There are two main classifications of metal bellow seals: edge welded metal bellows an formed bellows. Here we discuss the advantages and disadvantages of each so you can be confident in choosing the appropriate solution for your sealing challenge. The right bellows seal being one that addresses the application and its operating requirements, as well as key factors such as seal balance, materials selection, operating lengths and spring rates.
Overview: Edge Welded Metal Bellows Overview
Edge welded metal bellows are formed by miocroscopically welding the interior diameters of individual , stamped metal diaphragms into two-piece convolutions. These convolutions are then welded together on the outer diameter to form a welded bellows core.
Due to their high level of precision and durability, edge welded metal bellows are used in a variety of industries and applications where reliability is critical, including aerospace, oil and gas, semiconductor, and medical to name a few.
Overview: Formed Bellows
There are several types of formed bellows including cold-formed, hydroformed, and electroformed bellows. Cold-formed and hydroformed bellows are typically manufactured from either welded or seamless tubing and utilise extensive auomation for mass production. Electroformed bellows are created by electroplating metal onto an aluminium mandrel.
The process in which formed bellows are manufactured offers some cost benefit, however this is not without its limitations. Typically, the mechanical strength and operating temperature limits are much less than edge welded metal bellows. This limits the use of formed bellows in high temperature or harsh applications typically found in the oil and gas, cryogenic, and power generation industries.
A Side-by-Side Comparison
In order to make the right selection for an application, you must consider three key points: materials of construction, operating characteristics, and cost.
| FORMED BELLOWS | EDGE WELDED METAL BELLOWS |
Production | Uses hydraulic pressure to force metal tubes to expand inside a mold | Welding of male and female metal diaphragms to create a convolution, then welding a series of convolutions to each other to create a bellows core |
Materials of Construction | Limited to metals with high elongation properties such as brass and bronze | Can be produced from a wide range of materials including corrosion resistant alloys and specialty metals |
Characteristics | Thick, formed walls with inherent rigidity and stiffness. The process of forcing material into a form can create inconsistent wall thickness on both the OD and ID of teh bellows which present as points of potential failure | Individual metal diaphragm thickness is measured in mils. This characteristic allows for dramaticallly greater flexibility, range of motion, and space allowance. Preferred choice for harsh environments |
Spring Rate | Fomed bellows typically have a thicker convolution structure with higher spring rates and larger deviations than edge welded bellows, impacting precision loading | Edge welded bellows seals accommodate the addition or subtraction of individual convolutions, enabling a precise spring rate. Achieving the correct spring rate lightens the force affecting the faces which generates less heat, leading to longer seal life. |
Cost | Tyically less expensive per piece than an edge welded bellows unit when produced in high volume. Higher tooling and set up costs. | Typically less expensive than formed bellows for small volume runs. Actual costs are dependent on quantity, materials of construction and size. |
The Final Score
Not all metal bellows technologies are created equal. While price is a factor, it shouldn't be the primary factor in the selection of any mechanical seal. By ensuring the correct construction materials and operating parameters are met, potential seal failure and associated longer term costs, can be avoided. The two most important criteria are therefore suitability and reliability. With this in mind, the edge welded metal bellows is the frontrunner.
Different manufacturing methods and construction materials profoundly affect how a metal bellows seal will perform in terms of stroke, pressure capabilities, spring rate, and temperature response. These characteristics will impact how long a metal bellows seal will last in the field. The wrong bellows may operate for the short term, but fail to achieve the expected long term results.
Edge welded metal bellows have earned their place in today's fluid sealing market place because of their ability to deliver the desired results consistently and across a wide range of industries and sealing environments,
You can read more about edge welded metal bellows in our post "Introduction to Edge Welded Metal Bellows".
Specialised Flow Solutions is the Australian distributor for Flexaseal Engineered Seals and Systems, LLC - a leading U.S. manufacturer of custom engineered mechanical seals, welded metal bellows and the two-piece split cartridge seal. We provide expert technical advice and can assist you to identify and solve any sealing challenges.