My MK Martin single stage pto blower has 2 shear pins (bolts). One on the drive shaft that came with the blower, that protects the gear box in case something jams the fan and one on the auger input shaft, in case something jams the auger. I know they work because I have broken quite a few since getting this thing about 9 years ago. The first year I did some serious damage to the poor thing when I found some large rocks buried in the snow piles that were created from plowing. I have since learned to go over the driveway before the first snow fall and clear it of any rocks that may cause grief for the blower. Even with this procedure I still manage to find the odd rock that pops up after some freeze / thaw cycles. But I haven't had any serious jams since starting this process. If you get one make sure to use class 2 bolts for shear pins not class 5. The dealer I bought it from didn't have any class 2 bolts so I use carriage bolts, which are class 2.
By the way I am in the middle of converting the old girl to a front mount hydraulic blower. I just can't take another season of looking over my shoulder while a back up for hours at a time.