Nuclear Instruments and Methods in Physics Research A 678 (2012) 88–90

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Design modification in rotor blade of turbo molecular pump

Munawar Iqbal a,n, Abdul Wasy b, Dimitri Batani c, Haris Rashid a, M.A.K. Lodhi d

a Centre for High Energy Physics, University of the Punjab, Quaid-e-Azam, Campus, Lahore 54590, Pakistanb Department of Mechanical Engineering, University of Engineering and Technology, Taxila 47050, Pakistanc Centre Lasers Intenses et Applications, Universite Bordeaux 1, Liberation, 33405 Talence cedex, Franced Department of Physics, Texas Tech University, Lubbock Texas, 79409, USA

a r t i c l e i n f o

Article history:

Received 25 January 2012

Received in revised form

14 February 2012

Accepted 21 February 2012Available online 3 March 2012

Keywords:

TMP

RPM

Rotor

Blade profile

02/$ - see front matter & 2012 Elsevier B.V. A

016/j.nima.2012.02.030

esponding author.

ail addresses: muniqbal@yahoo.com, muniqba

a b s t r a c t

Performance of a Turbo Molecular Pump (TMP) is strongly related to the frequency of the rotor. As rpm

increases deflection in the rotor blades starts to occur. Therefore, quality of material and blade design

has been modified in order to obtain stable performance at higher speed. To reduce the deformation,

stiffer material and change in blade design have been calculated. Significant improvement has been

achieved in modeling the blade design using CATIA software. The analysis has been performed by

ANSYS workbench. It is shown that the modification in the blade design of TMP rotor has reduced the

structural deformation up to 66 percent of the deformation produced in the original blade design under

the same conditions. Modified design achieved additional 23 percent rpm which increased TMP’s

efficiency.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Turbo Molecular Pump (TMP) is widely used in vacuumtechnology in creating the high vacuum [1,2]. Its efficiencydepends on its rotor speed [3] and molecular weight of the gasto be evacuated [4]. It works on the principle of momentumtransfer. The rotor blades of the pump, hit the gas molecules,which are transferred to the next rotor stage, directed by thestator. TMP runs at high velocities that ranges to thousands ofrpm. Due to high velocity and high momentum impact, deflectionin rotor blades is likely to occur. The rotor blades fixed at one endacts like cantilever beams. The blades exert a pressure on the gasmolecules, and in response, they re-bounces in the oppositedirection. Due to the deflection produced in the rotor bladesand small clearance between stator and rotor blades, a crashmight occur between them. The deflection and crashing both startincreasing rapidly, particularly, when the rotors’ speed exceedsbeyond the design limits. As a natural consequence this alsointroduces different types of stresses like shear principal, equiva-lent stresses, etc. in blade roots of the rotor. These stresses anddeformation can be reduced using stiffer material and by modify-ing blade design.

TMP was invented in 1958 by Becker, based on the oldermolecular drag pumps developed by Gaede, Holweck andSiegbahn in 1913, 1923 and 1944 respectively [5]. In the begin-ning, due to high compression demand, multistage TMPs were

ll rights reserved.

l.chep@edu.pu.pk (M. Iqbal).

made by assembling individual stages on common shaft withbearings. This multistage assembly produced two problems: largevibration amplitude due to required high rotational speed andcontamination in vacuum system due to leakage, re-greasing andlubrication of bearings during operation. In later age, the com-plete TMP rotor was manufactured in single metallic piece usingelectrical discharge wire cut machine technology. Bearings withdry or solid lubricant were introduced, however; blade designwas gradually being modified by each manufacturing company bychanging the blade angle and chamfering the blade edge alonglength some time. One of the main manufactures named VarianInc., bent the blade tip from outer edge in its new models. Theyhave also introduced TMP with Macro Torr technology [4] inwhich additional three metallic disks were added that increasedits outlet pressure. All improvements in design were to increaseTMP’s performance, efficiency, compression ratio, rotationalspeed and to reduce the vibration and deflections of rotor bladesas minimum as possible.

This work is also an effort to improve the TMP design. Weinitially focus on reducing the blade deflection by improvingblade profile, during pump operation at high rotational velocity.Then by performing material analysis, it is further suggested touse stiffer material for rotor to further decrease the deflection ofthe rotor blade.

2. Modeling

TMP with optimized dimensions is modeled in software namedComputer Aided Three-dimensional Interactive Application (CATIA) [6].

M. Iqbal et al. / Nuclear Instruments and Methods in Physics Research A 678 (2012) 88–90 89

It is used to model 3D mechanical systems for sketching, model-ing, designing of parts, assemblies, mechanisms, etc. This softwareis widely being used in aerospace and defense industry. Designand modeling of TMP rotor is made under following postulatesobtained from the experimental results [7].

�

The blade length, blade angle and rotational speed are directlyproportional to the pumping speed. On the other hand, bladelength and blade angle are inversely proportional tocompression ratio.

The blade makes angle with vertical axis within design limitsthat ranges from 121 to 421. Beyond this limit, frictional affectsover load the motor, which may cause its failure.

3. Results and discussions

3.1. Structural design and discussion

Structural analysis of pump rotor has been done in ANalysisSYStems (ANSYS) workbench [8], for both original and modified

Fig. 1. Analysis of TMP rotor in ANSYS Workbench. (a) Analysis of rotor with

actual blade profile and (b) analysis of modified rotor blade design.

blade design as shown in Fig. 1(a) and (b). The profiles for originaland modified blades are shown in Fig. 2(a) and (b). The rotor withthe original and modified blade design is analyzed under sameenvironmental conditions that are velocity of 13,000 rpm and

Fig. 2. Blade profiles modeled in CATIA. (a) Original blade profile and (b) modified

blade design.

Table 2Deflection in rotor blade for different types of material.

Material Directional deflection in theoriginal blade (mm)

Directional deflection inmodified blade (mm)

Aluminum

alloy

2.876 1.91

Titanium 2.071 1.80

Table 1Applied pressures at different rotor stages with

cylindrical supports and magnitude of velocity

13,000 rpm along the z-axis.

Rotor Stages Pressure (MPa)

Stage 1 1.0�10–12

Stage 2 1.0�10�11

Stage 3 1.0�10�10

Stage 4 1.0�10�8

Stage 5 1.0�10�6

Stage 6 1.0�10�4

Stage 7 0.01

Table 3Deflection and stresses in original and modified aluminum rotor.

Parameter Orientation Modified blade profile Original blade profile

Minimum Maximum Minimum Maximum

Directional deformation z-Axis �1.42�10�5 m 1.91�10�3 m �2.43�10�5 m 2.88�10�3 m

Total deformation Global 0.0 m 2.19�10�3 m 0.0 m 3.31�10�3 m

Equivalent stress Global 34,599.77 Pa 4.55�108 Pa 75,927.84 Pa 5.95�108 Pa

Maximum shear stress Global 19,976.14 Pa 2.59�108 Pa 40,666.34 Pa 3.12�108 Pa

Stress intensity Global 39,952.27 Pa 5.18�108 Pa 81,332.68 Pa 6.23�108 Pa

Maximum principal stress Global �4.1�107 Pa 4.87�108 Pa �2.48�107 Pa 8.12�108 Pa

Normal stress x-Axis �1.12�108 Pa 4.28�108 Pa �1.77�108 Pa 6.76�108 Pa

Fig. 3. Decrease in parameters (deflection and stresses) for aluminum rotor.

M. Iqbal et al. / Nuclear Instruments and Methods in Physics Research A 678 (2012) 88–9090

pressure range from 1�10�12 MPa to 1�10�2 MPa in sevenstages as given in Table 1. We have used two different bladesmade of aluminum alloy and titanium in two different designs.The comparison of their performance under the same environ-mental conditions is given in Table 2. We have evaluated deflec-tions and stresses for Al alloy, which are given in Table 3.

In modified blade design, maximum shear stress becomes 83percent, maximum principal stress becomes 59 percent, normalstress becomes 63 percent and equivalent stress becomes 76percent to the respective stresses in the original blade design.

3.2. Rotor materials and discussion

At 13,000 revolutions per minute (rpm), the deflection in theoriginal blade design is 2.876 mm while in modified blade thedeflection is 1.91 mm. From this analysis, it could be inferred thatthe deflection in blades may exceed 10 mm at a speed of30,000 rpm, keeping the rotor material same. In this analysis,properties of the materials have been selected which are com-monly available [9]. The deformation and stresses at higherspeeds could also be decreased by selecting heat treated alloys.Aluminum (Al) alloy 6061 T6 is best suitable for manufacturing ofthe pump rotor. Al is light in weight, stiffer with respect tostructural steel (SS). The weight of Al rotor is about three timeslighter than the structural steel of the same dimensions. Selectionof Al 6061 is due to its better properties among other aluminumalloys, e.g., heat treatable characteristics [10].

4. Summary and conclusions

Rotor with modified blade design has less deflection than theoriginal blade design, for the same environmental conditions. ForAl alloy, deflection decreases from 2.876 mm to 1.91 mm which is66 percent of the original blade deflection. Maximum shear stress,maximum principal stress, stress intensity and equivalent stres-ses are also reduced in modified blade design as are shown inFig. 3. Results also show that when we use stiffer material, liketitanium, the deformation decreases. The titanium has lessdeformation than that of, Al alloy. It is also observed that, if werun the modified rotor at 16,000 rpm, it has a deformation of2.88 mm, which is the same as the original rotor has at13,000 rpm. Hence, an additional velocity of 3000 rpm can beachieved with modified blade for the same deformation limitswhich is 23 percent additional to the applied rotational velocity.As a result, it enhances the efficiency of a TMP. Thus, we suggest amodified blade profile at optimized design within the sameenvironment and compare the results using different rotor’smaterials like Titanium and Aluminum alloy.

Acknowledgments

We are pleased to acknowledge the role of the University ofEngineering & Technology of Taxila, for providing the software.We are also grateful for National Institute of Vacuum Science &Technology (NINVAST) for working one of the authors (AW)on TMP.

References

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New York, 1990, p. 1278.[6] J. Swider, et al., Journal of Achievements in Materials and Manufacturing

Engineering 23 (2007) 23.[7] S.C. Joshi, S.N. Singh, G.V. Kane, S. Chinnatambi, S.V. Kokil, J.A. Velavan,

A.K. Jain, A.S. Raja Rao, Bulletin of IVS 2 (1999) 69.[8] ANSYS, Inc., ANSYS FLOTRAN CFD Analysis Guide, Release 10.0, 2006, p. 1.[9] J.T. Staley, D.J. Lege, Journal De Physique IV 3 (1993) 179.

[10] ASM Hand Book, Heat Treating, vol. 4, 2004, p. 846.