Atherm Metal Bellows Assemblies- FAQ’s

Athermalizaton Bellows Assemblies






Q: What are athermalization 
bellows assemblies?
A: Athermalization (atherm) bellows assemblies are filled and sealed flexible bellows assemblies that translate changes in temperature into precise linear mechanical motion.

Atherm bellows assemblies take advantage of the flexible, expandable nature of precision electroformed metal bellows, and the steady volumetric expansion of incompressible fluids with changes in temperature.  These bellows assemblies have known temperature-dependent rates of length change depending upon the fluid used in the assembly.

Each end of the bellows connects to system components using stock fittings or custom fittings designed to meet customer requirements.  These fittings allow the dimensional change of the bellows to translate into mechanical axial motion that effects the desired change in the system.

Q: What is athermalization?
A: The term athermalization is most commonly heard in optics applications. Optical engineers use atherm assemblies to give their systems optothermal stability, meaning that the systems’ optical properties are immune to changes in temperature. Athermalization applies to nonoptical applications, too. More generally, then, an atherm is a device that uses components which undergo linear, temperature-dependent changes in length in order to affect or counteract temperature dependent changes in the system.

In athermalization systems, the rate of length change is determined by the assembly’s makeup. It is precise enough that engineers can use it in a variety of ways from adjusting optical focus to triggering electro-mechanical systems at specified temperatures.

Q: Where is athermalization helpful?
A: Atherms are useful in a variety of applications. They are commonly used in optical applications because of the precision required to maintain the properties of sensitive optics over the range of operating temperatures.

Other applications put temperature dependent length change to mechanical use, similar to the operation of a thermostat’s bimetallic strip. And atherm assemblies can shield delicate systems from the effects of temperature change.

Q: Where can atherm bellows assemblies be used?
A: Common athermalization applications like adjustments to the focal lengths of optical lenses or mirrors to prevent temperature-related drift are ideal for atherm bellows assemblies. In infrared applications they control the flow of liquid nitrogen or other fluids used to cool the black body that is used for IR reference.

Atherm bellows assemblies also work well in precision electromechanical applications that are sensitive to temperature. They can be used like thermostats to trigger heat or cooling system valves or other components when temperature rises or falls. In precise metering applications, they can adjust orifice openings so that the mass flow of gas or liquid is normalized independent of temperature.

These assemblies provide temperature-dependent mechanical actuation without the need for programming or electricity. Their
simple, robust design lets them operate through hundreds of millions of repeatable cycles without drawing power or requiring recalibration.

Thus, engineers use atherm bellows assemblies as back-ups to electrical controls that can continue to operate in the event of power loss. These assemblies can also entirely displace electrical systems with improved reliability and energy efficiency.

Q: How does the bellows assembly translate temperature change into mechanical motion?
A: The bellows assembly is filled with an incompressible fluid. The volume of the fluid sealed within the bellows assembly will expand or contract in response to changes in temperature.

The bellows in the assembly has a constant effective area. This constant effective area combined with the volumetric thermal characteristic of the fluid causes linear, axial movement of the bellows assembly in response to the temperature change.

For example, a bellows might couple to a rod that controls an orifice, a valve stem, or a lens. In each case, a different connector is needed. In fact, the majority of bellows assembly connectors are custom designed based on the needs of the application.

Q: How do the atherm bellows assemblies operate with respect temperature?
A: Atherm bellows assemblies can be designed to operate at temperatures ranging from -130° to 300°F. Each assembly is designed for a specific temperature range which corresponds to range of movement, expressed in length per degree of temperature (i.e. in/°F). The specific temperature range and deflection rate depends on the fluid selected.

For example, a stock atherm bellows assembly from Servometer is designed for a temperature range of 10° to 300° F. Within that range, it provides movements of 0.0002 in/°F. This makes it ideal for use in applications requiring reliable precision motion or adjustment in response to the temperature.

Q: How big is an atherm bellows assembly?
A: Precision electroformed bellows come in a wide variety of sizes. Their diameters range from 0.250 to 9 in, and they can be as long as 9 in. Atherm assemblies can be designed using any diameter within this range.

Suppliers such as Servometer stock components in the most common sizes, diameters ranging from 0.313 to 1.625 in. These sizes are readily available for prototype designs and proof-of-concept testing. Custom components can be designed for applications with needs outside the above range.

Q: How can I get the right atherm bellows assembly for my application?
A: The design of an atherm bellows assembly is typically application specific. Consequently, the most effective design will result from a collaboration between the application specialist and an engineer experienced in bellows design and athermalization. Engineers at suppliers such as Servometer can work with you to recommend the right assembly given your application’s temperature range and the movement required.