A hydraulic thumb is a lot different than a fixed mechanical thumb esp in
that all pivots need to be fully greaseable. So I welded up another one
similar to the earlier mechanical thumb (photo 1). The big difference is that
now I am using 1.000" ID DOM tubing with 1/4" walls. These will form
bushings for the 25mm pins, allowing a 0.4mm clearance. Cross tubes on
the cylinder will also be made of this material, along with threaded in zerk
grease fittings. I used an end-drilled main pivot pin with grease groove
and protected recessed zerk. I can get these chrome-plated 25mm pins
pins for very reasonable prices from the Prairie Dog backhoe manufacturer.
I will use cir-clips to retain them. These seem to work a bit better than
bolts, hair pins, or retention clips.
The cylinder tie bracket and end plate I
cut out of 1/4" plate so I could
customize a tie rod cylinder. I waited for a long time for a decent 2" diam
cylinder of at least 12" stroke to become available on Ebay or Surplus Center,
but I did not find any for under $80 or so. So I took a Grizzly 2x16 that I
had ($75 from AgriSupply) and
cut it up. If someone wanted to to use a
standard size cylinder, they could design their thumb around the cylinder,
rather than design their cylinder around their thumb. I prefer welded cyls
over tie rod cyls cuz I wanted welded cross tubes and the tie rods offer
more places for debris to get caught. 2nd photo shows the cyl intact;
3rd shows it all apart.