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Bolt breakage

Spanky

Moderator
Staff member
I have a problem shearing one of the bolts in my low-mount alternator bracket. It's happened twice. First time the custom chrome socket head bolt that came with the kit broke in the block; EZ Out got it out and replaced with a regular grade 8 bolt. Now it has failed again, but fortunately left enough thread that I could grab it with vice grips & extract it. There is a bushing in the bracket that should help with the shear loads, but I have to keep the belt quite tight since it squeals if I let it get slightly loose. Any suggestions? Would stainless offer more shear strength?
 
Is there any place you could add a brace to support the head end of the bolt? As per google....
Stainless steel bolts are rated for corrosion resistance. Bolt strength is rated in PSI (pounds per square inch). A stainless steel bolt has the same PSI rating as a grade 5 bolt (125,000 PSI). A grade 8 bolt has a stronger rating with a PSI of 150,000.
 
Most good quality socket head cap screws are grade 8. They usually come with a black oxide coating. Not too much for corrosion prevention but it should be much stronger than a grade 5 bolt.
 
The ARP Stainless is about equal to 300M material, hence why ARP calls it stainless 300, which is a little stronger than grade 8 and has corrosion resistance if that's required.

Finding a way to add a brace, as mentioned, to reduce the vibration fatigue is the real answer though.

At least you've been lucky enough to be able to get the stubs out pretty painlessly.
 
Spanky, I know you posted pics of your alt. mount elsewhere, but a refresher pic would be great!
 
Anytime you chrome plate a bolt you are causing hydrogen embrittlement.
Hydrogen embrittlement is a metal's loss of ductility and reduction of load bearing capability due to the absorption of hydrogen atoms or molecules by the metal. The result of hydrogen embrittlement is that components crack and fracture at stresses less than the yield strength of the metal

You never see military or aviation fastener that are chrome plated. Most all aircraft fasteners use to be yellow zinc plated then baked to help release the hydrogen build up. Now days most aircraft fasteners are color etched in a acid rinse. Most chrome shops don't oven bake parts. Chrome plated weldments are just as bad and should be baked if they are used in a stressed load application. There is a ton of information on this subject available on the internet. Breaking a alternator is one thing. Breaking a steering or suspension part is another thing all together. Just saying...
 
Are the outer plate and spacer both steel?

Could you weld a gusset in there?
 
How much play/space is there between the bolt and the spacer (diameter wise)? Can you get a shoulder bolt in there? or weld the bracket together so it doesn't move and twist.
 
G'Day Spanky,
Which pulley is the belt squealing on? A squealing pully is telling you there's not enough wrap on one of the pulleys. I've had to introduce an idler into a belt run B4 to increase the wrap. Upside is the belt stops squealing and you can drive the Alt and water pump using less tension on the belt. But yes, as stated above that bracket system needs a serious look at as there is a heap of stress on that bolt.
Regards,
 
Does the whole assembly put shear on this one bolt?
alt.jpg
I'm not sure a gusset would help in that case.
 
Can you drill and tap it to a larger size bolt? a 1/16" increase in diameter will give you a roughly 40 to 50% increase in strength depending on its present size.
 
There is a bushing in the bracket that should help with the shear loads, but I have to keep the belt quite tight since it squeals if I let it get slightly loose.
I don't think it's the shear load that's your problem, it's a bending and fatigue that makes the
bolt break. It will be very important that the bolt be torqued to the proper specs and not come
loose. I had the same problem with the pivot bolt breaking. I'm using a grade 5 bolt that is
might be less brittle than a grade 8 (Cannot find any data). This approach seems to be
controversial when I researched it. I have 5,700 miles on this setup without a problem. It seems
most likely that the grade 8 is really the better choice. Also eliminate the squealing belt.

Killing the myth of the Grade 5 bolt.

Summary (See attachment below for full article)
Fasteners play a critical role in many engineering struc-
tures, and their failure can result in significant conse-
quences. The most common cause of fastener failure is
fracture by fatigue, which is often created by inadequate
tension and clamping force upon installation. Contrary to
what may be intuitive, susceptibility to fatigue can be re-
duced by increasing the amount of tensile preload in
screws or bolts.


I run two v-belts and I didn't even know it was broke cause it was still enough to drive the alternator.
It wasn't until I was changing the harmonic damper that I found the broken bolt. This would be your
best change to eliminate your bolt fatiguing and failure.

FP02_FatigueFailureCloseup_00042.jpg
FP01_AlternatorBoltBroken_0038.jpg
.
 

Attachments

  • Fatigue of Threaded Fasteners.pdf
    461.8 KB · Views: 1
I run two v-belts and I didn't even know it was broke cause it was still enough to drive the alternator. It wasn't until I was changing the harmonic damper that I found the broken bolt. This would be your best change to eliminate your bolt fatiguing and failure.[/QUOTE said:
I agree.

3 inexpensive pulleys and your problem should be solved.
 
Just a little more interesting info.

I'm using two 3/8" v-belts.

Notice that a double 1/2" v-belts more than doubles (2.67 times) the
capabilities from 4.5 HP to 12 HP. So the 3/8" double v-belt should be
able to handle 9.3 HP, up from the single belt of 3.5 HP.

Belt_HP_Ratings.jpg
.
 

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