Terms Glossary

Bellows DD Form12-21B

Dimensional Requirements


  • Bellows OD, max./min
    The bellows acceptable outside diameter range for the application. This helps to define the acceptable size envelope for the bellows. It is critical for tool selection and to optimize performance.
  • Bellows ID, max./min
    The bellows acceptable inside diameter range for the application. This helps to define the acceptable size envelope for the bellows.
  • Effective Area
    The mean diameter between OD and ID squared and multiplied by .785. It is the equivalent piston area that will produce the same fluid displacement as the bellows for the same axial compression or extension.
  • Bellows Free Length
    The manufactured length of the bellows convolutions, with the bellows at a neutral at rest position with no applied forces acting on it.
  • Assembly Required
    This lets the design engineer know if assembly of the bellows to end pieces is required.

Environmental Backgrounds


  • Bellows Material
    The bellows material required for the application, if known.
  • Temperature, Max/Min
    The potential range of temperatures that the bellows or bellows assembly might be exposed to in the application.
  • Operating Temperature
    The temperature that the bellows or bellows assembly will experience during normal operation.
  • Media/Environment
    The type of environment or substances (gas, liquids, and materials) the bellows would be exposed to in the application.

Performance Requirements


  • Pressure Responsive
    What is the desired response with pressure changes?
  • Leak Test Required
    Determines whether the application requires the bellows or bellows assembly be leak tight.
  • Leak Rate
    If the bellows or bellows assembly is required to be leak tested, if known, give the required leak rate. The achievable leak rate will vary due to the design requirements (ID, OD, wall thickness, etc.)
  • Operating Pressure
    If the bellows needs to be leak tight, this is the differential pressure the bellows will be subjected to in the application. It is important to specify if the pressure will be applied internally or externally to the bellows.
  • Maximum Internal Pressure
    If the bellows needs to be leak tight, this is the maximum differential applied internally to the bellows, in the application.
  • Maximum External Pressure
    If the bellows needs to be leak tight, this is the maximum differential applied externally to the bellows, in the application.
  • Compression
    In the application this is the axial deflection from its nominal free length that the bellows will see in compression.
  • Extension
    The axial deflection from its nominal free length that the bellows will see in extension.
  • Lateral Offset
    The distance between the centerlines of the ends of the bellows that are parallel but not on the same line.
  • Angular Offset
    The angle between the centerlines of the ends of the bellows.
  • Spring Rate
    The force in pounds applied axially to a bellows, divided by the compression (or extension) in inches resulting from this force, and is a measure of stiffness. The value in the table is for small compression or extension not exceeding 10% of bellows convolutions length.
  • Cycle Life
    Number of cycles required for compression bending or offset movements. Loading is not a consideration.