pto post hole digger - auger connection question

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Hi all,I'm building my own pto post hole digger and so far, it's going a lot better then expected. I'm building this with some scrap steel that's laying around and an old car differential.Today I drilled a test hole.The auger I used for the test drill is some an auger that I had bought (25cm). In a later stadium I will build a bigger one myself.All went well during the drill except at the end at full depth. The auger's shaft, a coupling I had made out of a 20mm solid rod of S235JR steel, just snapped of !As you can see the rod is just demolished. The bolt of 8mm is still in one peace. You can also see that the hole that was drilled in the rod is shifted so the metal is not resistant to the forces applied to the auger. It kinda sheared off.The gound here is just sand. Due to lots of rainfall the last days the soil at 1m of depth was very sticky.So now I have to think of a better, stronger way to make the link to the auger. The rod was an "inside" connection. I think I have 3 options of what to do:1. I have some pipe laying here that will fit over the outside diameter but I'm not sure if the walls wille take the forces since they are only about 2.9mm thick. I could reinforce the walls with some flat where the hole needs to be.2. I could weld a plate to the end of the auger and put a piece of pipe with stiffeners on the sides3. Or should I just order some shaft that is stronger then S235JR (mild steel) ?Has anyone experience with this kind of things ?Thanks.Lincoln Electric Invertec 170SLincoln Electric Powertec 350C PRO
Reply:If it were me, I'd go with the outside pipe like you were thinking.i like random stuff like this Last edited by MinnesotaDave; 08-09-2014 at 07:14 PM.Dave J.Beware of false knowledge; it is more dangerous than ignorance. ~George Bernard Shaw~ Syncro 350Invertec v250-sThermal Arc 161 and 300MM210DialarcTried being normal once, didn't take....I think it was a Tuesday.
Reply:Google "Kelly bar" that's what you're trying to make. Most are pretty stout and  square or octagonal so the forces are exerted on the flats like a socket on a bolt spreading the forces out.
Reply:Most of the post hole augers I've seen have a 7.5-10cm center tube. I think yours needs to be bigger or you will keep twisting it off.Yeah, I know, but it'll be ok!Lincoln Square wave 255Miller Vintage mig30a spoolgunThermal Dynamics Pacmaster 100xl plasmaSmith mc torchEllis 1600 band saw
Reply:The augers I have experience with use a 2 inch drive bar with two perpendicular bolts two inches apart. The grearbox drives are hardened steel.Ranger 250 GXTSmith Gas Axe
Reply:Thanks for your opinions guys.I looked up a "kelly bar", it is indeed a thingy that looks very stout and will fit to stand up for the job. Except that they are very hard to find around here, second hand I mean. And buying a new one would hack my idea of making a solid but cheap post hole digger myself (around 1500$ for a new one vs about 100-150$ spend until now, fictive scrap costs included).A piece of hardened steel is also an option, my best score that would come close to that around here would a piece of "axle steel", also know as C45 graded steel.So I think I will first try the bigger pipe that fits over the outer auger shaft, maybe "upgrade" the pipe locally to reinforce the coupling. The pipe is what I have laying around.I would like to keep only 1 bolt for the coupling, IF something goes bad I would like the bolt to break, not the pto. You never know what is burried beneath the surface ...I'll keep you guys posted on the final solution.Lincoln Electric Invertec 170SLincoln Electric Powertec 350C PRO
Reply:Google "Jiffy Products" in Dallas Texas.  He has all of the components you need.  Your Kelly bar can be two inch sextagon cold rolled and readily available.  Your pipe can be used if the elastic in your underwear on this project is cheap.  All you need from Jiffy is the connectors.  But if you want a cheap project that works like a good product would then you will also need to talk to them about the cutter teeth, heads, etc.The advantage of the square or sextagon material is the load is carried by the shape and not bolts.  It's all about durability.I have a vise stand for the truck made from a piece of two inch bar with a Kelly connector.  That allows the vise to be positioned anyway I need.  BTW, if you end up with anything over five hundred dollars in time materials in your PTO unit you've went into ego land.Last edited by wroughtn_harv; 08-10-2014 at 06:39 AM.life is good
Reply:Agree with the bigger tubing.  You are right about using the bolt as a shear pin, that's exactly how my commercial PHD is built.  Around here, I buy the bolts by the pound and carry them with the PHD.   On some fence lines, I have averaged one shear pin per hole   What rpm are you running the auger at?   Tractor mounted PHDs turn fairly slowly and I always run the tractor at idle.
Reply:Originally Posted by wroughtn_harvGoogle "Jiffy Products" in Dallas Texas.  He has all of the components you need.
Reply:Originally Posted by k45On some fence lines, I have averaged one shear pin per hole.
Reply:it looks like you have a bolt flange on the drive head build an adapter plate and put some 2" pipe between the head and the auger with a couple of bolts or just weld it I don't no how thick the pipe you can get cheep is though but some sched 80 should handle it fineDo not argue with an idiot. He will drag you down to his level and beat you with experience.
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Reply:I am just curious, what are the chances of running into buried unexploded munitions in your area?That is something I hear about time to time.
Reply:Originally Posted by partsmanI am just curious, what are the chances of running into buried unexploded munitions in your area?That is something I hear about time to time.
Reply:So this is what I did. And it seems to work for this small auger.I took a small piece of pipe to extend the auger's length. at 1 and I welded a plate so it can be connected to the differential. On the auger's end I  welded a piece of smaller tube in the pipe, so that it fits the auger outer diameter pretty good.Welded and grinded.Lincoln Electric Invertec 170SLincoln Electric Powertec 350C PRO
Reply:Then where the shear bolt is placed I welded an extra thin piece of plate (6mm) on the outside to divide the forces.With this extension I drilled about 1m30 - 1m40 deep.The connection holded fine, broke 1 shear bolt. It was in 3 pieces, no damage to the connection pipe.The complete differential side.Now I have to take it all apart, clean, paint and she's ready to go for 80+ post to dig.I have some time now to order some stuff to make a decent connection for the bigger auger.Lincoln Electric Invertec 170SLincoln Electric Powertec 350C PRO