26-06-2012, 05:37 PM
AIR BEARING APPLICATION AND DESIGN GUIDE
Air Bearing Design Guide.pdf (Size: 2.14 MB / Downloads: 132)
Introduction
Bearing technology represents one of the age-old problems for mechanical engineers. Rolling element
bearings developed in the last century were a revolutionary improvement over the plain bearings that had
been pushed to their limits in applications like electric motors and automobile wheels. Similarly, rolling
element bearings are today being pushed to their technical limits by the demands of applications like
semiconductor manufacturing, high resolution scanning, and high-speed machinery.
Air bearings represent the next logical step in bearing design. Air bearings in general have a proven track
record having been employed in coordinate measuring machines for 20 years. The many technical
advantages of air bearings such as near zero friction and wear, high speed and high precision capabilities,
and no oil lubrication requirements are powerful advantages for today's machine designers. However,
these benefits have not been more fully utilized to date because air bearings are difficult to manufacture
and they have not been commercially available until recently. New Way Precision was founded ten years
ago to pioneer the use of porous media technology and make air bearings that are robust, simple to use,
inexpensive, and available off-the-shelf.
What is an air bearing?
Unlike contact roller bearings, air bearings utilize a thin film of pressurized air to provide a ‘zero friction’
load bearing interface between surfaces that would otherwise be in contact with each other (Figure 2).
Being non-contact, air bearings avoid the traditional bearing-related problems of friction, wear, and
lubricant handling, and offer distinct advantages in precision positioning and high speed applications.
The fluid film of the bearing is achieved by supplying a flow of air through the bearing face and into the
bearing gap. This is typically accomplished through an orifice or a porous media that restricts or meters
the flow of air into the gap, referred to in Figure 2 as R1. The restriction is designed such that, although
the air is constantly escaping from the bearing gap, the flow of pressurized air through the restriction is
sufficient to match the flow through the gap. It is the restriction through the gap, R2 that maintains the
pressure under the bearing and supports the working load.
Why use air bearings?
Because of their advantages over rolling element bearings, air bearings are a natural choice for
applications such as Coordinate Measuring Machines, precision machine tools, semiconductor wafer
processing machines, and other clean room, high speed, and precision positioning environments. The
main advantages of air bearings are listed below. Some of the specific concerns of the design engineer
(friction, wear, stiffness, and load capacity) are then discussed in more detail.
Stiffness
A common misconception about air bearings is that
they do not have the required stiffness for precision
applications. However, a 6” diameter bearing
running at 60psi actually has a stiffness greater than
2,000,000 lb/in with a load of 1,000 lbs. Put another
way, that’s less than one half of a millionth of an
inch deflection per pound of additional force. By
testing a bearing through a range of loads and
measuring the change in air gap, a stiffness plot can
be created like the one shown in Figure 4. Notice
that the slope is not linear and as the film gets
thinner the stiffness gets higher.