Several factors here. It's just like at the strip, a little slippage will help protect driveline parts.
The real killer is shock loading. Either improperly taking up slack or pulling steady then losing traction then suddenly gaining again.
We got into an M last year that we knew the pilot bearing was out. Found signs of abuse; the two halves of the differential had tried to move, the bolt bosses were fractured but bolts still there and tight. Also one of the bull pinion cages was fractured 80% of the way around. It was obviously old damage and the tractor was still in use! Dad had bought it from a club that was using it on plow days to pull a 10 foot disk! Their plan was to run it until it cratered. They hadn't had it too long and the damage occured long before they acquired it.
We also noticed that the torque tube was from a different year tractor and concluded that whatever event caused the internal damage probably fractured the torque tube and was obvious to some p.o.
I don't think plowing is "hard" for these size machines and they're actually plenty overbuilt for their power range. I've been into several of these tractors and you always find the same things worn out: Input shaft bearings, pilot bearing and mainshaft bearing. I frequently find evidence that inner axle bearing have failed at one time and were replaced. I haven't personally seen any gears or splines that failed on their own. The damage that I have seen on those parts was secondary after a bearing failed or due to shock loading. I have seen a cracked axle, the crack was right where it came out of the housing.
I think a lot of times someone finds an old tractor and gets it running good and then puts it back to work w/o considering that was parked in a fairly well to well worn condition. Bearing are already loose and worn and probably have some corrosion damage from sitting. Now it's put back work or put in a tractor pull and bearings finish wearing out and then fail.
I mean how many people really tear into a trans and rear and go through it to at least check before putting it back in operation? A lot of people on the forum do but I see and talk to a lot of people at tractor shows that I know don't have the time, resources or inclination to do so. And it's true that even a well worn tractor will do light duty.
Anyway, kind of long winded here. I've seen a couple of guys running v-8's, 383's, 454's through otherwise stock Super M drivetrains and they've held! I always see plenty of tire spinning in those classes though.
I say replace any of the standard ball bearing with a max type. More balls = more capacity, a lot more. Have a clean housing with clean oil and you won't have any failures with any power level you can make from stock parts.
As far as actual load carrying capacity, SKF and Timken both have the actual design load, in Newtons of the bearings. The inner axle bearing is a 214W (a max type)and the stock type outer is a 214D (shield on one side). Replace the outer with a 214WD (max type with shield) to up the capacity. Convert the bearing design load from Newtons to pounds and multiply by 4 and there's some kind of number you can use as a point of departure.
I'm not sure how you would go about figuring your actual pulling load vs. tractor weight and traction. Any weight from wheel weights or loaded tires is not actually on the bearings directly but is pushing directly on the tire. Assuming you can hook up enough to convert all tractor weight into pulling force you're looking at around 7000lbs of load.
The results of max pull results from the Nebraska tractor tests were quite a bit less than that. I haven't looked at those in a while but I remember at least some of them gave the data as to how much extra weight they piled on for the tests.
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