANKS DAVE!

These from the BX23S Workshop Manual WSM showing 3060 psi maximum:
View attachment 57597
View attachment 57598

That pressure looks like it is within the HST transmission, not an external pressure.

Hydraulic pump (1) Has a pressure relief valve directly on its output. I assume that PRV valve is normally set to open at 1850 PSI or so.

Dave's insight is interesting, and maybe possible, do not know. Seems like system pressure would still be held at the setting of the PRV on the output of the hydraulic pump , unless something external is happening within lines that are isolated by closed control valves, which does not seem to be the case with your testing , but I honestly cannot say.

The drawing seems to show the HST pump as separate from the pump that drives the hydraulic cylinders.

I take it you have verified that the guage you are using is measuring accurately?

Not sure what you mean about "PSI reading here are incorrect".

Just curious at this end...

 

[ruger1980]

@Henro is correct, those are your reliefs for the forward and reverse circuits in the HST.

The relief you are concerned with for the hydraulic pump is item 6

 

[foobert]

Today I learned the BX’s PTO clutch is engaged through a hydraulic circuit — items 8,9,10 on the schematic above. No wonder I’m not able to “feather” the engagement.

Thanks for posting that @Kubit

 

[Dave_eng]

My big point in all of this discussion is that I find it hard to accept that Kubota would have set the hydraulic pressure so high as the owner measured. i.e. 2,700 psi or far above 1,850. Most owners find the Kubota pressure setting to be at or below the spec'd level.

This in a 2019 model with 135 hours.

Is it not possible a mistake was made in installing the loader or third function valve which is leading to these high pressures being measured?

For me I am not comfortable suggesting changes based upon the known facts.

I do not know this specific machine and certainly defer to those that do.

Dave

 

[leveraddict]

First off why not take the reading in the correct place the correct way??? Remove the loader valve quick disconnect (its easy) put gauge on line closest to operaters seat, start machine bring RPM's up to 3000 and bury the joystick, check pressure reading! Easy peasy! Thats how my directions read that came with my gauge! Now if you get a reading of 3000psi checking it the correct way then you have a problem! Either your dealers mechanic is trying to break your machine? Maybe doesnt know any better? Reverse the blame back to you when problems arise, and they will! I dont think you could add enough shims to get that high a reading IF CHECKED CORRECTLY!

 

[DustyRusty]

First off why not take the reading in the correct place the correct way??? Remove the loader valve quick disconnect put gage on line closest to operaters seat, start machine bring RPM's up to 3000 and bury the joystick, check pressure reading! Easy peasy! Thats how my directions came with my gauge!

Real men don't read instructions!!!!!!!!!!!!!!! See #3

 

[ruger1980]

Turns out:
--Okay to take readings from the bypass valve
--Gauge is correct
--PSI ratings here are incorrect

Shout out: THANKS DAVE!

These from the BX23S Workshop Manual WSM showing 3060 psi maximum:
View attachment 57597
View attachment 57598

Please see the correct specification below.

 

[Dave_eng]

At the beginning of this thread I was trying to explain why caution needs to be taken when converting a mechanical thumb to a hydraulic one because of the hydraulic amplification which takes place when one hydraulic cylinder forces on another cylinder in a different circuit.

I recently watched a YANMAR youtube video on installing a hydraulic thumb on a mini excavator.

I took a screen shot of the end of the video where a special hydraulic relief circuit is set up to limit the pressure in the thumb cylinder even when its control lever is in the neutral position.

For anyone converting a mechanical thumb to a hydraulic one, providing this relief protection would be wise.

This relief is not unlike the safety valve (not relief valve) in the 3 pt hitch circuit on newer tractors to deal with pressure spikes when implements bounce on the raised 3 pt hitch.

Dave

 

[DustyRusty]

If you have either the Land Pride 3rd function valve, or the Kubota 3rd function diverter valve assembly, do you have to add the auxiliary circuit relief to the tractor, or has Kubota compensated for this in the design of the 3rd function valve. I was thinking of using the Land Pride 3rd function valve for the rear thumb hydraulic cylinder project on my BX23S. I am surprised that Kubota didn't offer an upgrade to the thumb on the tractors that have the manual thumb installed as an option from the factory.

 

[Dave_eng]

If you have either the Land Pride 3rd function valve, or the Kubota 3rd function diverter valve assembly, do you have to add the auxiliary circuit relief to the tractor, or has Kubota compensated for this in the design of the 3rd function valve. I was thinking of using the Land Pride 3rd function valve for the rear thumb hydraulic cylinder project on my BX23S. I am surprised that Kubota didn't offer an upgrade to the thumb on the tractors that have the manual thumb installed as an option from the factory.

To answer your question is the hydraulic overload relief protection as shown by Yanmar incorporated into 3rd function or diverter valves, the simple answer is NO.

The reason manufacturers provide a mechanical thumb as an option but not a hydraulic thumb: they are trying to keep cost down by not having to incorporate extra protection circuits. This is why "do it yourself" hydraulic thumb projects are so risky as owners only see the benefits but not the risks of severe damage. This is not meant as a criticism of owners doing these projects as it involves complex aspects of hydraulics that most owners never are faced with.

Remember, the 3rd function valve is just a simple solenoid valve utilizing the power beyond feature of the loader valve. The 3rd function valve relies on the loader relief valve for protection and this valve only works when a circuit is active be it loader operation or 3rd function operations. Same for the diverter valve.

Dave

 

[DustyRusty]

How would you plumb in a relief valve when converting a manual thumb to a mechanical thumb when using a 3rd function valve, or is it not possible to add this protection as an add on? thanks

 

[ruger1980]

How would you plumb in a relief valve when converting a manual thumb to a mechanical thumb when using a 3rd function valve, or is it not possible to add this protection as an add on? thanks

This would be fairly simple. You would only need to purchase a relief valve assembly for each circuit you would want protection on and connect the circuit to the valve and then run the return to a tank connection. There seems to be a connection to tank at the main control valve so easy to pick up that connection there.
You would really only need the barrel end of the cylinder protected. I would mount a relief valve on the backhoe frame, run the lines from the 3rd function or remotes if you have them to the cylinder with the line to the barrel end tee'd at the relief valve.