Backhoe Thumb Hydraulic Options

[avc8130]

I plan to copy the Kubota thumb pretty much directly since I already have the mount on the stick. There is no way the thumb will ever have leverage over the curl. As we all know, the curl is the strongest function on the hoe by far.

ac

 

[dfkrug]

The thumb definitely needs to stop when it pushes against the curl and the fold
back when the curl pushes against it.

If you design the thumb properly, neither the thumb nor the bucket need to retract in reaction
to force from the other, and you do not need work port RVs. That's how mine works. I have 2" cyls
for both, and both are well-made cyls that can handle the pressure from the other pushing on it. Part
of a good design requires calculating the max forces at the max leverage angles.

There is another way to control the thumb, too. Another single-spool lever valve works, of course, as Kubota
uses. An electric dual selector valve slaved off one of the hoe valves works, too. I run my thumb cyl in parallel
with my curl cyl, and put one small 12v spool valve on just one of the cross-over hoses. This way, when
I flick a toggle switch, my bkt and thumb act like a grapple. Flick it off, and the thumb is anchored in a fixed
position. A push button on the right joystick would also work here.

 

[J_J]

Antone want to try and compute tip force with this scenario.

Curl cyl 3 in bore @3000 psi

Pushing against the pin holes 10 in from the pivot point.

Length of bucket 30 in from pivot point to tip of bucket.

Thumb cyl 3 in pushing on the thumb at an angle of 45 degrees.

length of thumb is 30 in.

Which cyl will push the other at what tonnage/lbs

 

[Gary Fowler]

If you design the thumb properly, neither the thumb nor the bucket need to retract in reaction
to force from the other, and you do not need work port RVs. That's how mine works. I have 2" cyls
for both, and both are well-made cyls that can handle the pressure from the other pushing on it. Part
of a good design requires calculating the max forces at the max leverage angles.QUOTE]

I have the Kubota hydraulic thumb on my B26 and it works just like DFKRUG states. Neither will push the other back. They will connect tightly and then go to relief if you continue to apply hydraulic pressure but they wont push back in either direction. I like the foot pedal control, just wish I had a bit larger platform area than what is on the B26 because with the pedal folded down, it doesn't allow for much foot action

 

[J_J]

For some reason, I keep thinking that if I put two cyls head to head, lock down, and apply pressure, even with relief, the larger cyl will cause the other cyl to go into relief and retract.

If you really want to know, just tee a gage into each cyl circuit.

 

[avc8130]

I run my thumb cyl in parallel with my curl cyl, and put one small 12v spool valve on just one of the cross-over hoses. This way, when
I flick a toggle switch, my bkt and thumb act like a grapple. Flick it off, and the thumb is anchored in a fixed
position. A push button on the right joystick would also work here.

I'm not sure I follow exactly how that works, but it sounds really cool.

Any chance you could try to explain it to me again? LOL

ac

 

[J_J]

He is putting both cyl in parrallel and fluid is flowing to both cyl when he activates the 12v and when they get to relief setting on the BH valve, they relieve or he stops the flow.

The thumb cyl is at rest when the switch is off and the curl cyl will still work using the BH spool for the bucket.

 

[avc8130]

He is putting both cyl in parrallel and fluid is flowing to both cyl when he activates the 12v and when they get to relief setting on the BH valve, they relieve or he stops the flow.

The thumb cyl is at rest when the switch is off and the curl cyl will still work using the BH spool for the bucket.

Thanks for that explanation. I found his original thread about it:

http://www.tractorbynet.com/forums/build-yourself/125912-custom-hydraulic-backhoe-thumb.html

It looks like a pretty ingenious way to do it. Any objections?

I guess to "stow" the thumb the cylinders are left in parallel and spread until the thumb is in the stowed position and then the switch is thrown into "curl only"?

ac

 

[Coyote machine]

Very interesting. So how would I fix an existing hydraulic thumb where the curl of the bucket pretzled the thumb cylinder first time I had it hooked up, while testing the setup? I replaced the cylinder and was told by the manufacturer to move the thumb to the bucket, not the bucket to the thumb. I fear making a mistake and wasting another thumb cylinder. Any help appreciated. Sorry OP, not intending to hijack your thread, just looking for help along similar line to what you're asking....
TIA,
CM

 

[J_J]

Coyote machine,

What happened to you is what I was trying to say.

I would think a relief valve on the base end of the cyl and running the relief port to tank would prevent the excess pressure.

Just set it to a lower pressure than the BH valve or the tractor valve.

With no relief on the cyl, anything pushing on the thumb cyl will probably generate enough pressure to damage the cyl or hoses.

If you do the math, you can come close to figuring out the correct size cyl to use to oppose the curl cyl setup.

The solenoid valve used by the above person, is still not free from damage, because if he removes the 12 v, the force is against the thumb and the solenoid valve, and if it does not have a relief, either the cyl or the valve could be damaged.