Backhoe Thumb Hydraulic Options

[avc8130]

AC:

The above quotation from a maker of thumbs illustrates my earlier point. They are
marketing a design to be installed on multiple different and unknown hoes. The maker
has fixed some variables (like cylinder diameter) that can not be easily changed. This
is the antithesis of custom design where you optimize your thumb for your hoe.

Making and selling a generic hyd thumb is extremely difficult because you would really
really want a workport RV in the thumb valve. And you would have no knowledge of
the customer's hoe valve(s). That's one of the reasons that aftermarket hoes were so often
sold with PTO pumps: the maker had little idea of how to tie into the huge number of
hyd systems out there.

Along those lines, we were running a FACTORY hydraulic thumb on the Bobcat mini-ex we have here at work today. If we grabbed something in the thumb/bucket, as more "curl" was applied the thumb would fold back.

I THINK Kubota uses a work port relief on the valve for this. Is that what is visible at the bottom of the valve in this pic? The thumb circuit is the last valve in the stack on the left.



ac

 

[dfkrug]

Along those lines, we were running a FACTORY hydraulic thumb on the Bobcat
mini-ex we have here at work today. If we grabbed something in the thumb/bucket, as more "curl" was
applied the thumb would fold back.

I THINK Kubota uses a work port relief on the valve for this. Is that what is visible at the bottom of the valve in this
pic? The thumb circuit is the last valve in the stack on the left.

Hard to say from the photo what is hanging down from the left-most valve. Have you looked at the tractor's
parts diagram?

As for mini-Xes, I was told by a Kubota engineer that the newer ones use multiple HST-drives instead of
trochoid pumps. I wonder if your Bobcat had the thumb circuit on the same pump as the bucket? As
another wrinkle to the hydraulic "logic" used on a mini-X, the Takeuchi that I used had a spring-return
foot control for the factory thumb. Push the foot lever and the thumb extended; let go and it retracted.
I kept my hands on the joysticks, and there was no push-back when curling the bkt.

 

[avc8130]

Hard to say from the photo what is hanging down from the left-most valve. Have you looked at the tractor's
parts diagram?

As for mini-Xes, I was told by a Kubota engineer that the newer ones use multiple HST-drives instead of
trochoid pumps. I wonder if your Bobcat had the thumb circuit on the same pump as the bucket? As
another wrinkle to the hydraulic "logic" used on a mini-X, the Takeuchi that I used had a spring-return
foot control for the factory thumb. Push the foot lever and the thumb extended; let go and it retracted.
I kept my hands on the joysticks, and there was no push-back when curling the bkt.

I don't know how the Bobcat was plumbed, but I do know the control is electronic and 100% featherable.

ac

 

[dfkrug]

070 to 100 sure looks like a RV.

 

[J_J]

070 to 100 sure looks like a RV.

That would be the load check.

 

[avc8130]

That would be the load check.

What's that mean? That is what Kubota calls it.

What is its function?

ac

 

[J_J]

Keeps the load from falling as you shift the lever.

It also prevents the cyl fluid from entering the input flow.

 

[avc8130]

Keeps the load from falling as you shift the lever.

It also prevents the cyl fluid from entering the input flow.

OK, so it is NOT a pressure relief. That must be handled somewhere else on the stack?

ac

 

[J_J]

That is true.

The tractor relief will also act as the relief unless it is set differently.

If the BH has it's own pump, then the BH relief is the only relief.

However each function of the BH may have work port reliefs.

 

[dfkrug]

What's that mean? That is what Kubota calls it.

What is its function?

Your parts picture in post #53 went away, AC. If Kubota is calling this the load check valve,
then that is what it is. It sure looks smaller than what your photograph shows. This means
that Kubota has no work-port relief for the thumb circuit.

Now, how about the bucket valve?

In general, hoes use work-port RVs where you can induce large forces to a valve that
is closed. This can happen when you are using the dipper & bkt to pull on a root that suddenly
gives way, or when you lift something with the boom to its limit, then continue the
lift with the dipper. In the latter case, you can see the boom cyl drift down when the
boom valve WP RV opens.