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Old 04-19-2010, 06:50 PM   #1 (permalink)
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Metric Bolt Shear strength

Which grade of metric bolts has the highest shear strength without
being too brittle. Am I missing something? (
(8.8, 10.9, 12.9)

My alternator bolt recently sheared off at a track event.
I think it was stainless steel, which is very weak compared to
the aforementioned grades.

-Robert
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Old 04-20-2010, 03:44 AM   #2 (permalink)
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It is a common misconception to replace hardware with that of higher strength. It's doubtful that your alt bolt sheared because of inferior material. It's much more likely it sheared because of being over-torqued for the material used. I'd think there are virtually zero alt bolt failures on the street & the track doesn't really harshen the requirements. Why not call your friendly Lotus dealer, buy the bolt they sell and use a torque wrench on it. BTW None of the above are "brittle" as you put it. They do, however need their own specific torque to make them work properly. A high strength fastener under-torqued can be just as bad a a low strength one that's over-torqued. Bolt stretch is what it's all about as a bolt is a "spring".
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Old 04-20-2010, 06:26 AM   #3 (permalink)
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Why do you say it was SS? Factory bolts are not SS...
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Old 04-20-2010, 08:07 AM   #4 (permalink)
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if there is anything that will have sheer put on it(suspension, seat mounts seat belt mounts etc), then use AN hardware.
for the alternator there is not sheer being put on it, so you dont need the AN hardware, but if you want to be sure, then use it anyway.

there are measurements for length and grip on AN hardware.
pegasusautoracing sells a nice little measuring tool so you can see the exact bolt diameter and length you will need.
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Old 04-20-2010, 08:51 AM   #5 (permalink)
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12.9 should not be used outside the engine due to susceptibility to corrosion...

I would get a 10.9 from McMaster..
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Old 04-20-2010, 09:06 AM   #6 (permalink)
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AN Hardware is english sizes and threads, so it isn't going to do much good for the Lotus application since it is all metric.

I agree that it is unlikely that the bolt failed due to having the wrong material, provided it was the original bolt. If it was replaced at some point and also overtorqued, then who knows. If it was indeed stainless steel, then as turbophil stated, that was not an original bolt

For the record and to answer the question, the nominal mechanical properties for metric bolt classes are:

8.8: 800 MPa tensile
10.9: 1000 MPa tensile
12.9: 1200 MPa tensile
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Old 04-20-2010, 10:06 AM   #7 (permalink)
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One other word of warning about using "stronger" bolts than necessary.

Years ago, I had a engine with an aluminum block. The factory had missed installing one of the four mounting bolts for the air conditioner. That resulted in vibrations causing the other bolts to break. Fixed once under warranty, but they didn't replace the "fourth" bolt because it required removing the engine mount and lifting the engine. So they broke again. Still under warranty, they repaired it a gain, using "stronger" bolt.

The result?

The vibrations continued, only this time the mounting holes in the side of the engine block broke out (open hole into the engine). The dealer had to replace the entire engine to fix the air conditioner mounting bolt (instead of fixing it properly in the first place). The new engine/AC lasted 150,000 miles without any more problems since it had all four bolts.
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Old 04-20-2010, 10:35 AM   #8 (permalink)
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Bolts do not have to be prestressed to resist load in tension or bearing (which is really shear).

The reason to pretension a bolt is to get a clamping force that holds the pieces together through friction rather than bearing on the actual bolt. In structural engineering these connections are called "slip critical", the old term was "friction bolt". The capacity of slip critical bolts in shear is always less than the bearing capacity in shear because if you keep loading the bolt the frictional resistance from bolt pretension gives way and slips and the pieces bear on the bolt shaft. Bearing failure of either the bolt material or the connected material is always the failure mode of bolts loaded in shear.

This is all for loading in shear. For loading in tension, prestressing does not make sense because if you load the bolt to failure, it fails at its tensile strength regardless of how much prestress you put on it. Prestress is only a fraction of that amount or you would fail the bolt when you prestressed it.

Maybe things are different for mechanical type stuff, but this is how bolts work in structural engineering for buildings and bridges and stuff.
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Old 04-20-2010, 10:42 AM   #9 (permalink)
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...does tensile / yield strength even matter in an instance of shear? Surely the bolt wasn't stretched to failure.

EDIT: I suppose to be more specific, wouldn't its % shear in a crack resistance test (Charpy V-notch) be more applicable?
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Old 04-20-2010, 11:18 AM   #10 (permalink)
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bolt capacity is calculated from tensile strength, for both tension and shear, since so called "shear yield" is a function of tensile yield. Tihs is true even though many bolt materials dont have well defined yield points.

a typical value would be Tult for design would be 0.75*Fu, for Vult it would be 0.4Fu for threads included in the shear plane and 0.5Fu if threads are excluded from the shear plane.

If you are using Strength design, you would then apply a Phi factor of 0.75 on that to get a design capacity.
If you use allowable stress design, you would use a Factor of Safety and divide by 2.00 to get an allowable capacity.
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Old 04-20-2010, 11:26 AM   #11 (permalink)
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Quote:
Originally Posted by vittoreo View Post
...does tensile / yield strength even matter in an instance of shear? Surely the bolt wasn't stretched to failure.

EDIT: I suppose to be more specific, wouldn't its % shear in a crack resistance test (Charpy V-notch) be more applicable?
A good rule of thumb is that the ultimate shear strength of a material is 1/2 of its ultimate tensile strength. So when a bolt is put in "double shear" the effective area of the bolt material is twice what it is in simple tension. Since this is the area "A" in the P/A stress calculation, a bolt loaded in double shear will have the same ultimate strength as one loaded in simple tension. (Note also that any quoted bolt strength measure relates to ultimate (i.e., failure) strength, and, of course, a higher ultimate strength usually means lower ductility and toughness.)
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Old 04-20-2010, 12:05 PM   #12 (permalink)
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The discussion has turned to the definition of shear. I know that's the word the OP used but I didn't interpret it as an actual shear failure. If you look at a typical alternator mount it is highly unlikely for the bolt to fail in shear. I think the failure was tensile and misinterpreted as shear (or the word "shear" just casually coined).
BTW Yes PE. Same for mechanical eng'g. Fizix is fizix everyware.
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Old 04-20-2010, 12:07 PM   #13 (permalink)
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.... I think we're still just talking about a mounting bolt for the alternator, correct?...lol...

If it truly was a SS bolt, then it shouldn't have been. Would just about rather use fishing line than SS bolts on something like this. There's no reason you should have replaced that bolt unless a new bolt came with your SC kit. In which case, if it was SS, then shame on the provider. SS fasteners belong on boats and in toilet bowls... that's about it...

Most of the engine bolts are 8.8 or 10.9 bolts and should be replaced with such. Most of them have a nice flange and are of 10, 12, 14, 17 mm size heads and should be replaced as such when needed. If not, it requires too many tools to work on the car! There's probably a handful of OEM bolt suppliers (Fastenal is NOT one) in about every city. Just ask your local auto shop where they get their bolts. Call that company and get the fastener you need. If you have no luck doing this, call me up and I'll see if I have one on the shelf. Quite certain that I do

Cheers,

Phil
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Old 04-30-2010, 10:46 PM   #14 (permalink)
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Quote:
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Why do you say it was SS? Factory bolts are not SS...
Because every bolt on the vf2 kit is stainless steel. (except a monstrously huge one)
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