design, realization and experimental investigations into the performance parameters of air-bearings

7/23/2019 Design, Realization and Experimental Investigations Into the Performance Parameters of Air-Bearings

1/9

Design, Realization And Experimental Investigations Into The

Performance Parameters Of Air-Bearings

Kuldeep Sharma1

, A. John Vivian Prashant2& Dr. V. Radhakrishnan

3

1 & 2

(Bachelor of Technology, Department of Aerospace Engineering, Indian Institute of Space Science & TechnologyEmail: [email protected], Email:[email protected])

3(Emeritus professor, Department of Aerospace Engineering, Indian Institute of Space Science & Technology

Email: [email protected])

ABSTRACTThe design and implementation of air-bearings has to be

dealt with approximations and empirical relations due to

complexity of flow physics involved. For preliminary

design purposes simulation results from the

computational fluid dynamics softwares have been used.

The 3-D segmented configuration with reservoir hasbeen identified as the efficient simulation technique for

such geometries after many trials with various schemes

to capture the flow phenomenon in the most efficient

manner. The manufacturing drawings are also provided.

After preparing the actual model, parametric studies

have been performed to evaluate the effect of various

design parameters on the static and dynamic

performances of air-bearing platform and have been

explained through graphs. Also through a method of

trial and error an efficient configuration has been

deduced which consists of a slot at the bottom surface of

the Air-bearing. With this configuration it has beenobserved that the bearing provides the highest load

capacity and produces the minimum amount of vibration

noise due to the air flow. Then finally, four air-bearings

have been put together on a platform and integrated

which can be used for real time load transport purposes

with a capacity of 250 kilograms. Detailed design and

manufacturing details are provided for re-creation of the

air-bearings and related experiments.Keywords-Air-bearing, Parameter study, flow physics

simulation, Stiffness1. INTRODUCTIONThe fluid bearings consist of hydrostatic and aerostatic

bearings. The aerostatic bearings are further classified into

the orifice and porous media type air bearings. In the orifice

type bearings air flows out through the holes and grooves

whereas in a porous air bearings flow is naturally controlled

by the permeable nature of carbon materials [1],[2]. Air-

Bearings have various advantages when used in shop floors

for load lifting purposes [1]. To begin with they have zero

wear due to no contact between the bearing and the surface.

Being non-contact air bearings they average out the surface

irregularities and provide straighter motion. Also their

operations are silent and smooth[3]. And being fluid film

bearings they have a squeeze film damping effect resulting

in higher damping. This results in better stiffness and

controllability. They also result in high speeds at high

acceleration because there are no balls or rollers to slip at

high acceleration. This results in efficient use of power.

There have been many innovations in the design of air-

bearings since its conception. One such innovation is ofusing elastic orifices for pressurized gas bearings. The

elastic orifice allows the self control of the flow. The flow

rate either increases or decreases as the pressure drop across

the device increases[5]. As a consequence of this these

bearings tend to have greater stiffness.

One of the many commendable uses of air-bearing is in

spacecraft simulators

[6]. They are used for spacecraft

attitude determination and control hardware verification.

They offer one of the possibilities to the problem of

simulating a functional space environment. Though these do

not provide a gravity free space for spacecraft simulation,

they do provide a torque-free and force-free rotational and

translational motion. These environments achieved by airbearing are used to check the satellite control systems on

ground. It is very important to know the variouscharacteristics, specifications and certain conditions which

are essential for the optimum performance of air-bearings.

Many parameter influence the performance of air bearings.

And the effect of various parameters has to be verified with

experimentation due to the uncertainty involved with

empirical relations. An optimum bearing should have high

efficiency of load capacity, high stiffness and good stability

during levitation. In general Air-Bearings run on a

controlled film of pressurized air that is typically less than

0.3 mm thick[3].

While talking about air-bearings its very important to talkabout Pneumatic hammer instability. This phenomenon is

associated with the compressibility of gases and the

consequent delay between bearing clearance changes and

the response to this change through variation in pressure in

the orifice pocket[8].A long delay time and a large pocket

volume result in pressure increase due to which the bearing

clearance increases. This results in the reduction of pressure

in the pocket and again the clearance decreases. This results

in the eventual increase of pressure and hence the cycle

continues. This instability is to be avoided as it is a major

deterrent to the smooth and quiet operation of the air-

bearing. Air-bearings have only viscous friction associated


2/9

with the air-film layer being sheared during motion of the

bearing. Thus using high speed spindles the bearing gap

should be large enough to ensure that the friction power is

less than twice the pumping power [10].

2. Design Process

Considering the above mentioned pros and cons of various

types of air bearings we have decided our configuration tobe an Aerostatic bearing. It is a pocketed bearing with a

metal orifice nozzle. Once the type of Air-Bearing was

decided the iterative process of design commenced. To

begin with, fluent simulations were run to get an idea of the

pressure profile and load capacity by trying different pocket

sizes and nozzle diameters. Here it is essential to note that

fluent cannot give accurate results. This is due to the fact

that in the real scenario the surface finish of the air-bearing

can maximum goes up to three triangle finish since the

material is aluminum and grinding is not possible. Also the

surface on which the bearing is to be used cannot be of high

finish. But in fluent these factors cannot be incorporated.Analytical solution to this flow problem is also complicated

due to the axial flow which is followed by radial flow. Thus

the only possibility is to take the results from simulation just

as a guideline. Here we have tried different configurations

and once we were close to the load capacity we desired we

had locked on the initial parameters. Then with the help of

empirical relations the various geometry dimensions are

calculated and finally the manufacturing drawings were

done. Here also special features have been introduced so

that the bearing can be used again even if the nozzle is

damaged. To achieve this, dismountable features were

introduced so that the nozzle can be changed as in when

required.

3. Simulations

As stated previously the simulations cannot exactly capture

the flow phenomenon mainly due to the complex geometry

and also the surface roughness effect which cannot beincorporated in fluent software. To start with 3D simulation

was carried out with a pocket depth of 1mm and pocket

diameter of 5 mm. But it was noticed that the pressure

profile was not flat as per intuition, thats why we went for a

process of carrying out a 3D segmented simulation along

with a reservoir. This method yielded good convergence and

also the results comply with the flow physics of the

problem. It can be stated that 3-D segmented simulationwith a reservoir is a good method to simulate the flow in a

circular air-bearing. After establishing and achieving a

successful simulation method some other configurations

were also tried.

For all the above mentioned techniques employed various

pressure and velocity plots have been presented. Along with

that the grid independence has also been performed not only

by decreasing the grids but also by changing the whole

pattern itself. This is formidable evidence that the procedure

of simulation has been correctly performed. And this also

supports the claim of having encountered a good simulation

technique for air-bearing flow problems.

3.1 Boundary conditions

Table 1: Boundary conditions

Inlet total pressure 6 bar

Inlet static pressure 5.9999 bar

Outlet pressure 1.1325Simulations were carried out at T=288K.

3.2 Gridding & Grid Independence Test

Fig.1: Grid 1 & Grid 2 & Grid independence test for Grid 1

3.3 Simulation Results

Table 2: Air Gap = 1mm (Incompressible Solver) Grid 1

Zone name Pressure force (N)

Bearing 39.260757

Continuity equation satisfying criteria

Zone name Mass flow rate (kg/s)

Pressure outlet -0.0096766734

Upper hole 0.0096766874

Net 1.3969839 * 10-08 kg/s

Fig 2: Velocity profile in the air gap for grid 1

(Incompressible solver)


3/9

Fig 3: Velocity profiles in air gap for grid 1

(Compressible solver)

Pressure Contour at Z=0

Velocity magnitude plot along the Z- axis through the hole

Fig 4: pressure and velocity profiles at different z locations

in air gap for grid 2 (compressible solver)

3.4 Gridding with 3-D Reservoir

Fig 5: Typical segmented 3-D grid for air bearing

Fig 6: velocity profile in air gap

(1mm pocket depth 5 mm pocket diameter)


4/9

Fig 7: pressure profile in air gap


Fig 8: Velocity profile in the air gap


Fig 9: Pressure profile in the air gap


Fig 10: Velocity profile in air gap


Fig 11: Pressure profile in air gap


4. Empirical Relations Used For Design

The empirical relations given in Kenneth J Stout [1] have

been used to design our air-bearing. In case of pocket

compensated air-bearings that we have chosen the author

states that generally the predicted and the measured valuesgenerally agree with an accuracy of 10%.In addition to the

calculation of various parameters of the air bearing as per

the empirical relations certain factors have to be kept in

mind to avoid degradation in the performance of the air

bearings. First the advantage of selecting a pocketed orifice

is that it gives up to 1.5 times greater stiffness than annular

orifices. But with pocketed orifices one has to pay attention

to avoid pneumatic hammering. To prevent pneumatic

hammering the pocket geometry should be designed such

that the total volume enclosed in the pocket is to be less than

one-twentieth of the bearing land volume. Also the recess

depth should be equal to or greater than orifice diameter. As

a rule of thumb it should be ensured that the curtain flow

area is at least twice the orifice flow area. The design

equations used for pocketed bearing design have been given


5/9

for an outer to inner radius ratio of the main disc of 20.

Based on this the Ri and Ro where chosen to be 2.5mm and

50mm respectively. Then an air gap of 45.26 m was

chosen. The supply pressure was chosen to be 6 bar as per

availability in the lab. Form these values and using the

empirical relations all other parameters were calculated.

These values have been present in a tabular form. After thecalculation of these parameters checks were done to ensure

that there should be no pneumatic hammering as per thedesign values.

Fig 12: Schematic figure representing the parameters which

were decided on basis of empirical relations

Table 3: The Characteristic properties of the pocketed air-

bearing obtained using empirical relations

Max stiffness (N/m) =18.9895

Max load (N) = 477.5

Air flow rate (m3/s)

Mass flow rate (Kg/s)

= 4.837 * 10-4

Orifice diameter (mm)

Pocket depth (mm)

b= 0.905

Ro= 50 mm , Ri = 2.5mm , do= 1mm

Pa= 1.01325Bar , Po= 6Bar

Where, is supply pressure, is air film thickness, is

ambient pressure, is outer radius of bearing, is radius

of the pocket.

5. Drawing

5.1 Design Drawing

Fig 13: Top & Bottom part of air-bearing

Fig 14: Nozzle &Nipple

Fig 15: Top &Bottom part of the nozzle supporting disc

Fig 16: Common Connector Blown up view of all

components

5.2 Manufacturing Drawings


6/9

Fig 17: Manufacturing drawing of an air-bearing

Fig 18: Manufacturing drawing of the connector

Fig 19: Manufacturing drawing of base plate

6. Component Manufacturing And Integration

Firstly the main disc of the air-bearing, the connecting disc

and the nozzle are manufactured by turning process on the

lathe. Then the nipple is also manufactured by turning. The

complete assembled air-bearing is shown in the pictures.

After making four such bearing they are all connected to a

common air supply source. The four bearings are finally

attached to the platform via balls that allow the four of them

to adjust and be parallel to the surface. The whole sequence

is shown through actual pictures taken while assembly. Also

pictures of experimental apparatus used are shown.

Fig 20: Actual pictures describing the integration process

and experimentation in sequenc


7/9

7 . Experimental Results

7.1 Surface Roughness Measurements

The surface finish measurements suggest a three

triangle finish which is the recommended finish for

air bearings.

Table 4: Surface roughness values for bearing surface

Air-

bearing

piece

Material of

the air-

bearing

Ra(m) Rq

(m)

Rz

(m)

1 Aluminum 0.333 0.409 2.271

Brass 0.664 0.88 4.699

2 Aluminum 0.836 0.978 3.782

Brass 0.937 1.27 6.977

7.2 Maximum Stiffness For Various Configurations

Table 5: Stiffness measurements with variable pocket

depth, pocket diameter and nozzlePocket

Diameter

(mm)

Pocket

Depth

(mm)

Nozzle

(mm)

Maximum

Stiffness

(N/m)

5 1 1 19.62

5 1 1.5 9.81

5 1 2 9.81

5 1 3.3 13.08

8 1 1 9.81

10 1 1 13.08

12 1 1 19.62

17 1 1 1.96228 1 1 19.62

5 2 1 9.81

8 2 1 13.08

12 2 1 6.448

17 2 1 3.224

28 2 1 1.612

7.3 Friction Coefficient

For calculating the friction coefficient the bearing

was loaded with weights and a thread was connectedto it on which weights were added until the bearing

just started to move. The Normal force on the bearing

was 727.9 N and the frictional force was observed to

be 0.34335 N. Thus the friction coefficient which is

the ratio of lateral force to normal force is calculated

to be 4.71698 x 10-4

.

7.4 Air-gap and stiffness plots

The supply pressure was changed manually using a

pressure regulator. The change in air gap was

measured using a dial gauge having a least count of

1m. Loading was done using identical 4 kilograms

plates .The bearing was placed on a granite table.

Two V-blocks were used to avoid any lateralmovement of the air-bearing. Only the change in air-

gap was measured along with the loading of the

bearing with the loads.

Fig 21: Air gap variation with supply pressure

Fig 22: Air-gap Vs load with varying nozzle diameter


8/9

Fig 23 : Air-gap Vs load for pocket depth 1mm withvarying pocket diameter

Fig 24: Air-gap Vs load for pocket depth 2mm with

varying pocket diameter

Fig 25: Air-gap Vs load with varying taper

Fig 26: Air-Gap Vs load for the slotted

configuration

8. Discussions

Firstly we discuss about the surface roughness

measurements. Though the finish is of the order of

three triangle finish, still more superior finish can be

obtained by grinding. But for grinding we cant use

aluminum. Therefore we would have to go in for

some other material which can be ground. This would

result in a better distribution of the pressure profile

and hence help in improving the air bearing

performance.

Now we will look at the salient features in the

pressure and stiffness plots. Figure 21 displays an

increase in the air-gap with increase in the pressure.

This is in accordance with our intuition. But if thepressure is increased to vey high values it may result

in pneumatic hammering. The design supply pressure

for the bearing is 6 Bar and it has been pressurized up

to 8 Bar without any signs of pneumatic hammering.

Thus this whole range of pressure values may be

described as a safe range for the operation of the air

bearing. It may also be noted that the plot becomes

flatter after pressure values of 6 Bar. This shows that

there isnt much benefit in operating at higher

pressures.

Figure 22 shows that the air-gap decreases as the

nozzle diameter increases at lower loads but at higher

loads it shows a very scattered and unusual trend.The trend shown at lower loads can be explained with

respect to expansion of flow in the nozzle. Due to this

pressure decrease the air-gap comes down. But at

higher loads though the pressure is decreasing, there

is also some pressure buildup due to the air-gap

reduction. Due to this opposing effect a very

scattered pattern is obtain. Comparing Figure 23 and

Figure 24 we find that the pocket depth plays a major

role in the response of air-gap to load with varying


9/9

pocket diameter. When the pocket depth is 1mm a

continuous fall in air-gap is observed with increasing

pocket diameter. But in case of pocket depth 2mm

this trend is seen only up to a certain initial loading.

At higher loads the variations are complex. This

shows that a simple linear extrapolation would not be

sufficient to draw conclusions on the behavior of the

air-bearing stiffness. One more important conclusion

is that the hammering loads are different while

loading and unloading. A general trend has been

observed that while unloading the hammering

sustains even till very low values of loads.

Figure 25 shows that the air-gap increases with taper

angle. And the increase is still higher if there are

slots. Slots seem to increase the stiffness. Due to this

observation various slotted configurations were tried.

After some trial and errors the most efficient slotted

configuration in terms of high stiffness and less noise

production was obtained. This best configuration is a

slot of depth 0.2mm and width of 5 mm made at a

radial distance of 22.5 mm from the centre of thebearing. Figure 26 shows that the best configuration

has the highest air-gap at initial loads and there is no

hammering observed even unto a load of 70

kilograms.

9. Conclusion

The aim of design, realization and experimentation

has been successfully completed. The design and the

experimental values of stiffness have a deviation of

1.5% which is a very excellent result. The most

accurate technique for simulation of air-bearing

problems was found to be by a 3-D segmentedmethod with a reservoir. Both results of 3-D and 3-D

segmented with reservoir simulations have been

described so that they could be used as a guideline

for further research into this problem. The whole

process of integration is very clearly explained

through pictures. Stiffness experiments were done

which resulted in identifying the best configuration in

terms of air-gap, absence of pneumatic hammering

and silent operation. This best configuration is a slot

of depth 0.2mm and width of 5 mm made at a radial

distance of 22.5 mm from the centre of the bearing.

These experiments can be recreated to carry out

further studies.

REFERENCES

[1].Stout, Kenneth J. , Aerostatic Bearings.Trans Cambridge Philos, vol.22.

[2].Schwendeman, T.Witelski and P.Evans,Analysis of Pressurized Porous Air

Bearings. University of Delware : 20th

Annual Workshop on Mathematical

Problems in Industry.

[3].Byron R. Knapp, Brian P.OConnor andEric R.Marsh , Characterization of Porous

Graphite Air Bearings.Vol.119, August

1997, Journal of Manufacturing Science and

Engineering, pp. pp. 388-392.

[4].R.L.Kiang and P.M.Newgard, ElasticOrifices for Pressurized Gas Bearing.1966,

ASLE Transactions 9, pp. 311-317 .

[5].F.Sweeney and .J.Stout, Design ofAerostatic Flat Pad Bearings Using

Pocketed Orifice Restrictors. August 84. ,

Tribology International. Vol. 17 No.4, .

[6].6. . A.H, Slocum and Dearborn Michigan,Precision Machine Design: Society of

Manufacturing engineers, 1992.

[7].7. F.Sterry and Montgomery A. G, A simpleair bearing rotor for very high rotational

speeds,July 1955., AERE ED/R 1671.

design, realization and experimental investigations into the performance parameters of air-bearings

Documents