36 n BEARINGS, BELTS AND CHAINS March 2025 www.drivesncontrols.com Floating the idea of air bearings Air bearings and motion stages use a cushion of air to eliminate mechanical contact, thus mitigating problems associated with traditional mechanical bearings such as friction, wear, vibration, hysteresis e ects and particle generation. Air-bearing stages can be the ideal choice for precision, 24/7 automation and metrology applications. Air bearing surfaces are usually hardcoated and ground precisely to achieve exceptional tolerances. High sti ness is achieved by frictionless magnetic or vacuum preloading. The “surface averaging” e ect (see diagram, right) helps to provide good geometric performance, such as straightness and atness, because the large area of the bearing, together with the precision ground surfaces, can deal much better with small imperfections than ball or roller bearings. This e ect can be compared to the di erence between a hovercraft and a conventional truck riding over a road covered in potholes. The averaging e ect is also similar to the smoothness of a magnetically levitated train compared to a conventional train running on steel rails. With the absence of wear, and no need for lubrication or maintenance, air bearings o er considerable benets in high-speed applications and precise positioning, particularly in high-throughput automation installations that need high uptime and reliability. The advantages that air bearings o er for precision motion and positioning applications include: nAlmost unlimited, maintenance-free lifetimes Because air bearings work without mechanical contact, they do not deteriorate and thus do not need periodic inspections, maintenance or relubrication. Also, unlike cross-roller bearings, there is no risk of cage migration, especially when executing small repetitive motion cycles. Due to the absence of friction and lubricants, these systems are also suitable for cleanroom applications. nGood geometric performance with low straightness, atness and eccentricity errors Due to their precision manufacturing, air bearings deliver high accuracies. Because of the surface averaging e ect, linear air bearings o er exceptionally at and straight travel (0.75μm over 500mm) with minimal roll, pitch, and yaw errors (10μrad). Rotary air bearings are also superior in terms of eccentricity, wobble and tip/tilt. This makes them suitable for manufacturing and measurement processes such as optical inspection. The exceptional precision is important in advanced manufacturing processes where every nanometer counts – semiconductors, for example, are produced with line widths measured in nanometres, and even mechanical components in automotive engines can need sub-micron precision. n True planar multi-axis motion Planar XY stages and XY-theta stages cannot use traditional mechanical bearings. Here, individual axes are usually stacked on top of each other. In some instances, XY stages are combined into a single assembly and “planar stages”, however, the X and Y axes do not use the same reference plane. Building a multiaxis motion system by stacking individual axes is simple, but has several drawbacks. As the upper axis moves to extremes, it will generate torque loads on the lower axis, leading to geometric errors. With air bearings, fully planar XY and XYtheta positioning systems are feasible, with all degrees of freedom referencing the same base plane and supported over the full travel range. PI’s A-322 XY-theta stage, for example, uses one common plane for both X and Y motion. A exure joint on the cross-axis even allows for 1 degree of rotation around Z – a great advantage in applications where small alignment errors need to be corrected. nVibration-free motion with constant velocity and a high dynamic range Air bearings’ uid lms can accommodate high velocities. Some can even improve eciency at high speed, due to aerodynamic lift e ects. Some processes – such as semiconductor wafer scanning, 3D tomography and inertial sensor testing – demand constant motion at precisely controlled speeds, where mechanical Mechanical bearings and guidance systems have many drawbacks, especially for high-performance motion control. Air bearings can oer advantages in applications where high precision, long lifetimes, low vibrations and good repeatability, are needed. Stefan Vorndran, vice-president of marketing at Physik Instrumente (PI), explains. A planar XY air bearing stage which provides one degree of theta-Z motion, making it ideal for small corrections when, for example, a workpiece and a camera or laser system need to be aligned. It provides “exceptional” motion straightness and atness, while the granite base provides good temperature stability. Image: PI
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