Sajjad Z. Meymand

A survey of wheel–rail contact models for rail vehicles

ABSTRACT Accurate and efficient contact models for wheel–rail interaction are essential for the study of the dynamic behaviour of a railway vehicle. Assessment of the contact forces and moments, as well as contact geometry provide a fundamental foundation for such tasks as design of braking and traction control systems, prediction of wheel and rail wear, and evaluation of ride safety and comfort. This paper discusses the evolution and the current state of the theories for solving the wheel–rail contact problem for rolling stock. The well-known theories for modelling both normal contact (Hertzian and non-Hertzian) and tangential contact (Kalkers linear theory, FASTSIM, CONTACT, Polachs theory, etc.) are reviewed. The paper discusses the simplifying assumptions for developing these models and compares their functionality. The experimental studies for evaluation of contact models are also reviewed. This paper concludes with discussing open areas in contact mechanics that require further research for developing better models to represent the wheel–rail interaction.

Characterization of the time-variant behavior of a biomimetic beamforming baffle

Horseshoe bats can actively change the shapes of their noseleaves and outer ears on time scales that are comparable to the duration of the biosonar pulses and echoes. When the shape deformations and the emission or reception of the ultrasonic signals overlap in time, the result is a time-variant diffraction process. Such a dynamic process provides additional flexibility that could potentially be used to enhance the encoding of sensory information. However, such a function remains hypothetical at present. To investigate the time-variant properties of deforming baffles such as the outer ears of horseshoe bats, the acoustic behavior of a biomimetic microphone baffle modeled on these biological structures has been investigated. The methods employed to characterize this device included representations in the time-delay domain as well as in the time-frequency domain. It was found that characterization methods which do not employ Fourier transforms revealed even more substantial time-variant effects than were apparent from time-frequency domain characterizations such as beampatterns obtained for different times in the deformation cycle. Furthermore, conspicuous correlates of asymmetries in the time-variant physical shapes were found in some characterizations that could be used to link dynamic baffle geometry with acoustic behavior.

Interplay of static and dynamic features in biomimetic smart ears

Horseshoe bats (family Rhinolophidae) have sophisticated biosonar systems with outer ears (pinnae) that are characterized by static local shape features as well as dynamic non-rigid changes to their overall shapes. Here, biomimetic prototypes fabricated from elastic rubber sheets have been used to study the impact of these static and dynamic features on the acoustic device characteristics. The basic shape of the prototypes was an obliquely truncated horn augmented with three static local shape features: vertical ridge, pinna-rim incision and frontal flap (antitragus). The prototype shape was deformed dynamically using a one-point actuation mechanism to produce a biomimetic bending of the prototypes tip. In isolation, the local shape features had little impact on the device beampattern. However, strong interactions were observed between these features and the overall deformation. The further the prototype tip was bent down, the stronger the beampatterns associated with combinations of multiple features differed from the upright configuration in the prominence of sidelobes. This behavior was qualitatively similar to numerical predictions for horseshoe bats. Hence, the interplay between static and dynamic features could be a bioinspired principle for affecting large changes through the dynamic manipulations of interactions that are sensitive to small geometrical changes.

Design of a dynamic sensor inspired by bat ears

In bats, the outer ear shapes act as beamforming baffles that create a spatial sensitivity pattern for the reception of the biosonar signals. Whereas technical receivers for wave-based signals usually have rigid geometries, the outer ears of some bat species, such as horseshoe bats, can undergo non-rigid deformations as a result of muscular actuation. It is hypothesized that these deformations provide the animals with a mechanism to adapt their spatial hearing sensitivity on short, sub-second time scales. This biological approach could be of interest to engineering as an inspiration for the design of beamforming devices that combine flexibility with parsimonious implementation. To explore this possibility, a biomimetic dynamic baffle was designed based on a simple shape overall geometry based on an average bat ear. This shape was augmented with three different biomimetic local shape features, a ridge on its exposed surface as well as a flap and an incision along its rim. Dynamic non-rigid deformations of the shape were accomplished through a simple actuation mechanism based on linear actuation inserted at a single point. Despite its simplicity, the prototype device was able to reproduce the dynamic functional characteristics that have been predicted for its biological paragon in a qualitative fashion.

On the Application of Roller Rigs for Studying Rail Vehicle Systems

The primary intent of this study is to highlight the advantages and disadvantages of using roller rigs for engineering issues of importance to the railroad industry. Roller rigs have been in existence for more than a century for studying railway vehicle behavior. In contrast to field testing, roller rigs offer a controlled laboratory environment that can provide a successful path for obtaining data on the mechanics and dynamics of railway systems for a variety of operating conditions. Their use, however, imposes discrepancies from the field environment due to the nature of the commonly-used roller design. This study describes different rig configurations, including scaled and full-scale rigs. It includes the potential advantages and limitations of various rigs for conducting a wide range of studies concerning the dynamic stability of railcars, wheel–rail adhesion, wear and fatigue mechanisms, braking systems, and locomotive power.Copyright

The Development of a Roller Rig for Experimental Evaluation of Contact Mechanics for Railway Vehicles

This paper discusses the development of a state of the art single-wheel roller rig for studying contact mechanics and dynamics in railroad applications. The use of indoor-based simulation tools has become a mainstay in vehicle testing for the automotive and railroad industries. In contrast to field-testing, roller rigs offer a controlled laboratory environment that can provide a successful path for obtaining data on the mechanics and dynamics of railway systems for a variety of operating conditions. The idea to develop a laboratory test rig started from the observation that there is a need for better-developed testing fixtures capable of accurately explaining the relatively unknown physics of the wheel-rail contact mechanics and dynamics. Developing a better understanding of such physics would assist with designing faster, safer, and more efficient railroad systems. A review of the existing roller rigs indicated that many desired functional requirements for studying contact mechanics are not readily available. The Virginia Tech Railway Technologies Laboratory (RTL) has embarked on a mission to develop a state-of-the-art testing facility that will allow experimental testing for contact mechanics in a dynamic, controlled, and consistent manner. The VT roller rig is intended to allow for actively controlling all the wheel-rail interface degrees of freedom: cant angle, angle of attack, and lateral displacement. Two AC synchronous servomotors, accompanied with proper gearheads, accurately drive the rotating wheels. A novel force measurement system, suitable for steel on steel contact, is configured to precisely measure the contact forces and torques. The control architecture is developed based on the SynqNet data acquisition system offered by Kollmorgen, the drive-motor and actuator supplier. The Synqnet provides a unified communication protocol between actuators, drives, and data acquisition system; therefore eliminating any difficulty with data conversion among these units. Other auxiliary sensors and measurement systems are implemented to help with characterizing the contact mechanics and contact geometry. This paper will describe the main steps in the design process of the VT roller rig and the final design solution selected. It will also present the testing capabilities of the rig. The design analysis indicates that the rig can successfully meet the set requirements: additional accuracy in measurements, and better control on the design of experiments.© 2015 ASME

Design of a Bio-Inspired Smart Ear Prototype

The outer ears (pinnae) of many bat species are smart structures that undergo non-rigid deformations controlled through an intricate muscular actuation system. It is hypothesized that such non-rigid changes in the physical shape of the pinnae provide a substrate for adaption of the spatial sensitivity (reception beampattern) of the animals’ biosonar system on a short time scale. In the research presented here, a simplified biomimetic baffle shape was developed to investigate the functional properties of non-rigidly deforming baffles. This prototype had the shape of an obliquely truncated cone that was augmented with local shape features that aided in achieving a biomimetic deformation pattern and may also have direct acoustic effects on the device beampattern. The prototype was manufactured from a thin sheet of rubber and actuated parsimoniously through a single linear actuator. Despite its comparative simplicity, the prototype device was able to reproduce the deformation pattern seen in the ears of horseshoe bats qualitatively. Biomimetic baffle deformations resulted in profound, qualitative, and quantitative changes to the beampattern. Future research will investigate how the time-variant beampatterns relate to the specifics of the deformation patterns and how they could be controlled and used in an engineering context.Copyright

Vibration Analysis of a Coupled Multibody Dynamic Model of a Contact Mechanics Roller Rig

This study develops a detailed multi-body dynamic model of the Virginia Tech Roller Rig (VTRR) using multi body simulation software package SIMPACK. The Virginia Tech Roller Rig, a single-wheel roller rig with vertical plane roller configuration, is a state of the art testing fixture for experimental investigation of wheel-rail contact mechanics and dynamics. In order to have a better understanding of the dynamics at the contact, dynamic behavior and interaction of various components and subsystems of the rig need to be understood. In addition, it is essential to make sure that the measurements are only due to particular subject of study and not any intermittent source of disturbance. Any unwanted vibration at the contact needs to be compensated in the data measurements. To this end, a fully detailed model of the rig including all the components is developed in SIMPACK. The coupled multibody dynamic model represents all the major components of the rig and their interactions. The multibody dynamic model is employed for conducting noise, vibration, harshness (NVH) analysis of the rig. An Eigenvalue analysis provides the modal frequencies and mode shapes of the system. The modal analysis predicts the first natural frequency of the rig to be approximately 70 Hz, providing a relatively high bandwidth for evaluating the dynamics at the wheel-rail interface. Only dynamic that could have higher frequencies than the rig’s bandwidth is wheel-rail squeal. The model is also used to evaluate the performance of the contact force measurement system designed for the rig. The results show that the contact forces can be estimated precisely using the force measurement system.Copyright

Vibration Analysis for Improving Powertrain Design and Contact Measurements of a Roller Rig

This paper provides two vibration analyses on a scaled roller rig that is under construction at the Railway Technologies Laboratory of Virginia Tech (VT) for evaluating wheel/rail contact mechanics. The scaled vertical rig includes a wheel that is placed on a roller with similar profile of a U.S. 136 weight rail. Two independent AC servomotors enable controlling the relative speed of the disks to a high degree of precision. Linear actuators allow for adjusting the simulated load, wheel angle of attack, rail cant, and lateral position of the wheel with respect to the rail, including flanging. Rotation of each disk is dominated by internal dynamics of the motors, gearheads, couplers, and flexible shafts. As a result, dynamics of each component has direct effect on the relative speed of the wheel and the roller at the contact patch. On the other hand, it is essential to make sure that the measurements are only caused by the particular subject of study, and not any intermittent source of disturbance such as unbalanced rotation. Electromechanical models of the rig components have been developed in previous works of the authors for studying the overall behavior of the coupled drivelines. This study aims to fulfill the previous studies by analyzing the effect of incorporating compliant joints in the drivelines, as well as unbalanced dynamics in the disks. Appropriate consideration is given to providing an accurate mathematical model of each phenomenon. The mathematical models are solved numerically to carry out parametric studies that represent actual working conditions of the rig. The results of these studies indicate that incorporation of constant velocity joints in sensitive instruments like the roller rig, leads to inevitable axial vibrations that affect both driver and driven sides. This paper also provides a tool for filtering the undesired vibrations from the contact measurements due to unbalanced rotation or other sources of the same nature.Copyright

Modeling of a Roller Rig for Evaluation of Wheel/Rail Contact Mechanics

This paper provides a detailed dynamic model of the electromechanical system for a scaled roller rig that is under construction at the Railway Technology Laboratory of Virginia Tech (RTL) for accurate study of the mechanics and dynamics at wheel-rail interface in railway vehicles. Roller rigs are critical laboratory test equipment for studying rail vehicle dynamics, either as a full railcar or single component. The controlled laboratory environment will provide a successful path for obtaining data on the mechanics and dynamics of the system, including creepage and creep forces at the wheel-rail interface under various conditions. The single-wheel scaled rig under development at RTL includes a wheel that is placed on a roller with similar profile as a U.S. 136 weight rail. The test setup allows for adjusting the wheel load, the wheel angle of attack, the rail cant, and the lateral position of the wheel with respect to the rail (including flanging). The roller and the wheel are each powered independently by two AC motors that enable controlling their relative speed to a high degree of precision, i.e., 0.1 RPM, in order for precisely controlling and simulating various creep conditions that occur in practice. An essential step for the successful design and development of the test rig is modeling the motors and the roller/wheel drivelines. The model includes the electromagnetic dynamics of the AC motors, the compliances and damping of the drivelines, the inertial properties of the motors, shafts, couplers, and the rotating wheels, in a multi-domain (electrical, magnetic, and mechanical) lumped-parameter model. The model is used to determine the damped natural frequencies of the coupled system. The results of the study indicate that the compliances of the driveline mechanics is the most critical element in maintaining a prescribed speed at the driven wheel, and also controlling the relative creep between the wheel and the simulated (round) rail.Copyright