Apparently, some mowers are made with swing tip blades:
(From: Roderick Carmichael (email@example.com).)
"I use a top notch Supa-Swift with a gravity cast alloy chassis and swing-tip blades (Australian invention, no bent cranks on our mowers mate!)"
Why are these not common in America? Conspiracy to sell replacement parts by the small engine manufacturers? :-). You would have to really work at bashing a curb to bend a crankshaft with such a mower.
Another possible approach - applicable for both new lawn mower designs as well as field upgrades - would be to replace the heavy steel blade with one made of nylon with a molded-in steel edge. An auxiliary flywheel might have to be added (under the deck) to provide the needed inertia (normally supplied by the steel blade) for the engine to start and run properly and to help the mower plow through tall grass. However, since this additional flywheel could never be stopped abruptly due to hitting a rock or curb, its inertia would never contribute to a sideways bending force on the crankshaft. The greatly reduced mass and increased flexibility of a reinforced nylon blade should virtually eliminate the possibility of a bent crankshaft from such unfortunate incidents. See the section: Why you really don't want to attempt to move an immovable object.
(From: Mowerman (firstname.lastname@example.org).)
I have been reconditioning lawn mowers for many years and find it fascinating what design goes into some of this equipment. Like arrangements to prevent the crankshaft from bending if the blade hits something. Most engines have an aluminum key at the flywheel but this will not stop shaft bending most of the time. Mower manufacturer uses many methods. Lawn boy has a tapered shaft at blade level and so the shaft has no key and the blade slips if hit. Some other mowers had steel washers and fiber washers nearest the blade like a clutch I seen this on some antique mowers. The best idea is disc with small flail blades or short blade with flail at end. Manufacturers in New Zealand and Australia use this method on their domestic mower mostly and it works well. Victa of Australia was one of the first ones around here with this idea. I am sure you have them over there to no doubt. I hope this information will be to some help.