Metal Bellows Correctedn 7/11/00 2:40 PM Page 7
Hoop stress is what holds a bellows together like the hoops
Pressure
on a barrel. This stress must be held to a code stress level.
The customer should specify the code to be used.
The bellow’s ability to carry pressure is also limited by
bulge stress or EJMA stress S4. This is a stress that runs
longitudinal to the bellows centerline. More specifically, it
is located in the bellows sidewall and it is a measure of the
tendency of the convolutions to become less U-shaped and
more spherical.
Figure 2. Hoop stress or S2 runs in the
circumferential direction.
For bellows that are not annealed after forming, S4 is allowed by
EJMA to exceed the initial yield strength of the bellows material
by a large margin because it is cold worked. If a bellows is
Pressure
annealed after forming, S4 must be severely limited because the
bellows sidewall material is no longer cold worked.
The convolution
wants to take
shape
Accommodating a requirement for annealing will often result in
the addition of reinforcing rings or a much heavier bellows
material and more convolutions. Our standard policy is to
S4 or pressure
provide bellows in the as-formed condition to take advantage of
bending stress
the added performance that is imparted to the bellows through
cold work. Senior Flexonics Pathway will accommodate
Figure 3. annealing requirements on request.
Squirm
A bellows that is pressurized internally is similar in
many ways to a column loaded in compression. At
some loading, a long column will buckle. Similarly,
an internally pressurized expansion joint will
eventually buckle or squirm at some internal
pressure loading. It is the responsibility of the
expansion joint manufacturer to design the bellows
to avoid squirm during operating conditions or
pressure testing.
Squirm can lead to catastrophic failure of a bellows.
Stable bellows Squirmed bellows
Our design equations treat squirm conservatively. A
Figure 4. Bellows squirm is similar to column buckling.
hydrostatic test of the completed expansion joint
verifies stability. If hydrostatic testing is desired, it
should be specified at the time of order placement.
Cycle Life
When a bellows deflects, the motion is absorbed by bending of the sidewalls of each convolution. The
associated stress caused by this motion is the deflection stress or EJMA stress S6. This stress runs longitudinal
to the bellows centerline. The maximum value of S6 is located in the sidewall of each convolution near the
crest or root.
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