crankshaft

They have a main motor shaft (I don't think it's called a crankshaft unless it attaches to pistons in an ICE) which the blade attaches to and passes through the center of the motor. If you bent this, it would cause vibration and shakes when turned on.

First thing I would check is to remove the blade and turn on the motor with no blade. If it has no vibration/growling without the blade, then only the blade has the issue, not the motor shaft, and you might just need the balance the blade.

However, I suspect that chances are good the motor shaft is at least slightly bent, in which case it would probably be cheaper to buy a new mower than to have it fixed. (A motor shop could do it, but suspect they would charge more than a new mower would cost. If you happen to have a "spare parts" mower with a working motor, you could swap the motors to fix it....but otherwise, may be time to buy a new mower.)

Without rods and pistons, no crankshaft.  Chances are, there is only a motor shaft - or, if the motor is offset, and belt or chain drive, a driveshaft.  That said, the suggestion above of trying without the blade may be valid; if there isn't a safety mechanism that doesn't allow it.

Another way to tell, is to (with the battery removed), leave the blade ON, get a ruler, and rotate the blade 1/4th the way around taking a measurement at all 4 positions of how far the blade is from the deck (or ground - much harder to do).  If all distances are the same, it's not bent.  The length of the blade will actually accentuate the delta if there's anything bent.  Measure both ends of the blade at each position, and I think it will be evident if anything is out of whack.

If it is gear/chain driven, there is a chance you stripped a gear tooth or something similar to a situation like that.  Or, knocked the motor cockeyed in it's mount.  I would look for that "mechanically-inclined" friend and see if they'll be kind enough to take a look.  These things aren't too complex.  If you're near 33498, I'll take a look.  Best of luck.

Every time I see this thread, this is all I can think of. LOL



Note - this is the title character from the Crankshaft comic strip.

There is no crankshaft per se, but there is a shaft in the electric motor, and it can be bent, so same difference. When young, I was employed by my scoutmaster's family who owned a large industrial electric motor shop. We didn't work on small motors, which don't cost justify repair, but the principles are the same. Anytime a lawn mower blade strikes any object direct enough to bring the blade to an instant stop, a bent shaft is a real possibility. Once this happens, you are likely better buying a new mower - that's what I did, exact same model, keeping the old one for parts to be used in future repairs.

Here's a look at shaft repair, should you want to attempt it:

https://youtu.be/NVVcAt73C3c

Don't know it this is the case for you but I recall atypicalengineer??? said when you have a sudden impact on the blade the stator coils can break loose due to magnetic forces being interrupted. 

So no crankshaft to replace but an electric motor.

Mike