Scions of Triumph,
For our edification, and my boredom, I calculated the factor of
safety against shearing for the woodruff key on the crank pulley of a
Spit motor.
** I assumed the crankshaft to be 1" where the pulley mounts.
** I assumed the full torque of 150 lb-ft was applied from the crank
to the pulley through the woodruff key. (in reality, much less than
the full torque is transferred due to friction at the bolthead/pulley
interface).
** I assumed the woodruff key to have a x-sectional area of .155"
Plugging the math shows that the key sees 23,000 psi of shear. This is
about the shearing range of really weak steel. Shearing strengths for
higher strength steels can be 6 or 7 times this number.
Again, this analysis doesn't take any friction between the
bolthead and the crank pulley face into account. Any friction at that
interface would lessen the shear stress the key sees, so the real value
of shear stress at the key is more likely to be 15,000 psi or so.
In short, I wouldn't worry about the woodruff key failing if
the crank pulley were used to hold the crank when torquing the pulley bolt.
Greg
Greg Meboe meboe@wsunix.wsu.edu
Dept. of Mechanical and Materials Engineering
Washington State University, Pullman, Wa.
'85 XJ-12 H.E. (daily) '67 Spit-6 '74 TR-6
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