adding hydraulic thumb kubota B21

[rScotty]

EDIT - BTW, It seems reasonable that in order for the over pressure relief on this valve to protect the valve (?) the Tank port MUST go to tank, rather than rely on anything past the PB port, and be able to pass full pump output, which in this case is just under 9 GPM. If that made sense. (Ignoring that the relief MAY only be to protect against operating mishaps that "spike" the pressure due to hitting something during implement motion. . .)


joe a.

Yes! Good thinking! That's a good deduction. The overpressure valve must be able to dump immediately to some place without any back pressure. So it has to go directly through the return line and to the vented tank.

rScotty

 

[orangetree]

Yes! Good thinking! That's a good deduction. The overpressure valve must be able to dump immediately to some place without any back pressure. So it has to go directly through the return line and to the vented tank.

rScotty

worth noting the other difference with this "four port" (vs 5-port with tank drain) valve (also, "pressure-rated Tank drain" is another good description):

The most common cylinder valves route:

• pressure to one side of the cylinder (obviously) and
• the other end to tank

this guarantees that the cylinder can see/use the FULL pump pressure. If that tank line could be pressurized, the cylinder (might) not see full pressure.

 

[Doc Joe 237]

I am looking to incorporate the hydraulic thumb on my BX23s w/BH and would like to piggy back on this discussion. My BH has three hoses to connect it. P, T, & PB. Your discussion didn't mention how many hoses are used to connect and disconnect your BH. I had a older BX23 that had only two lines.
So in my research to add the hydraulic thumb to my BH, would be to disconnect the T line install a PB fitting into that last valve in the existing BH SCV. Then the PB would be plumbed to the new scv and the T line be connected to the T position on that valve. That is what I'm understand, and I have not been able to look at my BH to see if this can be done since it's located at my second home that I won't be going to until Thanks Giving.
So when I remove the BH I have to loop to P hose into the PB port to power the rest of the tractor.
Would be interested in your comments whether my thinking is correct. Thanks

 

[ericm979]

If your BH has a PB line it's likely already got the PB fitting installed.

 

[rScotty]

I'm not sure yet how you will end up doing this, but will say that I find having a hydraulic thumb is worth the effort. For an older guy, the BH and thumb is incredibly useful for picking and placing anything.

I guess the first place to start is with the factory mechanical thumb. Does yours have that? Looking at the photos of the BX23S with mechanical thumb, I don't see any reason why it could not be made to be hydraulic....but the fact is that Kubota didn't offer that and that should give us pause.

There will be sizing the cylinder to give the thumb a proper range of motion. For that, it will help to have a BX23S with the OEM mechanical thumb for making measurements of open/close length and pin size. If it were mine, I would start by mounting the OEM mechanical thumb and then mocking up a cylinder out out of PVC pipe, cardboard, and gorilla tape. There are probably some mechanical guys who can get it right the first time with measurements and pencil sketches - but for the rest of us a model works better.

When it comes to hydraulic hookups, there are several people here who can help. I believe you will be happier with the result if we have access to the hydraulic diagrams in the shop manual. A parts manual for fitting sizes helps too.

The thumb will have it's own control valve and also its own relief valve. The latter is generally set so that while the bucket rotation is overpowering the thumb there is a unrestricted path for the thumb cylinder fluid to exit to the tank.

While we are on that subject of tank return, you should leave your tank return line as it. The tank return flow is under very minimal pressure and you want to maintain that condition. Otherwise you get back flow in the tank line which leads to all sorts of odd control valve problems. So you don't want to be interrupting that tank line - also called the return line - with a valve. We will have to get the necessary thumb pressure from somewhere else in the system. Expect that the thumb control valve will have it's own return line, although it can be short and is often dumped directely into the main BH return line if the hose is sufficiently large.

We can kick this around some more as we get more info.

Good luck,
rScotty