Why do asymmetric bellows occur?

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Mechanical seals using welded metal bellows have seen increasingly widespread use due to their advantages such as simple structure, good expansion capacity, and high-temperature resistance. They are especially used in the sealing of high-temperature pumps, and in some cases have become the primary type of shaft seal. The sealing quality of welded metal bellows in China is not stable enough, and the main reason for this is the poor quality of the materials used to manufacture these bellows; most of them are made from 18-8 type stainless steel. This material is corrosion-resistant, but it has poor strength and elasticity; especially at high temperatures, it loses its elasticity after being used for a while. Abroad, it is mostly manufactured using AM350, which is a precipitation-hardening stainless steel that retains sufficient strength and elasticity at high temperatures, resulting in fewer instances of loss of strength. However, after long-term use, some defects were revealed, with cracks appearing near the welds of the waveplates. The reason is that under compression, the stress distribution across the axial cross-section is uneven, with severe stress concentration, which reduces its service life. In existing welded metal bellows, the geometric shapes and dimensions at the inner and outer edges of the cross-section (or known as the troughs and crests) are identical; hence they are also referred to as “symmetric” bellows. It has the aforementioned defects, and to overcome these, the “asymmetric” bellows was developed.