PNEUMATIC backhoe thumb

[Arc weld]

For a thumb air wouldn't work. A 2" hyd. cylinder at 2000 PSI has 6280 lbs. push. To get the same force from an air cylinder at 125 PSI, you'd need an 8" cylinder. Most air systems use a large tank so the cylinder can act fast. You would need a huge compressor if you planned to use your thumb for picking up a pile of logs or something. Also with air you don't have the same control you do as with using hyd's. Air works good for things like clamping jigs were the cylinder is basically open or closed. Very hard to control stroke somewhere in between. Hyd's make it so much simpler. I'm sure somebody could come up with an air system but it would be 100 times more complex and most likely 100 times more expensive. Even then, you couldn't get away from needing a HUGE cylinder and a huge compressor. Consider how long it takes a 12 volt compressor to fill a flat car tire to 32 PSI? Do you really want to wait an hour for your thumb to build up 125 or 150 PSI?

 

[PANHAPP]

If you use an air cylinder and a rock slips out be ready for an EXTREME BANG.
Pan

 

[bigdeano]

It would probably be easy to come up with a linkage which goes over center to lock the thumb in position with just a small air cylinder. For instance the cylinder which lowers the main landing gear on a 747 doesn't support the weight of the plane, it just moves the linkage.

 

[KYErik]

It would probably be easy to come up with a linkage which goes over center to lock the thumb in position with just a small air cylinder. For instance the cylinder which lowers the main landing gear on a 747 doesn't support the weight of the plane, it just moves the linkage.

Here's the perfect solution. A light duty 12 volt linear actuator might end up being less work (no plumbing or compressor).

 

[Arc weld]

A linear actuator? Have you priced one of those lately? A backhoe thumb is anything but light duty and you'd destroy a linear actuator in no time. If you don't want or can't afford a hyd. thumb, get a mechanical thumb.

 

[KYErik]

A linear actuator? Have you priced one of those lately? A backhoe thumb is anything but light duty and you'd destroy a linear actuator in no time. If you don't want or can't afford a hyd. thumb, get a mechanical thumb.

When I said "here's the perfect solution" I was meaning bigdeano's idea- which I quoted. Use the linear actuator to lock the lever over center (so it would never see the load put on the thumb)

 

[Arc weld]

I was looking at some small linear actuators and they were several hundred dollars.

 

[Brad_Blazer]

Does your JD310 have 3600 psi hydraulics and a bucket digging force of 11,000lbs like the current model? The thumb needs to be able to counter the full digging force of the bucket with a considerable safety factor or it will be quickly destroyed.

If you have an appropriate hydraulic cylinder, something like 4x24 rated to at least 3600psi, and it will only cost $300 in additional components I doubt you will get by any cheaper than that.

I used a $110 solenoid circuit selector from Solenoid Valves item 220-934 on my loader grapple and it works great. It is rated at 5000psi but the 13gpm flow rating mat be a little low for you.

 

[KYErik]

Here's a linear actuator for about $70 with free shipping:

Heavy Duty 10 inch Linear Actuator Lift Hood Adjustable Stroke 200lb 12 Volt DC | eBay

I used a similar actuator on my chicken house door to open/close it at dawn/dusk. They are slow but very strong.

 

[sparc]

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.

This sounds like a logical plan to me if you are going to use air. I too would be worried about the clamping force you can generate with air. I have worked on many pneumatic actuators used to operate valves and rebuilt many different styles and makes. Used in an enviroment such as construction or demolition the seals will wear pretty quick, and air being much thinner than water or hydraulic fluid will leak by sooner and faster.

but, using the air powered actuator to lock a rigid thumb in position would work well I think and require less air volume and pressure.