All bellows have a critical pressure at which they become unstable. Instability can occur in either of two modes, column instability (or squirm), or in-plane deformation of the convolution side wall. Squirm affects the bellows as a whole, while in-plane deformation only affects one or more convolutes individually.
Column Instability Column instability (or squirm), is the phenomena whereby the centerline of a straight bellows develops a sideways or lateral bow. This condition is most associated with bellows which have a relatively large length-to-diameter ration and is analogous to the buckling of a column under compressive load.
In-plane squirm is defined as a shift or rotation of the plane of one or more convolutions such that the plane of these convolutions is no longer perpendicular to the axis of an unreinforced bellows. It is characterized by tilting or warping of one or more convolutions. This condition is predominantly associated with high meridional bending stress and the formation of plastic hinges at the root and crest of the convolutions. It is most common in bellows which have relatively small length-to-diameter ratio.
How to Avoid Instability (or Squirm)
The test pressure should be less than or equal to 1.5 times the design pressure based on column or in-plane stability at ambient temperature material properties.