d. To determine the minimum length of threads
e. These calculations show that one full thread in
engagement would hold the design pressure of 2,000 psi
with a 50-percent margin of safety (built into the
formula). Therefore, there is no thread problem. The
thread is made long enough to provide room for sealing.
69. Minor Parts. a. The spacer, expander, stop ring,
Figure 64. Cap for outside tube.
and seals are considered minor parts. The spacer (fig.
65) is a ring located between the outside tube and the
telescoping tube. This part has 20 holes in it to permit
propellant gas to flow into the void between the outside
and telescoping tubes. It is provided with internal
threads to attach it to the telescoping tube.
b. The expander is an alloy-steel ring, and it is
located at the top of the outside tube in order to lock the
outside tube to the trunnion. The outside diameter of
the expander increases slightly at the top to provide a
wedge surface. When the outside h tube is threaded to
the trunnion, the expander wedges the outside tube
tightly against the trunnion, locking them together. The
expander also provides better sealing action in the
The base of the cap is made 0.23 - 0.01 inch thick
to provide sufficient thickness to prevent failure of
c. The stop ring (fig. 66) is a short, circular steel
the end piece.
section with external threads to accommodate the
c. The minimum thickness of the wall of the cap at
trunnion, and a shoulder to prevent tightening the ring
the undercut of the threads, as designed, is 0.045 inch.
too far into the trunnion. When it
One way this section could fail would be the result of
tension. The tensile force is equal to the internal
pressure times the area of the end disc, and the surface
resisting this force is equal to the thickness of the wall at
the undercut times the circumference of the cap.
Since the yield strength of steel is well above 39,600
psi, the cap is sufficiently strong as designed.
† Safety factor.