According to Carroll Smith's book, "Engineer to Win" (and my memory, because
my dad hasn't returned the book since I lent it to him), shot peening is
used to form a layer of a certain (specified) thickness in the surface of
steel or aluminum parts where the material is in compression. The idea is
that under applied tensile stress, the treated surface does not reach the
tensile stress level required for cracks to start. Since cracks must start
at the surface (barring inclusions), lowering the stress level at the
surface can improve the lifespan of parts quite a bit.
Another thing that he stressed (no pun intended) was surface preparation.
All corners and edges, convex as well as concave, are stress risers and
places where cracks can start. Therefore, all outside corners should be
radiused. On the fulcrum pins, the four places where the reliefs for the
mounting bolts are machined in are prime candidates. The inside radii of the
pins (where the step down to the threaded section is) should be polished,
with the tool orientation such that the polishing marks are made in a
longitudinal direction, along the direction of stress rather than across it.
Doing all this stuff is no substitute for regular inspection. We have 35
years of history telling us that these parts are stressed right to the
limit, especially in high-load applications. No amount of prep will save a
part that is used beyond its lifespan.
Theo
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