Description

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Specification

Material	Sus304,321,310S,316L,etc.
Parts	Flange, bellows, internal sleeve, tie-rod, pipe end, nut
Size available	Φ25mm~φ7000mm
Bellows layers	1-6 layers
Threads	DIN, BSPT, BSP, NPS, NPT, Metric, (ISO7/1, DIN 2999, ANSI B1.20.1)
Surface	Carbon steel flange galvanized or AL painted
Normal Specification	ID40~600mm,overall length:300~400mm
Application temperature	-50~300°C

The 3 Main Types of Metal Bellows Explained

Formed bellows

Formed bellows, sometimes called convoluted bellows, are a series of tubes created from a single sheet of metal. Manufacturers create formed bellows using a method called deep drawing, a process that pulls a sheet of metal into a forming die.
Then, one of two techniques is used to create the series of tubes that make up the formed bellows: cold forming or hydroforming. The forming die can turn various types of metals, such as copper or nickel, into formed bellows. Steel plants, cement plants, and refineries use them in their applications.

Welded Bellows

Manufacturers create welded bellows by welding individual diaphragms together. Generally, when choosing between formed and welded bellows, the buyer needs to consider both cost and performance.
For example, hydroformed bellows have a lower tooling cost when mass-produced, but their performance might suffer due to reduced flexibility from being so stiff. Welded bellows are both inexpensive to produce and exhibit some higher performance traits, although they aren't as strong as the welded joints.

Electroformed Bellows

Producers create electroformed bellows using electrical currents and positive and negative anodes. They place a mold into a solution of metal salts along with the alloy that will make up the bellows. An electrical current passes through the solution, with the positive and negative objects depositing the alloy of choice, such as copper or nickel, onto the mold.

Metal Bellows: Various Materials

Metal bellows are available in a variety of materials, including brass, bronze, beryllium copper, Monel, and stainless steel. Each material has its own advantages depending on the application.
Here is an overview of which material to use and when:

Brass Bellows

Brass is a traditional and popular bellows material with a low manufacturing cost. It soldered easily, even with today's lead-free solders. Suitable for operating temperatures up to 300ºF (150ºC), brass bellows are typically used in applications involving an air medium. Examples include mechanical altimeters and thermostatic assemblies.

Bronze Bellows

Bronze offers slightly higher tensile strength, better corrosion resistance, and better electrical conductivity than brass, while also maintaining a low manufacturing cost. Bronze can be soldered as easily as brass but can also be brazed for use in high-temperature applications. Bronze bellows are often used in high-volume applications, such as appliances and HVAC equipment.

Beryllium Copper Bellows

Beryllium copper (BeCu) has superior tensile strength and electrical conductivity compared to other bellows materials. It can be soldered or brazed. Age-hardened beryllium copper is often used in bellows assemblies, which can then be heat-treated for maximum dimensional stability. Beryllium copper bellows are favored in applications requiring a small package size and demanding lifecycle requirements, such as aerospace systems and instrumentation.

Monel Bellows

Monel, a nickel alloy, offers better corrosion resistance than brass, bronze, or beryllium copper. It is routinely welded but can also be brazed. With mechanical performance similar to bronze, Monel is often used in corrosive environments, such as those containing steam or saltwater.

Stainless Steel Bellows

Bellows can be manufactured from various stainless steels, with 300 Series steels being the most common. Stainless steel has excellent tensile strength, making it easier to maximize stroke and minimize package size. It also has excellent corrosion resistance in multiple environments and media. Typically brazed or welded, stainless steel bellows can withstand high operating temperatures. Applications include electrical interrupters, power transmission systems, and industrial controls.

Nickel Bellows

Nickel is a hard material with excellent corrosion resistance. Electrodeposited bellows are made from nickel, but the material is also useful as a corrosion-resistant plating for other types of bellows. Nickel is widely used in aerospace applications.

Prevalent Uses for Metal Bellows Expansion Joints

Metal bellows and expansion joints are commonly used to manage and balance thermal expansion, pressure surges, vibrations, misalignment, and other forms of displacement in pipelines and machinery. Some typical uses include:

Petrochemical Sector

Metal bellows and expansion joints compensate for movement and disturbances in pipelines, nuclear power plants, and other machinery.

HVAC Systems

In Heating, Ventilation, and Air Conditioning systems, these joints are valuable for accommodating thermal contraction and expansion caused by variations in pressure and temperature.

Aerospace Industry

Metal bellows expansion joints are employed in exhaust systems and fuel liners to provide resilience and flexibility under harsh conditions.

Power Generation

These joints are used to manage thermal expansion and vibrations in piping systems.

Medical Devices

Metal bellows expansion joints are crucial in medical equipment, including ventilators, anesthetic machines, and surgical robots, where precise motion control is required.

Semiconductor Industry

They are used to handle motion and vibrations in vacuum tubes and other machinery.

Benefits of Metal Bellows

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Constant elasticity

The tubes are corrugated by using forming tools at high pressure, and then are trimmed to the designed size. In the next stage, the spring-like metal cylinders are pressed together in accordance with the specified dimensions (length, outer and inner diameter). This creates the constant elasticity needed for the stable repeatability of the movement. In the final step, the bellows are degreased and passivated, i.e. given a protective coating against corrosion.

Seal-free functionality

In a pressure switch, the metal bellows is brazed or welded to the process connection. Its other end is in contact with the transmission mechanism. This means that the instrument has a fully seamless measuring system without the need for any rubber sealing materials, and the risk of leakage is eliminated. This feature makes pressure switches with metal bellows suitable for a wide range of applications. Process reliability is also guaranteed with liquid media.

Permanent accuracy

A metal bellows changes its length in line with the pressure. When a defined switch point is reached, it triggers the switching function via a transmission mechanism. If the pressure drops or rises again, the bellows returns to its original shape and the switch contact returns to its original position.

Metal Bellows Applications

Bellows, whether welded or convoluted (formed), are used in a wide range of industrial applications. Below are a few examples:

Load Cells

A load cell deforms when a certain load, in the form of pressure or strain, is imposed on it. This deformation is detected by a strain gauge through which a low-voltage direct current flows. The change in voltage is detected and displayed on a control panel. A bellows is mounted over the gauge to protect it from external influences.

Vacuum Interrupters

To switch very high voltages in transformer stations and avoid sparks, it is essential to prevent the surrounding atmosphere from exploding. This is achieved by removing oxygen from the area where sparks occur. Bellows are used to seal these confined volumes, and the inside of the bellows is either evacuated or filled with an inert gas.

Mechanical Seals

These seals are primarily used to close the inside of a pump from the outside world to prevent leakage. A mechanical seal is mounted on the pump shaft. As the pump shaft turns, a sealing element consisting of a stationary and a rotating ring is used. To ensure sufficient pressure on the two rings, one is fitted with a spring. This spring can also be in the form of a diaphragm (welded) bellows.

Pressure Gauges

When measuring the pressure of aggressive fluids or gases, the gauge needs to be isolated from the flow. For critical applications, a diaphragm seal is used instead of a Bourdon tube in the gauge. This provides greater security that the aggressive media will not leak. The diaphragm is a self-contained sensor that transmits displacement to the measuring device.

Sensors

In this application, a diaphragm or convoluted bellows is completely sealed and filled with a specific gas. Two electrical poles penetrate the inside of the bellows. By varying the current through these poles, the temperature inside the bellows can be regulated. The expansion or contraction of the bellows is used as an actuator to control a specific movement.

Valve Sealing

A bellows is used between the housing and the rising stem to seal the inside completely from the outside world.

Couplings for Stepper Motors and Servomotors

The flexible part, which compensates for misalignment, is made of a bellows. It ensures there is no angular positioning difference between the two coupling halves, which is essential for extremely precise positioning accuracy.

Exhaust Bellows

Running engines cause vibrations. To compensate for these movements and temperature differences that result in thermal expansion, bellows are used to connect the exhaust gas pipes to the funnel.

Piping Expansion Joints

In this application, bellows are formed in series to absorb thermal movement and vibration in piping systems that transport high-temperature media, such as exhaust gases or steam.

How Metal Bellows Are Made

Bellows Design
Metal bellows and bellows expansion joints are used in piping systems to absorb thermal growth. Each time the piping system heats up and cools down, the bellows go through one complete cycle. The manufacturing of a metal bellows begins with the design process. The desired life cycle of the bellows or expansion joint must be determined to specify the bellows' thickness and the number of plies required. A multi-ply bellows is generally a better choice for a longer cycle life. In applications with minimal movement or infrequent heating and cooling cycles, a single-ply metallic bellows may be satisfactory. A multi-ply metal bellows is preferable when vibration is present in the piping system, such as in marine exhaust, generator exhaust, blower exhaust, and pumps. To ensure you are purchasing bellows expansion joints from a reputable manufacturer, always request a design data sheet that follows the Expansion Joint Manufacturers Association (EJMA) 10th edition guidelines.

Making the Bellows Tube
The first step in manufacturing a metal bellows is making the tube or tubes that are to be corrugated. The coil or sheet stock is precision sheared to the predetermined size. For multi-ply bellows, each outer tube will be slightly larger than the inner tube. Typically, multi-ply bellows consist of 2 or 3 tubes (plies), but depending on the application, there may be 5 tubes (plies) or more.
Once sheared, the sheets are rolled into tubes. Care must be taken to ensure the tubes are perfectly round so they will fit over the bellows forming dies. For multi-ply bellows, each tube is carefully marked so they will fit together after welding.
After rolling, each tube is seam-welded using an automatic longitudinal seam welder. This is a critical step in the manufacturing of the bellows, as a poor seam weld may result in splitting during the forming process. After welding, the tubes are inspected, cleaned, and telescoped together.

Forming the Bellows
The process of corrugating the tube can be done in several ways, with the most common being mechanical die forming and hydroforming. Triad Bellows incorporates a mechanical forming process that is much less expensive than hydroforming and achieves similar results in terms of metal thinning. The tube is placed over the die on the bellows forming machine. The machine is programmed to form the bellows corrugation to the exact engineered diameter. The tube is rotated as the dies expand and contract, creating an even expansion without concentrated thinning. Once the preprogrammed number of convolutions is formed, this stage of the process is complete.

Roll Seam Welding
The roll seam welding process involves a continuous series of overlapping spot welds used on multi-ply bellows to join the tubes together at the bellows neck. Depending on the material thickness and the experience of the welder, this may or may not be necessary. A reinforcing band can be incorporated into the roll seam weld process as well. For example, if a welder needs to weld a 0.016" thick single-ply bellows to a pipe, this can be challenging. However, if a 0.062" thick reinforcing band is roll seam welded to the 0.016" thick bellows neck, the welder is effectively welding a 0.078" tube to the pipe, which simplifies the weld joint and reduces the chance of leaks.

Trimming the Bellows Necks
A special rotary shear is used to trim the bellows necks to the exact length. A circumferential line is scribed on the tangent of the bellows. The technician carefully guides the bellows into the rotary shear blades, following the scribed line.

Expanding the Bellows Necks to Size
Using special bellows-expanding machines, the bellows necks can be sized to fit over a pipe or inside a flange. Welders who fabricate metal expansion joints will attest that this is a crucial step for achieving the best welds. A tight fit allows the welder to produce a consistent weld size with minimal potential for leaks. A sloppy fit with gaps increases the likelihood of leaks significantly.

How to Select the Ideal Metal Expansion Joints for Your Specific Use

Choosing the right metal bellows and expansion joints is essential for ensuring reliability and durability in various industrial applications. Consider the following factors when selecting the material:
Temperature: The joint must be capable of withstanding the temperature range of the application. It is important to select high-temperature expansion joints that can endure the expected temperature range, as different materials have varying temperature limits.
Corrosion Resistance: In industries with corrosive substances, it is crucial to select a material resistant to corrosion. Materials like Inconel, Monel, Incoloy, and stainless steel are commonly used due to their corrosion resistance.

Metal Expansion Joints

Pressure Tolerance

The metal bellows expansion joint must be strong enough to withstand the working pressure. It is essential to choose a material that can handle the anticipated pressure variations, as different materials have different pressure ratings.

Flexibility

The material should be flexible enough to accommodate motion and vibrations. Metals such as Inconel, Incoloy, Monel, and titanium are known for their adaptability.

Cost

Cost is an important consideration. The selected material must be economically feasible for the application, as some materials are more expensive than others.

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FAQ

Q: What is a Bellows?

A: A metal bellows is a flexible, lightweight, spring-like component with ribbed or corrugated tubing, giving it an accordion-like quality. Metal bellows are precision components designed to perform various functions in machine components and assemblies.

Q: What is a Bellows' Function?

A: Metal bellows convert pressure, mechanical, vacuum, and temperature changes into linear or rotational motion. They can also be used in flexible electronic contact applications. Bellows provide a specifically defined dynamic response as part of a larger machine component or assembly, often offering a more precise, reliable, or cost-effective alternative to existing engineering solutions.

Q: How Are Bellows Made?

A: Our metal bellows are produced using one of three methods: electrodepositing, hydroforming, or edge-welding technology. More information on each of these processes, including a comparison and recommendations for different applications, can be found in our Metal Bellows Comparison Guide.

Q: What Materials Are Recommended for Metal Bellows?

A: Nickel, Copper, Silver, and Gold are recommended materials. These metals are generally suitable for forming through electroplating.

Q: How Long Do Bellows Last?

A: Bellows can be designed to have an infinite cycle life. Our design engineers will create custom metal bellows to meet your application specifications and work with you to optimize performance.

Q: What is a Bellows' Maximum Pressure Allowance?

A: This depends on the process used. Electroformed bellows can accommodate pressures up to 3000 PSI. Other methods, such as hydroforming and edge-welding, will have different results. The specific application requirements will help us calculate the actual maximum pressure. More information on each process, including maximum pressure, can be found in our Metal Bellows Comparison Guide.

Q: What Are Recommended Joining Methods for Bellows Assemblies?

A: In addition to soldering, our bellows can be welded or joined using various adhesives, including epoxies.

Q: What End Types Are Required for Welding Bellows to a Hub?

A: The preferred method for welding bellows is with an electron beam welder. Since this method does not use filler material, it is important to ensure there is no gap between the bellows and the hub at the weld joint. Ends such as Type A, D, or I may result in poor, incomplete, or porous welds due to difficulties in eliminating gaps and limited material for fusion. An end such as Type E provides a good weld joint because the flange can be pressed against the hub, eliminating any gap, and the beam can be focused perpendicular to the axis of the bellows, ensuring adequate material from both the bellows and the hub can be fused together.

Q: What Can Be Done to Protect Metal Bellows from Corrosion or Make Them Biocompatible?

A: Depending on the application and the media the bellows or electroform will be exposed to, they can be plated with a thin layer of gold or coated with parylene. Gold offers protection against corrosion in all climates and environments and is also an excellent conductor of electricity. Parylene is a polymer coating that provides moisture, chemical, and dielectric barrier properties, as well as dry-film lubricity. Both gold and parylene are biocompatible.

Q: What is Reverse Bending?

A: Reverse bending occurs when the bellows are operated in compression and moved through the free length into extension, or vice versa. Reverse bending reduces the cycle life of the bellows.

Q: What Metals Can Be Used to Make Edge-Welded Bellows?

A: Any metal that can be easily formed, blanked, and welded. Common materials used are:
Stainless Steels: 304L, 316L, 321, 347, AM350
Nickel Alloys: Inconel 600, Inconel 625, Inconel 718, Inconel X750, Hastelloy C276, Haynes 242
Titanium: Grade 5

Q: How Many Cycles Can an Edge-Welded Bellows Endure?

A: This depends on the customer's design criteria. Factors such as axial stroke, lateral and angular offsets, pressures, media, temperature, cycle rate, and environment affect cycle life, so it is important to consider all these variables before predicting cycle life.
A bellows made from 304L, 316L, 321, 347, or titanium, with vacuum inside, is typically designed for a cycle life of less than 1,000,000.
A bellows made from AM350, Inconel 600, Inconel 625, Inconel 718, Inconel X750, Hastelloy C276, or Haynes 242, with vacuum inside, is typically designed for more than 1,000,000 cycles.

Q: How Can I Reduce the Bellows Force for a Given Deflection?

A: Here are three ways to reduce force:
Reduce Material Thickness: The easiest method, though it may decrease the bellows' cycle life depending on performance requirements.
Increase Length: Adding convolutions makes the bellows longer, which lowers the force if the axial stroke length remains unchanged.
Change Material: Stainless steel bellows have similar force, but Titanium bellows will have about half the force of AM350 or 316L stainless steel bellows.

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