Air bearings offer numerous advantages over mechanical bearings such as profiled linear guides and crossed roller slides. But they are still considered by many designers and engineers to be a high-cost solution, primarily for niche applications that require extreme precision. However, their benefits include more than just high precision, and cost is being driven down by an increase in the number of manufacturers and advances in materials and manufacturing for the precision bearing surfaces.
To demonstrate how air bearings are becoming more common in industrial equipment and processes, here are some of the benefits they provide and the applications they serve.
Benefit: High positioning accuracy and repeatability
Air bearings have no mechanical contact and are virtually frictionless. This means they do not experience stick-slip effects or hysteresis, allowing them to achieve nanometer-level positioning accuracy and repeatability.
Applications: Ultra-precision machine tools; optical grinding
Benefit: Extremely straight and flat travel
With linear air bearing guides, pitch, roll, and yaw errors are virtually eliminated, as are tilt and wobble errors with rotary air bearings. This is due in part to the “surface averaging” effect, which occurs because an air bearing supports the load along its entire length, in contrast to mechanical rolling element bearings, which support a load only at the contact points of the balls or rollers. This allows an air bearing to average all the imperfections of the surface across the length of the bearing, where a mechanical bearing’s travel is influenced by every imperfection in the bearing surface.
Applications: Optical inspection equipment; medical device manufacturing
Benefit: Precise force control
The frictionless nature of air bearings, together with ironless linear motors or voice coils, create precision stages that can operate as force generators or transducers, even down to the milliNewton level.
Applications: Tensile and friction testing equipment
Benefit: Smooth, constant velocity
With no rolling elements to cause perturbations in motion, air bearings can maintain a highly consistent, controlled velocity.
Applications: Wafer scanning; digital printing
Benefit: Low particle generation
Because they have no contacting parts, air bearings (and stages built with air bearings and linear motors) produce virtually no particulates. And for use in the most stringent cleanrooms, where nearly undetectable particle generation is required, they can be operated with 99.9 percent pure nitrogen rather than with air.
Applications: Semiconductor wafer processing; medical device manufacturing; medical research and testing
Benefit: No change in performance characteristics over time
Friction is highly non-linear, and reducing or eliminating it means that settling times are shorter, as there is no need to wait for the (relatively) slow servo loop to overcome the effects of friction. This also means that the primary variables in performance become the servo bandwidth, payload, and machine resonances, making performance very stable and predictable over time.
Applications: Semiconductor manufacturing; equipment located in remote or difficult-to-access locations
Benefit: Virtually unlimited service life
Load, speed, acceleration, and other application parameters do not affect the life of an air bearing. In fact, the primary factor is wear due to contaminated air, which can cause corrosion. For an air bearing stage driven by a voice coil or linear motor, the cables and cable management – which are, in most cases, easily replaced – become the limiting factor for wear and life.
Applications: Processes that require continuous operation or extremely high duty cycles
Feature image credit: Dover Motion
Filed Under: Linear Motion Tips, Bearings, Motion control • motor controls