PNEUMATIC backhoe thumb

[npalen]

I'm not sure that locking the thumb into it's clamped position would do a good job of holding the load. The load is probably going to do some shifting while being moved requiring a constant resilient force against the thumb.
I don't know if it has any application here but air over hydraulic is used in industry to control the speed of an air cylinder. An example is the power feed on a production drill press where the power is supplied by a pneumatic cylinder which is mechanically attached to a small hydraulic cylinder with a needle valve to control the feederate. Hydraulic power is not required, just air pressure.

 

[Arc weld]

The easiest, cheapest and best way to a have a mechanical non rigid thumb is to just use the hydraulics already on the tractor! :banghead: It's like trying to reinvent the wheel.