Quote:
Originally Posted by jbollis
I don't know why everyone tries to scare people into thinking the aluminum rear ends are weak and cannot handle any kind of work load. I have never heard of anyone pulling the bolts out of the rear. I am not saying it has not been done, but I would bet if it has they were doing something they should not have been doing or the bolts were not tight.
I did some looking and figuring and this is what I found.
Fastener = 3/8-16 x.750. Using about .500
A 3/8-16 thread internal thread has around a .03392 shear area at .500 of use. I don't know the specs for the housing, so I used the lowest aluminum tensile strength of 26000 psi for this figure.
.03392 x 26000 = 881.92 psi per bolt, x 5 bolts = 4409.60 psi on only the bolts themselves would be needed to pull (strip) the threads out of the aluminum housing.
I understand that these numbers are not exact as I don't know the exact specs on the housing. But you can see that someone would have a very difficult time pulling the 5 bolts out under normal use. A person would have to be pulling more than 2 tons, that is more than you can legally haul in the bed of most pickups.
I can however see problems occurring if the bolts are not tight. But that is a another story.
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I have a few issues with your calculations. For starters, the threads will fail in shear, not tension, so you cannot use the tensile strength for this calculation; you must use the shear strength. Without knowing the alloy I'd want to assume a fairly conservative value of 11,000 psi. Multiplying that by the area you came up with (which I'm not going to try to verify at the moment) results in a force of about 430 lb (not psi) per bolt or using all 5, about 2150 lb. Now, that 11,000 psi shear strength is ultimate shear strength, as in the shear stress at which the material breaks. The yield shear stress (where permanent deformation occurs) is probably 50-60% of the ultimate shear strength, so that'll bring our load down to 1075 lb, as we don't want yielding to occur, either. Throw a factor of safety on there (I'd say a minimum of 2 for this, I imagine farm equipment is designed to at least that, but I don't know for sure) and we're down to just over 500 lbs. Something neither of us has taken into account is the preload on the threads from the bolt torque, so there is already a fairly substantial shear stress in the threads just from the bolt torque.
Something else you are not considering is fatigue. If this was a constant, steady load being applied, this could be okay, but the load varies with time, which will eventually cause failure of the part, even if the stresses are less than the yield strength of the material. This is why the aluminum connecting rods eventually break in our engines.
This is why I believe the threads can be damaged in the aluminum rears, why CCC never offered the IH 3-point on the aluminum rear tractors, and why xtrememotorworks has the improved lower 3-point brackets. It is unlikely the tiller will pull the bolts out because the tiller is pushing the tractor, whereas other rear implements may cause problems because they are all being pulled and are pulling on the bolts in the rear.
I'm using my #1 tiller on my aluminum-rear 782D with a Cat-0 hitch and the adapter package that I reverse engineered. I recently finished the whole thing but haven't had a chance to test it. Here are some pictures.
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