Metal Bellows Correctedn 7/11/00 2:40 PM Page 8
HOW A BELLOW WORKS (CONTINUED)
Expansion joints are designed to operate with a value for
S6 that far exceeds the yield strength of the bellows
material. This means that most expansion joints will take a
permanent set at the rated axial, angular or lateral motion.
Expansion joint bellows are rarely designed to operate in
the elastic stress range. Therefore the bellows will
eventually fatigue after a finite number of movement
cycles. It is important to specify a realistic cycle life as a
Convolution shape
design consideration when ordering an expansion joint.
before deflecting
An overly conservative cycle life requirement can result in
a bellows design that is so long and soft that it is subject to
squirm failure.
Multiple Ply Construction
S6 or deflection
The necessary amount of metal to contain pressure can be
bending stress
achieved with a single ply of bellows material or multiple
plies of material of reduced thickness. A bellows of
Convolution shape multiple ply construction often has a lower spring rate
after deflecting
than a single ply bellows for the same service. Thin
material experiences less strain than a thick material for
Figure 5. When the bellows compresses, the side the same deflection. That means a multiple ply bellows
walls bend to shorten the bellows.
may be shorter and it may have a higher cycle life than a
single ply bellows for the same application.
DESIGN VARIABLES AS THEY EFFECT BELLOWS DYNAMICS
VARIATION
Hoop Stress S2 Bulge Stress S4 Deflection Stress S6 Squirm Pressure External Buckling Pressure Cycle Life Rated Axial Rated Lateral Rated Angular Axial Spring Rate Lateral Spring Rate Angular Spring Rate Pressure thrust
Thicker Material –(1) –(2) +(1) +(3) + ––––+(3)+(3)+(3)S
Thinner Material +(1) +(2) –(1) –(3) – ++++–(3)–(3)–(3)S
Higher Convolute –(1) +(2) –(2) –(3) +++++–(3)–(3)–(3)+
Lower Convolute +(1) –(2) +(2) +(3) –––––+(3)+(3)+(3)–
Smaller Pitch – + – – +++++–––S
Larger Pitch + – + + –––––+++S
More Plies – – S + + SSSS+++S
Fewer Plies + + S – – SSSS–––S
Larger Diameter +(1) S S + – S S – – + + + +
Smaller Diameter –(1) S S – + S S + + – – – –
More Convolutions S S – – S ++++–––S
Fewer Convolutions S S + + S ––––+++S
LEGEND: + INCREASE; – DECREASE; S SAME
(#) INDICATES HOW STEEPLY THE VARIATION AFFECTS THE DESIGN VARIABLE, I.E., (1) MEANS THE CHANGE IS LINEAR; (2) MEANS THE DESIGN
VARIABLE CHANGES BY THE SQUARE OF THE VARIABLE; (3) MEANS THE DESIGN VARIABLE CHANGES BY THE CUBE OF THE VARIABLE.
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