Custom hydraulic backhoe thumb

[dfkrug]

This thread will describe what I went through in the design and build process
for a hydraulicly operated thumb for my Prairie Dog backhoe attachment.

Those who have seen my previous DIY thumb postings will note the many
similarities to my Fixed Mechanical Backhoe Thumb (1st and 2nd generations),
and my Articulated Mechanical Backhoe Thumb. Here is the link:

http://www.tractorbynet.com/forums/build-yourself/94333-another-backhoe-mechanical-thumb-fixed.html (Photo 1 is the fixed Gen 2 thumb.)

I did use a modified version of my Generation 2 thumb, but going hydraulic
meant substantial and laborious changes. Over the last several months, I
have been experimenting with cylinders and valves to achieve my design
goals. This hydraulic thumb is all new, and not a conversion of one of my
mechanical thumbs. I have recently completed the project and it works
great. Over the next few days, I will post 18 pix and share the process I
went through.

Two of my design goals were to be able to completely retract the thumb
when not in use, and to be able to grasp objects with more "dexterity" than
a fixed thumb. Both activities had to be performed from the operator's seat.

First, the experiments. As a mock-up, I took a 2x8 cylinder I had and
substituted it for the toplink I use for the mechanical thumb brace (2nd
photo). I connected this cylinder in parallel with the curl cylinder so
they operated simultaneously when the curl circuit is activated. This
functionality was very effective in grasping rocks and logs since equal hyd
pressure goes to both cyls. (3rd photo) I moved a lot of rocks and
logs with this setup.

My force calculations and thumb geometry showed that my 2" cylinder
would apply slightly greater force than the bucket at its maximum force
position. At full thumb extension, the bkt force became slightly larger as
the 2 moment arm lengths changed. A 2" cylinder would be best. The
stroke would need to be 11 inches to achieve the desired travel.

 

[dfkrug]

I next tried several kinds of valves in my quest for an economical solution.
I tried manual restrictor valves (photo 1, right), but 2 would be needed and
I would still need to get off the hoe to operate them, essentially turning
the thumb into a fixed thumb. The red valve is a solenoid-operated
poppet valve, which will cut off flow in only one direction. Again, 2 would
be needed.

I decided to go with a single solenoid-operated spool valve that would cut
off flow in both directions. I used this in preference to a solenoid-operated
diverter valve because those are a lot more money and they are much
larger, and require more hoses. By cutting off flow to only one side of the
thumb cylinder, I convert it to a fixed thumb, either fully retracted or
stopped anywhere in its travel. The 2nd photo shows the Hydraforce valve
I used. The 3rd shows my mock-up with the new valve. The valve was
about $40, but had to be ordered from a catalog from a reluctant distributor.

Next: the fabrication.

 

[RedDirt]

OH goody, goody. I look forward to your post.

A thumb is on my winter project list. I can see the value of a hydraulic operated thumb but did not want to go through the expense of a dedicated thumb control circuit. I like your concept of running the thumb parallel with the curl cylinder. I'm anxious to see your results.

 

[dfkrug]

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.

 

[dfkrug]

These pix show the cross tubes just after welding them to one end of
the cylinder and to the rod. I also drilled and welded a JIC female
fitting to the cylinder end and capped it for an air pressure test. I am
rarely a good enough welder to get it air-tight the first time. Hence the
air pressure tests. My cylinder is now a hybrid between a tie rod cyl and
a welded one.

Next, I wanted to use a hard line for the retraction hydraulic connection.
This end is closest to the action so I wanted to minmize risk of hose damage.
Ever have a hose under pressure get ripped out? What a mess! My third
photo shows how NOT to weld up a hard line. My hyd hose shop gave me
the tubing in the photo and I welded it carefully, but it was very weak
at the weld and easily broke. You can see the chrystalline ends of the
break in the photo. Clearly the wrong tubing...it was only .030 in thickness,
too. I went and bought some 3/8 .063 tubing at Home Depot and re-did
it. My next post will show the finished custom cylinder.

 

[dfkrug]

And here is the finished cylinder. The glossy black paint and lighting
conditions make the pix not that clear, but you can see the hard line
and the bracket for mounting the solenoid valve.

In the 3rd photo, with everything installed on the backhoe, you can see
the valve mounted on the cylinder and the electrical connections. I made
it so there is no hose or tubing between the valve and cylinder. Strain
relief is provided for the wires.

The 4th photo shows my thumb control switch, which is on the left side of
the control console. When ON, the thumb cylinder operates at the same
time as the bucket. When OFF, the thumb is fixed in the position it was
in when the switch is thrown. BTW, this is a water-proof switch...rarer than
hen's teeth.

 

[dfkrug]

Here is the completed hydraulic thumb showing fully open and fully
closed positions. The only thing I need to change is to have another
short hose made to replace one that has the wrong end fitting.

 

[mjncad]

Excellent work!

 

[RedDirt]

Man, that's a beauty! Great design and workmanship and an excellent job on the post. Thanks.

Two questions:
1. What is a cost estimate for the solenoid-spool valve?

2. When the thumb and bucket operate together do you have a noticeable slowing of the bucket speed compared to when the thumb is in a fixed position (and the bucket is getting the full flow of the pump)? It would seem the bucket would move at about half speed when moving the thumb (if the thumb and bucket cylinders were about the same size).

 

[dfkrug]

Thx, MJN & RED

1. What is a cost estimate for the solenoid-spool valve?

I quoted about $40 back in post #2, but allow me to expand: I live near
a big metro area that should have plenty of places to buy hydraulic stuff.
Other than hoses and fittings, I have had only poor experiences at retail
bricks&mortar places. Consequently, after Prince closed its direct sales
site, I have been relegated to Surplus Center and Ebay for valves and
cylinders. If I need something specific from a catalog, I can rarely get
a distributor to call me back. I keep PDF catalogs from numerous valve
suppliers (Parker, Eaton, Sterling, etc) and most carried what I needed.
After a month of waiting, I got a Hydraforce disti in Oregon to send me
a couple of their SV08-24-6T-N-12DS valves. This is a 12 volt 3000psi
4.5GPM unit with dual spade connectors.

2. When the thumb and bucket operate together do you have a noticeable slowing of the bucket speed compared to when the thumb is in a fixed position (and the bucket is getting the full flow of the pump)? It would seem the bucket would move at about half speed when moving the thumb (if the thumb and bucket cylinders were about the same size).

Yes, operating both cyls at once does slow down the movement of either
cylinder, but they are moving the thumb and bkt toward each other, so
the effect is not readily apparent. My hoe moves plenty fast anyway, since
my Kioti tractor delivers 8.3GPM max. I run at about 2000RPM engine
speed. I also deliberately made my welded-on fitting on the custom cylinder
somewhat restrictive. In actual use, the bkt and thumb experience different
forces when they meet a rock or dirt or whatever. The hyd fluid takes the
path of least resistance and pressure is always the same in each cylinder.
So what you see in operation is usually either the bkt or thumb moving by
itself until it touches something, then the other element moves. The orifice
restriction DOES provide some dynamic pressure differences in the cyls,
but hopefully you get the idea.