SAVITRIBAI PHULE UNIVERSITY OF PUNE

1.INTRODUCTIONIt has been a long time since the industrial revolution started in England. We have now entered in 21st century. As the industries have gone automatic, it is obvious that there has been tremendous advancement in the technology in every field of machine design. Bearings are no exception to it.

BEARINGS: - Bearing is a machine member, which supports moving parts & confines its motion. Bearings are broadly classified as Contact & non-contact bearings. Air bearings fall under category of non-contact bearings. Ball and roller bearings fall in category of contact bearings.There are several drawbacks in conventional ball & roller bearings due to factors such as friction, noise, dust collection, power loss, need of frequent lubrication etc. To overcome this drawback engineers started to think about non-contact bearings. That is how air bearings came into existence. Although the air bearings seem to be a new concept, they were invented way back in 1960s. However, their applications were very limited due to their delicateness. They had low strength and that is why they were incapable in most applications. However, due to vast advancement in the technology they are now considered as a replacement to other bearings. Accurate, precise, easy controlled machines are the requirement of the new industries. Quality of product is of utmost importance and hence imprecision, low accuracy, wear &tear, dust collection, play between the parts is intolerable in quality manufacturing environments. In such applications air bearings are gaining preference over conventional bearings such as roller & ball bearings.Even though ball and roller bearings can never be replaced in all the applications, there are many cases in which roller bearings are totally unacceptable because of their shortcomings. Hence, in these areas air bearings can be useful to designers.

2.Basic principle Even though ball and roller bearings can never be replaced in all the applications, there are many cases in which roller bearings are totally unacceptable because of their shortcomings. Hence, in these areas air bearings can be useful to designers. The general form of air bearing can be understood from the figure.

Layout of General Air Bearing One can visualize air bearings as two compressed surfaces separated by a small air gap running in micro inches. As the load goes on increasing, the air gap reduces in width and the effective pressure in of air increases linearly. A constant supply of air resists the two surfaces form touching each other.Bearings typically operate in pressures ranging from 200Mpa to 900Mpa according to the loading. The required pressurized air is obtained from compressors. Due to use of air film, the friction is negligible, which is the main feature of air bearings.

3.TYPES OF AIR BEARINGSA) ACCORDING TO ORIFICE STRUCTURE The air bearings are classified in two types, according to orifice structure. They are as follows.1) ORIFICE BEARINGS In orifice bearings, the air passes through standard diameter orifices, which are located where pressure location is most convenient. The orifice air bearings are classified as follows, SINGLE ORIFICE AIR BEARINGS In this type of air bearing the pressurized air passes through geometrically central orifice. In this type, the pressure is maximum at the hole as it supports major share of its weight. Load bearing capacity of it is very less.

MULTIPLE ORIFICE AIR BEARING Air passes through more than one hole & hence multiple site support is given to support the bearing. MATERAIL USED: Orifice air bearings can be made from metals, alloys and alloy steel. They are mostly made up from aluminum, duralumin. Material with high bending & shearing strength are suitable for orifice bearings.

2) POROUS AIR BEARINGS These air bearings overcome all the shortcomings of above air bearings. The porous air bearings have a honeycomb structure. Hence, the air passes through millions of pores having diameter of few microns. As these pores are distributed over entire bearing surface, the bearing provides support throughout the bearing area. Hence, pressure gets distributed over entire bearing area & the chances failure is also lessoned. MATERIAL USED: Porous air bearings are made only from graphite carbon or porous ceramic, as they require an effective in-built porous structure 3)AERODYNAMIC BEARINGSThese bearings are commonly known as self induced pressure bearings. This is because no external pressure is required. Bearings pressure is generated within the bearing due to velocity induced, viscous shearing of the air film and high speed of shaft. 4)AEROSTTIC BEARINGSAerostatic bearings require an external pressurized air source. This air pressure is introduced between bearing surfaces by precision holes, grooves, steps or porous compensation techniques. B) ACCORDING TO PRESENCE OF PRELOADAir bearings can also be classified according to the presence of reload.i) NON-PRELOADED BEARINGS In this kind of air bearings, no preload is used to stabilize the bearing. ii) VACCUME PRELOADED AIR BEARINGS (VPL)These bearings are incorporated with a vacuum port along with pressure port. Simultaneous application of vacuum and pressure at different areas stabilize the bearing.4.Some important featuresBearings typically operate in pressure ranging from 200Mpa to 900Mpa according to the loading. The required pressurized air is obtained from compressors. Due to use of air film, the friction is negligible, which is the main feature of air bearing.Oil cannot be use at very low temperature or for high temperature operating condition because oil is liquid at normal temperature then becomes solid at very low temperature (-100C) and gaseous at very high temperature (600C). This problem can be avoided by air bearings.Air bearings form of general bearing which uses pure air as a lubricant. Air has a viscosity, which is approximately 100 times less than that of kerosene. And nearly 1000 times less than industrial oil.Higher precision ,increase in accuracy, better resolution , clean room compatibility, longer life and improved reliability are all inherent characteristics of air bearings.

4.WORKING OF AIR BEARINGS PRINCIPLE The principle applied to all air bearings is same. Air bearings work on the principle that, whenever a thin film of lubricant is used between two mutually dynamic compressed surfaces, the friction is reduced. The difference between conventional bearings and air bearings is that the supporting layer is also the fluid film rather than contact support.In case of air bearing, the lubricant is nothing but air. In addition, its a well-known fact that the viscosity of air is less than hundredth of oil, which reduces the power loss. Using the principle there are many types of air bearings are in application today.

WORKING OF ORIFICE AIR BEARING The orifices may be accompanied by grooves. The air passes through orifices on bearing surface, which maintains a thin film between two surfaces. Working of orifice air bearings

WORKING OF POROUS AIR BEARINGSThe principle of working of air bearings is same as that of orifice air bearings. The only difference is that no. of pores run into millions, which are uniformly distributed over the area of bearing surface.The benefit of porous bearings is that the air flows through the bearings microscopic honeycomb structure. The result is a large bearing support surface and a significantly stiffer bearing with a small air gap between air bearing and granite or ceramic guide.

COMPARISON OF ORIFICE AND POROUS AIR BEARINGS

From above graph, we can easily understand the difference between the orifice and porous air bearings. The pressure distribution in both single and multiple orifice air bearings is concentrated at orifice locations. Whereas in porous air bearings the pressure is distributed over the entire bearing surface, providing more stability and stiffness.

WORKING OF RADIAL AIR BEARING These bearing can either be porous or orifice air bearings. These bearings give radial support to the rotating member. SPHERICAL BEARINGS These bearings are mostly axially supporting. The bearing surface is spherical in shape thats why the name. VACCUME PRELOADED BEARINGS The fundamental principle of Vacuum Preloaded Bearings (VPL) is to create a vacuum under the bearing surface. Simultaneously pressurized air applied to bearing surface prevents the surfaces from making physical contact. Air bearing lands or any inactive surfaces on same plane as the bearing face can serve as a vacuum seal.

Vacuum Preloaded Air Bearings It is relatively easy to create a vacuum of 20 in. of mercury (-10psi), almost two-thirds of a perfect vacuum. A large monolithic VPL (for example, 12 in.2) can create over 800 lb of preload force and a stiffness of well over 2 million lb/in., with only a single pound of payload

5) PREREQUISITES FOR AIR BEARINGS As such, there are no prerequisites for air bearings except the air used in it. The standard & quality of pressurized air supplied to air bearing must be maintained to increase the life of bearing and avoid early failure of bearing. An efficient system ensures minimum pressure loss, removal of contaminants such as water, rust and other foreign particles. Air should be clean i.e. free from particulate matter. Although the air bearings are self-purging, the air supplied to it must be made to go through a series of filters so that large sized particles i.e. particles more than 50 microns are removed. Moisture and oil: The air should be perfectly free from moisture and oil as oil vapour. The moisture content in air should be negligible as far as possible and the amount of oil/oil vapour should be below 10mg/m3. Bearings typically operate in a pressure range from 30 PSI to 120 PSI. A variety of compressor-dryers, which are ideal for this application, are available. Air bearings can also be operated from compressed dry nitrogen. Following are the filters used for various purposes. General-purpose filters are mostly used for cleaning air. Coalescing filter is used to remove oil and liquid wat

6. ADVANTAGES AND DISADVANTAGES OFAIR BEARINGS

ADVANTAGES ELIMINATION OF FRICTION RELATED PROBLEMSSemiconductor manufacturing, high-resolution scanning and high-speed machinery are just some of the applications pushing rolling element bearings to technical limits. One reason is friction. Friction variation has always been Achilles heel of precision positioning systems, particularly trying to initiate or stop the motion precisely. This is because; plane bearing and rolling bearing to a lesser degree have a higher static friction than dynamic friction coefficient. In other words, more force is needed to initiate the motion than to maintain it. The stick-slip action is more pronounced in plane way systems, though rolling element bearing suffer from it. Air bearings are not prone to this phenomenon because they are contact less bearings and friction due to air layers is negligible as compared to solid contact. Hence, stick-slip action is avoided and extraordinary precision in motion and stiffness is achieved.The difference between mst and mdy is the reason of stick-slip action, which increases inaccuracy and less control.

Elimination OF HEAT GENERATION DUE TO FRICTION AND THERMAL EXPANSION Friction generates heat, which in turn causes spindles and other components to thermally grow, compromising precision. Air bearings generate much less amount of heat in a given application than rolling element bearings in most cases. In fact, relative speeds should be more than 40m/sec before air bearings generate any significant heat at normal gaps. Air bearings excel in applications requiring tight velocity control, such as scanning and wafer inspection. Because they eliminate force ripples from recirculating balls bearings loading and unloading.

NO REQUIREMENT OF LIFE TESTINGAdvanced, high-speed, high reliability machines may run a billion cycles annually. It is therefore impractical to do accelerated life testing on such equipments. Engineers typically estimate bearing life based on speed, acceleration and load. Air bearings dont require such life calculations because bearing components dont touch each other. Air bearings will work as new after millions of cycles. The only mode of wear in air bearings is erosion from feed itself.

SAFE FROM PARTICLE GENERATIONLack of wear debris and no oil lubrication makes air bearings well suited for use in clean room, medical, pharmaceutical and food processing application. Air bearings also excel in dry, dusty and corrosive environments such as salt or sugar factories where oil lubrication would quickly become lapping slurry. Air bearings instead self purge and blow away light dry dust.

HIGH STIFFNESSMost people visualize the air bearings as a hovercraft and they erroneously conclude that air bearings floats on air and cant be very stiff. Actually, these bearings are many times stiffer than a ball or roller bearings. As the load is applied on an element in motion, the air film gets pressed creating sudden pressure in the orifice. As a result, the air pressure pushes the element back to its position. Hence, a stabilized, pressurized air film is created which remains uniform till the load is applied. This results in bearing stiffness of millions of pounds per inch for small deflections.Stiffness is linear and does not change with temperature. In contrast, ball or roller bearings almost have no stiffness unless heavily loaded. The stiffness of a ball bearing is not linear and varies considerably with temperature.

HIGH PRECISIONAir bearings can be made with a precision, which is 10 to 100 times better than any other type of bearing. The motion of an air bearing follows the average shape of rotor so that the precision of motion is better than the precision of surface of the bearings. Since there are no balls or rollers, the motion is completely smooth.

SILENT AND SMOOTH OPERATION Recirculating roller or balls create noise and vibration as hard elements become loaded and unloaded and change direction in return tubes. This is especially noticeable in resolution of scanning.

HIGHER DAMPINGBeing fluid film bearings, air bearings have a squeeze film damping effect resulting in higher dynamic stiffness and better controllability.

ELIMINATES OILAir bearings dont use oil lubrication, eliminating the problems associated with oil. In dusty environments (dry machining) ways are dry and bearings are self-cleaning, because positive air pressure pushes dust away. In contrast, oil lubrication becomes lapping slurry.

HIGH SPEEDS Air bearings have special excellence in high-speed applications. They can be successfully operated at speeds up to 3, 00,000 r.p.m. with only a slight temperature rise due to shear friction in air film.

NO DAMAGE BY SHEAR LOADS Air bearings are also more rugged than any other type of air bearings. Shock loads will flatten or brinnel the balls in ball bearing but air bearings has no points of contact and can usually withstand loads 10 to 100 times greater than its load rating. CORROSION RESISTANCEAir bearings are more resistant to corrosion than any other type of bearing. The constant flow air also serves to dry the bearing.

OPERATES IN DIRTY ENVIRONMENTSOne may think that such a high precision bearing would be sensitive to dirt or adverse environment, but just the opposite is true. The gap between the rotor and stator is larger in air bearing than it is in ball bearing. Therefore, air bearings can be operated in environments, which are too dirty for ball or roller bearings to survive.

SELF-PURGING The air bearings are capable of cleaning themselves. Because of pressure created due to air, light, dry dust is thrown away with force cleaning the orifices.

CAN SUSTAIN HIGH LOADBearings typically operate in a pressure range from 206Mpa to 824Mpa. Thus, we can imagine the amount of load the air bearings can sustain. Even if this is true, the air bearings are suitable only for lightweight, high-speed machines.

7) DISADVANTAGES AIR BEARINGS HIGH COST Air bearings are relatively costlier than other bearings. The high precision in manufacturing, costly material adds up to its cost. However, the cost is mainly high because of monopoly of western countries over the manufacturing.

NEED OF PRESSURIZED AIRRoller and ball bearings do not require the pressurized air. The air bearings need pressurized air. The infrastructure, instruments are required which increase the cost.

LOADING SHOULD NOT REACH MUCH ABOVE RATED LOADINGIf the loading exceeds rated loading the bearings can compensate the effect up to certain limit but if the load is too high above rated loading then the surfaces will touch each other and instantly get cold welded and an accident can occur.

7) APPLICATIONWith increasing awareness about air bearing and increasing strength, stability and load bearing capacity the range of application of air also increasing, many modern and advanced tooling equipment and machines are using air bearings as supporting and guiding members.Right now, the air bearings may not be globally used because of cost and the other factors. Nevertheless, as the time is passing by, the air bearings promise to be the replacement for conventional bearings.The applications of air bearings can be divided into two classes of machines and instruments. Precision machining High load applications1)PRECISION MACHINEMost of the companies, which are manufacturing goods, which require high accuracy, precision, and which were nearly impossible to achieve with roller and ball bearings, are employing air bearings. This is because air bearings provide high stiffness, stability, avoid frictional losses, friction related accuracy problems and phenomenal rigidity. Some of the precision-machining applications are as follows. Precision machine tools Coordinate Measuring Machines (CMM) High precision measuring equipments Semiconductor Wafer Processing Medical Machines and instruments Optical lens Production equipment Digital printers Diamond Turning Machines Material Testing Machines Spindles and Friction Testing Memory Disc testing Coating Rolls Bearing Testers Microscope Focusing Of mirrors and lenses

2)HIGH LOADING APPLICATION Radial and axial bearing in manufacturing machines and having heavy to medium loading Guiding Systems, Guiding Slides etc. Heavy Load moving platforms and Flat Air bearings Micro-turbines

8)AN INDUSTRIAL TRIAL Review of an Industrial Trial: To verify the results of air bearings let us study the boring machine as case study. An Industrial trial was conducted at Ampress Works of Wellworthy Ltd. at Lamington Hunts, USA. and the performance of the air bearing boring head within a normal production environment was observed Specifications of Boring Machine Operation Performed: finish boring Tool:-Diamond Tool. W/P:-Aluminum alloy piston Feed Rate:-0.007 per rev. Depth of cut:-0.005 Spindle speed:-1,700 rpm After each test the specimen was measured for roundness, surface finish and tolerance of environment1. Roundness produced:-A simple statistical analysis has been applied to the roundness results and the outcome of the analysis is presented in the following graph. It can be seen that the out of roundness produced by the ball bearing boring head is unto 50% greater than that produced by air bearing boring head. This difference reduces as the feed rate increases but at 0.004 in/rev. it is still 39%. Graphical results obtained represent less improvement margin that is obtained by using an air bearing boring head in place of ball bearing boring head.

2. Surface finish produced With the ball and roller bearing, balls and cage generate a cyclic variation of the axis of rotation of the spindle and this causes variation in the depth of cut produced by successive revolutions of spindle. The waviness is reflected in following graph in terms of C.L.A. values.

Roughness is 28in for air bearings boring head and 42in for ball bearing head. measurement on the chart indicate that the variation of tool position on successive revolutions is six times greater for the ball bearing boring head compared with that for the air bearing boring head. 3. Tolerance of environment The ability of the air bearing boring head to withstand the normal production environment is particularly important if expensive cleanliness precautions are to be avoided. There are no special precautions to protect the air bearings from swarf or slurry. No maintenance was necessary on the air bearing boring head or its air supply filter system during the period of test. CONCLUSION DRAWN FROM INDUSTRIAL TRIAL The feasibility of producing an air bearing boring head capable of satisfactorily performing fine boring operations is beyond doubt. Circularity of bore produced by such boring head will be at least 40% better than that achieved by a ball or roller bearing head. The surface finish in terms of C.L.A.and the surface geometry will be more regular and predictable. CONCLUSION Air Bearings are not a new technology, but they may be a technology whose time has come. That is because air bearings strike at the very heart of the environmental problem of oil lubrication. This paper, which covers the field of air bearings, will help the personnel in field of high precision machine designing. Air bearings have been a do it yourself technology, and they have not been that much easy to do it yourself. Recent changes in air bearing technology and the fact that they are now commercially available have conspired to make them more practical for broad industrial uses and easier for design engineers to use. As the technology is non-stationary, it is sure that the shortcomings of air bearings will soon be removed and the air bearings will be applicable in many more applications and fulfill the demands of machine designers. In short, the air bearings are on their way to become Future Bearings.

REFERENCES BOOKS1) Material Handling Handbook, Editor:-Raymond A.Kulwiec, Sponsored by.:- ASME, Page no.- 256-260 2) Bhandari V.B.:-Basic modes of Lubrication, Publication:-TMH Page no.456-4582) Mehta N.K.:-Air lubricated bearing Publication: -Tata McGraw Hill Publication, Page no.201-203, 266-270

TECHNICAL PAPERS1) G.T.F.KilmisterExternally pressurized Air Bearing Boring spindleUniversity of Southampton U.K.,Gas Bearing Symposium ,I.I.T. Bombay2) Michel KellyBearing System cuts Engine cost3)J. BoumanAn Air Bearing Carriage,University of Southmpton U.K.Gas Bearing Symposium , I.I.T. Bombay WEB SITES 1)http//www.hovair.com 2)http//www.grc.nasa.gov. 3)http//www.space-electronics.com 4)http//www.doverinstr.com COMPANY JOURNALS Hov-Air Bearing Ltd. Washington

RSSOER, Department Of Mechanical Engineering