How to calculate the Max torque on a Mower shaft when the blade hits a rock. 2

[jil1969]

Can somebody guide me on how to calculate the max torque the shaft of the shaft in the image below?
It's an brushless outside rotor motor, the max torque the motor gives is 25 N.m
I would like to calculate the max torque in the shaft if the blade a 5,000 RPM hits a rock and stops.

Thanks in advance.

 

[XR250]

There is the torque and also the bending moment to consider. The force imparted on the blade by hitting the rock will cause an equal and opposite force at the shaft. This will be acting some distance below the bearing of the motor causing a bending moment in the shaft. Having bent many lawn mower engine shafts, it is a thing.

 

[mfgenggear]

rebuilding many small engines , hundreds of lawn mowers, typically one does not want to hit a big rock, bad things happens. but most of the time, on gas motors the fly wheel key will shear and thats the just of it. it needs to be pulled and repaired. these old lawn mowers were built very tough and could take years of abuse. on and electric motor the blade hitting a rock would just stall the motor.
so calculate the torque produce by the motor at the shaft, then the inertia of the blade. and the torque it will produce. but to me is all mute due to the motor stalling.

 

[mfgenggear]

I think the possible solution for this case is a hub that slides over the shaft od the motor with an aluminum key. thus calculate the force to shear the key on impact.
much easier to remove and replace. held in pace with a single fine pitch high strength bolt. shaft gets bent it's scrap, or if the mounting is damaged. blades can easily be replaced.

 

[hydtools]

It's the rotor rotating inertia that drives the torque through the shaft to the stopped blade. (WR^2 * n)/(308*t) W rotor weight in pounds R rotor radius in feet n change in rpm t change in seconds

Ted