Ok guys I really need some help here. I have a John Deere 3720 44HP mid size compant tractor. I just had the dealer put on a 485 backhoe and install a custom mechanical thumb. I am sorry that I don't have the brand name of the aftermarket thumb but it's pretty sturdy. The whole rig cost me $9450 with tax ($1,100 extra for the thumb installed)
The rub here is that last weekend I was all excited to try it out and dig some medium size rocks (maybe 400 lbs) out of a path and I could hardly get the thumb to work and hold the rock. The weight of the rock was well within the lifting capacity of the backhoe it was just the bucket was having trouble locking in the rock against the stationary thumb. I called the dealer to explain and he said he has put many thumbs on that backhoe and I was the first to complain. I think the problem is that they welded the pivit of the thumb too close to the pivit of the bucket and when the bucket tries to pin the rock against the thumb it spits out. It's kind of like trying to pick up a 1" dowel with a pair of scissors...it will keep squirting out on you.
I think what has to happen is they need to grind off the welded thumb on the outer boom and and they reweld it about 4-6 inches further up the arm (away from the bucket pivot) to give it more opening capacity between the bucket and thumb. That way when the bucket pinches the rock against the thumb it will lock in the rock.
Anybody have any experience here? I really need some expert advice (or at least some moral support) here to get psyched up to insist the dealer to redo the install.
I have an aftermarket hydraulic thumb, and it pivots from the same point as the BH bucket. The Deere thumb also pivots from the same point. I don't think it is possible to mount a thumb any closer to the bucket than this, and yet I can pick up rocks in the 200-300 lb range quite easily.
What size is your BH bucket? Mine is 18" and most of the time rocks sit in the bucket with the thumb stabilizing them. If a rock is too big to fit in the bucket it is going to be difficult.
I had a self-made mechanical thumb on my JD 8A backhoe. It had two positions for the opening of the thumb - ~90 degrees and ~ 45 degrees - to the plane of the dipper. I used it for rocks and stumps. It worked very well.
I'm with CurlyDave......... the thumb on my Woods hoe pivots on the same pin as the bucket. They couldn't be any closer. I also don't think you realize what your backhoe is going to look like after they cut that thumb off and weld it elsewhere . How is the thumb shaped? Is it curved slightly toward the bucket at the end? How wide is your bucket? Does your bucket have teeth?
To answer the questions....I have an 18" bucket with teeth. I am surprised to ehar that others have the thumb and the bucket on the same pivot. I have to appologize as I will not be able to post the pictures for two weeks as I will not be back to Maine where the tractor is until about Memorial Day weekend.
I was really hoping to be able to pick and place rocks that were bugger than the bucket.
You should be able to pick up rocks that are bigger than the bucket! I used to be able to do that with the 18" bucket on my old 8A. (Although, it didn't take a very big rock to make the hoe really struggle!)
If you do some research here on TBN - you'll see a number of different thumb designs with differing attachment locations. The more elaborate and sophisticted ones have a shared pivot point with the bucket. But, the more basic designs will work just fine... note - the "seat-of-the-pants" example of a thumb I built.
I have the thumb on my 485 bachhoe mounted on my 3320. The thumb had to be moved foeward so it worked properly the dealer had it welded and re welded.The first stump I grabbed after digging out twisted the teeth frame enough so I had to use a rock to bend back and got the pin in and never used it again the teeth on the bucket jam up to it every time I close the bucket.
I have the thumb on my 485 bachhoe mounted on my 3320. The thumb had to be moved foeward so it worked properly the dealer had it welded and re welded.The first stump I grabbed after digging out twisted the teeth frame enough so I had to use a rock to bend back and got the pin in and never used it again the teeth on the bucket jam up to it every time I close the bucket.
It doesn't sound like the materials used to build the thumb were very strong. The frame and teeth for my thumb were made using 5/8" steel plate (some 3/8" plate on the dipper) and 1/4" wall thickness square tubing.
However, the position of the thumb is always less than the arc of travel that the bucket makes when closing on the dipper stick. Therefore, you could bend or break the thumb if you're not paying close attention to the amount of force you're exerting on it while working.
The long square tube that extends from the upper mounting bracket on the dipper stick is only 1 1/2" square tubing. It's the weakest link in the thumb. It could bend or deflect if the force from the bucket is great enough (before the hydraulics stalled into relief).
In the year and a half that I used the thumb I never bent it... and I dug a whole lotta stumps with it! But... I always tried to keep the load centered between the thumb teeth to avoid twisting and attempted to keep the bucket from stalling to relief (max pressure) when closing down on whatever I was picking up.
. How is the thumb shaped? Is it curved slightly toward the bucket at the end? How wide is your bucket? Does your bucket have teeth?