Design and Calibration of a Tri-Directional Contact Force Measurement System

Rizwan Ahmed,Christian Maria Firrone,Stefano Zucca

doi:10.3390/app11020877

Abstract

In low pressure turbine stages, adjacent blades are coupled to each other at their tip by covers, called shrouds. Three-dimensional periodic contact forces at shrouds strongly affect the blade vibration level as energy is dissipated by friction. To validate contact models developed for the prediction of nonlinear forced response of shrouded blades, direct contact force measurement during dynamic tests is mandatory. In case of shrouded blades, the existing unidirectional and bi-directional contact force measurement methods need to be improved and extended to a tri-directional measurement of shroud contact forces for a comprehensive and more reliable validation of the shroud contact models. This demands an accurate and robust measurement solution that is compatible with the nature and orientation of the contact forces at blade shrouds. This study presents a cost effective and adaptable tri-directional force measurement system to measure static and dynamic contact forces simultaneously in three directions at blade shrouds during forced response tests. The system is based on three orthogonal force transducers connected to a reference block that will eventually be put in contact with the blade shroud in the test rig. A calibration process is outlined to define a decoupling matrix and its subsequent validation is demonstrated in order to evaluate the effectiveness of the measurement system to measure the actual contact forces acting on the contact.

Highlights

In low pressure turbine (LPT) stages, high static loads caused by centrifugal forces and oscillating forces of working fluid result in highly pre-stressed forced vibrations [1]
The purpose of this study is to analyze the effectiveness of a cost effective and adaptable design configuration, calibration and decoupling process of a tri-directional force measurement system comprising of three uniaxial force sensors arranged orthogonally, that would be employed to measure static and dynamic shroud contact forces applied in three orthogonal directions simultaneously
In case of shrouded blades, the need to measure the contact forces at the shroud contacts must be addressed during the experimental campaign

Summary

Introduction

In low pressure turbine (LPT) stages, high static loads caused by centrifugal forces and oscillating forces of working fluid result in highly pre-stressed forced vibrations [1].In order to avoid high cycle fatigue (HCF), there is a need to estimate and reduce these vibration amplitudes. Friction damping is commonly utilized in turbomachinery [2] in the form of under platform dampers, snubbers, and shrouds to reduce the blade vibration amplitude at resonance due to its efficacy, simplicity, and reliability. The frictional contact surfaces are characterized by hysteresis loops that indicate the variation of tangential contact forces with respect to relative displacements. The area bounded by the hysteresis loop depicts the energy dissipation at the contact surface and the slope of hysteresis loop defines the contact stiffness. These two parameters, i.e., the dissipated energy and contact stiffness influence the dynamic characteristics of the shrouded blade assembly

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