A New Technique for Identifying Synchronous Resonances Using Tip-Timing

Abstract

Non-contact measurement of vibration at turbomachinery rotor blade tips using blade tip-timing has become an industry-standard procedure. Current research focuses on analysis methods for interpretation of the measured vibration data from a limited number of probes. The methods are classified by the form of the vibration they can identify. Identification of asynchronous response amplitude and frequency is well documented. Whilst a method for identifying maximum synchronous resonance amplitude has existed since the early 1970s, there is no published evidence of a method for directly identifying frequency or engine order using a small number of probes. This paper presents a new analysis method for identifying synchronous resonance engine order using two tip-timing vibration measurements. The measurements are made at different locations on the turbomachinery casing using a minimum of two probes. A detailed description of the method and results from its practical application are given. The potential of the method to identify the amplitude and frequency of close modes, not possible with current methods, is demonstrated. The effect of blade mistuning on the accuracy of the method is investigated. Existing synchronous response analysis methods and the new method presented here give the response amplitude and frequency after the resonance has been traversed. Real-time identification of synchronous response amplitude and frequency would allow tip-timing to be used as a safety monitor of all blades. Real-time methods, their limitations and practical application are discussed. The future use of tip-timing as the dominant vibration measurement system is discussed with reference to experience on measurements made solely with tip-timing on assemblies with undefined vibration characteristics. [S0742-4795(00)02602-8]