Dynamic Performance Characteristics Research Articles

Enhancement of dynamic stability of cantilever tooling structures

The least rigid components of machining systems are cantilever tools and cantilever structural units of machine tools (rams, spindle sleeves, etc.). These components limit machining regimes due to the development of chatter vibrations, limit tool life due to extensive wear of cutting inserts, and limit geometric accuracy due to large deflections under cutting forces. Use of high Young's modulus materials (such as sintered carbides) to enhance the dynamic quality of cantilever components has only a limited effect and is very expensive. This paper describes a systems approach to the development of cantilever tooling structures (using the example of boring bars) which combine exceptionally high dynamic stability and performance characteristics with cost effectiveness. Resultant success was due to: (1) a thorough survey of the state of the art; (2) creating a “combination structure” concept with rigid (e.g. sintered carbide) root segments combined with light (e.g. aluminum) overhang segments, thus retaining high stiffness and at the same time achieving low effective mass (thus, high mass ratios for dynamic vibration absorbers, or DVAs) and high natural frequencies; (3) using the concept of “saturation of contact deformations” for efficient joining of constituent parts with minimum processing requirements; (4) suggesting optimized tuning of DVAs for machining process requirements; (5) development of DVAs with the possibility of broad-range tuning; (6) structural optimization of the system; and (7) using a novel concept of a “Torsional Compliant Head”, or TCH, which enhances dynamic stability at high cutting speeds and is suitable for high rev/min applications since it does not disturb balancing conditions. The optimal performance and interaction of these concepts were determined analytically, and then the analytical results were validated by extensive cutting tests with both stationary and rotating boring bars, machining steel and aluminum parts. Stable performance with length-to-diameter ratios up to L/ D = 15 was demonstrated, with surface finish 20–30 μm with both steel and aluminum at L/ D = 7–11. Comparative tests with commercially available bars demonstrated the advantages of our system.

Elastohydrodynamic Studies of Three-Lobe Journal Bearings with Non-Newtonian Lubricants

A computer aided elastohydrodynamic study of the three-lobe journal bearing with non-Newtonian lubricants is presented for the static and dynamic performance characteristics. The three-dimensional momentum and continuity equations in cylindrical coordinates governing the flow of Newtonian lubricants in the clearance space of a three-lobe journal bearing have been solved using the finite element method. The non-Newtonian effect is introduced by modifying the viscosity term for the model iteratively. Three-dimensional elasticity equations are solved to obtain deformations in the bearing shell. Static and dynamic performance characteristics are presented for a wide range of values of non-dimensional load, deformation coefficient and non-linearity factor.

Dynamic performance assessment of selected sport shoes on impact forces

Few investigators have evaluated the performance characteristics of non-running sport shoes. The purpose of this study was to assess the dynamic performance characteristics of four different shoe models during landings. Five male subjects performed 25 voluntary hanging drop landings (60 cm) onto a force platform (1000 Hz) for each of four shoe conditions (C1 and C2 = basketball shoe, C3 = running shoe, C4 = volleyball shoe). Ground reaction force data were evaluated for maximum forefoot (F1) and rearfoot (F2) impact forces as well as the respective times of occurrence of these events (T1, T2). Results of the group data analysis indicated a preferential performance rank order of C1, C3, C4, C2 although significant interaction effects were observed, indicating a need for single-subject analyses. Three techniques were incorporated to assess individual subject condition differences, all of which elicited unique rank orders for the shoes although each identified C1 as the "best" shoe condition. The results of the study support the necessity for within-subject analyses conducted with an adequate number of trials when attempting to detect subtle performance differences that may exist between various sport shoes. Whether the observed statistically significant differences are biomechanically meaningful remains an important unanswered question.

Effects of fluid compressibility on the dynamic response of hydrostatic journal bearings

A theoretical analysis for the dynamic performance characteristics of laminar flow, capillary and/or orifice compensated hydrostatic journal bearings is presented. The analysis considers in detail the effect of fluid compressibility in the bearing recesses. At high frequency excitations beyond a break frequency the bearing hydrostatic stiffness increases sharply and is accompanied by a rapid decrease in direct damping. Also, the potential of pneumatic hammer instability (negative damping) at low frequencies is likely to occur in hydrostatic bearing applications handling highly compressible fluids. Useful design criteria for avoiding undesirable dynamic operating conditions at low and high frequencies are determined. The effect of fluid recess compressibility is brought into perspective, and found to be of utmost importance to the entire frequency spectrum response and stability characteristics of hydrostatic and hybrid journal bearings.

Elastohydrodynamic studies of two-lobe journal bearings with non-Newtonian lubricants

An elastohydrodynamic study is presented of static and dynamic performance characteristics of two-lobe journal bearings with non-Newtonian lubricants. The Navier-Stokes and continuity equations were solved for Newtonian fluids using the finite element method in the cylindrical coordinates representing the flow field in the clearance space of a two-lobe journal bearing. The non-Newtonian effect was introduced by modifying the viscosity term for the model in each iteration. Deformation of the bearing shell was obtained by solving the three-dimensional elasticity equations. The static and dynamic characteristics are presented for wide range of W ̄ , C ̄ a and K ̄ .

Elastohydrodynamic studies of circular journal bearings with non-Newtonian lubricants

A computer-aided study is presented for the static and dynamic performance characteristics of a hydrodynamic journal bearing with a non-Newtonian lubricant. The Navier-Stokes equations in cylindrical coordinates representing the flow field in the clearance space of a journal bearing using Newtonian fluids have been solved by finite element method using Galerkin's technique; the non-Newtonian effect is introduced by modifying the viscosity term for the model in each iteration. Deformation of the bearing shell is obtained by solving the three-dimensional elasticity equations. Using a suitable iterative solution procedure, the converged solutions for the lubricant flow and elastic deformation fields are obtained.

Three dimensional dynamic motor performance of the normal trunk

Traditionally, human strength has been described in terms of the maximum force one could exert under isometric conditions. However, in the past decade ergonomists have attempted to find strength measures that better represent the dynamic aspects of occupational tasks. Recent efforts have focussed upon dynamic motor performance capabilities of the human body that go beyond the traditional measures of isometric strength. The objective of this study was to document the three dimensional dynamic motor performance capabilities of the normal human trunk as subjects flexed and extended their trunks as fast as they could under 'sagittaly symmetric and asymmetric “back lifting” conditions. The results indicated that the range of motion, trunk angular velocity and trunk angular acceleration decreased in the sagittal plane as the trunk became more asymmetric. Dynamic motor performance characteristics increased in the frontal and transverse planes as the trunk became more asymmetric, however, these differences were also dependent upon the device used to measure trunk motor performance.

Walsh transform realization for microprocessor implementation of distance protection

Abstract In recent years there has been a growing trend towards application of digital methods to power line relaying. This has resulted in the design and implementation of very fast distance relays, operating in conjunction with modern circuit-breakers, to meet the growing system requirements of ultra-high-speed fault clearance. This paper describes simple computational procedures, based on the fast Walsh-Hadamard transform (FWHT), for real-time calculation of the fault impedance of transmission lines. These algorithms are implemented on an Intel 8086 microprocessor using 16 samples per cycle of current and voltage signals. Special attention has been paid to the reduction of errors produced by DC offset and by high frequency components in the currents and voltages. A quadrilateral characteristic has been used in the relay for trip decision. Other characteristics such as restricted admittance can also be realized easily, thus providing flexibility to the relay. The hardware and software architecture of the relay is described. The paper also presents the results of tests carried out on the relay using simulated fault data. A tripping signal is sent in less than three-quarters of a cycle of a 50 Hz system for different types of faults. The relay is also evaluated for its dynamic performance characteristics which include directional discrimination, variation of speed and accuracy with fault level, distance to fault, transient components, etc.

High resolution recording of asymmetry currents from the squid giant axon: Technical aspects of voltage clamp design

The design, realisation and performance of a voltage clamp system dedicated to recording asymmetry currents from the squid giant axon is presented. The design has been optimised with respect to dynamic response, signal-to-noise ratio and linearity. Analytical expressions are given for both the dynamic performance and noise characteristics which simplify the design and setting up of the clamp and provide excellent agreement between design theory and practice. A 0.5 cm 2 area of membrane can be voltage clamped in response to a command input voltage step within 10 μs with smooth settling and up to 100% of the effective series resistance being compensated. The noise contributions of the recording chamber, voltage sensing electrodes and clamp electronics have been reduced such that recording of gating currents with a more than 10-fold reduction in the number of averages required compared with previous clamp designs is possible. The overall system nonlinearity results in typically less than 1% contribution to the measured asymmetry charge.

Dynamic performance prediction of polyphase hysteresis motors

The dynamic performance characteristics of polyphase hysteresis motors are presented. A mathematical model to predict the starting and synchronization process of such motors under various operating conditions is given. The model offers a tool for studying the dynamic stability of the hysteresis motor for small-scale disturbances such as changes in load torque, supply voltage, and frequency. A parametric variation is provided to highlight effects of varying some of the critical quantities like hysteresis lag angle, recoil permeability, and rotor dimensions. Two experimental rotors made of 36% and 17% cobalt-steel alloys were used in this investigation. Simulations of the dynamic behaviors of the two motors are presented. There exists a reasonably close agreement between the simulated and measured results. >

Performance characteristics of an externally pressurized capillary-compensated flexible journal bearing

Effect of bearing shell deformation on the static and dynamic performance characteristics of a four-pocket capillary-compensated hydrostatic journal bearing have been studied. The finite element method has been used to solve the Reynold's equation governing the lubricant flow in the clearance space of a hydrostatic journal bearing and the three-dimensional elasticity equations governing the displacement field in the bearing shell. The performance characteristics are presented for a wide range of values of deformation coefficient which takes into account the flexibility of the bearing shell.

Elastothermohydrodynamic effects in elliptical bearings

In this paper, the effects of bearing liner elastic deformation and lubricant viscosity variation with pressure and temperature are studied for an elliptical bearing. The three-dimensional momentum and continuity equations and three-dimensional elasticity equations are solved to obtain pressure in the lubricant flow-field and deformation in the bearing liner. The energy equation and Fourier heat conduction equation are used to obtain the temperature distribution in the fluid film and the bearing body. Simultaneous solutions of fluid flow, elasticity and thermal equations are obtained using a finite element method and a direct iteration scheme. A range of values of the deformation coefficients are presented for various lubricants taking into account the flexibility of the bearing liner, the static and dynamic performance characteristics of an elliptical journal bearing operating at different eccentricity ratios. The lubricants presented fall into three categories: (i) isoviscous, (ii) piezoviscous, and (iii) thermopiezoviscous (viscosity dependent on both pressure and temperature).

An elastohydrodynamic study on two-axial-groove journal bearings

An elastohydrodynamic study of two-axial-groove journal bearings operating in laminar and superlaminar flow regimes is presented. The effect of transition is included in turbulence analysis. Static and dynamic performance characteristics are presented for several values of deformation coefficient and Reynolds number. Illustrative examples are cited to demonstrate the significance of the present investigation.

A study of steady state and transient performance characteristics of a flexible shell journal bearing

A computer-aided study of static and dynamic performance characteristics and linear and nonlinear transient motion analysis of a flexible shell journal bearing system is presented. Trajectories of the journal-centre motion have been drawn to predict the transient response of the system. It is concluded on the basis of results presented in this paper that the motion trajectories obtained using nonlinear equations of motion give a much higher stability margin in terms of critical mass than those obtained through linearized analysis.

Speed control of a DC series motor using an integral-cycle controlled single triac

The speed control of a DC series motor using an integral-cycle-controlled triac together with a bridge rectifier is studied. The triac is connected in series with one of the motor AC supply lines. Computer results for the motor dynamic and steady-state performance characteristics are presented. The saturation in the magnetic circuit, the iron losses, the mechanical losses and the variation of the motor self-inductance with the armature current are considered in the digital simulation of the motor model. The validity of the mathematical model is checked experimentally. A microprocessor-aided measuring circuit is used to measure the motor performances. >

Elastohydrodynamic effects in elliptical bearings

In this paper the effect of bearing deformation on the static and dynamic performance characteristics of an elliptical bearing is presented. The finite element method has been used to solve the three-dimensional Navier-Stokes and the continuity equations governing the lubricant flow in the clearance space of the journal bearing and the three-dimensional elasticity equations governing the displacement field in the bearing shell. The static and dynamic performance characteristics are presented at various eccentricities for a wide range of deformation coefficient values which take into account the flexibility of the bearing liner.

A study of elastohydrodynamic effects in a three-lobe journal bearing

The effect of flexibility of bearing liner on the static and dynamic performance characteristics of a three-lobe journal bearing was studied. The three-dimensional Navier-Stokes and the continuity equations governing the lubricant flow in the clearance space of the journal bearing, and the three-dimensional e elasticity equations governing the displacement field in the bearing liner, were solved by using the finite element method and an iteration scheme. The static and dynamic performance characteristics were calculated at various eccentricities for a wide range of values of deformation coefficients which take into account the flexibility of the bearing liner.

A method for presenting dynamic performance characteristics of highly damped vibration isolators

A method for presenting dynamic performance characteristics of highly damped vibration isolators is discussed. The method utilizes normalized stiffness versus load graphs obtained from laboratory test data and enables determination of dynamic stiffness and natural frequency as a function of isolator load and temperature. Isolators examined were grommets and bushings made from three PVC thermoplastics. Because similar types of isolators show similar isolation characteristics with respect to load and temperature, they can be categorized into families based upon material formulation and isolator geometry. This provides a means for presenting a large amount of performance information in an abbreviated format. Methods of data acquisition and reduction are discussed briefly with major emphasis on the determination of isolator effectiveness for basic noise and vibration control problems.

Transient Analysis of Partial Armature Short Circuit in an Electronically Commutated Permanent Magnet Motor System using an Integrated Nonlinear Magnetic Field-Network Model

The simulation of the dynamic performance characteristics of an electronically commutated brushless dc machine system with radially oriented permanent magnets, which is experiencing a partial short in one of its phases, is reported in this paper. The newly introduced integrated field network (IFN) method was used throughout this work. The IFN method, which is detailed in a companion paper, is based on simultaneously solving the dynamic equations of the machine system network, using machine winding parameters (inductances and emfs) which are determined from numerical solutions of the nonlinear magnetic field prevailing in the machine cores for the corresponding winding currents. These field solutions and corresponding machine parameters are updated at every time step of the solution of the dynamic equations. The results presented here document effects of the shorting of a portion of an armature phase winding on the dynamic performance of a 15 hp (11.2 kw), 120 volts samarium-cobalt permanent magnet brushless dc motor. A comparison of the current, inductance, enf, torque and power time profiles of the motor system with and without partial armature winding failure (short) is given here. These studies are of importance in motor system security and redundancy considerations. The dramatic change of the values of machine parameters upon occurrence of the partial short circuits demonstrate that conventional solution methods would have left much to be desired.

Dynamic performance characteristics of the liquid metal strain gage

Performance characteristics of the liquid metal strain gage (LMSG) were evaluated by both static and dynamic bench testing. Statically, the devices were found to have outputs closely proportional to engineering strains, up to strain levels of 40%. While individual gage factors varied appreciably (up to 50%), each of the gages studied showed excellent reproducibility of behavior. Dynamically, the response to sinusoidal strain inputs was frequency-independent up to 50 Hz, and there was no detectable phase shift. Similarly, the LMSG response to constant-speed displacement inputs was velocity-independent over the range of nominal strain rates from 20 s −1 to 0.02 s −1. The devices proved capable of maintaining stable outputs when held stretched to fixed lengths, even if such tests were performed immediately following stepwise displacement inputs. Thermal artifacts were found to be modest (0.185% apparent strain per °C), and there was no appreciable sensitivity to non-axial strains. When mounted on an in vitro ligament preparation, the LMSG measured apparent ligament strain similar to that detected by a video dimension analyzer. A protocol by which an implanted LMSG could be used to infer in vivo muscle forces was demonstrated, based on recordings of tendo-Achilles strains developed by a rabbit during slow hopping.