pivots. The trunnion is to be made of 4140 steel having
bending loads. However, a fillet is required to minimize
a tensile strength of 125,000 psi. Assuming that the
stress concentrations in the corner.
shearing stress is 60 percent of the tensile strength or
d. The outside diameter of the trunnion where the
75,000 psi, the Divot size is calculated as follows:
pivot is located must be designed to fit the mounting
piece in the aircraft without free play; otherwise, a
bending moment may be set up in the pivot. A fillet
should be provided where the pivot joins the trunnion to
Where:
avoid unnecessary stress concentrations.
S3 = shearing stress
F = load
A = area in shear
into the trunnion, but the skirt of the trunnion extends an
additional 3 inches along the outside tube. This design
If it is assumed that the full load may be carried by a
feature is incorporated because the bending moment
single pivot under certain loading conditions, a trunnion
caused by the eccentric loading of the seat ‡ acts on the
pivot 5/8 inch in diameter will be safe.
trunnion. If the trunnion stopped at the end of the
threaded section of the outside tube, the bending
c. Good trunnion design requires that the section
moment would be transmitted from the trunnion to the
of metal adjoining the pivots be sufficiently thick to
outside tube at the undercut of the threads, causing
prevent the pivot from being pulled out by the "roots".
excessive stress concentrations at this point. Since the
The size of the fillet radius must be a compromise
trunnion extends along the outside tube,
because the trunnion bearing loads must be taken close
in to the trunnion body so as to eliminate excessive
† 2 is the safety factor for structural members.
‡ The catapult is attached to the back of the pilot's seat, and, therefore, cannot direct its thrust through the center
of gravity of the seat. An additional moment is created by the aerodynamic force on the pilot and seat. This moment
opposes the Initial moment, therefore, bending may occur In either direction.
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