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As designed, the impeller drive on the Model 59 blower has a key and a 5/16" bolt to secure it to the drive shaft stub from the gearbox which then transmits power to the auger drive.

Unfortunately with this arrangement, the bolt will break, leaving only the key to secure the hub to the shaft. If debris does make its way through the augers, and binds the impeller in any way, the hub will crack in the key way. Sometimes you can salvage the hub by removing the key and using the bolt. In the Model 47 blower on my 420, though the hub was so badly damaged, that it had to be replaced, at a cost of $275.

Because of these problems in the past, I have removed the key relying on the bolt to secure the hub. Unfortunately, if you load the nose of the blower into a snowbank, the torsional load on the impeller will break the bolt. I have been using a Grade 5 bolt, but am wondering whether a Grade 8 bolt would hold up better.
 
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Grade 8 without a key is probably still weaker then grade 5 with a key.

I have seen split hubs, but I've sheared the key and bolt about 5 times and never damaged the keyway or hub.

I wonder if cracking the hub is caused by over tightening the bolt, causing a crack that blows out on the next failure. It's a shear bolt it shouldn't be tight against the surfaces.
 

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In my never humble opinion, I do not believe that this connection is designed to be a "shear" connection. I can understand how something could bind the impeller. There are several anecdotes on the Internet of these breaking and owners complaining about the high cost of replacement. However, the owner's manual for my #59 SB only references the two bolts that drive the augers as "shear" bolts. If one is worried about breaking the connection between the impeller and the two gear boxes, what other components are we trying to protect? It seems to me that if one is putting that much force on the impeller, there are other components, like the mid-PTO, that may be at risk. My walk behind snowblower has a similar setup - only the augers are connected via shear bolts. I think the intention is to protect the gearbox between the impeller and the augers. JD may be making the assumption the other components do not require that level of protection.

I bought my #59 SB from a dealer who was trying to get it off the lot because it was in such bad shape nobody wanted it. One auger had been trashed so badly part of the flight was cut off. The coupling you're referring to was completely worn out and I managed to rebuild it with parts from McMaster-Carr. Something had gone through the impeller and bent one of the "blades." However, the bolt and key were intact and the keyway did not appear to be damaged. There was more damage from rust so I cleaned up the shaft with some emory paper and used some shim stock to ensure a tight fit. No issues so far. I would offer that removing the key may allow to much rotational "play" and wear the holes in the components prematurely.
 

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All I know is it shears, and does so somewhat regularly. The huge chunks of solid ice that the plow redistributes into my driveway can do it. I'm pretty sure it's built that way to protect the impeller. The front auger can deliver ice pieces too big to fit up the chute.
 
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