Replacing a bolt on a snow blower drive shaft - how do you know the hardness?

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Old 03-22-18, 07:32 AM
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Replacing a bolt on a snow blower drive shaft - how do you know the hardness?

I have a Noma snowblower. Over the years, I had problems with propulsion. It turns out to be #3 in this drawing:

https://c.searspartsdirect.com/lis_p...3970-00009.png

that either breaks or falls out because the nut comes loose.

In a pinch, I scramble for whatever I have in the parts box to get the driveway clean and forget about it. I'm on a kick to do things right and want to get this bolt (and nut).

This page:

https://www.searspartsdirect.com/mod...8/1507200.html

has part number 73839MA. Me being me, I can't see spending $6 or more on a bolt from an online place with shipping. So I want to get it at Home Depot if I can.

All the places that sell this on line call it a 1/4-20x2.25 HH bolt. that size is likely at Home Depot. But I wonder is it a shear type of bolt designed to break or hardened? HH? Can't find what that means related to bolts.

This place I found shows the head with 6 lines = hardness 8?

How do you know the full specs of a part? The manufacturer wants to be vague to have you come back to them understandably.

The nut - from pictures it seems like a standard steel nut that I likely have loads of. I would have expected the plastic insert to lock it onto the bolt? Or you just crank it down on the bolt to keep it on? Or locktite?

thanks!
 
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Old 03-22-18, 07:41 AM
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Yes you need a grade 8 bolt for this application. It would be best it it were long enough so that there were no threads in the gear. you need a lock nut and do not tighten it the bolt must spin freely it is less likely to break this way. HH = hex head
 
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Old 03-22-18, 07:53 AM
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OOPs, forgot to include the link to the picture of the bolt head

https://www.partstree.com/parts/murray/parts/73839ma/

AJ - thanks. Great tips about allowing it to turn. THANKS!

HH - I was looking at pages from bolt distributors that listed all the abreviations and what they mean. HH wasn't on there. Guess it's too obvious : )
 
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Old 03-22-18, 08:02 AM
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HH = Hex Head..............
 
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Old 03-22-18, 10:35 AM
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6 lines means grade 8 bolt, correct.
 
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Old 03-22-18, 11:41 AM
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Another scheme to prevent the nut from coming loose is to use a longer bolt and two nuts tightened against each other. Since the bolt drives the axle. I think it should be softer than either the gear hub or the axle so the bolt wears and not the holes on the gear hub or axle since no compression is provided by the bolt. Cheaper to replace the bolt versus axle or gear.
 
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Old 03-22-18, 03:25 PM
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I think it should be softer than either the gear hub or the axle so the bolt wears and not the holes on the gear hub
That is what happens when using a soft bolt and not a shear pin, the soft even hard bolt will bend before it breaks like a shear pin will, it starts bending but stays in place, then starts wallering out the other pieces.

In the case of steel, hard means strong but brittle, soft means pliable which has more shear strength. Bolt hardness is meant as tensile strength not shear strength. A grade 8 bolt will have more shear strength than the correct shear pin, and can cause damage before it fails.

There is a reason as well shear pins do not have threads where they engage the moving parts.
 
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Old 03-22-18, 08:30 PM
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Blow away it's a grade 8 bolt!
Something far more expensive is going to break before the bolt will.
Any shear bolt I've ever seen is a grade 2 or max a 5.
Good luck finding a 2-1/4" long bolt, one longer will not be an issue.
Want to keep the nut from falling off then use an elastic stop nut, any hardware store will have them.
 
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