Drive shaft broke fcub

[BigBill]

Ok my fcub been sitting the drivevshaft broke at the input tranny bearing. There was two breaks a darker colored older one that’s around the outside of the shaft and the shiny final break on the inside of the shaft. Has anyone had this drive shaft break on them?

I’m thinking one of two fixes. The shaft flexes and bends at the point where it enters the tranny when it’s under load. Probably 30% to 45% of the length flexes to cause this shear.
A outer sleeve could be clamped on the shaft from the tranny bearing seal to 50% out on the shaft. Make the shaft so it can flex.?

The po must have abused this 52 fcub.

[Rick Spivey]

Get a replacement shaft, I've not heard of many breaking like that. A replacement shaft will last another 50 years under normal use, unless there is a special cause...

[Urbish]

If the shaft is flexing, it isn't in proper alignment with the engine. Has the clutch housing been damaged and repaired? Are there dowel pins where the bellhousing mates to the engine and where the clutch housing mates to the transmission? Are all the fasteners tight? You'll need to correct this, if there is indeed a misalignment, prior to replacing the shaft.

[halftonstude]

that shaft is in almost perfect alignment when all the pins and bolt holes line up. my old shaft had a 3/4" wobble in it when i removed it, put in a new one from TM and it slid right in absolutely no misalignment. Flex is an interesting concept on forged pieces. any weak point would flex more than another, but for a long shaft the stiffness ratio kind of keeps that flex covering the whole length of the shaft... a shoulder or splines won't concentrate it. and with 8 hp and a 1" shaft, how much flex we talking?

[brewzalot]

lathe operators- eye opener on bar whip

https://www.youtube.com/watch?v=C8ZJa2HSkYE

[Jeff Silvey]

WOW lucky someone didn't get hurt.

[halftonstude]

i'll have to put on my NDT hat now... I used to be the maintenance manager at Sperry, spent a lot of time looking at broken rails, and even got to break some with the 300 ton press out back. Two things are getting mentioned here, flex and whip. Whip occurs with an unconstrained end. You get the tiniest bit of misalignment and centrifugal force takes care of the rest. Flex would be torque along the length of the shaft. interesting note you mentioned darker and lighter. the darker was from a partial shear that didn't snap the shaft in half that also oxidized and caused the darker color. we used to see that all the time with railroad rail. If the steel was manufactured properly (vacuum degassing, no slag, etc) then you wouldn't see inclusions that would cause a break from the inside going out. However, it doesn't take much to start a crack on the outside. sharp edge on the shaft that was left from the machining process is a good place for that to start. then at some point the weakness caused by the first crack initiated the crack that finally broke the shaft. one thing we used to complain about was improperly drilled bolt holes for the rails. The track gang would use a gasoline engine powered drill with a huge carbide bit, and force it through hard with no lubricant - it was faster to drill it and move on if you didn't have to go slow and oil it. anyway, the bit would push through the other side, create a burr, and the burr invariably had a split in it... with the flexing of the rail at the joints, that little split would work its way back into the steel and bingo... bolt hole crack, and a chunk from the end could fall off. you could see the growth spurts it would get as the older parts of the crack were rusted more than the newer ones. Another defect was a transverse defect in the head, sometimes caused by a lip that had formed on one edge of the rail head, a little split like you get on the head of a splitting wedge from mushrooming over and you could see growth rings like a tree as the crack progressed through the head of the rail until the rail finally gave way and broke.
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