Edge-Welded Bellows Design

Cycle Life

This is the primarily the most important factor in good bellows design. In addition to the stroke, the cycle life required determines the number of convolutions needed in the bellows element and this also determines the amount of stress acting upon the bellows.

Stroke
Bellows can be operated in three basic ways, or any combination of these three :

1. Axial motion - Z axis movement
2. Lateral motion- X/Y axis movement
3. Angular motion (plane of angulation must be perpendicular to the bellow axis)

In all cases, the operational stroke of the bellows should be done in compression from the free length(cycle life can be reduced by upto 90% when stroking takes place in extension from the free length). In most applications designs can be optimised such that the stroke occurs between the free and closed lengths.

Deflection
The bellows stroke is governed by the deflection offered by the bellows convolution. This movement per convolution is determined by the material thickness/ bellows type and the radial span of the bellows and hence varies from size to size.

Pitch

Pitch is not a static dimension and changes as the bellows operates through a given stroke range.The extended length should not exceed the nested / static length for the bellows plus the maximum stroke. Exceeding this figure may lead to material deformation ultimately resulting in a significantly reduced operating life.

Pressure
This operating condition is critical in the design process since its governs material selection, wall thickness and cost. As a rule, edge welded metal bellows are 30% more resistant to internal pressure than to external pressure.

Where external pressure is concerned most applications with say a 1 Bar pressure differential (i.e. internal vacuum ) will work with no risk of squirming.

With internal pressure and with vacuum externally to the bellows, the bellow orientation is critical to bellows design. In a horizontal orientation over a long stroke, a bellows can be subjected to forces that cause the bellows to buckle/squirm (this squirm pressure can be calculated for a given stroke range and bellows size). In such situations the design must include a means for providing the unit with stability over its stroke range.