[Diy_efi] OT: torque question for the ME types

[Mr. Goat]

Before you go and throw those bolts in your rim a little bit of info on your
bolts...

12.9 is what is commonly referred to as a high tensile bolt. It has a
ultimate tensile strength of 1200Mpa and it's yield stress is 960Mpa. 12.9
bolts are tightened with a torque wrench right up to 90% of their ultimate
tensile. There is no other effective way to tighten a 12.9 bolt. If nuts are
to be captured then Loctite, safety wire or nyloc's must be used.

The stainless bolts are A2-70, I'm guess as that is the most common of the
metric stainless bolts. Ultimate tensile is 700Mpa and yield is 450Mpa. They
are about 10% weaker then 8.8 bolts.

The reason that your torque readings are higher on the stainless bolt is
because A2 bolts have approximately twice the k-value of the corresponding
steel fastener. This doesn't necessarily mean that stainless fasteners
require double the specific torque since they usually cannot achieve the
strength of a steel fastener.

I can babble for pages as to why the manufacturer spec'ed 10.9+ grade bolts,
since the correct design of a bolted joint is quite involved. Fortunately,
this isn't a machine design list so if you want my honest suggestion. Look
for chrome plated or zinc plated 12.9 grade bolts. Two popular brands are
"Unbrako" and "Holo-chrome".

Cheers,
M.
----- Original Message ----- >

> - wheels are 17"x8" and 17"x11"
> - bolts are M6x35mm, 1mm pitch, socket head cap screw (shcs)
> - 35 bolts per wheel
> - sections go together in such a way that there is no shear stress on any
> bolts (only tensile).
> - original bolts found on wheels were class 12.9 (and rusting like crazy)
>
> the manufaturer recommended at least grade 10.9, and to torque at ~12
> ft-lbs, and no greater than 15 ft-lbs. their torque spec seems pretty
> restrictive for a 10.9 bolt.
>
> my intention was to go with a stainless type of shcs, as any of the
> 10.9/12.9 bolt grades tend to corrode much easier, and in my application
the
> screw heads are high visibility items. I purchased some 302 stainless
bolts
> of the same size as those removed. upon inspection, the new bolts were
> stamped 'A2'. i did some destructive testing of these items, and was able
to
> reach 15 ft-lbs with no yield. yielding began at ~18 ft-lbs, and torque
> remained at ~19 ft-lbs for one full turn, at which point the bolt shears
> near its center.
>
> my guess is that the manufaturer is failing way safe on the bolt grade.
>
> can the A2 (stainless) bolts be sufficient for this task?
> perhaps the bolts i purchased are much stronger than they should be?
> is there something that i am overlooking?
>
> thanks for any assistance
> Al Malventano, ETC(SS), USN


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