As you progress from grade 5 up to grades 7 & 8, the commercial bolts

are even more

likely to fail catastrophically, as they are even more brittle. Hi

strength AN

hardware is much less likely to fail in this manner. In short, AN

hardware provides

a certain "safety net".

Let's see........ all six of my AN propeller bolts (1/2 VW-Wood Prop) failed
catastrophically/simultaneously.
Fortunately,  I was about 2500' over the airport when my prop splashed down
into the Ohio river, never to be seen again.  I was able to do a Bob Hoover
right into the hangar with the MiniMax.  No brakes on it either!

My six Grade8 bolts have now held tight for several years.

Bob Moore

Bill Freeman wrote:

This is a fairly common problem with wood props, esp where the humidity
changes a lot.  The wood expands in humid air enough to crush, since the
bolts won't give enough, then when it dries out, it shrinks back and the
preload is lost.  The bolts will very rapidly fatigue when the preload is lost.
I suspect that you may have been more careful about checking bolt torque,
and/or have less humidity variation, or in many cases a prop crushes a certain
amount and then stops.  Thin layer laminate props do this less than 6-7
laminate props due to the high glue content and inability of moisture to flow
as easily, plus higher compressive strength of the 1/16" laminate "plywood".

This has nothing to do with the bolts, any bolt will fatigue with a repetitive
load above about 50% of yield strength.

Bill Freeman

 
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