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Gas Turbines: Structures and Dynamics

Effects of Bending Moments and Pretightening Forces on the Flexural Stiffness of Contact Interfaces in Rod-Fastened Rotors

[+] Author and Article Information
Jin Gao

School of Energy and Power Engineering  Xi’an Jiaotong University 710049 Xi’an, Chinagaojin1985@stu.xjtu.edu.cn

Qi Yuan

School of Energy and Power Engineering  Xi’an Jiaotong University 710049 Xi’an, Chinaqyuan@mail.xjtu.edu.cn

Pu Li

School of Energy and Power Engineering  Xi’an Jiaotong University 710049 Xi’an, Chinaphililplee@stu.xjtu.edu.cn

Zhenping Feng

School of Energy and Power Engineering  Xi’an Jiaotong University 710049 Xi’an, Chinazpfeng@mail.xjtu.edu.cn

Hongtao Zhang

 Harbin Turbine Company Limited Harbin, Heilongjiang, P.R. Chinazhanght@htc.com.cn

Zhiqiang Lv

 Harbin Turbine Company Limited Harbin, Heilongjiang, P.R. Chinalvzq@htc.com.cn

J. Eng. Gas Turbines Power 134(10), 102503 (Aug 22, 2012) (8 pages) doi:10.1115/1.4007026 History: Received June 18, 2012; Revised June 20, 2012; Published August 22, 2012; Online August 22, 2012

The rod-fastened rotor (RFR) is comprised of a series of discs clamped together by a central tie rod or several tie rods on the pitch circle diameter. The equivalent flexural stiffness of contact interfaces K c in the RFR is the key concern for accurate rotor dynamic performance analysis. Each contact interface was modeled as a bending spring with a stiffness of Kc and a hinge in this study. The contact states of the contact interfaces, which depend on the pretightening forces and bending moments (static), have effects on Kc . The approach to calculating Kc in two contact states is presented. The first contact state is that the whole zone of the contact interface is in contact; Kc is determined by the contact layer, which consists of asperities of the contact surfaces. Hertz contact theory and the Greenwood and Williamson (GW) statistical model are used to calculate the equivalent flexural stiffness of the contact layer Kcc . The second contact state is that some zones of the contact interface are separated (when the bending moment is relatively large); the equivalent flexural stiffness of the rotor segment Ksf (not including Kcc ) decreases, as the material in the separated zone has no contribution to the bending load-carrying capacity of the rotor. The strain energy, which is calculated by the finite element method (FEM), is used to determine Ksf . The stiffness Ksf is equivalent to the series stiffness of the discs of the rotor segment with flexural stiffness of Kd and a spring with bending stiffness of Kcf in the location of the contact interface, so Kc is equal to the series stiffness of Kcc and Kcf in the second contact state. The results of a simplified RFR indicate that, for a fixed pretightening force, Kcc decreases with bending moments in the first contact state, whereas increases with bending moments in the second contact state. In addition, Kcf and Kc decrease abruptly with the increase of bending moments in the second contact state when the rotor is subjected to a relatively large pretightening force. Finally, the multipoint exciting method was used to measure the modal parameters of the experimental RFR. It is found that the experimental modal frequencies decrease as the pretightening force decreases.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 12

Finite element model for modal analysis

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Figure 13

Connection element of contact interface

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Figure 14

(a) Modal frequency versus dimensionless load α (α = 1/γ11 ). (b) Modal frequency versus dimensionless load α (α = 1/γ11 ). (c) Modal frequency versus dimensionless load α (α = 1/γ11 ).

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Figure 15

Mode shapes of the experimental rotor

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Figure 16

Relative errors between the calculated fifth order modal frequencies and test results (Kcc0  = Kcc for M = 0)

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Figure 1

Typical structure of a RFR

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Figure 2

Structure of a RFR segment

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Figure 3

Schematic for the contact surface

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Figure 4

Schematic diagram for calculating Kcc

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Figure 5

Structure of the experimental RFR

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Figure 6

Finite element model of the experimental RFR

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Figure 7

Contact stiffness of unit area of nominal contact surface versus normal pressure

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Figure 8

Equivalent flexural stiffness of contact layers Kcc versus dimensionless load γ11 (Kcc0  = Kcc for M = 0)

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Figure 9

Equivalent flexural stiffness of contact interface (calculated by FEM) versus dimensionless load γ11

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Figure 10

Equivalent flexural stiffness of contact interface versus dimensionless load γ11

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