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Simpleton7016

Keyed shafts -vs- Shear pins

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Simpleton7016
I spent the day yesterday tearing apart a Terra Tiger. It was a major pain in the dupa to get the axles out. The sprockets were frozen solid to the axles, but I only found that out after spending WAY to much time trying to get the shear pins out (also rusted solid with no easy access). I ended up having to cut all six axles off....too bad too b/c they were all pretty straight minus the pointy sprockets. Upon reassembly, I am going to gob my parts with antiseize like you have never seen before. I am going to have my machinst make all new axles for me, but before I get to that phase, I wanted some opinions on if I should go with keyed shafts or shear pins? The early model Terra Tigers had only a 10 horse engine and were equipped with shear pins holding the sprockets to the axle shaft. I understand that they are designed that way to be the sacrificial part in the case of a catastrophic failue. But Allis Chalmers themselves upgraded from shear pins to keyed axels on their later model 18 horse model Terra Tigers. This seems counter-intuitive to me. Wouldn't you want shear pins to go along with the increased horsepower? Anyway this tells me that maybe they were having problems with too many shear pins snapping? I don't think this question is unique to Terra Tigers but to anything with an axel. :D Right now I am leaning heavily toward keyed shafts just because the shear pins were such a major endeavor to remove....but I would like the engineering experts (i.e. Rod) to opine. I trust your expertise over my own. Thanks in advance.

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steve-wis
My Allis 810GT has pins on the rear wheels for driving them. The newer, higher horsepower ones have keys. I would say keys are the way to go. I don't think there will be any advantage as far as getting them off when rusted tight tho. Either way will be a bear. As a machinist, I have seen shafts in various applications twisted off, and the keys and keyways were still driving. Some are designed with smaller keys so they will shear before the shaft, some aren't. You will need to decide how you want it to be and then size the keys accordingly. Standard size keys for whatever size shaft should shear before the shaft breaks, assuming a mild steel key. Steve

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HubbardRA
Erik, Without knowing more than you have told me, I can't say what would happen with a key instead of a pin. I helped a friend out who kept shearing the 1/4 inch bolts that keyed the driveshaft on his Pro Stock Cub Kadet pulling tractor. I designed him a driveshaft without keys or shearpins. After that he broke two axles and ruined two sets of differential gears. In most cases, if there is a shearpin, then it is designed to protect something else. When they went to keys, they probably beefed up the other part that the shearpins were protecting. Just my opinion, sorry I can't give you a direct answer.

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Simpleton7016
Very good advice guys. There are some moderate differences in the thickness of the hubs that hold the sprockets on. I can't remember what they were though. I will have to re-examine in the daylight. Makes total sense....thanks Rod.

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