New Way Air Bearings ® is proud to offer a variety of frictionless motion solutions to meet all of our customers’ needs, and today we want to discuss two different methods of preloading, available to you in our linear slide product line.
Preloading an air bearing system increases stiffness and accuracy, and is applicable across every kind of porous media bearing. Preloading air bearings follows the rules of fluid dynamics and more specifically, the squeeze film effect of compressible fluids. Because air is a compressible fluid, it possesses its own spring rate, or stiffness. Higher pressures effectively act as a preload on the “air spring”, and if we think of the air column as a spring of arbitrary height, compressing or shortening the spring will increase its stiffness as the air attempts to “push back”. Stiffness in an air bearing system is a product of pressure in the air gap, thickness of the air gap and the projected surface area of the bearing.
Intuitively, it may be difficult to conceptualize how an air bearing could have stiffness commensurate with a roller bearing, which is in physical contact with the guide surface, however it should be noted that a point or even a line has, theoretically, no area. Such minimal contact areas create very high local stresses, and so require hardened materials to avoid large deformations of the roller or race elements. In air bearings, the load is transmitted through an air gap projected over an area several orders of magnitude larger than rolling bearings. The wide air gap is what enables the squeeze film effect, providing the positive correlation between stiffness and damping which is entirely unique to air bearings, and which is highly advantageous to precision systems.
Air bearings may be preloaded through numerous methods, including added mass, magnets, vacuum, or mounting two air bearings in opposition to each other on either side of a guide rail.
Magnetic and oppositional preload are the most common varieties used for linear slides, when especially high compliance is required. Via the geometry of the dovetail slide for example, the air bearings on the lower surface act as a preload to the air bearings on the upper surface of the stage.
Magnetically preloaded air bearing systems are a new innovation from New Way, and offer even more control versus oppositional preload. Magnets are placed into the stage in threaded holes, and by adjusting the magnets in their threading, can easily and precisely control the amount of preload force along multiple axes. Because magnetic preloading is not dependent on the air supply which floats the bearing, this allows for greater preload force, especially for diamond turning machines which require high damping.
Oppositional preloading is available on the full product line of dovetail air slides, and magnetic preloading will soon be available as a custom option on all compatible linear slide products, in either steel or aluminum.
