The measure of the bellows flexibility is known as the spring rate of the bellows and is normally expressed in terms of force required to move the bellows through a specified movement. (N/mm) or (Nm/deg) for angular movement. Apart from pressure thrust the unrestrained bellows expansion joints also exert spring force on the anchors or equipments adjacent to them Which is equal to
Spring Rate and Spring Force of Metal Bellows
The spring rates for Axial, Lateral and Angular movements are design considerations and are provided by the manufacturer for the piping engineer to take them in to consideration for piping flexibility and stress analysis.