Backhoe Building a thumb for a model 48 Backhoe

[texasranger556]

Very professional, and always fun getting new tools. I understand the desire to limit modifications to the backhoe frame, but it sure is going to make it annoying to change the bucket sharing a pin like that. Of course if you only have one bucket then I guess it doesn't matter much!

As for the hydraulics, an electric pump is going to suck up a lot of current. Your tractor, like mine, has a 40 amp charging system. Can't run the pump very long before killing the battery. An electric pump is going to provide maybe 20% the flow your tractor pump does, so very slow, and you have to find space to mount the pump/reservoir. Also run a heavy gauge wire to the battery.

I think you would be much happier with a diverter valve on one of the outrigger circuits, or get someone to help you install an extra valve upstream of the backhoe if you don't feel confident in your abilities.

 

[DanMc77]

Yeah, I only have one bucket and don't see that I'll be needing a different one. It's already a pain to take the bucket on and off. The first time I took it off it was a huge pain because one of the pins was stuck.

As for the electric pump, I was thinking that I could get way without adding another spool valve, but as I look into this, It looks like I still may need to add the valve. So maybe the electric route may not be as "easy" as I thought.

I'm expecting that I would only run it when I have to move the thumb. That's not very often and it would be much better than having a fixed thumb where you have to get off the machine to reposition it. I haven't bought any parts yet, so I am still open to all approaches. I have pretty much gone full circle on the hydraulics side of this. The only thing I'm committed to is the cylinder, which has been in my shop since last summer.

 

[texasranger556]

The bucket pin was frozen on mine too. I had to cut the end out of the dipper to get enough heat on the cross tube and pound on it for an hour before it finally came out 1/64" at a time.

When I cut the end out of the dipper water came out, so I suspect it sits in there and eventually works it's way in and gaulds the pin. I drilled a drain hole to prevent it from happening again.

 

[DanMc77]

When I bought the backhoe, it had very little run time on it, but it had sat outside for a while. The dipper pin was frozen in the boom and it was pivoting on the bosses of the dipper. Several people said just run it, but I couldn't do that even though I probably would never wear it out. I hired a guy to come over with a thermal lance. He blew a hole down the middle of the pin and that caused it to shrink enough that he could then bang it out. I got a new piece of TGP steel rod and machined a hole down the middle of it, with grease channels milled into the outer surface and added a zurk fitting on the end. Now i can grease the dipper pivot from the end of the pin and grease gets squirted into the center of the boom and outward along the little channels. I'll do the same thing with these pins once I do the final assembly.

I'll have to drill a hole in the bottom of the dipper to drain any trapped water in there!!!!

 

[texasranger556]

It's unfortunate JD didn't use that type of pin from the factory, a much better design. And they still want over $70 for a replacement.

 

[DanMc77]

'$70 for a replacement."

They are in business to make money and they do that well, unfortunately for us!

Any time take a pin out, I'll replace it with one that I've gone over and put in a Zerk. So much easier to get to the fittings too. The pin that goes to the links, I'll have to change to a snap ring on the end so I can get grease through to the middle.

 

[texasranger556]

Here is an example of a diverter valve if you decide to go that route. You could either use one of the outrigger spools, or the bucket curl spool. The advantage of using the bucket curl spool is you could mount the diverter near the thumb and not have to fish extra hoses through the boom, only the wires. I ran 3/8" hoses on mine and only one would fit inside the boom, the other is exposed along the side of the boom. With 1/4" hoses you may be able to fit both inside. Drawback is you can't move the bucket and thumb simultaneously, if you care.

12/24 Volt DC 13.2 GPM SAE 8 Solenoid Operated Double Selector Valve | Selector Valves | Hydraulic Valves | Hydraulics | www.surpluscenter.com

 

[ovrszd]

Assumption on my part here. Assuming you are going to push off your hoe stinger with the thumb cylinder? The chances of the thumb cylinder being an equal match to the power of the bucket cylinder is pretty small. How are you going to protect the thumb cylinder from damage when curling the bucket?

 

[DanMc77]

Assumption on my part here. Assuming you are going to push off your hoe stinger with the thumb cylinder? The chances of the thumb cylinder being an equal match to the power of the bucket cylinder is pretty small. How are you going to protect the thumb cylinder from damage when curling the bucket?[/QUOTE

There will have to be a bypass valve to protect that cylinder. I also have to pay attention to radius of the thumb cylinder pivot, but ultimately, the bypass will have to be there to limit the pressure from curling the bucket.

 

[dfkrug]

There will have to be a bypass valve to protect that cylinder. I also have to pay attention to radius of the thumb cylinder pivot, but ultimately, the bypass will have to be there to limit the pressure from curling the bucket.

By that theory, you would also have to have a work-port relief valve for the bucket cylinder. Not all hoes
come with those on the bucket cylinder circuit.

Instead, the thumb design can be done where the forces from one do not exceed the capacity of
the other. My own hyd thumb has worked well for hundreds of hours without any added RV.

Three sure ways to overpower your thumb cylinder: 1: Use a cyl with diameter smaller than your bkt
cyl. 2: design the thumb's cyl pivot to be too close to the dipper-thumb pivot. 3: use a cheapo tie-rod
cyl that is rated only 2500psi.

All of these forces can be easily calculated before cutting any steel.

I may soon be making another hyd thumb, this time for a mini-ex I have ordered.