Introduction to air bearings and their advantages
Low Friction
Of the more important advantages which are offered by gas lubrication, the low viscosity of
gases as compared with liquids can be exploited to special benefit. The extremely low static
friction which externally pressurized bearings can offer, finds its uses in torque measuring
equipment, dynamic balancing machinery, semiconductor positioning systems, micro or zero
gravity trajectory simulators and other instruments requiring near-static conditions.
High Accuracy
The high accuracy of motion that can be obtained with air bearings is equally important in
some applications. Considerable differences in motion accuracy exist between rolling element
bearing supports and air bearing supports. In linear slides, for example, rolling
element bearings
witness noise error (or rumbling) due to the ways’ surface roughness and/or eccentric rotation of
the rollers or balls.
On the contrary, air bearings do not suffer from this difficulty. The reason for this
lies in the absence of surface contact between the bearing parts and the averaging action of the air film over the various local surface irregularities present in the machined surfaces.
Even the
finest of rolling element bearings are orders of the magnitude less accurate than air bearings.
In
rotating air bearings, this effect produces high orders of rotational accuracy and smoothness of
travel. Typical T.I.R. for air bearing spindles are less than 1.0 µinch.
For linear slides, pitch, roll
and yaw errors of much less than a fraction of an arc second are attainable and straightness of travel errors on
the order of tens of nanometers have been achieved.
High Stiffness
At zero speed, air bearings provide considerably high stiffness characteristics.
This same effect
is seen at zero or low loads. For properly designed and manufactured aerostatic bearings, it is
not uncommon to measure stiffness on the order of several million pounds per inch.
Zero Wear
The advantage of zero wear can be seen greatly in externally pressurized or aerostatic bearings
and to some large degree in self-acting or aerodynamic bearings. Although some properly
designed rolling element bearings can achieve practical wear rates, none can match the zero
wear characteristic of aerostatic bearings. With aerodynamic bearings, starting and stopping
causes some rubbing within the bearing clearance, but this can be alleviated by introducing a
pulse of air just as the bearing begins translation. Furthermore, as compared with rolling
element bearings, air bearings do not suffer from increased wear rates as the speed or load is
increased. With proper care and maintenance, infinite life can be expected from air bearings.
Contamination
Gas lubrication has found a place of particular importance in circumstances where it is necessary
to keep the environment free from contamination by conventional lubricants.
Such situations
arise in semiconductor wafer handling systems. In these situations, it may be costly or
impractical to manufacture a system which can effectively seal off contaminants from oil
lubricants used in roller slides. The externally pressurized air bearing lends itself well to harsh
environments where liquids, dust and contaminants are present. The air bearing’s great
resilience stems from the fact that with positive pressure existing inside the bearing, all foreign
matter is repelled away from the critical bearing surfaces. Externally pressurized bearings
could
operate while completely submerged in a liquid. Unlike some rolling element bearing supports
that require periodic maintenance, cleaning, the addition of oil lubricants and sometimes the
replacement or re-surfacing of guideways, the air bearing’s self-cleaning nature allows it to be virtually maintenance-free.
Wide Temperature range
Perhaps the most exclusive quality of gases as lubricants is their potential for operation over
extremely wide ranges of temperature. In fact, it is the shortcomings of the solid components of
the machine, not that of the lubricant, which will set performance limits when simple gases are
used for high temperature lubricated applications. No difficulty is foreseen, for example, at the
hot end of the scale, in operating the bearings of small steam turbines. Also, gas bearings see a
performance increase with a reduction in viscosity due to temperature increase. In the case of
liquid bearings, the performance declines with reduced viscosity from temperature increase.
Externally pressurized bearings have been operated at temperatures of up to 900
°C and at
speeds of up to 65000 rpm.
