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Angular Rotation: The displacement of the longitudinal centerline of a bellows from a straight line into a circular arc. Sometimes confused with torsional rotational - see Torsion.

Angular Spring Rate: The moment required to displace the ends of a bellows out of plane and the centerline into a circular arc. Normally measured to in in.-lbs./degree. The angle is measured as the included angle between the planes of ends.

Axial Deflection: The longitudinal centerline of the bellows remains straight with the ends parallel and the convoluted length compressed or extended.

Axial Spring Rate: The force required to compress or extend the ends of a bellows with the longitudinal centerline straight and the ends parallel. Normally referred to in lbs./in. In most cases the spring rate referred to is the "theoretical axial elastic spring rate". Bellows are not elastic throughout their range of deflection. The "working spring rate" is sometimes used which takes into consideration deflection. For a complete discussion see Section C-4 of the Standards of the Expansion Joint Manufacturers Association.

Bands: In order to increase the thickness of the bellows neck for reinforcement or welding, an outside or inside band can be added. The bands are normally fused to the neck by resistance (roll) welding. The maximum band thickness is normally 2 to 3 times the neck thickness. Bands are not recommended for high vacuum applications.

Bellows: Formed metal bellows are made from tubing by the application of internal pressure. The convolutions are formed in parallel planes that are perpendicular to the longitudinal centerline of the bellows. The tubing is normally made from sheet or coil that is rolled and longitudinally welded. Seamless tubing is available for smaller diameters.

Conned: When you are convinced to buy another manufacturer's product.

Convolution or Corrugation: Each formed shape of the cross section consisting of a root and crest is a convolution or corrugation. With parallel sides the gap at the root and crest are equal, and referred to as "U" shaped. If the inside radii at the root and crest are equal but the gap between the sides is reduced, the cross section has an Ω shape and referred to as omega shaped.

Convolution Crest: The semicircular segment of the convolution at the outside diameter.

Convoluted Length: For the purposes of this catalog the convoluted length is measured between the convolution side walls at each end of the bellows to allow an actual physical measurement to be made. For analytical purposes the convoluted length is measured between the centers of the radii of the end convolutions.

Convolution Root: The semicircular segment of the convolution at the inside diameter of the convolution.

Cycle Life: The cycle life or fatigue life expectancy of a bellows is based of the number of complete pressure and displacement cycles that result in metal failure. Cycle life data in this catalog is based on the design methods specified by the Standards of the Expansion Joints Manufacturers Association.

Effective Area: The cross-sectional area of the bellows based on the Mean Diameter of the convolutions. This area multiplied by the pressure equals the Pressure Thrust Force (Lbs.)

Expansion Joint Manufactures Association: An organization of leading manufacturers of metal bellows expansion joints that publishes the Standards of the Expansion Joint Manufacturers Association , the worldwide standard for metal bellows design.

External Pressure: Bellows used for applications with internal vacuum or enclosed in a pressure vessel and externally pressurized do not squirm but must be analyzed for collapsing pressure. Any bellows rated for a maximum pressure greater than 15 psig is suitable for full vacuum service.

Inside Diameter: The nominal inside diameter of the convolutions and neck of a standard neck trim bellows. The inside diameter of the convolutions and neck can be specified separately.

Laminated Bellows: Bellows can be manufactured with up to four (Hyspan's standard) plies or laminations i.e. material thicknesses. The maximum pressure, spring rate, and stability pressure are increased in direct proportion to the number of plies but the axial deflection is the same as a single ply. Multiply designs permit a lower spring rate and higher cycle life than a single ply configuration for an equivalent pressure. This type of construction is effective to design high pressure rated bellows, and is recommended for applications involving vibration or rapid cyclic movement because of the inherent internal damping provided by the relative movement of the plies. Multiply designs are not recommended for vacuum applications because of possible out-gassing from undetectable leaks in inner plies.

Lateral Deflection: The displacement or offset of the ends of the bellows perpendicular to the longitudinal centerline with the ends remaining parallel.

Lateral Spring Rate: The force required to displace (offset) the longitudinal centerline of a bellow with the ends parallel. Normally referred to in lbs./in.

Material Thickness: The original thickness of the tubing used to form the bellows. For multiply or laminated bellows it is the thickness of the individual plies.

Maximum Convoluted Length: Since most bellows are made from sheet or coil that is rolled and welded into tubing, and the original tube length is approximately three times longer than the completed bellows, there are restrictions on the convoluted length based on available material sizes. There may also be limitations on the manufacturers welding or forming equipment.

Maximum Pressure: For the purpose of Hyspan Catalog 974 there is a maximum pressure stated that is the lowest recommended internal or external design pressure of a bellows based on stresses created by pressure with allowance for a test pressure. This pressure does not consider the Squirm Pressure. For internally pressurized applications the working pressure should not exceed the lower of these two pressures.

Mean Diameter: The diameter of the bellows convolutions calculated by adding the convolution inside diameter and outside diameter and dividing by two.

Multiply: See Laminated

Neck: The neck or tangent is the straight tube at each end of the bellows. See page 3 on the printed catalog or Neck Trim for available neck trim configurations.

Outside Diameter: General references to outside diameter refer to the outside diameter of the convolutions.

Ply: References to "ply" refer to the thickness of the material used to manufacture a bellows.

Pressure Thrust Force: If the ends of a bellows are capped and the assembly pressurized, there is a total resulting force that is equal to the applied pressure times the mean diameter of the bellows. The only force opposing the pressure thrust results from the axial spring rate that is generally insignificant compared to the pressure thrust.

Side Wall: The portion of the convolution that connects the crest and the root.

Squirm Pressure: Internally pressurized bellows become unstable at a critical or squirm pressure. Bellows which are long relative to their diameter tend to buckle much like a long column under compression. This generally occurs when the length is 2-2 times greater than the inside diameter. Another type of squirm occurs referred to as in-plane occurs when the individual convolutions deviate from parallel planes. Either condition represents the maximum pressure, and failure will occur if the pressure is increased.

Stability Pressure: See Squirm Pressure

Stock Number: A five digit alphanumeric number that specifies the diameter and material thickness of Hyspan Series 7500 standard bellows. To completely specify a bellows configuration, dash numbers must be added to specify the convoluted length and neck length. See page 18 of the printed catalog or Ordering Instructions for complete part numbering.

Tangent: See Neck.

Thinning: Most bellows are formed by application of internal pressure to tube with a diameter approximately equal to the final convolution inside diameter. The material is drawn from the length of the tube. As a general rule the original tube length is approximately three times longer than the finished part. Thinning may occur at the Root and Crest of the convolutions depending on the forming method used. The maximum thinning for Hyspan bellows is 5%. Most bellows performance data is based on the "as formed" condition.

Torsion: A moment or displacement (twisting) around the longitudinal centerline of the bellows. Although bellows can react a limited amount of torsion they are not designed for torsional displacement, or to react torsional moments.

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