Hydraulic thumb

[ridefst]

Here's some drawings for a hydraulic thumb I'm working on.
All laser cut 1/2" steel (good to have friends)
2" x 16" cylinder
1" pins throughout, also using three pins welded in to space/strengthen the tips. No bracing in the bottom half of the thumb, so the cylinder will fit in the middle when I'm not using it.
Mostly want the thumb to handle that pile of firewood logs in the background.
I've got the hoe to install a bunch of drain tile in the back yard and pasture.

Doesn't seem to be any real good way to design this, unless I'd go for a common pin with the bucket (which seems like too much trouble). Since it's just logs, I'm not too worried about precise control of the tips to the bucket teeth or anything, so I think this'll be okay.

Already got the aux hydraulic lines to the boom, so I should just need to cut/weld this up, and run some short lines.

See anything I'm missing or screwed up?

 

[magicheater]

Something looks out of scale on your drawings. seems like the thumb is awfully long. Most I have seen are shorter but maybe you are on to something. Good luck with your project.

 

[CurlyDave]

If you can get a close look at a commercial hydraulic thumb for your machine, take some measurements, some pix, and then copy it.

There is a lot involved in getting just the right geometry, cylinder sizes, etc. A copy of a known good one would be your safest bet.

 

[ridefst]

I agree that it is kinda long, mainly due to the pivot point being a little far up on the stick.
I'll probably cut out a paper/cardboard version before cutting steel, maybe I'll be able to move the pivot down a bit. That'd help with the geometry overall anyways.

Not sure where to find a weld-on thumb around here. The local rental shop has a couple with the common-pin mount, but that doesn't really help me much.

 

[mwb]

I think the bucket curl will buckle the thumb cylinder. If the three pins (two cylinder, on thumb) are almost in line then you will not have any mechanical advantage to oppose the bucket curl cylinder. You might want to look at the curling force of the bucket and work backwards make sure the thumb and cylinder can handle it.

Here is a sketch that might help illustrate this. I moved the rod end pivot point out this gives the thumb cylinder some leverage. If the lever distance is shorter (than the bucket) then you will probably have to increase the cylinder size or risk busting it.

Does this make sense?

 

[ridefst]

Makes perfect sense, I'll have to make this change.
The downside is that it will not "store" as nicely when I'm not using it, but if I can make it shorter, it might stay out of the way anyways.
I should've noticed that when drawing, but I was focused on making it fold up as small as possible, rather than actually functioning well:ashamed:

Guess that's why I posted it up, thanks for the catch!

 

[sizzami]

just a thought , but I built one once for this guy who had me make it just short enough to fit in the bucket far enough so that he could use it to bust tops and brush.

 

[niemeyjt]

Have you thought about not having the thumb hydraulic - like the grip talon shown here:

Excavator attachments, Ripper Teeth, Clay Spades, Tilt Ditchers, Land clearance rakes, Grapples, Grip Talons

Some of what they are showing looks similar to your requirements.

It swings out of the way when not required.

J

 

[ridefst]

I did originally think about a fixed thumb, but figured that if I can make it hydraulic for only about $200 extra, then why not?

Not sure about making the thumb significantly shorter than the bucket, I can see the advantage if you want to "bite" something off, but I don't think that is right for me.

I certainly appreciate all the input though:thumbsup:

 

[dfkrug]

Not sure about making the thumb significantly shorter than the bucket, I can see the advantage if you want to "bite" something off, but I don't think that is right for me.

RE the length of the thumb.....you may want to make it short enough
not to hit the boom when it is perpendicular to the dipper. I made mine
just long enough to clear the boom, minus about 1/2".

MWB is correct that if your thumb cyl stroke it long enough to extend the
thumb out straight, you risk overpowering it with the bkt curl force. You
can avoid that by limiting the extend stroke of your cylinder, keeping the
retract stroke long enough so that the thumb is up against the dipper
when retracted.

Another thing to consider: when using weld-on thumb pivot backets at
the end of the dipper, you must allow for the extreme tension forces
pulling on the dipper stick. The metal is not usually thick there. My solution
was to wrap the bracket around the dipper before welding.