Peter Cowan/Lexi Cameron wrote:

Paul, would you mind digging out some info for me on bolts?
I am making a temporary change to my motor mount for a short test flight. Currently
all 4 firewall bolts are 3/8. For various reasons I want to go to 5/16 on the top
and on the bottom stay at 3/8 but also end up with a 3/8 in shear. (400 lb
Lycoming)
How does 5/16 compare to 3/8 in tensile and shear?
Where does aircraft material sit wrt grade 5 and  8 designations? I can buy grade 8
at brafasco for a lot less than AN.

Many thanks
Peter.

That is a very good question. Apparently there are so many variables that
there is no simple answer I am sorry to say. The ME and machinist handbooks
avoid the question. Much depends on the torque applied to the bolt
and of course how well the threads are lubed when you apply that torque.

AN bolts are in the range of 125,000 to 140,000 psi and grade 8 are
150,000 psi minimum tensile. If you calculate the areas (pi*R^2) and
apply the max tensile strength of the bolt material used in psi the basic
strength should become apparent.

A 3/8 is .375 dia. or .11 square inches or about 13,000 pounds at 125K psi.
A 5/16 is .3125 or .0766 square inches or about 11,490 pounds at 150K psi.

I suspect the grade eight's are somewhat more brittle and quality control
more lax (the FAA would have us believe).

Looks great doesn't it? I once designed a motor mount for a big
block Chevy in an endurance racer and specified two 1/4  inch bolts.
One on each side of the engine. Worked great until the mechanics
torqued them off. :-) Hey a 150K psi 1/4 inch bolt will take 7,000 pounds
in single shear each. A 700 pound engine with a 4.5 G vertical bump is
only 3000 pounds total. Plus some for torque of course. There is a famous
poster of the car completely airborne over a bump at the Nurburgring.

Machinery's Handbook has a lot of formulas for this sort of thing
but they give no stress values for grades of bolts. Apparently the bolt
manufactures complained that the material they used was better than their
competitors or something like that. ARP claims 200K psi and above
for their bolts.

I have this book called Aircraft Hardware Standards Manual & Engineering
References which is also surprisingly noncommittal on the subject.

I think the bottom line is you are taking a chance. Sorry to be
the bearer of bad news. Don't execute me :-).

Paul Lamar

It would also be good to remember that with the engine cantilevered 2 ft
out from the firewall, the top bolts aren't the ones you want to
downsize. They are likely to carry Much More than 1/2 the load as the
mount pushes against the firewall at the bottom & pulls at the top.

Charlie

 
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