Ronald D. Flack

Accuracy Evaluation of Experimentally Derived Dynamic Coefficients of Fluid Film Bearings Part I: Development of Method

The goal of many experimental studies of fluid film bearings is the determination of the eight linearized stiffnes and damping coefficients. These coefficients cannot be measured directly; they are derived from a series of other measurements. In many cases, the coefficients are found by applying known steady-state sinusoidal loads to excite a journal on a rigid rotor and measuring the resulting displacement, velocity, and acceleration orbits. This paper relates how uncertainties in these measurements affect the accuracy of the derived coefficients. The analysis can also be used for the proper selection of test procedures and transducers to measure the primary variables and estimate results beforehand

Feedback control applications in artificial hearts

Artificial hearts are a commercially interesting technology which relies heavily on feedback control, particularly with centrifugal pump implementations using magnetic bearings. Several examples of feedback control in these pumps were explored in this paper, including parameter estimation for position sensing, observer based motor commutation, position control of the rigid impeller subjected to highly uncertain fluid dynamic forces, and pump discharge rate regulation (physiological control) to achieve uncertain objectives (due to insufficient experience) using difficult measurements on an extremely complex plant. Such problems are typical of an expanding range of biomedical applications for automatic control.

Effects of Volute Design and Number of Impeller Blades on Lateral Impeller Forces and Hydraulic Performance

A comparison is made between the characteristics of the measured lateral impeller forces and the hydraulic performances of a four- and a five-vane impeller, each operating in a spiral volute, a concentric volute, and a double volute. The pumps rotor was supported in magnetic bearings. In addition to supporting and controlling the rotor motion, the magnetic bearings also served as active load cells and were used to measure the impeller forces acting on the pumps rotor. The lateral impeller force characteristics, as a function of a normalized flow coefficient, were virtually identical in the four- and five-vane impellers in each respective volute type. The measured impeller forces for each volute type were compared with correlations in the literature. The measured forces from the double volute configurations agreed with the forces from a correlation model over the full flow range. Single volute configurations compared well with the predictions of a published correlation at high flow rates, ϕ/ϕng0.5. Concentric volute configurations compared well with a published correlation at low flow rates, ϕ/ϕnl0.4. The head-versus-flow characteristics of the four-vane impeller in each volute type were stable over a greater flow range than the corresponding characteristics of the five-vane impeller. At higher flow rates in the stable region of the heads characteristic curves near the best efficiency point, the five-vane impeller produced higher head than did the four-vane impeller in each volute type.

Effects of Three-Lobe Bearing Geometries on Rigid-Rotor Stability

The stability of a rigid rotor mounted in three-lobe bearings is theoretically studied for which the preload factor, offset factor, and load orientation are systematically varied. Rigid-rotor stability maps are presented. Results indicate that bearings with offset factors of 0.1 or less and moderate preload factors are often more stable than those with larger offset factors. Results also indicate that, for a given preload and load orientation, a particular offset factor exists which produces minimum stability (such bearing geometries should be avoided). Finally, increasing the preload factor was found to increase the rigid-rotor stability. Presented at the 36th Annual Meeting in Pittsburgh, Pennsylvania, May 11–14, 1981

Accuracy Evaluation of Experimentally Derived Dynamic Coefficients of Fluid Film Bearings Part II: Case Studies

Six case studies are presented on the predicted uncertainties of derived bearing coefficients rsulting from uncertainties in experimentally measured displacements, velocities, accelerations, and forces. In a previous paper the full set of equations, including eight linearized bearing coefficients, were derived from which bearing coefficients can be calculated based on measured quantities. Different algorithms for obtaining and reducing data are developed and presented for the cases. To demonstrate the method, bearings with known coefficients are chosen and uncertainties are theoretically applied to the measurements. The «measured» coefficients are then predicted and compared to the known values

Temperature, pressure and film thickness measurements for an offset half bearing

Abstract A fluid film journal bearing test rig with a shaft, 70 mm in diameter, was developed and an offset half journal bearing was tested at five vertical loads and two rotational speeds. The bearing had a length-to-diameter ratio of 1.00, a pre-load factor of 0.51 and an offset factor of 1.0. Loads from 0 to 4450 N and speeds of 2250 and 1650 rev min−1 were tested. On-rotor instrumentation and amplification are used to measure continuous fluid film pressures around the bearing, film thicknesses and journal temperatures. Slip ring telemetry is used to transmit the data to a real time data collection system. External instrumentation is used to measure film pressures and temperatures in discrete locations on the bearing surface, thus, allowing comparison with the on-shaft measurements. The eccentricity ratio and attitude angle are pressure measurements obtained indicate agreement within 5% between the results recorded by the on-shaft and the external pressure transducers. The experimental film thickness, temperature and pressure profiles and journal eccentricity agree with the expected behavior for this bearing type and are presented for Sommerfeld numbers from 0.41 to 2.2.

Experiments on the Stability of Two Flexible Rotors in Tilting Pad Bearings

Two rotors were tested in two sets of five shoe tilting pad bearings. One rotor had three inboard disks while the second rotor had two inboard disks and one overhung disk. Both rotors had two critical speeds below 10 000 RPM. One set of bearings had ball pivots and preload factors of 0.00; the second set had line pivots and preload factors of 0.50. All destablizing mechanisms were eliminated from the systems except for bearing cross coupling coefficients. The first system went unstable at 11 200 RPM while the second went unstable at 10 400 RPM. Both systems went unstable due to whip. Possible causes of the instabilities are discussed. Presented at the 42nd Annual Meeting in Anaheim, California May 11–14, 1987

Experiments on the Stability and Response of a Flexible Rotor in Three Types of Journal Bearings

Three sets of bearings were tested on a three-mass flexible rotor: axial-groove, pressure-dam and preloaded three-lobe, and the instability thresholds and unbalance responses were determined. The pressure-dam bearings were designed for maximum rigid rotor stability; the three-lobe bearings had preload factors of approximately 0.75. Rotor first and second critical speeds were observed at 2550 and 9800 rpm, respectively. The instability thresholds and unbalance responses at the first critical speed for the axial-groove and three-lobe bearings were found to be strongly dependent on the load orientation. Maximum stability limits of 6550, 7400 and 10 400 rpm were found for the axial-groove, pressure-dam, and three-lobe bearings, respectively. Thus, the preloaded three-lobe bearings were the only set that allowed this particular rotor to operate above the second critical speed. The experimental data were also compared to theoretical predictions. Differences in instability thresholds of up to 37 percent were rea...

Effect of Relative Impeller-to-Volute Position on Hydraulic Efficiency and Static Radial Force Distribution in a Circular Volute Centrifugal Pump

The hydraulic performance and radial hydraulic force characteristics of a circular volute centrifugal pump are strongly affected by the impeller to volute relative position, For a typical design configuration the geometric center of the impeller will be coincident with the volute geometric center. However, assembling a circular volute pump with the impeller center eccentric from the volute center can radically alter both the hydraulic performance and the radial hydraulic force characteristics. In particular, at the design flow coefficient an optimum impeller to volute relative position exists where the efficiency is maximized and the resultant radial force is minimized. At the optimal relative position a 5 percent and a 3.5 percent increase in the efficiency was realized compared to the centered positions for the circular and spiral volutes, respectively. In addition the nondimensional resultant radial force at the design flow coefficient was reduced from 0.045 at the centered position to 0.005 at the optimal position for the circular casing

A Hydrodynamic Journal Bearing Test Rig with Dynamic Measurement Capabilities

A bearing lest rig was developed to characterize the static and dynamic properties of hydrodynamic journal bearings. Static measurement capabilities include operating eccentricity, pressure and thermal boundary conditions, and continuous circumferential pressure and film thickness profiles at multiple axial planes. Dynamic stiffness and damping coefficient measurements are achieved using steady state harmonic excitations generated by a two-axis shaker system. All essential data for a complete understanding of one particular bearing can be collected simultaneously. To ensure high quality results, the rig was designed to minimize the influence of measurement uncertainties on the derived dynamic coefficients. Complete details are described. The rig is configured to lest bearings with 70 mm bore diameters at speeds up to 2500 rpm and loads up to 6000 N. The data reduction technique is presented along with typical results at one steady operating position for a three-lobe bearing which demonstrate the reproduci...