The Adaptive Precision of Foil Air Bearings

Andrew Emerson

Princeton, New Jersey

08540, USA

In systems where thermal stability, dynamic load capacity, and maintenance-free operation are critical, foil air bearings offer a technically advanced alternative to conventional rolling element and oil-lubricated bearings. OAV Air Bearings creates precision foil bearing systems that operate without any kind of mechanical contact or external lubrication, leveraging compliant foil structures and hydrodynamic air films to support loads with high stiffness and damping.

Operating Principles and How They Work 

OAV’s foil bearing systems are particularly well suited for cleanroom environments, vacuum chambers, aerospace components, and other applications where conventional lubrication would introduce unnecessary risks. The bearing structure is broken down into 2 parts:

- Top Foil: This is a flexible layer that distributes pressure evenly across the bearing and conforms to movement of the attached shaft.

- Sub-Foil: This is a substructure that provides mechanical support and passive damping. It acts as a spring-damper system, absorbing transient loads and thermal expansions.

This multi-layer design allows the bearing to absorb shock loads, compensate for misalignment, and operate under wide thermal gradients. The result is a platform that maintains its performance even in aggressive or variable operating conditions.

This configuration is illustrated in the diagram above, which shows a cross-sectional view of a compliant foil bearing. As shown:

• The yellow region represents the rotor, which begins at rest in contact with the top foil. Upon rotation, pressure builds in the gap between the shaft and the top foil (the black line), lifting the rotor and initiating non-contact operation.

• The orange structure is the sub-foil (also referred to as bump foil), which acts like an evenly distributed array of springs. This configuration creates compliant support and localized damping  - essential for accommodating shaft misalignment and dynamic loads.

• The blue outer ring is the fixed housing of the bearing, which constrains the foil layers and provides the necessary mechanical boundaries

• The radial load vector W illustrates the direction of applied force, and the shaft eccentricity ε shows how the rotor displaces under load. The angular variables θ and ϕ describe the position of the load and the shaft within the bearing coordinate system.

Once the bearing reaches sufficient speed, the self-generated air film maintains separation between the shaft and the top foil. The bump foil deforms elastically to adapt to pressure gradients and rotor motion, ensuring consistent stiffness and damping across a wide range of operating conditions.

Design Tradeoffs: Clearance vs Load Capacity

The load-carrying capacity of foil bearings is strongly influenced by both rotational speed and radial clearance. The graph below, based on performance data for the FB-J40-L35, OAV’s 40 mm Foil Journal Air Bearing, at an eccentricity ratio of 0.8, illustrates this relationship:

• Bearings with tighter radial clearance (10–15 µm) provide higher load capacity across the speed range due to increased hydrodynamic pressure in the air film.

• Bearings with larger clearances (20–25 µm) are more accommodating to thermal expansion and shaft misalignment, but show reduced load capacity at equivalent speeds.

• At 250,000 rpm, the load capacity ranges from  approximately 1387 N (10 µm) to 1310 N (25 µm), confirming strong performance across clearance variations.

• This data enables designers to optimize clearance based on operating temperature, alignment precision, and required load margins.

Understanding these trends is necessary for electing the right bearing configuration in applications with varying speed and load requirements, such as turbomachinery. 

Tailored for Demanding Environments

Air foil bearings are already being deployed in a wide range of high-performance machines, from air cycle machines in aircraft to blowers, expanders, and high-speed motors in industrial systems. Their oil-free operation is a significant advantage in vacuum applications, clean manufacturing, and environments where conventional lubrication would degrade or outgas. OAV designs its foil bearings with materials that withstand wide thermal fluctuations and high-speed operation, ensuring they meet both performance and environmental standards in critical systems.

Because they generate their own lift without external pressurization, foil bearings simplify system design while increasing reliability. There are fewer components to fail, and the absence of lubrication systems reduces both weight and complexity. For engineers tasked with building compact, high-efficiency machines that operate in demanding environments, foil bearings offer a future-proof solution.

Proven Performance in Turbomachinery

Air foil bearings have become a critical component in high-speed turbomachinery, including microturbines, turboexpanders, and air cycle machines.. These systems benefit from the bearing’s ability to operate at extreme rotational speeds without contact-based friction or oil-lubrication infrastructure.

The self-pressurizing nature of OAV’s foil bearings allows for integration into sealed environments without need for an external air supply or reservoirs. This simplifies system architecture and increases overall reliability -  addressing another pain point in the engineering of these systems.

In applications such as power generation and energy recovery, foil bearings help minimize losses and enable higher operating temperatures. The graph below shows the correlation between speed, radial clearance, and power loss in  OAV’s 40 mm Foil Journal Air Bearing, a typical component in turbomachinery systems:

As shown:

• Power loss rises with rotational speed due to increased shearing in the air film.

• Bearings with larger clearances (20–25 µm) exhibit lower power losses, making them more suitable for thermal efficiency and long-duration operation.

• Conversely, tighter clearance bearings offer higher stiffness and load capacity (as seen in the Load vs Speed graph), but at the cost of increased power loss and heat generation.

  
  

This performance data helps engineers balance power efficiency with load capacity, depending on the requirements of their machinery design. Foil bearings also support rapid spool-up/shutdown and require no maintenance over thousands of operating hours - making them ideal for aerospace compressors, waste heat recovery systems, and other uptime-critical platforms.

A Core Component of Clean, Contactless Rotation

Air foil bearings represent a mature and continually evolving technology that provides smooth, wear-free motion in systems where traditional bearings fall short. OAV Air Bearings leverages decades of expertise in non-contact bearing technology to deliver foil bearings that perform with consistency, precision, and long-term stability. As industries continue to push for higher speeds, lower maintenance, and oil-free operation, air foil bearings are proving to be a foundational component - quietly enabling motion where it matters most.