Friction Draft Gear Research Articles

Assessing dynamic characteristics for friction draft gears with worn wedge shoes

A friction draft gear model was developed to assess the degraded dynamic characteristics of worn draft gears in the vehicle impact condition. It presents an approach to simulate multiple typical wear profiles of wedge shoes and reflect their effects on the draft gear performance. Based on the rated design indexes and existing overhaul regulations of MT-2 type friction draft gears in China, the safety wear limits for wedge shoes with different wear profiles were classified and assessed. The results indicate that wedge shoes with uniform or concave wear can significantly worsen draft gear performance, and the existing maintenance limit of 1 mm only applies to wedge shoes with uniform wear but not to the most common wedge shoes with concave wear. Also, for wedge shoes with concave wear, when the wear area covers 50%, 75%, and 100% of the total main friction surface, the corresponding safety wear limits are 0.5 mm, 0.8 mm, and 1.3 mm, respectively. Therefore, it is proposed to design multiple graded and dynamic maintenance limits to ensure the service safety of MT-2 type friction draft gears.

Parallel co-simulation of heavy-haul train braking dynamics with strong nonlinearities

A new wave of research interests on train braking dynamics has been raised which requires more accurate and faster train braking dynamics simulations. Simulations with fully nonlinear fluid dynamic air brake models and nonlinear friction draft gear models are more accurate, however these are not yet available. This paper developed a model that has combined both of these aspects. Simulations were achieved by using a parallel co-simulation method which requires minimum working loads to merge legacy codes. A case study was carried out to simulate a heavy haul train braking commencing with a minimum service brake and then followed by a full-service brake before the minimum service brake was fully executed. Such transient simulations were not able to be accurately conducted by using empirical air brake models, but only by using fluid dynamic air brake models. Influences of co-simulation synchronization frequencies on computing time and results of the simulations were also investigated. Simulation results have recommended a synchronization frequency of 100 Hz. The corresponding simulation was 43% faster than real-time for a 152-vehicle train.

Assessment of main phenomena and patterns to ensure quality of reinforcing materials of surface layer of rolling stock elements

According to the principle of hierarchy, the interconnection and subordination of the structural elements of the technical system (in this case, the rolling stock) determines the quality criteria both for each part and for their materials. So, for example, when planning the quality of rolling stock elements, such as parts of an automatic coupler, draft gear, etc., it is necessary to take into account the requirements for the material of the surface layer, which determines the reliability of the element. In this case, an indicator of the reliability of the elements is the wear resistance of the material, which, from the point of view of the consumer, is conveniently defined as the wear rate. In this case, it is necessary to take into account that the wear of the surface layer is of a probabilistic nature, and the wear rate has deviations from the calculated value. This fact is associated with the influence of various, constantly changing factors. On the other hand, in the manufacture of technical systems, the occurrence of disturbing factors is associated with a significant number of technological processes of various kinds, the implementation of which affects the service characteristics of each product, and, as a result, consumer properties that characterize its quality. In order to ensure the required level of service characteristics of the surface layer of the friction draft gear elements, the paper proposes to plan the wear rate using stray fields, taking into account the factors that determine its change during the entire technological sequence of working surface formation. At the same time, the principles proposed in the article are only the first stage in the process of assigning quality criteria for materials of the surface layer. No less difficult, the second stage is the establishment of cause-and-effect relationships of the influence of various factors on the dissipation of the wear rate.

Comparison of friction draft gear models for simulation in longitudinal train dynamics

The friction draft gear with hysteresis characteristics is one of the typical nonlinear elements in longitudinal train dynamics (LTD). Due to the nonlinear hysteresis of friction damping, the discontinuity between loading and unloading envelopes becomes a mathematical integration issue when adopting look-up table methods to establish draft gear models. It might cause integration divergence and wrong results if ignored. To solve this discontinuity, three different transitional strategies (velocity-dependent, intermediate slope, and smoothing approximation) are applied to building draft gear models, and the one-dimensional (1D) longitudinal train model adopting the ‘Zhai’ integration method is used. To ensure accurate results, the international benchmark cases of LTD are studied, and three draft gear models with different transitional strategies are compared in this paper. The results show that these three models can successfully work with ‘Zhai’ method in the LTD simulation to achieve the force agreement with benchmark cases. Compared to the other two models, the velocity-dependent model doesn’t exhibit good buffer deflection agreement and requires a smaller step size to achieve convergent results. Therefore, it is recommended that adopting the intermediate slope method or smoothing approximation approach avoids discontinuity problems of modelling draft gears in LTD.

Characterising stochastic friction in railway draft gear

The draft gear is a key component in a railway train system. The stochastic feature of dry friction in the draft gear has been observed by many researchers, however, characterisation studies in this regard have not been published. This paper used a state-of-the-art laboratory testing rig to measured draft gear forces and deflection responses to sinusoidal force inputs. Measured data were then inversed into values for Coefficient of Friction (CoF). The inverse method was developed based on a friction-wedge-spring draft gear model. The results show that CoF values have a range from 0.22 to 0.47; however, the majority of values are located above 0.34. Normal distribution fitting of the CoF values show that they have a mean of 0.42 and a deviation of 0.02. Weibull distribution fitting shows that they have a scale parameter of 0.43 and a shape parameter of 22.48. The normal distribution stochastic friction was used to model a friction draft gear. A simulation was conducted, and the results show that simulations using the stochastic model can better match the measured data than using conventional deterministic friction.

Simulation and time-frequency analysis of the longitudinal train dynamics coupled with a nonlinear friction draft gear

AbstractTrain safety and operational efficiency can be improved by investigating the dynamics of the train under varying conditions. Longitudinal train dynamics (LTD) simulations performed for such purposes, usually by utilising a nonlinear time-domain model. This paper covers two modes of LTD results corresponding to the time domain and frequency domain analysis. Time-domain solutions are essential to evaluate the full response used for parameter optimisation and controller design studies while frequency domain solutions can provide significant but straightforward clues regarding system dynamics. An advanced draft gear model, which works under a four-stage process is constructed considering all structural components, geometric relationships, friction modelling and dynamic characteristics such as hysteresis, stiffening, state transition, locked unloading, softening. Then, this model is parametrically reduced and implemented into an LTD simulation. The simulation in the time domain is conducted assuming a locomotive connected with a nine wagon via “ode3” fixed-step solver. The transfer function among the first wagon acceleration (output) and the locomotive force (input) estimated through system identification methodology. Then, the identification results interpreted by investigating step-response characteristic and best response giving parameter set is selected. Next, the modal and spectral analysis performed to reveal the behaviour of the in-train forces and the effects of vibration. This paper discusses a reliable methodology for the longitudinal dynamic analysis of the multi-bodied train in time and frequency domain and clarifies in-train vibration behaviour under the existence of sophisticated draft gear.

Friction dampers—Everything old is new again

The title is suggested by a delightful song and dance scene in the film “All That Jazz” (1979). Due to the author's lack of grace, rather than repeating that performance, the presentation will explore a concept suggested by the friction draft gear patented by George Westinghouse in 1888. It and variants are in wide use throughout the North American railroad system. The idea to be examined exploits the fact that the normal forces on the faces of a wedge are greatly magnified, thereby enhancing the friction force associated with the Coulomb friction model. Because of this behavior a simple oscillator whose moving mass is a spring-loaded wedge can absorb much more vibrational energy than other passive techniques. Dynamic analysis of a one-degree-of-freedom model leads to an algebraic expression for the energy dissipated per unit cycle. That formula indicates that critical damping ratios much greater than one are readily obtainable if the springs are precompressed sufficiently. The simplicity of the underlying physical system implies that it is scalable over a large range, from MEMS to seismic structures. A particularly interesting idea is to fabricate a metamaterial consisting of an array of wedge dampers.The title is suggested by a delightful song and dance scene in the film “All That Jazz” (1979). Due to the author's lack of grace, rather than repeating that performance, the presentation will explore a concept suggested by the friction draft gear patented by George Westinghouse in 1888. It and variants are in wide use throughout the North American railroad system. The idea to be examined exploits the fact that the normal forces on the faces of a wedge are greatly magnified, thereby enhancing the friction force associated with the Coulomb friction model. Because of this behavior a simple oscillator whose moving mass is a spring-loaded wedge can absorb much more vibrational energy than other passive techniques. Dynamic analysis of a one-degree-of-freedom model leads to an algebraic expression for the energy dissipated per unit cycle. That formula indicates that critical damping ratios much greater than one are readily obtainable if the springs are precompressed sufficiently. The simplicity of the underlyin...

An improved dynamic model of friction draft gear with a transitional characteristic accounting for housing deformation

ABSTRACTA white-box friction draft gear model has been developed. All components of the draft gear are considered. The distinctive feature of the model, as compared to its predecessor, is the transitional characteristic, which accounts for the effect of elastic deformations of the draft gear housing on the position of the friction wedge system components under loading. The adjustment of the model parameters for improved agreement with experimental data is discussed. The new model can be used in the simulation of shunting impacts for single cars and car groups represented by detailed finite-element models. An example of the simulation is presented and compared with experimental data obtained using a shunting hump test stand.

Freight cars shunting impacts analysis using an improved dynamic model of friction draft gear

ABSTRACTIn this study, simulations of shunting impacts for groups of freight cars that include up to six cars are considered. The simulation technique employs a white-box improved dynamic model of friction draft gear considering all its components and detailed finite element models of the freight cars. The key differences between the one-to-one shunting impact and the impact of long groups of cars in terms of features of the draft gear deflections and the coupler force time history are discussed. We present an example of dynamic finite element analysis for the car body using the coupler force time history obtained from the shunting impact simulation.

An integrated draft gear model with the consideration of wagon body structural characteristics

ABSTRACTWith the increase of railway wagon axle load and the growth of marshalling quantity, the problem caused by impact and vibration of vehicles is increasingly serious, which leads to the damage of vehicle structures and the components. In order to improve the reliability of longitudinal connection model for vehicle impact tests, a new railway wagon longitudinal connection model was developed to simulate and analyse vehicle impact tests. The new model is based on characteristics of longitudinal force transmission for vehicles and parts. In this model, carbodies and bogies were simplified to a particle system that can vibrate in the longitudinal direction, which corresponded to a stiffness-damping vibration system. The model consists of three sub-models, that is, coupler and draft gear sub-model, centre plate sub-model and carbody structure sub-model. Compared with conventional draft gear models, the new model was proposed with geometrical and mechanical relations of friction draft gears considered and with behaviours of sticking, sliding and impact between centre plate and centre bowl added. Besides, virtual springs between discrete carbodies were built to describe the structural deformation of carbody. A computation program for longitudinal dynamics based on vehicle impact tests was accomplished to simulate. Comparisons and analyses regarding the train dynamics outputs and vehicle impact tests were conducted. Simulation results indicate that the new wagon longitudinal connection model can provide a practical application environment for wagons, and the outputs of vehicle impact tests agree with those of field tests. The new model can also be used to study on longitudinal vibrations of different vehicles, of carbody and bogie, and of carbody itself.

Study on longitudinal force simulation of heavy-haul train

ABSTRACTThe longitudinal dynamics model of heavy-haul trains and air brake model used in the longitudinal train dynamics (LTDs) are established. The dry friction damping hysteretic characteristic of steel friction draft gears is simulated by the equation which describes the suspension forces in truck leaf springs. The model of draft gears introduces dynamic loading force, viscous friction of steel friction and the damping force. Consequently, the numerical model of the draft gears is brought forward. The equation of LTDs is strongly non-linear. In order to solve the response of the strongly non-linear system, the high-precision and equilibrium iteration method based on the Newmark-β method is presented and numerical analysis is made. Longitudinal dynamic forces of the 20,000 tonnes heavy-haul train are tested, and models and solution method provided are verified by the test results.

Comparisons of draft gear damping mechanisms

ABSTRACTThis paper compared three types of draft gears with different damping mechanisms: friction, polymer and friction–polymer. Impact characteristics, frequency responses, in-train characteristics and fatigue damage for these draft gears were compared. Wagon impact simulations show that the friction draft gear, polymer draft gear and the friction–polymer draft gear can withstand impact speeds of 9, 12 and 11 km/h, respectively. Comparisons of frequency responses show that the friction draft gear has higher stiffness at lower working velocities, while the polymer draft gear has lower stiffness at lower working velocities. The response of the friction–polymer draft gear is less sensitive to the working velocities as it blends the friction and polymer damping mechanisms and offsets their influences on draft gear forces. Train dynamics simulations show that the in-train characteristics of friction draft gears and polymer draft gears are similar to their static characteristics and very different from their impact characteristics. The in-train characteristics of the polymer draft gears have lower stiffness and lower damping abilities than the other two types of draft gears. The friction–polymer draft gears achieved better vehicle acceleration performance than the friction draft gears. The polymer draft gears achieved similar maximum accelerations throughout the train and the maximum accelerations were significantly lower than that of the other two types of draft gears. The polymer draft gears achieved 23.4% less fatigue damage than the friction draft gears; the fatigue damage of the friction–polymer case is only half that of the friction case.

Modelling polymer draft gears

ABSTRACTThis paper developed a new and simple approach to model polymer draft gears. Two types of polymer draft gears were modelled and compared with experimental data. Impact characteristics, in-train characteristics and frequency responses of these polymer draft gears were studied and compared with those of a friction draft gear. The impact simulations show that polymer draft gears can withstand higher impact speeds than the friction draft gear. Longitudinal train dynamics simulations show that polymer draft gears have significantly longer deflections than friction draft gears in normal train operations. The maximum draft gear working velocities are lower than 0.2 m/s, which are significantly lower than the impact velocities during shunting operations. Draft gears’ in-train characteristics are similar to their static characteristics but are very different from their impact characteristics; this conclusion has also been reached from frequency response simulations. An analysis of gangway bridge plate failures was also conducted and it was found that they were caused by coupler angling behaviour and long draft gear deflections.

Parallel Computing Enables Whole-Trip Train Dynamics Optimizations

Due to the high computing demand of whole-trip train dynamics simulations and the iterative nature of optimizations, whole-trip train dynamics optimizations using sequential computing schemes are practically impossible. This paper reports advancements in whole-trip train dynamics optimizations enabled by using the parallel computing technique. A parallel computing scheme for whole-trip train dynamics optimizations is presented and discussed. Two case studies using parallel multiobjective particle swarm optimization (pMOPSO) and parallel multiobjective genetic algorithm (pMOGA), respectively, were performed to optimize a friction draft gear design. Linear speed-up was achieved by using parallel computing to cut down the computing time from 18 months to just 11 days. Optimized results using pMOPSO and pMOGA were in agreement with each other; Pareto fronts were identified to provide technical evidence for railway manufacturers and operators.

Advanced dynamic modelling for friction draft gears

A white-box friction draft gear model has been developed with all components of the draft gear and their geometries considered. The conventional two-stage (loading and unloading) working process of the friction draft gear was detailed as a four-stage process. A preliminary work called the ‘base model’ was improved with regard to force–displacement characteristics, friction modelling and transitional characteristics. A set of impact test data were analysed; five types of draft gear behaviour were identified and modelled: hysteresis, stiffening, change of stage, locked unloading and softening. Simulated comparisons of three draft gear models were presented: a look-up table model, the base model and the advanced model.

A review of dynamics modelling of friction wedge suspensions

Three-piece bogies with friction wedge suspensions are the most widely used bogies in heavy haul trains. Fiction wedge suspensions play a key role in these wagon systems. This article reviews current techniques in dynamic modelling of friction wedge suspension with various motivations: to improve dynamic models of friction wedge suspensions so as to improve general wagon dynamics simulations; to seek better friction wedge suspension models for wagon stability assessments in complex train systems; to improve the modelling of other friction devices, such as friction draft gear. Relevant theories and friction wedge suspension models developed by using commercial simulation packages and in-house simulation packages are reviewed.

Longitudinal dynamics and energy analysis for heavy haul trains

Whole trip longitudinal dynamics and energy analysis of heavy haul trains are required by operators and manufacturers to enable optimisation of train controls and rolling stock components. A new technology named train dynamics and energy analyser/train simulator (TDEAS) has been developed by the State Key Laboratory of Traction Power in China to perform detailed whole trip longitudinal train dynamics and energy analyses. Facilitated by a controller user interface and a graphic user interface, the TDEAS can also be used as a train driving simulator. This paper elaborates the modelling of three primary parts in the TDEAS, namely wagon connection systems, air brake systems and train energy components. TDEAS uses advanced wedge-spring draft gear models that can simulate a wider spectrum of friction draft gear behaviour. An effective and efficient air brake model that can simulate air brake systems in various train configurations has been integrated. In addition, TDEAS simulates the train energy on the basis of a detailed longitudinal train dynamics simulation, which enables a further perspective of the train energy composition and the overall energy consumption. To demonstrate the validity of the TDEAS, a case study was carried out on a 120-km-long Chinese railway. The results show that the employment of electric locomotives with regenerative braking could bring considerable energy benefits. Nearly 40 % of the locomotive energy usage could be collected from the dynamic brake system. Most of tractive energy was dissipated by propulsion resistance that accounted for 42.48 % of the total energy. Only a small amount of tractive energy was dissipated by curving resistance, air brake and draft gear systems.

A review of dynamics modelling of friction draft gear

Longer and heavier trains mean larger in-train forces and more complicated force patterns. Practical experience indicates that the development of fatigue failure of coupling systems in long heavy trains may differ from conventional understanding. The friction-type draft gears are the most widely used draft gears. The ever developing heavy haul transport environment requires further or new understanding of friction draft gear behaviour and its implications for train dynamics as well as fatigue damage of rolling stock. However, modelling of friction draft gears is a highly nonlinear question. Especially the poor predictability, repeatability and the discontinuity of friction make this task more challenging. This article reviews current techniques in dynamics modelling of friction draft gears to provide a starting point that can be used to improve existing or develop new models to achieve more accurate force amplitude and pattern predictions.