Precision bellows have extremely low spring rates, making them a better option than traditional coil springs in many mechanical and electrical devices
Metal bellows are used for hermetic seals, volume compensators, pressure and temperature sensors, and flexible connectors in countless applications where dependability and long life are critical requirements. Electrodeposited metal bellows, one version of these products, are a viable alternative to metal springs in a variety of mechanical and electrical devices, improving performance and durability while lowering costs. Here’s a look at how they’re made and used.
Electrodeposited bellows are manufactured by first machining an aluminum mandrel to match the internal geometry of the desired bellows. The mandrel is then plated with layers of metal (typically nickel) to the required thickness. The tightly controlled deposition process produces walls as thin as 0.0003 in. The ends of the plated mandrel are scored through the plating to expose the aluminum and define the end trim dimensions. Next, caustic chemicals dissolve the aluminum mandrel, leaving behind the thin metal shell. Finally, finishing operations such as coating and testing complete the manufacturing process.
This proprietary electrodeposition process turns out helium-leaktight bellows that are highly sensitive, responsive, and reliable. A bellow’s responsiveness and spring rate are a function of wall thickness. Thus, electrodeposited metal bellows can be produced with low spring rates and, therefore, are capable of significant deflection with a minimal applied force. Couple this with excellent sealing capability yields a bellows that is both sensitive and responsive to changes in pressure.
By comparison, springs drawn from standard thickgauge wire require more force to generate the same motion and, by their nature, are not helium leaktight. Rubber and polyurethane bellows are usually quite flexible, but they typically have limited cycle life and sealing capability compared to metal bellows.
Mechanically formed (or hydroformed) metal bellows are limited in their minimum wall thickness; they cannot match the thin walls of deposited bellows. As a result, they do not have the same flexibility and responsiveness.
Electrodeposited-nickel bellows typically provide just one-fifth to one-tenth the spring rate of hydroformed brass bellows of the same size. The force required to compress them is especially low and stays consistent among bellows of the same size and type.
Bellows as spring replacements
In electrical devices, springs are commonly used as electrical interfaces for temporary connections. Metal bellows are a reliable alternative to springs in such applications.
To increase conductivity, a layer of gold is plated over the bellows to ASTM B488. The actual shape of the bellows is a key factor in the design. Because the signal travels along the walls of the bellows, rather than the circuitous path of a spring, bellows have lower dc resistance, along with minimal self-inductance and insertion loss.
This makes bellows well suited for testing electrical circuits. The small size and many available shapes let bellows, acting as electrical contacts, easily pair with through-holes and pads. Soldering the bellows to a probe makes for easier handling during manual testing.
For example, a semiconductor-chip maker used a bellows electrical contact to test miniature components as they traveled down an automated production line. The bellows assembly was less than 1 mm in diameter and 1 mm long, with a conical front end to match the shape of the components being tested. The electrical contact was soldered to a probe and the flexible bellows contact compensated for offsets and vibration.
With a wide operating temperature range, the bellows withstood heating and cooling cycles inside the test chamber, as well as ultrahigh vacuum. Despite aggressive liquids and gases used within the chamber, the bellows electrical contact resisted attack. Its seamless design ensured media would not build up and contaminate the equipment, which can be a problem in spring and pin configurations.
Two for one
In certain valves, metal bellows act as both seals to contain pressure and springs to dampen motion. The bellows convolutions ensure proper flow of media within the valve, whereas a coil spring could trap particles and lead to clogging.
In other cases, with the bellows sealed to atmosphere or filled with liquid, it responds to changes in pressure and temperature. In this case, the bellows assembly itself becomes an actuator.
How long a valve can operate often determines its success or failure. Electrodeposited metal bellows can be designed for “infinite” fatigue life, letting engineers maximize life expectancy while meeting an application’s requirement for spring rate, flexibility, and size.
In one liquid-dispensing application, a bellows 0.5-in. long and 0.049in. in diameter with extremely thin walls was used. This produced a spring rate low enough that the actuation of the valve compressed the bellows, yet high enough that the bellows could overcome nozzle pressure when dispensing. In addition, the bellows provided sufficient spring force to securely shut the nozzle tip when the valve’s solenoid was not actuated.
The dispensed liquid also had to be heated to lower its viscosity. The elevated temperatures did not affect the metal bellows. The bellows was attached to the solenoid pin at one end and to the valve block at the other. Cycle tests at up to 350 Hz showed the metal bellows could operate the nozzle valve at high speeds and repeatedly dispense precise volumes of liquid. Overall, the bellows reduced the number of components and size of the assembly, while improving valve reliability.