air bearingSeminar Report

Air Bearings – Next Generation Bearings

1.

INTRODUCTION

It 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 noncontact 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

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Air Bearings – Next Generation Bearings 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. 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.

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2. a)

TYPES OF AIR BEARINGS

ACCORDING TO ORIFICE STRUCTURE The air bearings are classified in two types, according to orifice

structure. They are as follows. i)

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 of member to be.

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Air Bearings – Next Generation Bearings MULTIPLE ORIFICE AIR BEARING Air passes through more than one hole & hence multiple site support is given to support the bearing.

ii)

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.

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Air Bearings – Next Generation Bearings b)

ACCORDING TO PRESENCE OF PRELOAD Air 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.

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3.

MATERIAL USED IN AIR BEARINGS

FOR ORIFICE AIR BEARINGS 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.

FOR POROUS AIR BEARINGS Porous air bearings are made only from graphite carbon or porous ceramic, as they require an effective in-built porous structure.

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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, it’s 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

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Air Bearings – Next Generation Bearings WORKING OF POROUS AIR BEARINGS The 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.

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Air Bearings – Next Generation Bearings 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 that’s why the name.

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Air Bearings – Next Generation Bearings 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.

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Air Bearings – Next Generation Bearings 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 water.

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5.

APPLICATIONS

With 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 applications

Precision machining Most 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

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Air Bearings – Next Generation Bearings  Medical Machines and instruments  Optical lens Production equipment  Digital printers  Diamond Turning Machines  Material Testing Machines  Crystal Pulling  Spindles and Friction Testing  Memory Disc testing  Coating Rolls  Bearing Testers  Microscope  Focusing Of mirrors and lenses

HIGH LOADING APPLICATIONS  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

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6. ADVANTAGES AND DISADVANTAGES OF AIR BEARINGS ADVANTAGES ELIMINATION OF FRICTION RELATED PROBLEMS Semiconductor manufacturing, high-resolution scanning and highspeed 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.

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Air Bearings – Next Generation Bearings 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 TESTING Advanced, 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 don’t require such life calculations because bearing components don’t 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 GENERATION Lack 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.

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Air Bearings – Next Generation Bearings HIGH STIFFNESS Most people visualize the air bearings as a hovercraft and they erroneously conclude that air bearings floats on air and can’t 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 PRECISION Air 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.

STRAIGHTER MOTION Rolling element bearings are directly influenced by surface finish.As the air bearings are non-contact, the irregularities on the surface finish can’t influence the motion irregularity.

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Air Bearings – Next Generation Bearings 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 DAMPING Being fluid film bearings, air bearings have a squeeze film damping effect resulting in higher dynamic stiffness and better controllability.

ELIMINATES OIL Air bearings don’t 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.

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Air Bearings – Next Generation Bearings CORROSION RESISTANCE Air 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 ENVIRONMENTS One 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 LOAD Bearings 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.

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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 AIR Roller 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 LOADING If 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.

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7. CONCLUSION This article, which covers the field of air bearings, will help the personnel in field of high precision machine designing. From the report, we can predict that air bearings will soon replace the conventional bearings in high speed, low loading applications because of their excellence in overcoming the friction related problems. 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’.

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REFERENCES 1)

Types of bearings, working and parts of bearing assemblies. Sundarajmoorthy T.V., Shanmugan N., Machine Design.

2)

Types of Air Bearings, Working of VPL s, Prerequisites of air bearings Website ® newwayprecision.com

3)

Advantages of air bearing, types of air bearings Website ® spaceelectronics.com

4)

Working of orifice and porous air bearing, materials used in air bearings Website ® starretmaterial.com

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Air Bearings – Next Generation Bearings 2)

Working Advantages And statistics of bearings. Devitt Drew, Founder Chief Technology Officer, New way Precision Inc. Machine Design May 8, 2004, Machine Design, August 1, 2005.

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Air Bearings – Next Generation Bearings

ABSTRACT This report has been prepared to assist engineers and designers in the field of machine design, especially the precision machines with high speed and low loading. The information has been compiled on the reports of machine design magazines and the websites of leading companies that are manufacturing the various types air bearings. This report represents the current trend in the bearing and guiding assemblies. Contents on the types & working of various air bearings are given. Principle of air bearings, their superiority over conventional bearings is discussed.

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CONTENTS

Sr. No.

Topics

Page No.

ABSTRACT 1. INTRODUCTION

01

2. TYPES OF AIR BEARING

03

3. MATERIAL USED IN AIR BEARING

06

4. WORKING OF AIR BEARING

07

5. APPLICATIONS

12

6. ADVANTAGES AND DISADVANTAGES OF AIR BEARINGS

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7. CONCLUSION

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REFERENCES

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