Maurice L. Adams

Development of an experimental capability to produce controlled blade tip/shroud rubs at engine speed

An experimental capability using an in-ground spin-pit facility specifically designed to investigate aeromechanic phenomena for gas turbine engine hardware rotating at engine speed is demonstrated herein to obtain specific information related to prediction and modeling of blade-casing interactions. Experiments are designed to allow insertion of a segment of engine casing into the path of single-bladed or multiple-bladed disks. In the current facility configuration, a 90 deg sector of a representative engine casing is forced to rub the tip of a single-bladed compressor disk for a selected number of rubs with predetermined blade incursion into the casing at rotational speeds in the vicinity of 20,000 rpm.

Numerical Study of Some Nonlinear Dynamics of a Rotor Supported on a Three-Pad Tilting Pad Journal Bearing (TPJB)

The use of tilting pad journal bearings (TPJBs) has increased in the recent past due to their stabilizing effects on the rotor bearing system. However, in this paper two mechanisms capable of producing instabilities in terms of subharmonic and chaotic motions are suggested. The first one is that of a centrally loaded pad with rotor unbalance excitations. The second one represents a concentric rotor (or a vertical rotor) acted upon by centering sprigs and large unbalance excitations. Extensive numerical experimentation shows, for certain parameters, subharmonic, quasi-periodic, and chaotic motions. The pad state space trajectory, in many cases, resembles that of the two-well potential case as in Duffing’s oscillator. Time trajectories, Poincare maps, fast Fourier transform (FFT) plots, and the max Lyapunov exponent are utilized to examine the periodicity (order) of the nonsynchronous rotor orbits and pad trajectories. The TPJB problem belongs to a family of nonlinear rotor-dynamical phenomena that are potentially of a considerable value as diagnostic tools in assessing rotating machinery condition monitoring.

Simulation of Engine Blade Tip-Rub Induced Vibration

An analysis has been developed to simulate the time-transient vibratory motion of a general configuration engine turbomachinery free-standing blade when subjected to in-service blade-on-casing tip-rub events. The analysis imports the at-speed stress-stiffened blade stiffness matrix and lumped mass matrix from a finite element model of the actual blade. Formulation and computational approaches are presented. Correct characterization of the blade tip-surface rub mechanics tribology models necessitates using empirical information that is currently being acquired from single-blade spin-pit tests now in progress in a parallel companion phase of this research. Output results for validation cases are presented. The analysis efficiently simulates complete transients involving multiple successive incursions (blade on casing hits), tracking the blade tip contact force distribution and blade motion throughout the simulated time frame including blade motion during, between and after successive casing hits.Copyright

Mechanical and Electrical Run-Out Removal on a Precision Rotor-Vibration Research Spindle

Experimental determination of both steady-state and dynamic performance of a journal bearing requires the use of a high precision spindle with a vanishingly small range of run-out. This was achieved by first eliminating the mechanical run-out of the spindle by grinding the journal specimen while rotating in place. Once the mechanical run-out was removed, the electrical run-out sensed by the displacement proximity-probe-transducers was also removed. Using this procedure the mechanical and electrical run-outs of a research spindle were reduced to less than 0.2 micron (10 μin.), which is better than the resolution of the data acquisition system, 1 micron (50 μin.).

Analysis of the Elastic Wave Behavior in a Cracked Shaft

A new method currently under development for rotating shaft crack detection is presented. The underlying approach is to utilize the impact inherent in the once-per-revolution closing of a shaft crack. The axially traveling elastic compression wave, which is initiated by this impact, propagates to both ends of the shaft at the governing acoustic velocity. Provided suitable measurement near the shaft ends can detect the wave’s arrival, then extracting both the crack location and size is thereby feasible. Proof-of-concept for this new method for shaft crack detection utilizes one-dimensional wave propagation simulations and a newly designed test apparatus, which are presented.Copyright

Development of an Experimental Capability to Produce Controlled Blade Tip/Shroud Rubs at Engine Speed

Development of an in-ground spin-pit facility specifically designed to investigate aeromechanic phenomena for engine hardware rotating at design speed is reported in this paper. The purpose of this paper is to describe the facility design and operation and to demonstrate utility by providing typical results from a recently completed measurement program. The facility is designed to allow insertion of a segment of engine casing into the path of single-bladed or multiple-bladed disks. In the current configuration, a 90-degree sector of a representative engine casing is forced to rub the tip of a single-bladed compressor disk with predetermined blade incursion into the casing for rotational speeds in the vicinity of 20,000 rpm.Copyright

Chaos concepts as diagnostic tools for assessing rotating machinery vibration signatures

Chaos content in measured vibration signals is of some practical importance in rotordynamical systems. Of particular interest is the relationship between the occurrence of determinsite chaos and the diagnosis of mechanical failures in rotating machinery. Two nonlinear rotordynamical systems were studied using simulation and various forms of subharmonic, quasiperiodic and chaotic vibrations were observed. Different routes into and out of chaos show important signs for wear assessment and failure prediction. Experimental test facilities are currently under development and the next steps involve experimental verification of the simulation results and the development of signal processing techniques for extracting the dynamical features of the vibration signatures from measured time series data.

Design, Analysis and Testing of an Inside-Out Tilting-Pad Journal Bearing With Real-Time Controllable Preload and Stiffness

It has recently been shown that high-speed grinding can be applied to the finishing of ceramics with considerable improvements in throughput and quality. This will require new high-speed high-power centerless grinding spindles (7,000 RPM, 50 HP), with high-stiffness of three hundred and fifty million Newtons/meter (2 million lb/in). To meet these requirements a novel inside-out, three-pad, pivoting-pad oil-fed hydrodynamic journal bearing has been devised, built and tested. One of the three pad’s pivot point is supported by a hydraulically-actuated radial-motion loading piston. This provides real-time controllable preload to all three bearing pads, thereby controlling bearing stiffness, providing less-stiff spindle bearings for initial rough grinding and very high stiffness spindle bearings for precision finish grinding. Extensive bearing test data compare favorably with theoretically predicted bearing performance.Copyright

Successful Vibration Trouble-Shooting of Large Power Plant Rotating Machinery

This paper presents several recent case studies where the application of cutting-edge measurement and computer modeling have been successfully combined to correctly diagnose and fix power plant rotating machinery excessive vibration problems. These case studies include large turbogenerators and feed water pumps. Each case presented here shows an overall trouble-shooting strategy appropriate to the specific problem symptoms, and describes the measurement and computer modeling phases that successfully resolved the specific problem in each case.Copyright