Natio.iai Conference on Advancernems ·,nd Futuristic Trer j : in Mechanical and Mnrci ial-, Engineering (Februarv J9 20. 201m

CFD Analysis of Air Bearing

Mandar M. Kulkarni a " S.T. Chavan b, B.B. Ahuja c

a) Department of Mechanical Engineering, Bharati Vidyapeeth college of Engineering, Pune, Indiab) Department of Mechanical Engineering, Bharati vidyapeeth college of Engineering, Pune, India

c) Department of Mechanical Engineering, College of Engineering Pune, India• Corresponding author e-mail:-mandarmk2002@yahoo.com

ABSTRACT

Bearing technology represents one of the age-old problems for mechanical engineers. In AirBearings to facilitate the smooth. working, the fluid film is used by supplying air through the bearinggap. Conical bearing design is the new step by us to facilitate the smootlt working of the rotationalshaft. The air cushioning will provide the smooth motion, and the axial stiffness to sustain the loadscoming on the bearing. To analyze this problem analytically or experimentally is a time consumingprocess. We have used CAD and CFD codes to analyze this FSI (Fluid structure interaction)problem. Our paper explains about the CFD simulation of this bearing. How we have used the CADto model the bearing, the assumptions done by us to simulate this FSI problem in CFD code,simulation for the correlation with the actual testing. This kind of CFD requires lot of accuracy andcomputational capability.

Keywords: Air Bearing, CAD, CFD,FSI

1. Introduction

A bearing is a device to allow constrained relativemotion between two parts, typically rotation or linearmovernentj l]. Bearings may be classified broadlyaccording to the motions they allow and according totheir principle of operation as well as by the directionsof applied loads they can handle. Rolling elementbearings developed in the last century were arevolutionary improvement over the plain bearingsthat had been pushed to their limits in applicationslike electric motors and automobile wheels. Lowfriction bearings are often important for efficiency, toreduce wear and to facilitate high speeds. Essentially,a bearing can reduce friction by virtue of its shape, byits material, or by introducing and containing a fluidbetween surfaces[ 1-2].• By shape, gains ad anlage usually by using

spheres or rollers, .• By material, exploits the nature of the bearing

material used. (An example would be usingplastics that have low surface friction.)

• By fluid, exploits the low viscosity of a layer offluid, such as a lubricant or as a pressurizedmedium to keep the two solid parts fromtouching.

• By fields. exploits electromagnetic fields, such asmagnetic fields, to keep solid parts fromtouching.

Air bearings represent the next logical step inbearing design. Air bearings in general have a proventrack record having been employed in coordinatemeasuring machines for 20 years. The many technicaladvantages of air bearings such as near zero frictionand wear, high speed and high precision capabilities.and no oil lubrication requirements are powerfuladvantages for today's machine designers [I].However, these benefits have not been more fullyutilized to date because air bearings are difficult tomanufacture and they have not been commerciallyavailable until recently.

I Rolling Bearings II Air Bearings I

)

" - . Pedonnance ;;;S~ x Accuracy \

Figurel: Types of Bearing vs. cost [I]

2. Air Bearing

2.1 CONCEPT

Unlike contact roller bearings, air bearings utilizesa thin film of pressurized air to provide a 'zero

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2.3 Preprocessing- MeshingMeshing is an integral part of the CAE analysis

process. TIle mesh influences the accuracy,convergence and speed of the solution. Moreimportantly, the time it takes to create a mesh modelis often a significant portion of the time it takes to getresults from a CAE solution. Therefore, the better andmore automated the meshing tools, the better thesolution[3-4].

National Conference on Advancements and Futuristic Trends in Mechanical and Matcrrals Engineering (February 19 20,2010)

friction' load bearing interface between surfaces thatwould otherwise be in contact with each other. Beingnon-contact, air bearings avoid the traditional bearing-related problems of friction, wear, and lubricanthandling, and offer distinct advantages in precisionpositioning and high speed applications. The fluidfilm of the bearing is achieved by supplying a flow ofair through the bearing face and into the bearing gap.Authors have tried to explain about the new design ofthe Conical Air bearing and study of the fluiddynamics involved in this bearing[I-2].

2.2 Design and CAD

Modeling and assembly is done usingIDEAS II NX software. General construction of AirBearing is as shown in the figure 2, I. As we havediscussed earlier, the bearing has following parts

o Shafto Sleeveo Busho Bodyo End caps

The following figure shows the assembly of the airbearing.

Figure2: Section view of the Conical Air Bearing

The air is passed through the circumferentialgrooves provided on the bearing bush. Through thesegrooves the air circulates around the shaft, This airpressure developed between the sleeve and bushprovides the lubrication.

Figure3: Section view of Bush and Sleeve

For problem we are have to mesh the air cavity forCFD analysis.This meshing is done in two parts.

o Cavity extraction,o Meshing air flow cavity and air gap between

Bush and Sleeve.

Figure4: Cavity Surface extraction

We have used tetra meshing for the air cavityshown in the above figure.Flow through the Bush grooves and the air gapbetween bush and Sleeve are meshed using Hexaelements in CFD software.

As the air gap is very small, to capture the detailsand 10 maintain the element quality Hexahedralelements are the best. The Hexahedral mesh of thebush is done.

using blocking technique. In this method we createthe mesh by building blocks. Following figure showsthe blocking method of meshing.

Figure5: Meshing of Bush Using BlockingMethod

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Figure6: Mesh Cavity - Air Flow Path

The above figure shows the meshed cavity for airflow. We have meshed only half cavity tosimulate symmetric boundary condition.

The analysis will be done only on half part of thebearing; the pressure which we get will be sameon both sides of the bearing.

2.4 CFD AnalysisAs we have discussed earlier, real problem of air

bearing is of FSI (Fluid Structure interaction). Thatmeans it involves solution of structural deformation,Rigid body movements with fluid analysis evaluatingpressure fluctuations on bearing surfaces.Our problem is highly nonlinear and transient innature. So, we are using different approach to solvethis problem which is quasi static approach. We areusing MRF ie. Moving reference frame technique .Inthis method, we rotate the shaft and the viscous forceswill be simulated using turbulence modeling[3].

Steps to solve this problem with quasi staticapproach are.I) Keep zero eccentricity and input certain pressure

through pressure inlet, solve the problem withCFD code [Fluent]. We get static pressure data onthe outer surface of the sleeve.

2) We add different eccentricity and solve theproblem using CFD to get contours of staticpressure on conical surfaces of Sleeve ie.Pressure data surface.

3) Using these types of iterations at differentpressures and eccentricities, we get differentpressure patterns on the pressure data surface.Use this pressure distribution to get reactions atthe bearing surface.

4) We use this pressure data for further analysis, ie.To calculate the reactions at the bearing surfaces.We use structural analysis for this purpose.

2.4.1 AssumptionsThere are following assumptions done for this

analysis of the air Bearing.

I) The Model of Air Bearing is symmetric; we haveconsidered the model as symmetric for the CFDanalysis. We have modeled only half part of thebearing .The inlet pressure and boundary conditionsare applied only on the half part of the bearing. Thisalso reduces the computation time.

2) The air is used as the working fluid used in CFDanalysis of bearing.

3) For analysis of the bearing and computation ofpressures at different eccentricities, we have assumedthe shaft at different positions. ie. Concentric,Eccentric 0.1 mm, Eccentric 0.2 mm.

4) Air is considered as incompressible. As the Machnumber is more than 0.3, we have can consider it asincompressible.

2.4.2 CFD Model Building1) Air cavity is meshed using block meshing and

completely meshed using hexahedral element.

2) Symmetric boundary conditions are considered forreducing model complexity and reducecomputational time.

3) Air inlet boundary condition is used as pressureinlet in Fluent. As we are modeling only half partof air bearing assembly there are two outlets to air.

4) The pressure outlet Boundary condition to beapplied at two outlets, in order to do that the airdomain is extended ahead @ outlet in order tosimulate real life pressure outlet boundaryconditions (Air outlet to atmosphere).

2.4.3 Boundary Conditions and load cases.As we have discussed earlier .In this analysis we

have considered the shaft location at differentpositions.

I. Concentric

2. 0.1 Eccentric

3. 0.2 Eccentric

The load cases are designed in order to study theeffect of inlet pressure with varying the followingparameters.

1) Velocity inlet

2) Static Pressure

3) Reynolds number

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3. Results & Discussion

3.1 Fluid Behavior at different positions ofshaft

Here we have kept the shaft at concentric position.In each case we have kept the inlet velocity constantand at each inlet velocity, the pressure drop at thesmaller and the larger ends are studied againstdifferent static pressures. Static pressure is thepressure measured at any point in the fluid. The actualpressure of the fluid, which is associated not with itsmotion but with its state, is often referred to as thestatic pressure, but where the term pressure alone isused it refers to this static pressure.

-- --------------,5.00

4.50

4.00

1i 3.50+-----------~~--__;~ 3.00

~ 2.50i2.OOi------~~-;, I.SO -

1.00t--='~=----------___;0.50i---=------------___;0.00-1-- - --'

0.00 1.00 2.00 3.00'" 4.00 5.00 6.00 7.00

Static pressure(Bar-g}

Figure7: Plot for Static pressure Vs. Pressure Drop atConcentric position

,--1

-+-Vel·2

-.-VeI-3

-+-Vel-5..-.- .. 1Ie/-ID

Thus if we compare the Static pressure vs. Pressuredrop for O.2mm eccentric shaft, it is observed that thepressure increases and pressure drop reduces.

Pressure drop (PO·1)

6.00 ---------- ..--.-----.-----.--------,

'C" 5.00 i-------------/-=------!l

e 4.00 ---- --. =>": - -1!300-·- ---/ - --....;

! !: 2.00i-------;;>~--------_i~

-+-Vol·2

-a ..V94.3

-.-Vel·5---VeI-l0

i

10.00-I--_--_-~- __ -_-_--i0.00

»:1.00 ---;:::-..-<---

1.00 2.00 3.00 4.00 5.00 6.00 7.00

Static pressure(Bar-g)

Figure8: Plot for Static pressure Vs. Pressure Drop ateccentricity O.2mm

Thus the various parameters were studied for differentpressures and different eccentricities.

As the shaft deviates from its concentric position, itis observed that the fluid tries to push the shafttowards the centre.

The behaviour of the shaft is verified by doing theexperimental setup.

Figure9: Experimental setup

4. Conclusions

TIlls study shows that as the shaft deviates from itsconcentric position, the fluid tries to push the shafttowords the centre. Further this study proves thecapability' of the CAD and CFD softwares forsimulation of the fluid behavior.

References

I. Air Bearing applications and design guide- New llUY bearing.

2. Design and development of orifice type aerostatic thrustbearing. J.P. Khatit , W. Lin , W. J. Lin.

3. Internet sources for Fluid Dynamics.

4. "Practical Finite Element analysis" by - Finite to infinite.

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