The Air Bearings Engineering Engineering Essay

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Abstract

This report presents the development of research work carried out for the topic of Air bearings. Advances in engineering and technology frequently place severe new demands upon the support systems for machine components required to be in relative motion both in regard to improved performance and increasingly exacting operating conditions. However, in a growing number of applications it is becoming necessary to achieve a result which is either technically or economically impracticable with conventional lubricants. Air lubrication offers certain unique advantages, as in their own manner and place do the other alternatives, but likewise air bearings themselves have their own inherent limitations which must be clearly recognized in advance if the aspiring user is to avoid unwarranted failure. In particular it must be recognized that air bearings of practically every type are somewhat more prone to instability difficulties than are liquid bearings, although most of these instabilities are to a huge extent the same general type that occur in liquid-lubricated systems.

This report performs the study of comparisons with other conventional bearings with respect to their economic benefits, structural feasibility and design parameters. Also investigates the working technology and their applications in different fields over the conventional bearings to facilitate better performance.

Acknowledgement

The euphoria and joy accompanying the successful completion of my research report work would be incomplete without the mention of those people who rendered help and guidance throughout.

I would like to thank god for giving me the strength and health to complete this report. I take this opportunity to express my sincere heartfelt gratitude to my supervisor, Dr. Vojislav Ilic who with his zeal and outstanding patience helped making this endeavour a success.

My heartiest thanks go to my parent's, without their overwhelming positive influence on my life, I would not have been able to achieve my goals.

List of figures

Figure 2.1: Cutaway of an air bearing PCB drilling spindle.

Figure 3.1: Orifice and porous media air feeding.

Figure 4.1: Porous media technology.

Figure 5.1: Linear air bearings.

Figure 5.2: Rotary air bearings.

Figure 5.3: Spherical air bearings.

Figure 7.1: Stiffness graph for air bearing and rolling element bearing.

Figure 8.1: Stiffness and preload in CMM machines.

Contents

Introduction to air bearings………………………………………..

Literature review……………………………………………………

2.1 What are air bearings?

2.2 Why use air bearings?

Design parameters needed for air bearings………………………

3.1 How does air get into the bearing?

3.2 Geometry requirements for working of air bearings

Air bearing technology…………………………..…………………

4.1 Aerostatic bearings

4.2 Aerodynamic bearings

4.3 Orifice and porous media technology

Various configurations of air bearings…………….

Applications of air bearings…………..………………………………………

Exclusive Advantages of using air bearings………………………..

Designing of air bearings……………………………………………

8.1 Air bearing guides

8.2 Stiffness and preload

Factors affecting the performance of air bearings……………………………

Conclusions……………………………………………………………………

Recommendations……………………………………………………………..

References……………………………………………………………………

Air lubricating bearings

Aim: To understand the working of air lubricating bearing which is a typical type of bearing compared to other conventional bearings. Also investigate relative merits and operation as well as cost of air bearings compared to more conventional bearings.

1. Introduction:

Bearing technology represents one of the long standing challenges 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 as in the semiconductor manufacturing and high-speed machinery.

Air bearings represent the revolutionary advancement in bearing design. Air bearings have been employed in co-ordinate measuring machines for over 20 years and proven to have no better alternative. The numerous 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. In this report let us know about the working, the air bearing technology, types of air bearings, applications and advantages of using air bearings.

2. Literature review

2.1What are air bearings?

An air bearing is a system where an air film acts as the lubricant that separates the two surfaces in relative motion. Unlike conventional (Journal bearings, ball bearings etc.) bearings, air bearings utilize a thin film of pressurized air to provide a non-contacting load bearing interface between the two surfaces in relative motion. Being contact free, air bearings provide an edge of overcoming usual problems in conventional bearings like wear, friction and lubricant handling. It also offers a distinct advantage in high speed machinery applications and precision positioning.

The air gap that acts as a lubricating film of the bearing is achieved by supplying an air flow through the bearing face and into the bearing gap. This air gap is achieved with the use of an orifice or a porous media that restricts or controls the air flow into the bearing gap. The design of restricting the air flow is such that, even though the air is continuously escaping from the bearing gap, the restriction of pressurized air through the gap is sufficient to have enough air that maintains the gap. It is this gap of restricted pressurized air under the bearing that supports the operational load. If there were no restriction of pressurized air then the floating load would reach higher than required resulting in improper functioning and higher air consumption. This controlled flow of pressurized air is referred to as air bearing compensation. It is employed to adjust the bearing with respect to height of lift, working load and stiffness.

Figure 2.1

2.2Why use air bearings?

Fluids are of two types namely liquids and gasses. In terms of lubricating bearings the difference between the two is essentially the viscosity. We know that liquids have much higher viscosity than gasses. When we consider the fact that gasses having very low viscosity than liquids, it can be applied in lubricating bearings. This also implies that lower the viscosity, lower will be the load bearing capacity. But considering the extremely low viscosity of gasses let us have air as lubricating agent in this case. Air lubricating bearings operate with near zero static and running friction, whereas liquid lubricating bearings have much higher friction in the bearings which result in heat generation. Heat generation in bearings is a major concern which can be avoided using air bearings.

As discussed earlier air bearings require a controlled pressurized air for proper operation which means that a very high accuracy can be obtained during its motion. Using air as a lubricating fluid the cleanliness of the operating surroundings can be achieved. The other exclusive benefits of using air bearings are elaborated in further chapters.

3. Design parameters needed for air bearings

3.1 How does air get into the bearing?

Once the air enters the bearing and being routed through internal passages, the next stage is to get the pressurized air to the bearing gap. There are two basic ways to accomplish this, orifices and porous media. For orifices, the air flows through a small opening (typically .004" to .015" dia.) into the bearing area. Porous media bearings use a porous material (typically bronze, carbon or steel) which the air enters through into the bearing gap.

http://www.nelsonair.com/images/basics_img_NOXPOR.jpg

Figure 3.1

The relative benefits of each method are appreciable. Orifices typically do not generate a uniform pressure profile compared to that a porous media can achieve. Pressure profile can be improved using a technique called "pocketed compensation". Porous media feeding provides more inherent damping than orifices, however proper sizing of the orifices can adjust damping. Orifices can become plugged if a very large particle gets into the air supply (this is extremely rare), where the porous media acts as a filter for the air. However, because of this filtering effect, over time the porous media itself can become clogged (oil vapour in the air is particularly bad) and a clogged porous bearing is much more difficult to clear than a clogged orifice.

3.2 Geometry requirements for working of air bearings.

As air bearings require a very small gap for its working, the most important factor to be considered is flawless geometry. Any type of variation such as part size, improper edges, and straightness or out of roundness will lead to closing of the gap and will cause the bearing to lose its frictionless nature as well as accuracy and load bearing capacity. In addition, the part geometry is what determines the accuracy of the final bearing motion - even though local deviations are averaged out by the air film, the overall accuracy is dictated by how well the parts are made.

4. Types of Air Bearing

There are three kinds of air bearing technology:

4.1 Aerostatic bearings:

Aerostatic bearings work with an external pressurized air source. This air pressure is introduced into to the bearing gap by precision grooves, holes, steps or porous compensation techniques as discussed earlier. As aerostatic bearings have a pressurized air source they can maintain an air gap when there is no relative motion between the bearing surfaces.

4.2 Aerodynamic bearings:

Aerodynamic bearings are always dependent on the motion between the bearing surfaces and normally some type of spiral grooves to draw the air between the bearing gaps. This bearing action is very similar to hydroplaning in our automobile on a puddle of water at high speed. In just this way, aerodynamic bearings require relative motion between the surfaces, when there is no motion or when the motion is not fast enough to generate the air film the bearing surfaces will come into contact. Aerodynamic bearings are often referred to as foil bearings or self-acting bearings. Examples of this type of bearing include the read-write head flying over a spinning disk, crankshaft journals, camshaft journals, and thrust bearings for electrical generator turbines.

4.3Orifice and Porous media technology

Air bearings are typically classified as 'orifice' or 'porous media' bearings. In orifice bearings the pressurized air is supplied to the bearing surface through a small number of precisely sized holes. Porous media air bearings are different that the air is supplied through the entire surface of the bearing (Figure 4.1). The porous material controls the airflow in the same way an orifice bearing would do if it had millions of miniature holes across its surface. Broadly speaking there are two techniques for achieving the compensating effect in air bearings. Orifice compensation is traditionally the most widely used method, but porous surface compensation is rapidly emerging as the method of choice due to its many advantages and increasing availability.

Figure 4.1

5. Various Configurations for Air Bearings:

Figure 5.1

The above figures show the different configurations of linear air bearings.

Figure 5.2

The above figures show the different configurations of rotary air bearings.

Figure 5.3

The above figure shows a spherical air bearing with three degrees of freedom.

6. Applications of air bearings

6.1 Optical Fiber Manufacturing

To manufacture optical fibers a smooth, precise bearings are required to withstand the harsh, abrasive environment. As conventional oil bearings quickly damage the accuracy of machining. The self-cleaning nature of linear air bearings allow the process of manufacturing optical fibers for much longer periods without losing critical tolerances.

6.2 Optical Grinding

Optical grinding requires extremely accurate motion in order to achieve demanding tolerances for surface figure. In addition to excellent accuracy, the outstanding stiffness of air bearings allows the bearing to withstand forces without degradation in accuracy.

6.3 Optical Spectrometry

High resolution laboratory spectrometers need custom linear journal air bearings to provide extremely precise, tilt-free, smooth motion for their scanning mirrors. The noncontact nature of these assemblies also delivers the reliability and minimal maintenance today's laboratory users demand.

6.4 Coordinate Measuring Machines

Most coordinate measuring machines (CMMs) are built with air bearings because they allow for infinite resolution. Because air bearings actually float on a pressurized film of air there is no physical contact. This means only the shear of the molecules contributes to friction. The static and dynamic coefficients of friction at start-up are identical and there is no stick-slip effect. This minimizes lost motion and reversal errors around the triggering of the probe. And because air bearings are more repeatable and smoother than rolling element bearings error correction is more effective.

6.5 Testing Equipment

Many tensile and friction testing machines can be influenced be the friction in rolling element bearings. Wear in the bearings can also result in inconsistencies with testing processes. For this reason, many of the most accurate friction testing machines use air bearings to eliminate mechanical contact friction. Many testing machines require very accurate force control. The elimination of friction dramatically increases resolution of the instrument. Fatigue testing will often cause 'fretting' in rolling element bearings. Being non-contact, air bearings are insensitive to high frequency short travel applications.

6.6 High Speed Equipment

Machines designed today have moving elements that may cycle as many times as one billion cycles per year. The best alternative is to switch the mode of wear by changing the bearing technology from roller bearings to air bearings. In air bearings the speed or distance the bearing travels does not affect wear. The mode of wear in air bearings is erosion, so the amount of particulates in the incoming air is the determining factor in how long a bearing will last. The calculated life of an air bearing is measured in centuries regardless of whether it is moving at one billion cycles a year or remaining stationary. The dynamic coefficient of friction increases with speed and will only contribute heat problems at over 20 meters per second and then only in confined rotating applications.

6.7 Ultra accurate machine tools

Many of the most accurate machine tools in the world employ air bearing technology. The zero static coefficient of friction allows for unmatched performance during stage reversal in contouring applications. Very accurate velocity control and elimination of perturbations in the stage movement allow for the turning of optical quality surface finishes that are measured on the angstrom level. Errors in geometries on manufactured parts are often on the order of several millionths of an inch.

7. Exclusive Advantages of Air Bearings

The advantages air lubrication can offer is from the properties of gases: they are chemically stable over a wide temperature range and they have inherently low viscosities.

Low Friction

The use of gas allows a torque of magnitude smaller than that achieved by liquids, but a low-torque bearing with suitable large clearance can be made using gas lubrication. Air is employed as the exhaust can be released to the surroundings as it causes no damage to the atmosphere. High speed linear slides can operate at very high speeds for thousands of hours or repeatedly because of low friction aspects of air bearings. Any other rolling element bearing would never be able to achieve such a requirement. This low friction also finds uses in torque measuring equipment, dynamic balancing machinery, semiconductor positioning systems, micro or zero gravity trajectory simulators and other instruments requiring near-static conditions.

7.2 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 surface roughness and/or eccentric rotation of the rollers.

On the other hand, 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.

7.3 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.

Figure 7.1

Zero Wear

The advantage of zero wear can be seen greatly in externally pressurized or aerostatic 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 relieved by introducing a boost of air as the bearing begins translation. 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 free

Air lubrication has a particular importance in conditions where it is necessary to keep the environment contamination free from conventional lubricants. Such situations arise in semiconductor wafer manufacturing 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. Unlike some rolling element bearing supports that require periodic maintenance, cleaning, the addition of oil lubricants and sometimes the replacement or re-surfacing of guide ways, the air bearing's self-cleaning nature allows it to be virtually maintenance-free.

Wide Temperature Range

The most exclusive quality of gases as lubricants is their potential for operation over extremely wide ranges of temperature. Complex gases on the other hand will have decomposition limitations at the upper end of their usable temperature range. No difficulty is seen, for example, at the hot end of the scale, in operating the bearings of small steam turbines or circulators upon superheated steam, and, at the cold end, gases approaching their liquefaction temperatures could be employed to lubricate the bearings of, for instance, gas liquefying turbines. In both examples considerable simplification of design could thus be achieved in some situations. It is noted that whereas with liquids bearing performance falls off with rise of temperature due to fall in viscosity, in the case of gases, the load-carrying performance will in general improve due to a rise in viscosity.

7.7 Unlimited Service Life

The absence of contact means absence of wear, McCarthy says an air bearing will operate without change for decades. He notes that these bearings have been deployed in the thousands in digital printing systems worldwide over the last ten years, and has demonstrated intrinsically high production reliability. We can also see applications in which high precision is not required, and air bearings are being selected strictly for their unlimited, maintenance-free service life.

8. Designing of air bearings

8.1 Air Bearing Guides

Air bearings can be run on different types of guide material. Common guide surfaces include granite, hard-coated aluminium, ceramics, glass, stainless steel, and chromed steel.

8.1.1 Guide Surface Considerations

Surface finish, local flatness issues and possibly even holes in the counter surface need to be taken into consideration.

8.1.2 Surface finish

The recommend surface finish of 16rms is an optimal finish. Rougher surfaces may be used. The down side here is that the surface roughness must be considered as part of the gap so this influence is greater when designing for small gaps. Also damage is more likely to occur on the bearing face during a touch down while in motion.

8.1.3 Local flatness

The local flatness, which is the flatness under the bearing at any one time, should be less than 50% of the design air gap. This is a worst-case scenario and in reality it is relatively easy to keep this number less than 10% of the air gap.

8.1.4 Holes in the guide

With respect to holes in the guide surface it should be remembered that orifice bearings simply do not work flying over holes, it is very convenient that porous bearings work well flying over perforated surfaces. For example a 80mm bearing flying over an optical table with ¼ 20 threaded holes on one inch centres will have about 50% of its normal load capacity at 10 microns. The higher the air pressures in the gap the higher the efficiency losses from the holes.

8.2 Stiffness and Preload

Stiffness is an important factor when designing any precision motion system, with bearing stiffness being a significant factor in overall performance. The higher the stiffness, the less compliance there will be when loads are applied. Preloading is a method of increasing bearing stiffness that is used for all types of bearings.

Preloading in roller bearings follows the rules of Hertz Ian contact stresses. Basically, as a ball bearing is pressed against its race, the point or line of contact becomes larger as the load becomes heavier. The larger contact area leads to higher stiffness. In roller bearings this desire to increase stiffness must be weighed against higher friction and wear from the preloading.

Preloading in air bearings follows the rules of fluid dynamics. As air bearings are loaded, the air gap gets smaller and the pressure in the air film rises. Because air is a compressible fluid it has stiffness. If we think of the air gap as a column with a uniform stiffness it is seen that the shorter the column, the higher its stiffness will be. The factors that determine stiffness in air bearings are the pressure in air gap, the thickness of the air gap, and the projected surface area of the bearing.

Practically it may be difficult to comprehend how an air bearing could ever have stiffness as high as a roller bearing, which is in physical contact. It should be remembered though that a point or even a line of contact has theoretically no area. Such minimal contact area creates very high local stresses and so requires very hard materials to avoid deformation. In air bearings the load is transmitted through an air gap that is projected over an area several orders of magnitude larger than roller bearings. This wide air gap is also an important function in damping, which can be very advantageous in precision systems.

Air bearings can be preloaded using added mass, magnets, vacuum, or by mounting 2 air bearings on opposite sides of a guide rail. Adding mass often runs counter to the requirements for high acceleration and fast settling time. Magnetic preloading requires that the guide surface be metal under each axis, so metal strips would need to be inserted on a granite base, adding to the complexity and cost of the structure. Air bearings are most frequently preloaded by configuring air bearings opposite each other as shown in Figure 8.1. This requires a significant amount of space, requires two parallel surfaces, and doubles the total mass of the bearing components. Vacuum preloading offers an elegant solution that minimizes bearing mass and height, and can be utilized on granite as well as metallic guide surfaces.

Figure 8.1

8.2.1 Now on to ways of preloading air bearings:

8.2.1.1 Air bearings can be loaded with mass. An example of this might be moving a large object about on the surface of a precision granite surface plate or optical table. This is usually best on three bearings as three points describe a plane. When a known mass is to be supported, the bearings should be sized so the load that each bearing carries drives the air Gap to the desired area of the lift load curves or stiffness.

Sometimes stiffness is not an issue, for instance there may not be a change in load. Or their might be a desire to fly the bearings on surfaces that are less than flat. For instance in the case of an aluminium optical table with quarter 20 holes on 1 inch centres we'll often find raised high spots. Here, selecting an oversize bearing would be wise as a will result in the higher flying height and withstand more abuse.

8.2.1.2 Air bearings can be preloaded with vacuum. Air bearing lands that are finished at the same plane as the bearing face can be used as a vacuum seal. It is an additional factor that an air gap which is being pressurized with air can be a seal for the vacuum, but it works effectively. When we consider that a vacuum preloaded bearing may consume less than 5 cubic feet of air per hour and that only half of that will find its way into the vacuum chamber it is easier to see. Ambient pressure groves between active air bearing areas combined with seal lands dramatically reduce even that small flow into the vacuum. The vacuum load is created in the centre area where vacuum is drawn. By drawling vacuum in this area, outside atmosphere actually presses down on the Bering creating a pre load force equal to the projected area of the vacuum pocket times the pressure differential. This pressure differential can easily be two-thirds of a perfect vacuum, which is 20 in. of mercury or negative 10 PSI. The advantage of vacuum preloading is that it creates a Preloading force on the bearings without adding mass. Also preloading is possible over a plain in X and Y without having the guide surface be metal as would be required with magnetic preload. Large monolithic vacuum preloaded bearings say for instance 12in. square can create over 800 pounds of preload force of the and stiffness well over 2 million pounds per inch, with only a single pound of payload. This technique is used to advantage when high acceleration stages need fast settling times. By guiding in X and Y off of a precision plane the Abby errors and tolerance build-ups from stacked linear axis are eliminated providing for exceptional flatness of motion.

8.2.1.3 Magnets can create a preload just like vacuum without adding much mass. Metal strips parallel to the guide surfaces can be used for preloading linear axis. But in order to preload in X and Y the whole guide surface must be metal. Magnets are often used between two air bearings, mounted in some sort of a threaded holder which allows adjustment of the magnets to the metal strip. By adjusting the Clarence the preload force can be optimized.

8.2.1.4 Air bearings are most frequently preloaded by another air bearing. In this case it is possible to double the stiffness of the assembly as the stiffness of the two bearings are additive. It is usually best to try to preload bearings directly opposite one another to avoid structural damage.

9. Factors affecting the performance of air bearings

Bearing plays a rather important role in our industries. There are various factors related with it such as the environment, preload, and stiffness, etc. All these various factors may have much effect on the bearing performance. Air bearing is a special kind of product. The above mentioned factors can also have much effect on them. It is quite necessary to study these factors in detail.

Firstly, let us see the effects of environment on the air bearing performance. Temperature is an important element for every product. Room temperature is suitable for them. A variance of +/-30°F is acceptable in almost all applications. We should pay much attention to thermal effects on the structure of products. Besides, oil dripping and water dripping can cause many problems to these bearings. They will fill the air gap and create drag. Therefore, they should be avoided on the guide.

Secondly, stiffness is a significant factor in overall performance. It is an important factor when designing any precision motion system. The higher the stiffness, the less compliance there will be when loads are applied. There are many factors which can determine stiffness in them such as the pressure in air gap, the thickness of the air gap, and the projected surface area of the bearing.

There are various ways of preloading air bearings. They can be preloaded by using added mass, magnets, vacuum, or by using another air bearing. For instance, they can be preloaded with vacuum. Air bearing lands or inactive surfaces which are finished at the same plane as the bearing face can be used as a vacuum seal. It is counter intuitive that an air gap which is being pressurized with air can be a seal for the vacuum, but it works quite nicely.

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