Tractor backhoe - homemade

[rScotty]

The standard in real construction machinery (10 ton and above, so excluding compact machinery and jobsite wheelbarrows like TLB's) is hardened pin and hardened bushing with grease grooves.

Yes, and so we have a situation where 10 ton and up are made with one type of structural philosophy but 10 ton and smaller machines are made differently. We know that now. The question then becomes, "Why?"
I don't know the answer for sure, but can hazard a theoretical guess based on a lifetime of engineering. My guess is that they differ because this is not a linear design problem, and that ten tons is approximently where the required yield strength vs load/size slope begins to rise very rapidly.

After doing some thinking about the the loads on large machinery - but only a few back of envelope type calculations - My GUESS is that in ten ton and up lifting machines, the mass of the booms plus loads mean that their pivot construction is going to be constrained to any reasonable boom end joint size & pin diameter by the ultimate material strength of the pin. And if so, the designers may have no choice but to use high carbon alloy steels for that pin in spite of their requiring special fabrication techniques to retain the high strength without the danger of cracking. With that much loading, one would normally use hardened replaceable bushings as well.

In worksite "wheelbarrows" such as the 10 ton and down TLBs - JD310 & 410 series being representative - we now know that the pins tend to be mild steel. My "GUESS" as to why is that we haven't yet started up the steep slope of the non-linear boom size versus pin diameter problem. I'm hand-waving now; and haven't actually sat down and done these calculations...although it wouldn't surprise me to find that both Renze and BFreaky have done so.
Anyway...My "gut" calculations are saying that in these smaller machines it is possible to use boom ends the same size or even smaller than the boom body and that those ends are large enough to hold a mild steel pin with a diameter and strength that is more than sufficient for the job. That would mean that the strength of the pin is no longer the immedidiate constraint condition for the design. Replaceable bushing design follows by the same reasoning.

Whether the pin is hard or soft makes little difference to the outer support bushing construction. It's always easier to work with mild steel, but in either case, welding the outer support flanged bushings of high strength abrasion-resistant hard steel into place can proceed without special techniques or the necessity of line boring.
Enjoy,
rScotty

 

[dfkrug]

My "GUESS" as to why is that we haven't yet started up the steep slope of the non-linear boom size versus pin diameter problem.

Well, theoretically, you can always upsize your pins and bearings to accommodate larger stresses in larger equipment. Then the weakest link moves to perhaps the boom or dipper thickness or cross-section. I am sure that engineers inside Caterpiller, Kubota, Deere, etc. have done calculations and lab experiments. Stronger steels surely allow for smaller sizes of structural elements, but designs can certainly be based on mild steels. Part of the analysis and choice/sizing of materials is economic.

It is also interesting to see what happens in high-hour industrial machinery. A guy I hired in the 90s has a Case 580 with over 30K hours on it, and it was interesting to see where metal fatigue failures started to show up. His was a very well-maintained TLB with few pin or plain bearing failures.

I wonder: do large excavators with giant pivot pins generally have replaceable pivot bearings?

 

[Kyle_in_Tex]

Well, theoretically, you can always upsize your pins and bearings to accommodate larger stresses in larger equipment. Then the weakest link moves to perhaps the boom or dipper thickness or cross-section. I am sure that engineers inside Caterpiller, Kubota, Deere, etc. have done calculations and lab experiments. Stronger steels surely allow for smaller sizes of structural elements, but designs can certainly be based on mild steels. Part of the analysis and choice/sizing of materials is economic.

It is also interesting to see what happens in high-hour industrial machinery. A guy I hired in the 90s has a Case 580 with over 30K hours on it, and it was interesting to see where metal fatigue failures started to show up. His was a very well-maintained TLB with few pin or plain bearing failures.

I wonder: do large excavators with giant pivot pins generally have replaceable pivot bearings?

LOL, the pin I made for my post hole digger to hinge on, was 17-4 PH stainless, (type 630), hardened to H900. Which gives it a nice hardness and yield. So, expense is a driving factor.

 

[BFreaky]

Guys it all comes down to how much money you want to spend. For me it's logical to give extra money for the pins and put a replaceable bushing because longterm it gets cheaper and faster to repair. Others may think otherwise and no problem. We all have our ways of thinking. Both solutions are ok

 

[GLyford]

Just remember that grease is cheaper than pins AND bushings!

 

[Renze]

Yes, and so we have a situation where 10 ton and up are made with one type of structural philosophy but 10 ton and smaller machines are made differently. We know that now. The question then becomes, "Why?"
I don't know the answer for sure, but can hazard a theoretical guess based on a lifetime of engineering. My guess is that they differ because this is not a linear design problem, and that ten tons is approximently where the required yield strength vs load/size slope begins to rise very rapidly.

It all comes down to the cost per tonne of material moved. Under 10 ton are compact machinery, mostly used to be available when needed, which isnt 24/7, not even 8/5
Tracked excavators up to 20 ton usually have a design life up to 15.000hrs, up to 90 ton generally 20.000hrs, and above that, mining machines are designed to run 30 to 35.000hrs between rebuilds. That translates into Cat C32 engines putting out just 1000hp in an excavator: In a 90 ton excavator you will find a 750hp 18 liter.

Generally speaking, there are three design grades:

Compact grade up to 10 ton: Designed for 8-10.000hrs operating life. Usually used for odd jobs around the worksite, their performance is evaluated in an abstract way, they have to be a jack of all trade and usually carry materials around the jobsite more than they do excavation in ton/hr
Construction grade up to 90 ton: designed for 15-18.000 hours operating life. These machines performance is measured in ton/hr but they also finish grade a jobsite.
Mining grade 120 to 900 ton: Designed for 30 to 35.000hrs between rebuilds. Front shovels are usually retired after 80,000hrs. A whole fleet of haulers depends on them, so uptime is key, no time to replace pins and bushings every 3000hrs, which in 3 shift mining means at least twice a year. They dont do jobsite cleanup, they are too expensive for that work because a whole mining operation depends on them. They only do full buckets, year after year.

There might be different design lifes used for 800kg mini diggers than for an 8 ton midi digger, i dont know because ive never worked on anything smaller than 10 ton. I can just tell, in the 11 to 18 ton wheel loaders weve allways put 42CrMo4 hardened and then ground pins, with nitrated hardened bushings with cross cut grease grooves, and when i started at my current job they had the same supplier of bearing bushings, we do modifications and special booms (high reach demolition, long reach, material handling) on excavators of 20 to 200 ton.


Whenever the operating hours per year increase, it pays to use higher quality pins and bushings. Generally TLB's, compact loaders under 10 ton, and small excavators dont run 1500 operating hours per year, and when they do, they usually carry stuff around the jobsite, not pushed hard because there is a line of haul trucks waiting.

Whenever the hourly cost of the machine goes up, so does the price of downtime, and the cost of longlife parts drops in relation to the total build price of the machine.

 

[4570Man]

Guys it all comes down to how much money you want to spend. For me it's logical to give extra money for the pins and put a replaceable bushing because longterm it gets cheaper and faster to repair. Others may think otherwise and no problem. We all have our ways of thinking. Both solutions are ok

How much do you plan on using this machine and how big is it? The soft pin hard bushing setup on my 2400 hour JD 310 is in perfect condition. I see backhoes pretty often with twice that many hours with minimal wear. I’m guessing a full size TLB is also stronger than what your building. And I’m not hating on your build. I think it’s great and it’s definitely the best homemade backhoe I’ve ever seen.

 

[Renze]

. I’m guessing a full size TLB is also stronger than what your building.

The boom profile hes building, is comparable to a Caterpillar M314 mobile excavator, which is considerably stronger than a TLB. Unless its a Huddig:

HUDDIG schafft Lagerflache fur Stammholz - YouTube

SMALL CHANGES MAKE A BIG DIFFERENCE | Huddig

These beasts weigh 12.5 ton without buckets, which means a 15 ton operating weight, and are built to take a beating in harsh scandinavian frost. It has the same ZF axles as a 13 ton wheel loader like a Case 621 and arent comparable to any of the regular backhoes, both in durability as well as in price.

 

[Kyle_in_Tex]

But would not welding in the bushings warp them? It seems to me that to use the precision shell type bushings, that everything should be line bored after welding to control circularity/roundness.

What is the standard process of mfg using the replaceable bushings? I think they are a great idea, I just don't know what type of tolerances are required to install and use them.

Mostly, I'm thankful you guys speak such good English.

 

[Kyle_in_Tex]

But would not welding in the bushings warp them? It seems to me that to use the precision shell type bushings, that everything should be line bored after welding to control circularity/roundness.

What is the standard process of mfg using the replaceable bushings? I think they are a great idea, I just don't know what type of tolerances are required to install and use them.

Mostly, I'm thankful you guys speak such good English.

Maybe you could weld in the bushings and then get a good Sunnen hone, driven with a heavy duty hand held drill. They will straighten up a hole.

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