Backhoe Building a thumb for a model 48 Backhoe

[DanMc77]

Hello!
I am embarking on a project to build a thumb for the model 48 backhoe on my JD 4600 tractor. Here's a pic of the model held in place by magnets.


Problem is, the tractor is not near by, so I can't measure this. I need to know what the diameter is of the bucket pivot pin. I want the thumb to use the same pivot point as the bucket, so I'll need to get a longer pin. Need to know what diameter.

One more question: What grade of steel would this pin be?

Thanks!
Dan

 

[texasranger556]

My 48 has a 1.25 pivot pin, approx 11 inches long. I replaced mine with 1144 stress proof because the original one gaulded in place making it impossible to change buckets. I started a thread about it here a year or so ago.

Edit: I realized I never followed up on that thread and just did so it should be at the top of this sub forum.

 

[dfkrug]

I don't know what the 48 or 448 hoe uses for the bkt pivot pin, but you may
want to rethink sharing it with thumb. The extra long length, and high forces
involved will likely break it.

I suggest a separate pivot next to it on the dipper stick.

 

[DanMc77]

Thanks for that input about using a separate pin. I have seen both approaches used and sharing a common pin simplifies installation and keeps the thumb in line with the bucket at all angles. I will consider the separate pin and discuss it with the machinist that will do much of the design and fabrication.

 

[DanMc77]

This weekend I met with the machinist who will do most of the heavy metalwork of this thumb project. He said that I had a good start but needed to go back home and build a more accurate wood model that demonstrates more accurately the geometry. He predicted that if we built off of my partially completed foam model, that ultimately we would be cutting things apart to shift things around. I realized that I could eliminate the need for building several parts if I bought a hydraulic ram rather than use a fixed mechanical link. He added that his backhoe has a thumb that's plumbed into one of the stabilizers and has a hydraulic switch to select between running the stabilizer ram and the thumb ram. That looks like an excellent approach. So, as time permits, I'll be working toward this approach. I will also use the bucket pivot, which is what his backhoe does. Keeping the thumb and bucket on the same pivot makes the whole arrangement so much better as they maintain their alignment throughout the range of rotation. If I end up breaking or shearing a pin, I'll deal with it.

I ordered a 48" piece of 1.25" dia 4140 annealed steel rod to make all the pivot pins. I'll post a pic of the wood model once it's done. The 4140 was the best material considering strength, cost and availability.

 

[DanMc77]

I found a John Deere design for a mechanical thumb.

 

[texasranger556]

Not sure about your backhoe but my stabilizer valves are fairly stiff because the handle is short. Are you planning to change the handle or even make it foot controlled?

Another option is add another valve. I used a loader valve because I got it cheap and wanted the second circuit for twist on my log splitter project. Mounted it between the two existing sticks.

 

[dfkrug]

Keeping the thumb and bucket on the same pivot makes the whole
arrangement so much better as they maintain their alignment throughout the range of rotation. If I end
up breaking or shearing a pin, I'll deal with it.

Since you have elected to control the hyd thumb independently from the bkt cyl, the thumb
will close against a stationary bkt. If the thumb and bkt pivot about a different
point, that will make them contact each other at a different point, depending on the position of
the bkt. This is not a problem at all. I have used mine that way for years.

As for the shared bkt pin, it will not only have much greater bending forces on it, you
can not really grease the part that the thumb pivots on. You may not have considered
that.

 

[texasranger556]

Another thing to consider is any time you want to change the bucket you'll have to fight to line up the thumb and any washers you add.

 

[downsizingnow48]

You will need to provide grease fittings on the thumb bosses for the shared pin.