The EJMA standard covers the subject of bellows cycle life in depth. Badger recommends that system designers refer to the EJMA Standards when specifying cycle life
Metal bellows movements are normally designed to operate in the plastic range of materials and therefore will take a permanent “set” at the bellows rated movements. When a bellows compresses, extends, or angulates, the movement is absorbed by deformation of the sidewalls of the bellows convolutions. The stress caused by the sidewall deformation is defined as the “deflection bending stress, EJMA S6”. This stress is highest at the “root” and “crest” of the convolution.
Metal bellows are designed to operate with a deflection bending stress that greatly exceeds the yield strength of the bellows material and therefore operate in the plastic range of the material. Because of this, the bellows will eventually fail after a finite number of movement cycles. Bellows designed to the EJMA standards have proven reliability of cycle life, and realistic cycle life should be stated in the specifications. Refer to the chart on the next page, “higher” cycle life results in “weaker” (pressure) bellows. Optimum bellows design is based on accurate-real-world cycle life, pressure, temperature, and bellows movement data.
Metal Bellows Design Variables
The EJMA standard covers the subject of bellows cycle life in depth. Badger recommends that system designers refer to the EJMA Standards when specifying cycle life.
The chart below will help to understand the relationship of pressure and cycle life in bellows design.
Note: The red values in the chart indicate that when cycle life is higher, squirm pressure is lower
Knowledge Base Handbook for Metal Bellows Expansion Joints
Note: The red values in the chart indicate that when cycle life is higher, squirm pressure is lower