Backhoe Building a thumb for a model 48 Backhoe

[dfkrug]

In all my thumb experiments and installs, fixed and hydraulic, the things I have bent
are pins less than 25mm in diameter and the sides of my bkt when grabbing a
rock. The forces are high and a rock now and then has pushed the sides out a bit on my 12"
bucket. Not a problem, really. Only happens with certain rock shapes that are a bit
bigger than 12" wide.

 

[DanMc77]

I got some time last night to finish up on the bucket/thumb pivot pin. I chose to counter-bore the end of the pin and use a regular zerk fitting. I'm thinking that maybe I'll make an aluminum cap with an O ring that press-fits into the counter-bore to keep out the dirt. Heavy-duty external retaining rings from McMaster-Carr (98585A127) will keep the pin in place. These rings are a hefty .093" thick. I also picked up some comparable stainless steel rings. I was thinking of possibly using these to avoid rust. These stainless rings are about half the thickness of the heavy ones, so I could have made the slot in the pin just a tiny bit wider and slipped in two of them. I think the single heavy ring is probably stronger than the two thinner stainless ones. I left enough space on the outside of the pin to widen the slot and add a second heavy duty ring next to the first one if one ever fails.

The zerk feeds a 7 1/2" deep 3/16" hole that runs about to the center of the pin and comes out to feed grease channels that will carry grease out from the center, so the whole length of the pin will see grease from two sides.

 

[dfkrug]

The zerk feeds a 7 1/2" deep 3/16" hole that runs about to the center of the pin
and comes out to feed grease channels that will carry grease out from the center, so the whole length of
the pin will see grease from two sides.

That snap-ring retainer design is similar to all the main pivot pins on my Bulgarian-made backhoe attachment.
The biggest difference is no washers at each end, and the grease slot is radial around the center hole,
versus your longitudinal slot.

My 2 concerns with this design is that the pins really should be constrained from spinning, and
any grease slot weakens the pin. This pin is under the most stress on my hoe, and several
have broken in the middle on other Prairie Dogs.

I am very good about greasing, so none have spun in their end bosses yet.

 

[downsizingnow48]

Curious how you matched the width of your groove cutter to the width of the M-C snap rings? I considered snap rings for my pins but ended up with the boss on one side and the collar on the other because it was simpler for me to do.

 

[DanMc77]

... the grease slot is radial around the center hole,
versus your longitudinal slot.

My 2 concerns with this design is that the pins really should be constrained from spinning, and
any grease slot weakens the pin. This pin is under the most stress on my hoe, and several
have broken in the middle on other Prairie Dogs.

I don't understand the thing about broken pins. I can see that they could bend, but not break. Well, maybe if I took it and had it hardened. This is 1 1/4" 4140 TGP steel rod. The original John Deere pins were not hardened.

For the tiny amount of material that was removed for that little grease channel, I don't see that it will reduce the strength significantly. The longitudinal channel is about .05" deep by 1/8" wide. If anything, my concern is that it will get plugged up with gunk because it's so shallow. I think it will be fine, but if it's not, then I'll find out and have to make a new one that's stronger. I just don't want it freezing up with rust like the original pin did. It was pretty stuck and hard to get out because it was starting to rust in the dipper. The improvement with grease delivery and using marine grease should solve that problem.

 

[dfkrug]

I don't understand the thing about broken pins. I can see that they could bend,
but not break. Well, maybe if I took it and had it hardened. This is 1 1/4" 4140 TGP steel rod. The original
John Deere pins were not hardened.

Yeah, JD believes in "soft" pins, while Kubota uses hard pins. My own hoe uses chrome-plated "soft" pins,
which I think are hydraulic cylinder rod stock. There are defenders of both design choices. Pins are
easy to replace, however, and sleeve bearings are not. Some hoes have plain DOM tubing with no sleeve
bearings, and those are horrible to replace. BTDT.

On my hoe, the maker upgraded everything in its second generation to 30mm pins. All EXCEPT the bucket
pivot, which is the easiest to break. That was cuz of all the bkts already made that used 25mm pins. 30mm
or 1.25" like you have will do much better.

How did the pins break? The radial grease channel, tho shallow, did them in. Metal fatigue from small
bending forces. Your channel will do less weakening, but why do you suppose no one does it that way?
Perhaps that channel becomes the only channel for grease, and it does not distribute well around the pin?

 

[texasranger556]

I'm no ME but I've been told that failures in metal start at the surface, which is why processes such as shot blasting improve strength by putting the surface of metal into compression.

Aircraft are another example, surface scratches get blended and sharp corners rounded to keep cracks from forming.

I doubt that is the primary factor in hinge pin design though. I'd bet it is an optimization of material cost, labor, and "good enough" for the expected service life of a home owner grade implement.

 

[DanMc77]

Curious how you matched the width of your groove cutter to the width of the M-C snap rings? I considered snap rings for my pins but ended up with the boss on one side and the collar on the other because it was simpler for me to do.

Hi Pat,
I used a caliper to measure the thickness of the snap ring. Then I went around my shop thinking about the easiest way to come up with a cutter that would make that same thickness for the groove. I got lucky and had a HSS cutoff blade for the lathe that matched it exactly. It's the normal parting tool that I use. I was expecting that I'd have to grind a cutting tool to the right dimension. I really like these snap rings so far. I may think differently if one pops off and I bend something up. But I don't mind the challenge. It's all in the learning.

 

[DanMc77]

Your channel will do less weakening, but why do you suppose no one does it that way?
Perhaps that channel becomes the only channel for grease, and it does not distribute well around the pin?

I'm guessing that it's the cost of milling two shallow slots about 12" long. As for JD, they just drill a hole on the outside and probably use a self-tapping zerk. Zip-zop-done.

I just realized that I have a defect in my greasing strategy. I was depending on that little channel to get grease out to the thumb pivot. But the bosses in the bucket have square holes milled in for the retaining pins that I am not planning to use. If those little channels happen to be over those holes, the grease will go no further. I could bore out the bosses in the bucket and slip a sleeve in there, or just stick zerks in the thumb bosses. But I wanted a single-point to squirt grease in there and get everything. Need to think on this one.

 

[downsizingnow48]

Many thanks. That is exactly what I did except I could not find a cut off tool that matched the M-C rings I was looking at. So decided to stick with what I know rather than (attempt) to grind a cutting tool to match. Anyway all the factory B21 pins use 3/8" through bolts, so the pins I made match up.