Edited by Paul J. Heney, Editorial Director
Compensating for expanding fluid protects equipment and prevents leaks
In many fluid power and fluid-handling applications, the gas or liquid inside equipment and lines can expand beyond normal design limits. This can be due to machine operations or reactions to changes in the environment—including temperature and pressure.
Expanding fluid can lead to trouble. It typically increases internal pressure, which can result in blown seals, broken sensors and instrumentation, leaks and premature wear. Usually, the cost of downtime, repairs and environmental clean-up exceeds that of the replacement parts themselves.
To compensate for fluid expansion, accumulators with rubber bladders are an option. But they’re often not suitable, due to space or weight constraints, temperature extremes, or corrosive operating environments. Engineers can consider edge-welded bellows assemblies as an alternative. They maintain proper flow during normal operation and expand and contract to compensate for volumetric changes of the liquid or gas.
An edge-welded bellows is, in effect, a precision bubble. It consists of a series of profiled metal diaphragms that completely collapse under external pressure or internal vacuum, maximizing the movement of the assembly. Thus, it can compensate for significant volume changes in a relatively small space. Other types of metal bellows have shorter strokes for a given size, thus offering less volume compensation. For instance, hydraulically formed (hydroformed) bellows typically travel about 20% of their free length, compared to 90% for an edge-welded metal bellows. Thus, edge-welded units can provide the same volume compensation with smaller and lighter assemblies.
Edge-welded bellows are made by welding stamped, metal diaphragms into a long, flexible assembly. Strips of sheet metal are stamped into the shape of the diaphragm (typically round, or oval) with the pressure, stroke length, spring rate, and temperature helping to determine the thickness and material required to meet application demands.
The shape of the inside and outside edges and ripples in the diaphragm are crucial to performance. The ripples must be consistent to ensure accurate spring rates and long cycle life. For nested ripple-edge welded bellows (a version commonly used for volume compensation), diaphragms are positioned back-to-back (male to female) to pair the inside-diameter holes. Once in contact, they are welded together. Deep weld penetration is key to a long-lasting, leak-tight joint. Depending on the manufacturer and material, bellows can be welded using plasma, laser, arc, or electron-beam welding processes. Machine-vision systems are increasingly used to improve weld accuracy and consistency.
To size edge-welded bellows for applications, first define the operating conditions, the operating pressures, temperatures, and maximum volume the bellows must compensate for. Size can be determined by knowing the amount of volume to be compensated for, and understanding the size envelope that the bellows compensator needs to fit into. The mean effective area for the bellows can be analyzed to determine the height, diameter, stroke length and length requirements.
Because edge-welded bellows respond to internal and external pressure, the specific application should determine if the bellows will extend or retract. For example, a high-pressure application would lend itself to a design with external pressure on bellows inside a thick metal housing. This lets the housing contain the pressure while the bellows compress and nest. In lower-pressure applications, the bellows could see internal pressure and expand as needed.
Pressure can be a limiting factor, although a two-ply bellows (two diaphragms stacked and welded together), or liquid or gas-filled assemblies are possible solutions. For example, if the outside of the bellows is exposed to hydraulic fluid at 5000 psi, the inside of the bellows can be pressurized with nitrogen to reduce the differential pressure and stress on the bellows.
The rate of expansion can also be important. Violent pressure spikes or extremely fast volume changes can alter design requirements, compared to applications with slow cycles and a volume that changes with ambient temperature or altitude.
The bellows and housings can be constructed of the same material if they will be exposed to the same media. Typically, machined housings offer the most design flexibility and ensure the proper wall thickness to handle pressure spikes and extremes. Building guides into the ends of the bellows prevents side-to-side movement and increases cycle life. Guide materials should be compatible with the media or specially coated.
Custom fittings can also be integrated into the assembly. Threaded ports can be added to one or both sides of the bellows. The application, along with industry norms, determines the thread type.
SAE and UNF O-ring and coned metal-to-metal seals are commonly used in automotive and aerospace applications. Here, engineers should research O-ring material compatibility and temperature capabilities. Metal-to-metal seals are typically one-time use, but sealing cones in various materials might also be an option.
If the bellows must compensate for barometric pressure, they can be manufactured with a vent hole. They can also be filled with liquid for cooling or, as mentioned previously, to alter the differential pressure and increase pressure capabilities. While additional ports on each side are an option, a central tube can be as effective, lower costs, and increase reliability by reducing welding and potential leak paths.
To monitor conditions, sensors such as potentiometers and LVDTs can be included in the design. The data can be used to adjust valves or other devices further downstream. Bellows can also act as a sensing element, where a volumetric change alters the length or displaces a liquid inside the bellows.
Due to the bellows all-metal construction, engineers can select materials that ensure media compatibility without temperature limitations. Titanium is the metal of choice among aerospace engineers where every ounce is critical to the overall design. While low-end temperature ranges are limited, titanium bellows can be used inside aircraft that have temperature controlled areas. It is ideal for applications requiring high strength, lightweight construction, and media compatibility. Other choices include 316L stainless steel, AM350, Inconel, and Hastelloy.