The fundamental difference between air bearings and conventional bearings: a paradigm shift from contact-based support to non-contact support.

I. After a shaft is suspended

The spindle of a precision grinding machine rotates at 350,000 rpm, with radial runout no greater than 25 nanometers—roughly one three-thousandth the diameter of a human hair. Supporting this spindle is not a steel ball or an oil film, but rather an air bearing typically just 5 to 20 micrometers thick.

Understanding Air bearing and Plain bearing The distinction must be approached from the most fundamental physical level: Is there solid contact? This seemingly simple difference determines the fundamental distinctions between the two types of bearings in terms of precision, speed, service life, stiffness, and even the overall system architecture.

II. Working Principle: Two Radically Different Underlying Logics

The supporting principle of conventional bearings is “solid‑to‑solid load transmission.” Whether it is a ball bearing, a roller bearing, or a fluid‑film sliding bearing, the load path invariably follows this sequence: rotor → rolling elements or oil film → bearing housing. Although oil‑film lubrication eliminates direct metal‑to‑metal contact, the pressure generated by the oil film essentially arises from the hydrodynamic pressure effect of the liquid medium within a confined space.

Air bearings replace oil or rolling elements with a gas. Based on the mechanism by which the gas film generates pressure, air bearings are classified into three major types: Air static-pressure bearings, air dynamic-pressure bearings, and squeeze-film bearings Among them, hydrostatic bearings are the most widely used in industry.

Air-static-pressure bearing It relies on an external air supply. Compressed air enters the bearing clearance through a restrictor—such as a small orifice, a narrow slit, or a porous material—thereby establishing a hydrostatic air film between the journal and the bearing. The restrictor is the “heart” of this system: when the load varies, it adjusts the gas flow rate to maintain a stable air-film thickness, ensuring reliable operation across the entire speed range, from zero speed to high speed.

Air dynamic pressure bearing In such bearings, the gas film is entirely “pumped” by the rotor’s own rotation. A wedge-shaped clearance forms between the rotor journal and the bearing liner; due to viscous effects, the gas is entrained into the converging region, where it is compressed, generating hydrodynamic load‑carrying capacity. Herringbone‑grooved, stepped, tilting‑pad, and foil bearings all belong to this family. Hydrodynamic bearings do not require an external gas supply, but during startup and shutdown they lack gas‑film support and thus necessitate auxiliary supports or wear‑resistant coatings.

This classification is important because “Air bearings” are not a single type of bearing, but rather a class of bearings. The performance characteristics, applicable scenarios, and technical challenges of static pressure and dynamic pressure differ significantly; these distinctions will be consistently maintained throughout the following discussion.

III. Performance Comparison: Systematic Differences Across Eight Dimensions

3.1 Accuracy and Resolution

In precision positioning applications, the air bearing’s most celebrated advantage is its “infinite resolution.” This is not mere marketing rhetoric but a direct consequence of the inherent limitations of contact‑type bearing designs.

In rolling-element bearings, static friction (stiction) between the steel balls and the raceways can cause overshoot and creep under light-load, short-stroke operating conditions, thereby limiting system resolution due to mechanical hysteresis. Meanwhile, slight variations in ball size and inconsistent velocities of the balls within the raceways give rise to multidirectional parasitic motions, which amplify the guideway’s inherent straightness errors. Porous air bearings, by contrast, eliminate contact entirely, eliminating static friction; their resolution is limited only by the feedback system and the stability of the air film.

In terms of rotational accuracy, the radial runout of gas bearings can be as low as 0.025 micrometers Moreover, in conjunction with the “smoothing effect” of the air film—namely, the ability of gas pressure distribution to mitigate local surface imperfections—the transmission of rotor geometric errors can be further suppressed.

3.2 Speed Ability

This is the dimension where the differences are most pronounced. Rolling-element bearings are limited by the contact stress between the steel balls and the raceways, as well as by centrifugal forces; the upper limit of linear velocity is typically around… 3–5 m/s Magnitude. Because air bearings are entirely non-contact, their rotational speeds can reach 350,000 rpm or higher In the field of hydrostatic air‑bearing electric spindles, domestic research groups have already achieved the commercial development of products capable of 350,000 rpm.

The physical root cause of the speed disparity lies in the fact that heat generation and wear in rolling-element bearings increase linearly, or even super‑linearly, with speed, whereas air bearings exhibit extremely low gas‑film shear losses, and the low viscosity and compressibility of gases result in much smaller temperature rises at high rotational speeds compared to oil‑lubricated systems.

3.3 Load-Carrying Capacity: The Structural Weakness of Air Bearings

It must be frankly acknowledged: The load-carrying capacity of air bearings is significantly lower than that of rolling bearings and fluid-lubricated bearings of the same size. The load-carrying capacity of a gas lubrication film is approximately one to two orders of magnitude lower than that of an oil‑film bearing. In aerostatic bearings, the supply pressure typically does not exceed 0.6 MPa; beyond about 600 kPa, the gas film becomes turbulent, and the gains in stiffness and load capacity decline rapidly.

In applications requiring high cutting forces or heavy loads, rolling bearings still offer irreplaceable advantages. The optimal operating range for air bearings lies at the intersection of “high speed, light load, and high precision.”

3.4 Stiffness and Damping: An Underappreciated Paradox

Stiffness is the most delicate parameter in air bearing design. Unlike rolling bearings, whose stiffness is primarily determined by material elasticity, the stiffness of an air bearing depends on… Rate of change of air‑film pressure distribution with respect to gap height , is a quantity in fluid mechanics.

There is a fundamental trade-off here: Increasing load-carrying capacity typically reduces stiffness, and vice versa. Gas supply pressure, the throttling method (whether using a small orifice or porous material), bearing area, and gas‑film clearance collectively determine the bearing’s position on the stiffness–load curve. Increasing the permeability of porous materials enhances gas flow and load-carrying capacity but reduces the pressure gradient, thereby lowering stiffness; conversely, decreasing permeability increases stiffness but may trigger aerodynamic hammer instability.

For high-precision metrology and testing platforms, a stiffness‑first strategy typically employs a lower permeance and a higher supply pressure (400–500 kPa); for heavy‑load metrology applications, permeance is increased while pressure is reduced to achieve greater load‑carrying capacity.

3.5 Stability: The Real Challenge at High Speeds

If load-carrying capacity is the “inherent weakness” of air bearings, then… High-speed stability is its “post‑production challenge.”  。

The lubricant in gas bearings is a compressible gas, which gives rise to dynamic stiffness and damping characteristics that differ fundamentally from those of oil-lubricated bearings. When the rotor speed reaches a sufficiently high level, the gas film loses its damping capability, leading to self-excited vibrations—namely, Air‑film whirl and whirl instability This instability phenomenon constitutes a fundamental barrier to the widespread application of gas bearings.

The mechanism of vortex motion and the gas bearing’s… Cross stiffness Closely related. When the rotor is eccentric, the gas film not only generates a restoring force but also a tangential force perpendicular to the displacement direction; this tangential force induces whirl motion of the rotor about its equilibrium position. When the whirl frequency approaches the system’s natural frequency, the amplitude increases sharply, leading to system instability.

Engineering mitigation strategies include: inserting an O‑ring seal between the bearing housing and the machine frame to achieve a flexible mounting, thereby leveraging the rubber’s damping to absorb whirl energy; adjusting the supply air pressure to alter the coupling between the system’s natural frequency and the whirl frequency; and employing an elastic foil‑type structure that provides additional damping through Coulomb friction between the foils. Studies have shown that, under appropriate supply‑air pressure and ramp‑up rates, low‑frequency vibrations can be effectively suppressed, and the stable operating speed of the shaft system can be significantly increased.

3.6 Lifespan and Maintenance

The service life of rolling bearings is limited by fatigue spalling, and maintenance involves grease replacement and wear monitoring. Air bearings, provided the air supply is clean, Theoretical lifespan is infinite. — Without contact, there is no wear, nor any mechanism of fatigue failure. The core of maintenance has shifted from “replacing bearings” to “ensuring a reliable air supply.”

However, this very requirement constitutes the hidden cost of air bearings: the air‑supply system must deliver clean, dry, oil‑free compressed air with a filtration accuracy of 0.25 µm, a dew point below 0°C, and a pressure maintained at 0.4–0.6 MPa. The capital and operating costs of the air‑supply system must be factored into the selection process.

IV. The Core Logic of Selection: It’s Not About “Better,” But About “More Suitable”

The relationship between air bearings and conventional bearings is not a linear ranking of technological sophistication, but rather… Division of Labor at the Application Boundary 。

The technical advantage range of air bearings They are concentrated in the intersection of “high speed × light load × ultra‑high precision.” High‑speed spindles for semiconductor wafer thinning and dicing equipment, precision and ultra‑precision machine tools, PCB drilling machines, ultra‑precision measurement instruments, and high‑precision aerospace machinery—all share a common characteristic: relatively low loads but nanometer‑level accuracy requirements, extremely high rotational speeds, and strict avoidance of oil contamination.

The technical advantage range of standard bearings This covers the vast majority of industrial applications. From automotive wheel hubs to industrial gear reducers, and from household appliances to heavy machinery, the load-carrying capacity, cost-effectiveness, and ease of use of rolling bearings make them indispensable foundational components.

Notably, the latest advances in the industrialization of air‑bearing technology have drawn significant attention. In November 2025, China unveiled the industrial‑scale application of hydrodynamic air bearings, marking a breakthrough that has lifted the long‑standing technological blockade imposed by foreign entities in the ultra‑precision bearing sector. Leveraging this technology, AI‑powered supercomputing server cooling modules and ultra‑high‑speed micro‑blowers have been introduced, signaling that the pathway for air bearings to transition from laboratory research to mass‑produced products is accelerating.

V. Conclusion

The fundamental difference between air bearings and conventional bearings boils down to a matter of “boundary control.” Conventional bearings regulate force transmission between solid surfaces, with engineers focusing on materials, heat treatment, and lubrication. In contrast, air bearings govern the fluid’s pressure field and damping characteristics, requiring engineers to excel in fluid‑mechanics modeling, restrictor design, and stability control.

Understanding this distinction means that, when selecting a bearing, you no longer ask “which type of bearing is better,” but rather “what kind of support logic does my operating condition require?” When precision requirements reach sub‑micron or even nanometer levels, when rotational speeds exceed the 100,000 rpm threshold, and when oil contamination becomes an unacceptable factor—air bearings are no longer just one option; they become the only viable choice.