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.

 

[DanMc77]

Thanks guys for all of your input:
On the greasing of the thumb pivot - I have considered how to supply grease. I see two options: the first is to put zerks into the pivot bosses. The other is to use shallow grease channels in the pivot pin with a zerk on each end of the pin. As I mentioned elsewhere, I had to replace the dipper pin because it was rusted tight in the boom. When I got the new pin, I put it in the lathe and drilled a 3/16" hole from one end all the way into the center and then drilled another small intersecting hole so grease can exit the pin in the center of the boom. Then I milled two shallow channels from the intersecting hole so the grease can pass back outward about 1/3 of the way back out. This way, grease is well distributed over the span of the pivot in the boom. I'm thinking that for this bucket/thumb pin, I'd put in a zerk on each end of the longer pin and drill/mill the channels so grease can be injected into the dipper and then outward to the thumb.

On the stiff stabilizer control: I agree that the stabilizer control levers are stubby and stiff. I would much prefer a separate independent control for the thumb, especially one that is up near the other two controls. I guess it will depend upon cost and how much it is to install. I'm very curious about the splitter twist update!

 

[downsizingnow48]

There are about half a dozen detailed threads on adding hydraulic thumbs that would be worth looking up for info on valve options, reliefs, modifying pins, thumb configuration, etc. I would not favor milling a grease slot on the surface of the pin because that would increase the likelihood of the kind of failure dfk has mentioned. Flush face fittings on the thumb bosses are easy and safe.

 

[DanMc77]

There are about half a dozen detailed threads on adding hydraulic thumbs that would be worth looking up for info on valve options, reliefs, modifying pins, thumb configuration, etc. I would not favor milling a grease slot on the surface of the pin because that would increase the likelihood of the kind of failure dfk has mentioned. Flush face fittings on the thumb bosses are easy and safe.

Downsizing: I feel a bit foolish for not thinking of this before. I searched the top level forum for "thumb" and found a treasure trove of threads with photos of thumbs, mostly fixed mechanical. Lots of ideas there to help with working out the design.

 

[downsizingnow48]

yep tbn is an amazing resource.

 

[DanMc77]

This weekend I spent some time building a wood model of a thumb. This is just to work out the general geometry. I know that I need to add some support for the pin where it ties into my wood hydraulic cylinder.

I was pretty happy with this until a friend of mine sent me a photo of his thumb. His thumb tucks in very nicely around the hydraulic cylinder and shields it from damage while digging. Now I'm going to have to modify my design to incorporate this type of configuration.

 

[beenthere]

Building a wood model is a great way to design before fabrication. :thumbsup:

 

[downsizingnow48]

There are various ways to protect the rod end of the cylinder.

 

[DanMc77]

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.

Hey TexasRanger,
Now I'm putting some thought into the hydraulic side of this. From what I hear, a foot pedal is a great way to run a thumb, and that would mean adding another spool control valve. I spent a bit of time reading up on hydraulic control valves this weekend. You have already done this. Can you post some pics showing how you tied in your valve for your log splitter? That would be priceless!
Thanks!

 

[texasranger556]

The hydraulics are all hooked up, but version 1.0 of the twist design using scrap I had around didn't work out so I have to order the correct stuff. Once I get that finished I'll post more details of the build with pictures.

Basically my tractor hydraulic pump feeds the loader, then the power beyond feeds either the 3 point hitch or the backhoe if it is installed. From my investigation I couldn't determine if my 48 backhoe valve has a power beyond option (only 2 hoses from factory, and no visible place for a 3rd) and I didn't want to risk damaging it, so I plumbed ahead of it with my new valve. So hydraulic pump feeds loader, then my valve, then the backhoe. The backhoe return hose dumps to reservoir like it always did, but is now supplied by the power beyond port on my new valve. New valve also has a return to reservoir hose for the low pressure oil returning from the splitter cylinder. Clear as mud? I can sketch it out if it's not clear.

 

[DanMc77]

Crystal clear! My 48 backhoe is on a JD4600 with the same power beyond connection. So when the backhoe is lifted into place and pinned using the 3-point rockshaft, then the tractor is shut down and the loader control run through the motions to clear out any hydraulic pressure. Then the power beyond line on the back of the tractor is disconnected and the two backhoe lines are connected in.

So your valve has a power beyond port enabling you to insert it into the high pressure line that feeds the backhoe. That makes a lot of sense. So the tractor's power beyond line runs first to your new valve, then you added a new line from your valve's power beyond port to the backhoe. The backhoe return line is unchanged. Easy as pie!!

I took off the cover that reveals the top of my backhoe spool valve. I couldn't see much other than the 6 control rods coming down, and a lot of pine needles and other associated crud obscuring everything. I didn't have any time to take off any other things to look down below.

I found a spool valve that looks like an excellent candidate for this, but I'm not sure about the max flow rate and I have to figure out what pressure the hydraulic pump is running at: Looking at the JD4600 Service Manual, the pump has a working pressure of 2500 PSI and it lists "Front Pump Capacity" of 4.8 gpm and "Rear Pump Capacity" of 9.9 gpm. The front pump is for power steering, the rear pump is for implements. This valve has max flow rate of 8 gpm, so it is not be usable. Drinking from a firehose learning going on here!
1 SPOOL 8 GPM PRINCE MB11B5C1 DA VALVE | Directional Control Valves | Hydraulic Valves | Hydraulics | www.surpluscenter.com