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Farmall & IHC Tractors Discussion Forum
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strength of bolts

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garry in TN

05-05-2008 12:22:25




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I regularly shear the Grade 8 bolt in my bush hog PTO shaft. Is there such a thing as a grade 12 bolt? here do you get them? Thanks




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tesmith66

05-08-2008 03:55:21




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 Re: strength of bolts in reply to garry in TN, 05-05-2008 12:22:25  
We use fasteners that are rated up to 190,000 psi in the aerospace industry every day. There are even stronger ones, but you really don't want to know how much those cost!

Aircraft supply houses like Wicks or Aircraft Spruce will have these. Look for MS, AN or NAS series fasteners



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bc

05-05-2008 20:11:16




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 Re: strength of bolts in reply to garry in TN, 05-05-2008 12:22:25  
I meant to add that if you are like me, your mower sits around for years without checking the gear fluid. Just like any other gear box, you can get condensation in the box with resulting rusting and pitting of gears and bearings plus the watered down oil allows the metal to get hot and break down. Just something else to consider.

I watched a nascar program on Speed tv Sat that dealt with rear ends. Lube pump on one of those goes out and the pinion gear just melts.

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Jiles

05-05-2008 15:06:00




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 Re: strength of bolts in reply to garry in TN, 05-05-2008 12:22:25  
Most allen-head bolts are grade 12. I do a lot of heavy cutting with my 5' and I never use anything tuffer then a standard grade bolt "no marking on head". I usually shear about 2 or 3 per year. They are 1/2"



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Dave41A

05-07-2008 19:17:45




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 Re: strength of bolts in reply to Jiles, 05-05-2008 15:06:00  
For the archives...
There is a "class 12.9" but this is a metric designation that is roughly equivalent to a SAE grade 8. Metric head stampings are also different from SAE, and usually indicate the tensile strength of the bolt in metric units. Both bolts are rated with an ultimate yield strength of about 150,000 psi.

I have not yet come across a bolt rated higher than this, but am willing to learn if there is in fact someone who manufactures such an item.

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bc

05-05-2008 14:57:42




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 Re: strength of bolts in reply to garry in TN, 05-05-2008 12:22:25  
I'm with the others on this. Shear pins are supposed to shear before you break the gears and cause other damage. You might check your bolt/pin holes. If they are wallowed out of round with a lot of play and wiggle room then that will help them shear easier. Sharpen your blades and make sure the mower has plenty of gear lube in it. Dull blades creates more drag and lack of lubrication can do the same. Your mower gears could be binding up from a lack of lubrication or the bearings could be going out causing them to bind up. You can see how easy it turns by hand.

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MN Mike

05-05-2008 13:06:19




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 Re: strength of bolts in reply to garry in TN, 05-05-2008 12:22:25  
I run grade 5 bolts for my shear pin and only shear off 1 or 2 a year. I agree with the others that grade 8 might be too brittle on shear strength.



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Janicholson

05-05-2008 12:36:39




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 Re: strength of bolts in reply to garry in TN, 05-05-2008 12:22:25  
I agree with Damp.
The issue is whether gears, housings, and drive shafts cost more than grade 5 bolts. Sharpening and making sure the front edge of the mower is lower than the trailing edge make all the difference. JimN



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Damp;Dservice

05-05-2008 12:27:39




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 Re: strength of bolts in reply to garry in TN, 05-05-2008 12:22:25  

garry in TN said: (quoted from post at 12:22:25 05/05/08) I regularly shear the Grade 8 bolt in my bush hog PTO shaft. Is there such a thing as a grade 12 bolt? here do you get them? Thanks


grade 8 bolts are too hard, they will shear faster than a soft bolt due Grade 8 bolts being strong in clamping force or tourque but they are brittle in shear, especially if they are not tight.

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Dave41A

05-05-2008 13:17:00




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 Re: strength of bolts in reply to Damp;Dservice, 05-05-2008 12:27:39  
Link

No..."8.2" is as high as they go. That said, it's probably not advisable to use anything that strong (and brittle) unless the original design called for it.



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