Backhoe thumb

[JoelD]

I have the divit issue with mine, I'll be extending.
I figure you you end up with a problem with the bolt through you can always weld to the stick.

Your thumb looks very well built and I doubt you'd have any problems with the thumb it self.

Nice work,

Joel

 

[mboulais]

the reason thumbs are usually longer is that when the boom is fully extended the thumb can reach around the object allowing the bucket to swing in without scooping up dirt my guess is that , this set up may pull some divets, and I have to agree about through bolting ,it is a little sketchy a small rock will peel it right off , also it may split right at the teeth nothces , I am not trying to rain on your parade either but I have built quite a few thumbs. just friendly advice,saying it wont be overloaded is well you know..

What do you consider "small" when talking about rocks? I would worry about the pivot holes egging out before anything else on that fails. The most likely failure would be swinging the boom and striking the thumb against an immovable object. Sideloading will certianly twist that steel. Those notches are insignificant stress risers in a 3" wide piece of flat plate given the availble force of a compact tractor backhoe. The John Deere model 7 backhoe has a bucket roll force of 2400 lbs, but only 200 lbs boom lift capaicty at full extension. I am not concerned about overloading based on the available power in the backhoe. I think it will outlast the ones he priced online at $400+. The day I gave it to him, I offered to make a longer jaw for it if/when he wants one, but he likes it the way it is now.

 

[sizzami]

sorry you arent understanding the issue of the rocks, it isnt the weight of the rock that will cause failure it is the leverage enhanced crushing power of the bucket curling back against the thumb causing the plate bolted to the boom to shear the bolts off. the object will have to be the exact right size to form the perfect hinge point for the shear to happen so it probably will take a while but I bet dollars to donuts it will. the wieght of the rock is irrelivant . and by the way the job IS nice

 

[mboulais]

sorry you arent understanding the issue of the rocks, it isnt the weight of the rock that will cause failure it is the leverage enhanced crushing power of the bucket curling back against the thumb causing the plate bolted to the boom to shear the bolts off. the object will have to be the exact right size to form the perfect hinge point for the shear to happen so it probably will take a while but I bet dollars to donuts it will. the wieght of the rock is irrelivant . and by the way the job IS nice

Thank you for the compliment. I think I understand all the forces involved. The failure you are describing sounds more like a tensile load than a shear load. You would have to be prying on an immovable object. Force applied near the end of the jaw would make the thumb bracket try to pivot away from the backhoe at the jaw end of the mounting bracket. The bolts neast the jaw end of the bracket are under the greatest tensile stress, the bolts furthest from the jaw are under the least tensile stress. The bolts should all be sharing the shear stress equally. Quick math says each of those bolts will safely hold 2,600 lbs of shear load and safe tensile load on each of them is around 4500 lbs. The bucket curl motion is the most powerful motion on the backhoe due to the mechanical advantages provide by the geometry of the cylinder , bucket pivot point, and linkage. The bucket can produce 2400 lbs of force at the tip of the bucket. I am not going to do all the math but with 15 years of mechanical engineering experience in heavy industry, my gut feeling is that it will hold up.

 

[workinallthetime]

1 redneck with a torch and welder, has takin 1 mod since i built it and that was to add bracing to the tip of the "teeth". I wish that i would have made a way to fold it up but other than that it has picked up rocks that the bh would only lift 6" off the ground, pulled lots of trees out of the ground and only cost me 30 bucks, and a few hours welding.

 

[Volp]

I made an even simpler thumb from scrap iron which folds away easily when not used. I am holding quite a heavy stone with it here.

 

[sizzami]

what a coincidence I have 15 years of fixing stuff designed by "engineers" I say this , through bolting box tube isnt a good idea. it just isnt.take a scribe and mark the boom of your hoe and the thumb plate then pick up a rock and look at the marks do they still line up?