PNEUMATIC backhoe thumb

[Lapeer20m]

I am in the process of building a thumb for my backhoe. While a hydraulic thumb is much more useful/convenient than a manual thumb, the hydraulic version also requires a lot more money.

I am a master tinkerer, but i'm cheap. I have enough scrap steel laying around to build the thumb, but I think conservatively, it would cost an extra $300 to purchase the components to convert a manual thumb to hydraulic, not including the cylinder that i already have on hand.

Instead of hydraulic, why not power the cylinder with air? In my case, i already have a suitable 12 volt air compressor, storage tank, hoses, and valve. I will likely have to buy some fittings and a few other odds/ends, but in my case a pneumatic powered thumb would be nominally free.

If i were to use a cylinder with a 4 inch ID, it works out to about 12.5 square inches. In essence, for every pound of pressure applied to the cylinder, the cylinder can lift (push) 12.5 pounds. At 150 psi, the cylinder will have the force of approximately 1,875 pounds.

Do you folks think that is enough force to hold the thumb against the bucket for picking up trees and rocks?

If i made it slightly more complex and utilized a few links that made the cylinder move twice as far as the thumb i could double the force of the thumb to 3,750 pounds.

Can anyone think of a reason to not use air?


this photo is basically what i plan to build.....

 

[6sunset6]

Air is compressible. I don't think it will hold the load. Plus it will leak by the seals.

 

[Mendonsy]

I don't see anything wrong with the concept, but the details might get you.
A 4" cylinder probably wont give you enough force. Something more like 6" would be better.
Going bigger on the cylinder will use more air, maybe more than your 12 volt compressor can deliver.

 

[Mathias NY]

I would also be very leery about lifting anything with a compressible clamp.

If a rigid thumb will do most of what you need, consider using the air to deploy/retract the thumb and find a way to make it lock in place pneumatically. This would allow you to have all of the strength of a rigid system with the convenience of a hydraulic system. You would be able to use a smaller cylinder, you wouldn't need a complicated scissor mechanism to amplify the force, and if your air pressure failed you could still use it as a mechanical thumb.

Good luck.

 

[Kernopelli]

I too am in the process of making a backhoe thumb. I don't think pneumatics would be reliable or nearly strong enough for the job, especially on a 310. A rigid design is simplest but I ran across one in Farm Show magazine that got me thinking...Amulet makes a mechanical thumb that uses linkages that use the bucket cylinder as the thumbs actuator. Amulet Manufacturing Company - HoeClamp . A little more searching and I found that several of our TBN members had also come up with even simpler designs. http://www.tractorbynet.com/forums/build-yourself/94333-another-backhoe-mechanical-thumb-fixed.html I actually have auxiliary hydraulics and an extra cylinder I could use but I like this idea so much that I am using it to make my thumb.

 

[jim_wilson]

Air is compressible. I don't think it will hold the load. Plus it will leak by the seals.

What he said.

There's a reason why hydraulic fluid is used and not air. There's also a reason why there is the term "compressed air" - and not "compressed hydraulic fluid".

Hydraulic systems in multiples of the pressure that the typical air compressor puts out. I'm thinking if you put a rock in between the backhoe thumb powered by hydraulic fluid at 2000 psi - and a thumb powered by compressed air at even 200 psi - I'm thinking that the hydraulic side of the equation is going to win.

 

[Xfaxman]

Have you considered how much force the bucket will transfer to the air cylinder when picking up something?

I think it might exceed the pressure rating of the hoses, because the thumb will move, increasing the pressure in the cylinder.

The fact that you can't compress oil is why it is used.

 

[Gale Hawkins]

I could not see air being an option for reasons already stated. While I would like to have our manual 310 thumb set up powered it just does not make $$$ on our old rig. Picking up 300-400 pounds creates a lot of force on the parts. Our pin is a hollow pipe and it it is somewhat deformed already. I look at it as my fusible link.

Dad would say everything thing needs a weak link that is easy to replace and cheap.

 

[dstig1]

I'd agree with everything said above, but would also add that it will take a LOT of air to run a large cylinder up to pressure and you are proposing to run it off a 12v compressor, which I'm guessing is probably just made for filling tires. I doubt you will have enough air supply to keep up with serious work, or you will burn out the compressor unless is it rated for heavy service. Filling the surge tank to the needed pressure may be more than the compressor could handle, too. I wouldn't trust air for this application.

 

[dfkrug]

If i were to use a cylinder with a 4 inch ID, it works out to about 12.5 square inches. In essence, for every pound of pressure applied to the cylinder, the cylinder can lift (push) 12.5 pounds. At 150 psi, the cylinder will have the force of approximately 1,875 pounds.

Four inch cylinder? That's huge.

I like the idea that MATHIASNY proposed. I can picture a mechanical thumb that had it's rigid brace replaced with a
hinged one. The hinged link would lock into position like the knee on your leg. When the brace is folded, the
thumb retracts out of the way. To articulate such a hinged link, I would build a linkage to a small pneumatic
cylinder that would be mounted above the existing hyd curl cylinder. All linkages would have to be mounted
to the outside sides of the dipper, so I envision this approach would have TWO locking thumb braces.

Here is a link to a video my articulating thumb that Daryll referred to:
mechanical articulating backhoe thumb - YouTube

If you try anything with pneumatics, I would like to see what you come up with.