Backhoe Thumb Hydraulic Options

[avc8130]

I'm looking at my L45 and I would really "like" a thumb. Of course the Kubota kit is $1k+ for the valve part and $1500+ for the actual thumb. I want to focus on the valve side for now as I am pretty sure I can get the thumb side covered with some steel and a plasma cutter.

Kubota adds to the backhoe spool stack and uses a foot pedal to operate the additional valve. That seems "cool", but I would have to fabricate the whole pedal and linkage assembly.

I've been thinking about using a diverter valve. I could tap into one of the other functions on the hoe to control the thumb while still using my hands on the controls.

Anyone done this? Any fatal flaw to my idea?

ac

 

[J_J]

No, just get this valve and use one of the outrigger circuits . Add a switch and make the grapple the default mode.

Surplus Center - 12 VDC 13.2 GPM SAE 8 DOUBLE SELECTOR VALVE

http://www.ebay.com/itm/18-HEAVY-DU...avy_Equipment_Attachments&hash=item20d6ce1740

 

[avc8130]

No, just get this valve and use one of the outrigger circuits . Add a switch and make the grapple the default mode.

Surplus Center - 12 VDC 13.2 GPM SAE 8 DOUBLE SELECTOR VALVE

18" Heavy Duty Thumb for Backhoe Mini Excavator | eBay

Do you think the outrigger is the "right" place to borrow the function from? I was thinking one of the functions on the actual control sticks so I wouldn't have to move my hands. Just hold a button on the stick.

I'm not scared of the thumb portion. My hoe already has the cylinder mount on the arm and I will figure out the rest.

ac

 

[J_J]

You can put the selector on any spool in the valve. I just selected a lever that was not used that much.

If you did a 3rd function on the thumb, you could put the switch on any lever, and put a toggle fwitch for grapple extend or retract..

 

[avc8130]

You can put the selector on any spool in the valve. I just selected a lever that was not used that much.

If you did a 3rd function on the thumb, you could put the switch on any lever, and put a toggle fwitch for grapple extend or retract..

You've sold me on the ability to "feather" things now so I think I will do a diverter and think long and hard about which function to piggy back off of.

ac

 

[J_J]

You might keep this in mind about the cyl size.

Depending on the cyl size and mechanical advantage, one of the two cyl, the one on the bucket or the grapple cyl, will have more power than the other and cause the other cyl to compress and cause some damage.

An example might be a 10 ton cyl pushing against an 5 ton cyl, the 5 ton cyl will give on any pressure that will develop power above 5 ton. You might consider a relief valve across the cyl or maybe your BH has work port reliefs which would be a good thing. .

 

[avc8130]

You might keep this in mind about the cyl size.

Depending on the cyl size and mechanical advantage, one of the two cyl, the one on the bucket or the grapple cyl, will have more power than the other and cause the other cyl to compress and cause some damage.

An example might be a 10 ton cyl pushing against an 5 ton cyl, the 5 ton cyl will give on any pressure that will develop power above 5 ton. You might consider a relief valve across the cyl or maybe your BH has work port reliefs which would be a good thing. .

Yes, I definitely need to look into that. The thumb definitely needs to stop when it pushes against the curl and the fold back when the curl pushes against it.

ac

 

[MtnViewRanch]

Yes, I definitely need to look into that. The thumb definitely needs to stop when it pushes against the curl and the fold back when the curl pushes against it.

ac

The simple way to deal with this is to just get a smaller diameter cylinder for the thumb. Then it will never over power the curl cylinder.

 

[J_J]

That will not help as the larger cyl will usually develop more pressure than a smaller cyl. In order to be even close to equal, the cyl should be the same size.

Someone would have to work out the geometry to make them equal, so the next option is to put a relief valve across the thumb cyl, so it will relieve if the curl cyl over rides the thumb cyl.

In some cases, a locked down cyl would be better so the bucket could push against a solid bar.

There was one poster awhile back that said he could see the thumb cyl being compressed by the bucket, and it finally blew a hose.

If the BH has work port reliefs, the spool using the thumb should be adjusted to relieve before the curl cyl does.

 

[MtnViewRanch]

Why does it have to be equal? If the weak unit puts 6000lbs of pressure (which is plenty) against the curl cylinder that has 14000 lbs, unless the grapple cylinder has a huge leverage advantage what difference would it make? I will agree that the best situation seems like the grapple would have it's own relief.

 

[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.