Series 7500 Formed Metal Bellows

Bellows Nomenclature

Bellows Movements

AXIAL Extension or compression from the manufactured length along the longitudinal centerline with ends parallel.		LATERAL Displacement perpendicular to the longitudinal centerline with the ends parallel.

ANGULAR Rotation of the longitudinal centerline about the perpendicular axis.		COMBINED Axial, lateral, and angular movements can be combined within the rated movements.		TORSION OR TWISTING AROUND THE CENTERLINE (Not recommended)

Standard Tolerances

Neck Trims

STANDARD TRIM I.D. NECK The neck and Convolution I.D. are equal to the nominal I.D		STYLE A O.D. NECK The neck O.D. is equal to the convolution O.D.
STYLE B EXPANDED NECK The neck I.D. or O.D. can be specified within the range of the I.D. to the convolution O.D.		STYLE C O.D. TRIM The trim line is at the tangent point of the convolution O.D.

STYLE D I.D. TRIM The trim line is at the tangent point of the convolution I.D.		INSIDE BAND		OUTSIDE BAND

Series 7500 bellows with Standard, Style A, or Style B neck trims can be specified with inside or outside bands which increase the thickness for welding or added reinforcement. The band thickness is approximately 2.5 times the bellows thickness. The bands are attached by resistance seam welding for sizes 1″ through 48″ and by edge welding (GTAW) for larger sizes.

Caution: The use of bands is not recommended for vacuum applications requiring a mass spectrometer helium leak test.

Multiply or Laminated Bellows

Any standard-size bellows can be fabricated with multiply construction up to four plies of equal thickness. The maximum pressure, spring rate, and stability pressure are increased in direct proportion to the number of plies, but with the same axial deflection as a single ply. Multiply design permits a lower spring rate and a greater cycle life than a single ply configuration for an equivalent pressure. This type of construction is recommended for applications with vibration or rapid cyclic motion because of the inherent damping provided by the relative movement of the plies. Multiply construction in not recommended for vacuum application requiring a mass spectrometer helium leak test because of possible out-gassing from an undetectable leak in an inner ply.

Refer to the individual topics in the Explanation of Tabulated Data for the methods used to calculate multiply bellows parameters.

All dimensional data is applicable to tabulated single and multiply configurations with standard and optional materials. The tabulated performance parameters are for type 321 stainless steel at room temperature; however, they apply to all austenitic stainless steels with minor variations. Variations for other materials are given for each column explanation.

Materials & Properties
The performance data tabulated in Columns 7 through 11 (pages 6-17) was calculated for type 321 stainless steel at 70ºF. These values are a function of the Modulus of Elasticity, strength, and fatigue life. If the service conditions are substantially different or an optional material is used, contact the factory for assistance.

Optional materials can be substituted which include but are not limited to alloys 600, 625, 800, 825, Cb-20 and C-276, Nickel 200, Beryllium Copper and weldable Aluminum alloys.

Use of Tabulation
The data included in this catalog was computed by generally accepted analytical methods and empirical data derived from tests. This data should be used as a design guide only. Consult the factory if you have an application requiring close control of these values.\

The following paragraph numbers refer to the column numbers of the tabulated data on pages 6 through 17.

1. Stock Number
   A four or five digit number identifies the nominal size (inside diameter), and a letter corresponds to the material thickness. In order to fully specify the desired configuration refer to the Ordering Instructions on page 18.
2. Inside Diameter
   Nominal Inside diameter of the convolutions and the standard neck trim.
3. Convolution Outside Diameter
   Nominal outside diameter of the convolutions and maximum diameter of Style A neck trim.
4. Effective Area
   The cross-sectional area of the bellows based on the mean diameter of convolutions. This area multiplied by the pressure equals the pressure thrust force (Lbs.).
5. Material Thickness
   The values tabulated are representative designs for each diameter. Other thickness may be available – consult the factory.
6. Maximum Convoluted Length
   These values are established by manufacturing limitations. Substantially longer lengths must be obtained by splicing two or more elements. Any shorter length can be specified – refer to the Ordering Instructions.
7. Maximum Pressure
   The highest internal or external pressure recommended with a corresponding test pressure of 1.5 times this value. This pressure is exclusive of the squirm or instability pressure (Column 8).The working pressure for internally pressurized bellows is determined from the pressure tabulated in Column 7, or the squirm pressure calculated from Column 8 – whichever is lowest. For convenience Column 7 and 8 include a test pressure factor of 1.5, i.e., a bellows can be pressurized to 1.5 times the pressure tabulated in Column 7 without permanent set, and the anticipated squirm pressure is greater than 1.5 times the value calculated from Column 8. The working pressure for internally pressurized bellows which are guided or supported to prevent squirm is determined from Column 7.The working pressure for externally pressurized bellows is determined from Column 7 – squirm or instability does not occur. All bellows rated at a pressure (Column 7) exceeding 15 psi are satisfactory for Full Vacuum (internal) applications – Caution – This pressure applies to the convoluted sections only and the standard neck trims. Long unsupported neck trims may collapse. Multiply or Laminated Construction – multiply the values of Columns 7 and 8 times the number of plies.
8. Squirm or Instability Pressure
   Internally pressurized bellows are unstable at the critical or squirm pressure. In most instances this condition is characterized by the centerline of individual convolutions deviating from a common centerline – analogous to buckling of a long column under compression. This condition occurs when the convoluted length is long relative to the inside diameter – over 2 or 2.5 times. There is also a less familiar type of squirm which occurs when the plane of the individual convolutions deviates from parallel planes. Either condition represents a maximum pressure and failure will occur if pressure is increased.The tabulated values (Column 8) are the maximum recommended internal pressure to avoid squirm for one inch of convoluted length. The values are based on a test pressure 1.5 times the tabulated pressure. The pressure for other lengths can be calculated by dividing the tabulated value by the square of the convoluted length.The Column 8 values do not need to be corrected for other materials which have a modulus of elasticity in the range of 28 – 30 x 106 psi. For multiply construction multiply the tabulated value by the number of plies.
9. Axial Spring Rate
   The force (Lbs.) per inch of axial extension or compression resulting from the material and configuration spring constant is tabulated for one inch of convoluted length. The spring rate for other convoluted lengths is determined by dividing the tabulated values by the convoluted length. Multiply this value by the number of plies for multiply construction.
10. Lateral Spring Rate
    The force (Lbs.) per inch of the lateral offset resulting from the material and configuration spring constant is tabulated for one inch of convoluted length. This movement occurs when the ends remain parallel to each other and perpendicular to the longitudinal centerline, but the centerline displaces laterally. The lateral spring rate for bellows other than one inch convoluted length is calculated by dividing the cube of the convoluted length. For multiply construction multiply by the number of plies. Caution – The tabulated values must be multiplied by the 1000 except as noted to obtain the true spring rate.
11. Axial Deflection
    The allowable axial extension or compression for one inch of convoluted length is tabulated for 2000 cycles of movement from the nominal length to the tabulated value. The movement for other convoluted lengths are obtained by multiplying by the required length. Since these values are determined by the metal fatigue due to the bending stress produced by movement it is unchanged for multiply construction.
    The tabulated values can be corrected for other numbers of cycles (at 70ºF.) by applying the following factors:

500	1.40
1,000	1.20
2,000	1.00
4,000	0.85
8,000	0.75
10,000	0.70

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