Modular Air Bearings for Motion Control

Over 100 years ago Westinghouse applied for a patent that employed air bearings to support a steam turbine. Today, air bearings’ high precision make them candidates for applications such as coordinate-measuring and lithography machines. They are also frequently used for circuit-board drilling and waferdicing spindles because of their capability for working at high speeds.

Recently a type of air bearing called a modular-radial air bearing has become more widely available. These devices were once generally created only for custom orders. Today, they are standard products available from several suppliers.

Radial air-bearing modules do not constrain rotary motion using a 360° housing as is the case with rolling-element bearings. Instead they are positioned in combinations of three or four segments to support a rotor over just a small percentage of its circumference.

Here, a 1-m-diameter rotor is constrained using four radial and eight axial air bearings. There are four sets of bearings, each set mounted using gimbaled mounting screws through a horseshoe-shaped piece of aluminum plate. The radial bearings are preloaded on the rotor using tension; in this case four aluminum bands are used on each side. They connect to the horseshoes through pins located on a smaller radius than the spherical mount at the back of the radial bearing. This makes for inherent stability and — because only two of the horseshoes hard-mount to the base plate — the preload force on all four bearings is always the same. The preload force in this configuration can be as much as 1 ton, which results in a surprising amount of axial stiffness for the rotor. This is an example of how air bearings, which mount on discrete points, are consistent with kinematics. It also shows they can be used in combination to create lightweight, high-speed, open-aperture rotary-bearing systems.

Radial-bearing modules can be constructed with either of the two basic approaches for air bearings. One uses a surface comprised of numerous small orifices which feed air from a reservoir to the bearing surface. This produces a film of air which supports the bearing load. The second approach uses a porous media (usually carbon but sometimes ceramic for clean rooms or superhard surfaces). This media distributes clean dry air evenly through millions of submicron-sized holes across the surface of each bearing. These holes are much smaller than the orifices found in the more conventional approach.

When it comes to radial air-bearing modules, the porous-media approach tends to be most widely used. Porous-media air bearings are robust compared with orifice-based devices and can withstand repeated crashes (loss of air pressure) even at high speeds. It is possible to lap them in place by reducing air pressure until the bearing is dragging intentionally and then repeatedly flushing with alcohol. The porous carbon is a sintered material and will not “pick up or spall,” even on a soft material like aluminum. And porous-media bearings can support a rotor with extremely high precision.

Air versus rollers
It is interesting to compare radial air-bearing techniques with rolling-element bearings of today. For starters, rolling-element bearings have huge load capacities, far greater than the (current) capabilities of radial air bearings. Yet they are often oversized to provide for long life even in applications that are lightly loaded. This is because, as contact devices, they wear.

Air bearings, on the other hand, can carry surprisingly high loads. But, because they are noncontact devices, they do not suffer from contact wear. It does not matter if they are heavily or lightly loaded. In fact, load and speed are not significant wear factors in air bearings.

Another attribute of radial air bearings is the near-zero levels of friction they provide. Simply, there is no start-up friction or stick slip as is known in hydrodynamic/ plain bearings and heavily preloaded roller bearings. Further, the friction at high speed is still basically zero, so it takes much less current/energy to keep an object rotating. This property certainly qualifies radial air bearings as “green” bearings. In addition, they use no fossil-fuel-based lubricants, and they run “silent.”

Finally, radial air bearings can handle extremely high speeds thanks to their lack of friction. A general rule of thumb is that an air bearing will have only 10% of the load capacity of similar-sized rolling-element bearing but will have 10 times the speed capability and 100 times less friction.

Radial comparisons
Typically, radial rolling-element bearings have an inner and outer race that goes 360° around the rotating body. These races and the rolling elements they contain must be fit with a high degree of precision. The ability to manufacture parts with this type of precision has been the exclusive domain of bearing manufacturers.

Radial air bearings built as modules change this paradigm dramatically. Modular-radial air bearings support the rotor from a small percentage of the circumference. So the relative size between the rotor and the radius on the bearing is not critical. This means you can buy the radial air bearings and machine races on the rotor yourself, if need be. In the case of fans and turbines, many designs could benefit from this sort of support around the perimeter as such a construction would provide a clear aperture in the center (where a rolling bearing would go).

Modular air bearings themselves mount on spherical-ball gimbal seats, so they self-align to the rotor. The gimbals are on threaded studs and are adjustable. There is no need for precision features on the stator to mount the bearing, just a threaded hole for the studs. This contrasts with the 360° flat-mounting surface equipped with numerous tapped holes required by a large roller bearing or slewing ring.

Rotor manufacturing is simplified as well, because surfaces on the rotor can serve as a race and are supported directly by the radial bearings. This means it’s not necessary to drill mounting holes or deal with distortions from tightening bolts on the rotor.

Modular-radial air bearings also have the advantage of being kinematically correct. That is, they are consistent with exact constraint theory; so three radial air bearings can be used to constrain an axis of rotation for the same reason that a three-legged stool cannot rock.

Modular-flat air bearings can be used to constrain axial motion in the same way. Also, the exact force paths are known through the bearings and into the structure. This fact simplifies any needed FEA analysis because the solution will be given by neat closed-end equations.

The design flexibility of radial air bearings allows for stators that use tension as a way to make light and strong stator structures. An accompanying figure provides an example of such a stator design, using bands in tension to preload the air bearings. It is interesting to note that by tightening any one of the air bearings, the force on all the bearings rises equally, an additional example of how air bearings enable exact constraint design.

Another advantage of having only one rotating part is that such a configuration makes it easier to obtain a higher degree of precision. In spindle metrology, the errors in an axis of rotation can be classified as synchronous or asynchronous. In the simplest terms, synchronous errors are the same with each rotation; asynchronous errors are different each time around. Synchronous errors predict the ultimate geometry that a spindle is capable of generating, as with, say, the roundness of a part made on a lathe. In contrast, asynchronous errors would indicate the surface finish that a spindle may be capable of producing while single-point fly-cutting, turning or grinding.

Asynchronous errors are characteristic of rolling-element bearings and are directly linked to the errors of races and rolling elements precessing about at different speeds relative to each other. Using radial air bearings — with only one rotating element — virtually eliminates asynchronous error and reduces synchronous errors by a factor of 10. This makes radial air bearings excellent candidates for large machine-tool worktables or spindles.

In a similar way, and for the same reasons, radial air bearings provide a significant advantage in both the manufacture and application of rolls. New manufacturing processes for displays, photovoltaic cells, LED lighting, printed batteries, and even street signs rely on roll-type imprinting of fine structured surfaces on thin films. Such processes require a high degree of precision when producing relatively large objects. Radial air bearings enable improved precision in mastering the roll.

Also, by supporting the roll on the same surfaces used to master it, concentricity dramatically improves.

Interestingly, radial air bearings can also eliminate backup rolls in such applications because they can support the sag in the center, between the journals, in a noncontact manner to prevent deflation of the roll from process forces. Because they are gimbal mounted, they so self-align to deflections of the roll.

Loading performance
Modular-radial air bearings for a 2-m-diameter rotor can carry 2 tons of load at speeds over 500 rpm; that is, over 50 m/sec relative surface speed. Such rings with large spinning diameters see large centrifugal forces (easily 10s of gs). Loads that are not always evenly distributed around the circumference and under significant centrifugal forces can change the shape of the ring itself. The design of the rotor should consider these effects.

These high-speed applications also highlight another dynamic characteristic of radial air bearings. Although an intermittent load (oval-shaped rotor) may exert a force in excess of the bearing’s static capacity, the bearing will support the load for the short time it is on the bearing. This is because the load cannot force the air out of the gap in such a short time. The effect is much like what happens when your car hydroplanes on a puddle of water. Obviously, your tires could cut though the water at zero velocity. But tires don’t have time to touch down when you are hydroplaning.

This dynamic characteristic is called a hydrodynamic bearing effect. There are many classes of equipment — from supermarket scanners to utility-sized turbines — that employ hydrodynamic bearings. The steel industry uses hydrodynamic bearings in the most-demanding steel-rolling applications because rolling-element bearings cannot stand the abuse.

Kingsbury Inc., Philadelphia, has been manufacturing such bearings for over a hundred years. New radial air bearings avail themselves of similar modular, gimbaling-pad construction for radial and thrust force, and for the hydrodynamic effects. What is new is that externally pressurized air bearings also feature the hydrostatic effect of being able to support the rotor at the same height at zero velocity.

Another important advantage of radial air bearings is that even modular components 2 m in diameter are available in a matter of weeks instead of years. Because air bearings average the force of loads over large areas, special “bearing steel” is not needed to deal with the Hertzian contact stresses of balls and rollers. Nor will vibration during shipment damage or “Brinell” radial air bearings. This opens up opportunities to make rotors and bearing races from aluminum, fiber composites, ceramics, or other materials impractical for roller bearings.

Finally, a shift to perimeter bearing support and direct drive is consistent with history. We started with water wheels with shafts and belts for power takeoff. Today we have distributed electric motors but they still connect to leadscrews for linear motion, or belts and shafts for rotary motion. Just as leadscrews are being replaced by linear motors, we can expect that rotary motion will see more direct drive applications at the perimeter. Radial air bearings are right in line with that historical trend as well.