Vortex-Type Restrictors for Stable Low-Clearance Aerostatic Bearings

Aerostatic bearings are widely used in precision motion systems because they support moving components on a thin externally pressurized air film rather than through direct mechanical contact.  In precision manufacturing equipment and high-accuracy measuring instruments, this mode of support is attractive because it can provide clean, smooth, and highly repeatable motion. Pressure must be generated and distributed in a controlled manner, the air film must sustain external load, and the flow introduced through the restrictors must not disturb the stability that the bearing is intended to provide. A conventional aerostatic flat bearing often uses pocketed orifice-type restrictors to feed high-pressure air into the clearance. This arrangement can improve static support, yet it also introduces a difficult fluid-dynamic problem near the inlet region. Air supplied through an orifice perpendicular to the guideway surface impinges on the working surface and then turns sharply as it enters the bearing film. This sudden change in direction is associated with local acceleration, pressure depression, vortex formation, and turbulence. They can contribute to self-excited micro-vibrations, reduce the effective high-pressure region, and influence both load-carrying capacity and stiffness.

It must meter the supplied air so that a supporting pressure field is formed, and it must do so without creating unstable local flow structures that disturb the air film. Previous efforts to improve aerostatic bearing stability have therefore examined the geometry of the orifice chamber, the size and arrangement of orifices, and the influence of recess configuration on vortex behavior and micro-vibration. These studies point to a common physical concern: the flow structure near the restrictor shapes both the local pressure distribution and the dynamic steadiness of the bearing. In a recently published research paper in International Journal of Precision Engineering and Manufacturing Mr. Dong Zhang, PhD candidate  Senyu Yang, Pengfei Cao PhD candidate, Lubin Wang & led by Professor Weishi Li from Hefei University of Technology developed a vortex-type restrictor for aerostatic flat bearings in which two guideway-parallel, tangential orifices drive rotational airflow inside a circular recess before the air enters the bearing clearance. They also developed paired clockwise and counterclockwise restrictor configurations to compensate the torque generated by the rotating recess flow.  They evaluated the new design through CFD modelling and validated experimentally against a pocketed orifice-type bearing with matched main structural parameters.

Briefly, the researchers evaluated how the vortex-type restrictor alters the internal flow field and the bearing response and compared a circular aerostatic bearing with four conventional pocketed orifice-type restrictors against two vortex-type bearing configurations. Both vortex configurations used paired clockwise and counterclockwise restrictors, because a single vortex-generating recess produces torque; arranging opposite vortex directions allows that torque to be compensated while maintaining a symmetric bearing layout.

The flow simulations used ANSYS-Fluent with a realizable k–ε turbulence model and non-equilibrium wall functions. Their choice of modelling aligned with the problem being studied: the restrictor creates rotating and separating flow inside a small recess, and the pressure and velocity gradients near the inlet region are central to the bearing’s dynamic behavior. The computational domain used bearing symmetry to reduce cost while retaining the relevant flow structure, and the researchers treated the air as an ideal gas under supply pressures from 0.4 to 0.6 MPa. They focused in their comparison on bearings with the same main structural parameters except for the restrictor, so that the consequences of changing inlet geometry could be isolated. The simulated pressure field separated the two designs clearly. In the pocketed orifice-type bearing, the authors found the maximum pressure concentrated near the orifice, followed by a pressure depression at the orifice outlet. On the other hand, in the vortex-type bearing, the recess showed a more uniformly distributed high-pressure region, without the sharp local peak and depression seen in the conventional design. The velocity comparison was equally important. In the conventional bearing, the airflow velocity increased abruptly from 26 m/s to 205 m/s near the orifice outlet, whereas the vortex-type bearing kept the recess velocity lower and reached a maximum of 43 m/s at the recess outlet. The design choice of using tangential orifices to generate a controlled recess vortex therefore had the scientific consequence of reducing sudden pressure and velocity changes before the flow entered the bearing film.

The team performed streamline analysis and noticed the pocketed orifice-type restrictor, the jet impinged on the working surface, changed direction abruptly, and produced multiple vortices that were carried away and dissipated by the main flow. In the vortex-type restrictor, the air rotated along the recess wall and then moved into the gap more smoothly. The simulations also showed that torque from a single vortex-type restrictor increased with supply pressure, from 1.24 Nm at 0.4 MPa to 1.90 Nm at 0.6 MPa, which explains why paired opposite vortex directions were built into the bearing configuration rather than treated as an afterthought.

The experimental program tested bearings with four pocketed orifice-type restrictors and four vortex-type restrictors. Tests were taken for load-carrying capacity, stiffness, flow behavior, and micro-vibration under different supply pressures, film thicknesses, recess diameters, recess depths, and orifice heights. The low film thickness region received particular attention because the study identifies it as the operating condition of practical concern in precision equipment. The static results confirmed the main numerical trend. Increasing the recess diameter of the vortex-type restrictor substantially improved load-carrying capacity, especially at low film thickness, and the bearing with a 2.0 mm recess diameter exceeded the pocketed orifice-type bearing. Stiffness also improved under the conditions where the film thickness was below 5 μm, with the maximum reported stiffness increase reaching 286.6% relative to the pocketed orifice-type bearing. Recess depth had a limited effect on load-carrying capacity but influenced stiffness, while orifice height produced almost overlapping curves, indicating little effect on the measured static behavior.

The team also conducted vibration measurements and found that with a 2.0 mm recess diameter, the vortex-type bearing reduced micro-vibration amplitude by more than 60% at film thicknesses of 5 to 6 μm compared with the pocketed orifice-type bearing. With a recess depth of 0.6 mm, the reduction exceeded 70% in the same low film thickness range. Orifice height again had little influence between 3 and 8 μm. No pneumatic hammer phenomenon was observed for either bearing type during testing. The experimental evidence therefore connects the altered recess flow, the reduction in pressure depression, and the improved low-clearance stability in a consistent way.

The findings of Professor Weishi Li and his research team have direct relevance for the design of aerostatic bearings used in high-precision manufacturing equipment and high-precision measuring instruments, where motion stability at very small film thicknesses is essential. The study shows that the restrictor geometry itself can be used as an engineering tool to improve these characteristics, rather than treating the restrictor only as a passive air-supply element. The vortex-type restrictor is especially applicable where self-excited micro-vibration limits the useful operating range of aerostatic flat bearings.  The proposed design changes the inlet condition so that air rotates inside the recess before entering the bearing clearance, producing a smoother transition into the film and reducing the flow instability associated with the conventional configuration. For precision equipment operating at low film thickness, the reported improvements are particularly important. The same design approach also improved static performance: increasing recess diameter enhanced load-carrying capacity, and stiffness increased substantially at film thicknesses below 5 μm. These results suggest that the restrictor can be tuned to support both stability and load performance in the narrow-clearance regime where precision machines often operate. The work also provides practical guidance for bearing design. Recess diameter appears to be the most influential geometric parameter for improving load capacity, stiffness, and vibration suppression, while orifice height has little effect over the tested range.  At the same time, the paper notes that the vortex-type bearing consumes more air because each restrictor contains two orifices. For engineering implementation, this means the design is most suitable where improved stability and stiffness justify the higher air consumption, particularly in precision motion platforms, measuring systems, and manufacturing devices requiring low-vibration noncontact support.