Types and Introduction of Bellows

Release time:

2022-07-11

Bellows made of plastics and other materials play an irreplaceable role in fields such as medium conveyance, electrical wiring, machine tools, and home appliances. So, let’s take a closer look at the different types and introductions of bellows!

Types and Introduction of Bellows

　　Bellows: A load cell used in pressure testers. In the case of cylindrical thin bellows with numerous lateral corrugations, the bellows exhibit elasticity and can undergo displacement under the influence of pressure, axial forces, lateral forces, or bending moments. Plastic bellows are categorized into two types: one is completely airtight and watertight—used for applications such as propellant delivery in launch vehicles, gas transmission, and water heaters; the other is made by continuously wrapping strips of adhesive tape. Typically wound bellows are employed to protect cables in telephones, machine tools, and other equipment. During the manufacturing process of table-lamp bellows, steel wires are embedded within them.

　　Plastic corrugated pipes have high compressive strength and can effectively prevent corrosion caused by stray currents. Metals are excellent conductors of electricity; in contrast, plastic corrugated pipes exhibit remarkable resilience—they can withstand being stepped on without damage and are not easily crushed by vibration. They also offer superior sealing and leak-proof performance compared to metal corrugated pipes, making them ideal for vacuum pump casting applications. Moreover, plastic corrugated pipes are non-conductive, which further enhances the fatigue resistance of prefabricated components. The raw material used to manufacture plastic corrugated pipes is HDPE.

　　Plastic corrugated pipes offer superior corrosion resistance compared to metal materials. They are immune to corrosion by acids and alkalis, do not corrode themselves, and can effectively protect prestressing tendons from corrosion. When the waterproofing of post-tensioned precast components fails, leading to seepage through microcracks or blockage/ineffectiveness of drainage systems, many prestressed structures are exposed to severe external hazards such as molten salts and saline solutions. These pipes can prevent harmful substances from leaking through and contaminating the environment, thereby safeguarding the prestressing tendons from potential corrosive damage. Plastic corrugated pipes provide a natural barrier that offers far better protective performance for prestressing tendons than traditional corrugated pipes, ensuring that post-tensioned prestressed structures maintain excellent service performance.

　　In the structural design of plastic corrugated pipes, after the outer diameter and inner diameter have been determined, the wall thickness, wave pitch, and wave thickness—among other parameters—are established in proportion to either the inner or outer diameter. Typically, the inner or outer diameter of the corrugated pipe serves as the basic dimension, while other structural parameters are expressed as relative dimensions. The specific structural parameters include the wave-depth coefficient, wave profile, corrugated pipe thickness, wave pitch, and wave thickness.

　　1) The wave-depth coefficient k—also known as the bulging coefficient—is the ratio of the outer diameter to the inner diameter of a bellows (i.e., K = D/d). It is an important parameter that determines the geometric shape of the bellows. When determining the inner diameter d, the larger the value of k, the higher the wave height will be. The value of k affects both the performance of the bellows and the forming process used to manufacture it; as k increases, the difficulty of forming the bellows also rises.

　　2) The wall thickness of the bellows is also an important geometric parameter. The key characteristics of a bellows—such as its stiffness and working hoop stress or hoop rigidity—depend on its geometric dimensions, especially its wall thickness.

　　3) The wave profile refers to the pattern and shape of the wave when it is cut along the axial direction. Based on their geometric shapes, bellows can be classified into spiral, U-shaped, C-shaped, S-shaped, V-shaped, and other types. The wave profile of a bellows significantly influences its stiffness, displacement capacity, and compressive strength.

　　4) The wave pitch and wave thickness are important parameters of bellows and corrugated structures. The ratio of wave pitch to inner diameter decreases as the inner diameter increases. These parameters significantly influence the effective length and performance of the bellows. The trend of wave pitch variation shows that its magnitude increases with the increase in inner diameter. Both the wave pitch and wave thickness determine the radii of curvature at the corners, peaks, and valleys of the corrugations. The wave thickness refers to the axial width of the outer corrugated portion, which is relatively larger. Once the wave pitch is determined, the wave thickness directly affects the radii of curvature at the corners, peaks, and valleys of the corrugations.

　　Classified by forming process

　　Hydraulic forming is a commonly used method for manufacturing bellows. In this process, the tube blank is compressed to the desired length. This method is often employed for bellows with small diameters. By utilizing the hydraulic pressure of a liquid contained within the tube blank, the blank expands inside a restraining ring until it reaches the yield point along the ring’s circumference. The roll-forming technique for bellows is primarily used for manufacturing large-diameter bellows. In this method, shaping is achieved by rolling a forming wheel positioned on the tube blank. Single-wave roll-forming is commonly used, and some equipment can even produce multiple bellows in a single pass.

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