John M. Vance

Experimental Evaluation of a Metal Mesh Bearing Damper

Metal mesh is a commercially available material used in many applications including seals, heat shields, filters, gaskets, aircraft engine mounts, and vibration absorbers. This material has been tested by the authors as a bearing damper in a rotordynamic test rig. The test facility was originally used to support the design of a turboprop engine, developing squirrel cages and squeeze film dampers for both the gas generator and power turbine rotors. To design the metal mesh damper, static stiffness and dynamic rap test measurements were first made on metal mesh samples in a specially designed nonrotating test fixture, These property tests were performed on samples of various densities and press fits. One sample was also tested in an Instron machine as an ancillary and redundant way to determine the stiffness. Using the stiffness test results and equations derived by a previous investigator, a spreadsheet program was written and used to size metal mesh donuts that have the radial stiffness value required to replace the squirrel cage in the power turbine. The squirrel cage and squeeze film bearing damper developed for the power turbine rotor was then replaced by a metal mesh donut sized by the computer code. Coast down tests were conducted through the first critical speed of the power turbine. The results of the metal mesh tests are compared with those obtained from previous testing with the squeeze film damper and show that the metal mesh damper has the same damping as the squeeze film at room temperature but does not lose its damping at elevated temperatures up to 103°C. Experiments were run under several different conditions, including balanced rotor, unbalanced rotor, heated metal mesh, and wet (with oil) metal mesh. The creep, or sag, of the metal mesh supporting the rotor weight was also measured over a period of several weeks and found to be very small. Based on these tests, metal mesh dampers appear to be a viable and attractive substitute for squeeze film dampers in gas turbine engines. The advantages shown by these tests include less variation of damping with temperature, ability to handle large rotor unbalance, and the ability (if required) to operate effectively in an oil free environment. Additional testing is required to determine the endurance properties, the effect of high impact or maneuver loads, and the ability to sustain blade loss loads (which squeeze films cannot handle).

Effect of Fluid Inertia on Squeeze-Film Damper Forces for Small-Amplitude Circular-Centered Motions

Fluid-film forces generated by squeeze-film dampers (SFD) in response to small-amplitude centered motions are of special interest for stability analyses of rotating machinery employing SFDs with strong centering springs. They form the basis for calculation of linearized damping and inertia force coefficients obtained by subjecting the journal center to very small perturbations in velocity and acceleration. The analysis considers the fluid flow in an open-ends SFD due to small-amplitude circular-centered motions. Simplified governing equations are derived; and regarding the flow to be stable and laminar, the linear flow equations are solved exactly for arbitrary values of the Reynolds number. Exact damping and inertia force coefficient are then derived for open ends SFDs with arbitrary L/D ratios. Presented at the 41st Annual Meeting in Toronto, Ontario, Canada May 12–15, 1986

Experimental Measurement of Alford’s Force in Axial Flow Turbomachinery

Abstract : In 1965, J. S. Alford published a theory and mathematical model (1) which predicts that circumferential variation of blade-tip clearances in axial-flow turbomachinery will produce cross-coupled (normal to the eccentricity) aerodynamic forces on the rotor. Ever since then, the theory has been used (without experimental verification) by rotor dynamicists as one of the few mathematical models available to calculate the cross-coupled aerodynamic stiffness coefficients required for rotordynamic stability analysis. This paper presents the results of experimental measurements made on a small, high speed, axial flow test apparatus to verify the existence of Alfords force and to investigate the validity of his mathematical prediction model.

Actively Controlled Bearing Dampers for Aircraft Engine Applications

This paper describes some of the requirements for bearing dampers to be used in an aircraft engine and briefly discusses the pros and cons of various types of dampers that were considered as candidates for active control in aircraft engines. A disk type of electrorheological (ER) damper was chosen for further study and testing. The paper explains how and why the choice was made. For evaluating potential applications to aircraft engines, an experimental development engine (XTE-45) was used as an example for this study. Like most real aircraft engines, the XTE-45 ran through more than one critical speed in its operating speed range. There are some speeds where damping is desirable and other speeds where it is not. Thus, the concept of a damper with controllable forces appears attractive. The desired equivalent viscous damping at the critical speeds along with the available size envelope were two of the major criteria used for comparing the dampers. Most previous investigators have considered the ER damper to produce a purely Coulomb type of damping force and this was the assumption used by the present authors in this study. It is shown in a companion paper, however, that a purely Coulomb type of friction cannot restrain the peak vibration amplitudes at rotordynamic critical speeds and that the equivalent viscous damping for rotordynamics is different from the value derived by previous investigators for planar vibration. The type of control scheme required and its effectiveness was another criterion used for comparing the dampers in this paper.

Force Coefficients for Open-Ended Squeeze-Film Dampers Executing Small-Amplitude Motions About an Off-Center Equilibrium Position

Fluid-film forces generated by squeeze-film dampers (SFD) in response to small-amplitude motions about an off-center equilibrium configuration are of considerable importance for stability analyses of rotating machinery employing SFDs with weak or no centering springs. This approach is the basis for calculation of linearized force coefficients obtained by subjecting the journal to small perturbations in velocity and acceleration. The analysis considers the fluid flow in an open-ended SFD due to small-amplitude motions of the journal center about the static equilibrium configuration. Simplified governing equations are derived from the full Navier-Stokes equations by order of magnitude tests. Approximate damping and inertia force coefficients for finite-length SFDs are derived by implementing a correction factor to the long SFD model solution. This finite-length correction is of extreme simplicity and shows excellent agreement with a numerical solution to the differential equations for the pressure including...

Experimental Evaluation of a Metal Mesh Bearing Damper in Parallel With a Structural Support

In a previous ASME paper experiments were reported on metal mesh bearing dampers (MMD) that were tested in a power turbine rotor at speeds up to 12,000 rpm. They were made of 0.229 mm stainless steel 304 wire mesh, compressed to 57% density, which is close to the maximum density that was economically available. After balancing, a level of vibration was achieved similar to that previously observed with squeeze film dampers. These experiments showed that the MMD could suppress vibration amplitudes of the 22.7 kg rotor at critical speeds of 4,000 rpm and 9,300 rpm. Much of the testing showed the rotor having little or no response to unbalance on coastdown through the critical speeds. The donut-shaped MMD in those tests were the only bearing supports; no squirrel cages were used. A question was raised about the feasibility of using MMD in parallel with a squirrel cage bearing support so that the stiffness can be controlled independently of the damping. This paper presents experimental results for metal mesh dampers with a squirrel cage as a parallel bearing support. Experiments with copper mesh as seal elements (on another project) had indicated that copper mesh has higher damping than stainless steel, so copper was chosen for these experiments. Both a linear viscous damping model and a hysteretic damping model were investigated. Some hysteretic damping models predict that damping depends on stiffness. A different hysteretic model turned out to be useful and promising as a prediction model for two reasons: a) it fits the measured data, and b) it predicts that the damping is not lost if the MMD is put in parallel with a steel structure such as a squirrel cage bearing support. The measurements reported here support the validity of that prediction.Copyright

A Gas-Operated Bearing Damper for Turbomachinery

A gas-operated bearing damper for turbomachinery has been designed, analyzed, and experimentally investigated in the laboratory. The damper utilizes air bled off from the compressor to power an actuator through orifices with area modulated by the vibratory displacement at the bearing support. The design objective for this passive device is to make the actuating dynamic gas pressure phase lead the vibratory displacement by 90 deg. Several variations of the basic concept have been tested. An analysis was performed to guide the experiments. All of the designs tested to date can produce positive damping, and one particular design has produce a damping coefficient of 8,756 N-s/m (50 lb-sec/in.) with a power penalty of 5.2 kW (7 hp) at 310 KPa (45 psi). This design was installed on a laboratory rotor with flexibly supported ball bearings, and significant damping of the critical speed response was demonstrated. The experimental results to date suggest that further research can produce significant improvements in performance, and the device appears to be especially adaptable to high-temperature applications for aircraft engines.

Design Equations for Wire Mesh Bearing Dampers in Turbomachinery

In a previous ASME paper the second author reported experiments on wire mesh bearing dampers (WMD) incorporated in a power turbine rotor-bearing system in order to enable a direct comparison between WMD and squeeze film dampers (SFD). The results showed that both WMD and SFD perform equally well for reducing the rotordynamic amplitudes of vibration. Moreover the WMD were found to have significant advantages over SFD. The damping provided by the wire mesh is independent of temperature changes and presence of turbine oil. Experiments by another investigator showed that WMD are capable of sustaining more than twice the unbalance as compared to SFD, which promises possible application to withstand blade loss loads. This paper presents empirically developed non-dimensional design equations for WMD, capable of predicting stiffness and damping for a wire mesh ‘donut’ subject to changes in various design, installation, and operational parameters.Copyright

Experimental Evaluation of Hybrid Damper Seals With Brush Elements: Effect of the Bristles on Power Dissipation and Rotordynamics

A previous paper described measurements of damping and leakage of a brush hybrid seal in a nonrotating test apparatus. In those experiments most of the damping came from the brush. This paper describes further experiments with this seal in a rotordynamic rig designed to determine if the bristles will still contribute significant damping at large amplitudes after wearing in to a line-line contact. These tests continued to show much more damping and much less leakage than a labyrinth seal of the same dimensions. The damping performance is even better after the brush is worn in. In the rotating experiments it was discovered that the brush elements also produce a surprising amount of friction torque and consequent power loss, especially at high pressure drops. During the experiments it was found that the brush friction is destabilizing to backward whirl, and that support asymmetry suppresses the instability.Copyright

A Gas Operated Bearing Damper for Turbomachinery

A gas operated bearing damper for turbomachinery has been designed, analyzed, and experimentally investigated in the laboratory. The damper utilizes air bled off from the compressor to power an actuator through orifices with area modulated by the vibratory displacement at the bearing support. The design objective for this passive device is to make the actuating dynamic gas pressure phase lead the vibratory displacement by 90 degrees.Several variations of the basic concept have been tested. An analysis was performed to guide the experiments. All of the designs tested to date can produce positive damping, and one particular design has produced a damping coefficient of 8756 N-s/m (50 lb-sec/in) with a power penalty of 5.2 KW (7 HP) at 310 KPa (45 psi). This design was installed on a laboratory rotor with flexibly supported ball bearings, and significant damping of the critical speed response was demonstrated.The experimental results to date suggest that further research can produce significant improvements in performance, and the device appears to be especially adaptable to high temperature applications for aircraft engines.Copyright