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A torsional vibration absorber for automotive transmission, based on the MagnetoRheological effect of an elastomeric spring, is proposed to attenuate the torsional oscillations responsible of many vibro-acoustic disturbances. The spring element of the damper is constituted by a Magneto-Rheological elastomer, at which a suitable magnetic field can be applied in order to change its modulus and so the dynamical characteristics of the damper. The magnetic field can be varied by controlling the current in a coil surrounding the elastomeric element. In particular the damper can be tuned to operate in a desired frequency range as requested by the operative conditions of the transmission drive-line. Some numerical simulations of the transmission dynamic behavior are presented to describe interesting performances of the device in order to reduce annoying effects of the undesirable torsional vibrations, such as the gear rattle noise in the gear box. A technique based on the relieves of the gears accelerations has been applied on the numerical results, in order to evaluate the effectiveness of the torsional vibration damper in reducing the angular irregularities. Moreover an index based on the Wavelet multi-resolution analysis has been applied to confirm the goodness of the device in reducing the gear rattle.

The “gear rattle” phenomenon is a research topic of great interest for the NVH (Noise, Vibration and Harshness) automotive sector, concerning driveline noise and vibrations coming from the manual gear boxes. It is due to the internal combustion engine variable torque producing impacts and rebounds, and consequently noise, between the teeth of the unloaded gear pairs of the gear box because of the unavoidable presence of backlashes. Discrete Wavelet Transform (DWT) is used to decompose the angular relative motion signal and the wavelet decomposition details are adopted to analyze the dynamic behaviour of gears under rattle conditions. The DWT has been chosen because it is a particularly suitable instrument to recognize discontinuities, such as jumps or instantaneous changes in the slope of the signals, due to their localization in the time domain. Wavelet analysis allows to characterize an event by the localization, the kind, and the amplitude of the event. This technique enables, moreover, to define new indices for metrics of gear rattle especially useful in order to conduct comparisons among different operative conditions. Some examples of application of the proposed technique are reported in the paper. The theoretical investigations regard comparative analysis with respect to the amplitude and to the frequency of speed fluctuations and to the gears lubrication regime.

The results of an experimental activity carried out with the aim to investigate on the adhesive behaviour of visco-elastic materials in sliding contact with road asperities are presented. Experiments are carried out using a prototype of pin on disk machine in which pin is constituted by a specimen of rubber coming from a commercial tire, while different disks are realized in glass, marble, steel and in abrasive paper of different roughness. Tests are performed in both dry and wet conditions. Roughness of the test surfaces is evaluated by a rugosimeter, while pressure is evaluated, off-line, analysing the extension of the contact patch left by the pin on a sheet of graph paper under known applied loads. Slide velocity is imposed by an inverter controlled motor driving the disk. Basing on well known theoretical hypotheses, adhesive component of friction coefficient is estimated making the specimens slide on surfaces characterized by low values of macro-roughness, in order to underline the differences in rubber behaviour respect to micro-roughness surface variations. The results confirmed adhesion dependence on pressure and sliding velocity in both cases of smooth surfaces, where the main friction mechanism is the adhesive one, and of rough surfaces, where the main friction mechanism is the hysteretic one. Analysing various surfaces roughness it is possible to notice a maximized adhesive contribution on flat surfaces; it reduces with increasing roughness, while hysteretic friction comes over instead of it because of asperities penetration into rubber sliding surface. Moreover in the case of rough surfaces the separation between static and dynamic friction coefficient is evident and the static coefficient is greater than the dynamic one. On the other hand in case of smooth surface the absence of indentation phenomena doesn’t allow to recognize, in the measured force time history, the “classical” peak usually associated to the static friction coefficient. Dry and wet tests performed on different micro-roughness profiles highlighted that friction coefficient in dry conditions is greater on smoother surfaces, while an opposite tendency is shown in wet condition, when asperities are greater enough to break the thin water layer, providing a certain degree of indentation. A proposal for a methodology able to estimate the only adhesive friction component, developed thanks to wet contact tests, is expressed in the end of the paper.

Static transmission errors and engine torque fluctuations are two major issues that characterize the dynamic behaviour of gears. Torque fluctuations are typical, as an example, in internal combustion engine. Static transmission errors, such as tooth profile errors, tooth “spalling” defects, or wheel assembly eccentricity, are present in every actual gear pair. This paper introduces a technique that uses a wavelet-based signal processing procedure, applied to the signal coming from two meshing gears. Discrete wavelet transform (DWT) is used to decompose the meshing signal, and the wavelet coefficients variations are analyzed to describe the dynamic behaviour of gears. Some examples of application of the proposed technique are described adopting, as input data, the meshing signals acquired by two high resolution incremental encoders on a gear pair test rig. The ability of DWT in detecting either gear defects or torque fluctuation is shown. The analysis demonstrates that, by means of wavelet transform, it is possible to identify and evaluate the presence of specific anomalies in the meshing signal. The analysis also suggests the possibility of adopting such DWT based technique in an on-line control context. Finally, in the field of gear rattle, the technique can have also a practical relevancy in the definition of new rattle indices useful to the NVH departments of automotive industries.

In this paper an experimental test rig aimed to characterize a pneumatic tyre, together with the first experimental results, is presented. The objective is to determine some tyre mechanical characteristics useful to physically model its behaviour; in particular: the normal interaction characteristic, the radial stiffness, the total stiffness and the longitudinal hysteretic cycles. To this aim two different kind of tests have been executed: radial and longitudinal tests. In the radial test the load is statically applied, along the vertical direction, to the tyre by means of an hydraulic press and it is measured together with the consequent radial deformation, so allowing the estimation of the tyre normal interaction characteristic and of its radial stiffness. Different radial tests can be conducted for an assigned tyre varying the inflation pressure. The longitudinal tests are conducted applying, under an assigned constant vertical load, a variable horizontal strain to the tyre by means of a linear actuator, two profile rail guides and a system to transfer the horizontal motion to the contact patch of the tyre, opportunely placed on a moving steel plate placed on the two linear guide rails. During the tests the horizontal load and the resulting deformations are measured and acquired so allowing the estimation of tyre total stiffness and of its longitudinal hysteretic cycles. Longitudinal tests can be conducted varying the assigned vertical load, the horizontal displacement law in terms of frequency and amplitude, the tyre inflation pressure. All the different types of rim can be mounted on the test rig thanks to a universal quick flange.

The paper reports an experimental analysis of the "gear rattle" phenomenon in the automotive gear boxes. The study is conducted considering a single gear pair moved adopting suitable speed time histories in order to simulate the law of motion of an internal combustion engine. Some correlations between experimental and theoretical investigations about the influence of the oil lubricant between the teeth on the "gear rattle" reduction are reported. The analysis concerns in particular the effect of the oil viscosity. Moreover the dissipative effects due to losses in the bearings supporting the gears shaft have been considered in the study.

A novel experimental set-up is presented to study the complex behavior exhibited by gear systems. Speci¯cally, using an appropriate model of the experimental rig, the coexistence of several periodic orbits characterized by di®erent numbers of im- pacts per period is shown to be a fundamental mechanism in organizing the system dynamics. The presence of damping due to squeeze oil e®ects is taken explicitly into account. The aim is to uncover the e®ects of such damping on the complex behavior exhibited by the system. Preliminary two-parameter bifurcation diagrams will be presented together with phase space trajectories to support the analysis.

The article reports an investigation about the “gear rattle” phenomenon occurring in the automotive manual transmissions. In particular it has been studied the influence that may have the oil lubricant, and the various lubrication mechanisms, in reducing this undesirable event. The experimental analysis has the aim to validate a theoretical model, developed by the authors, that keeps in count the presence of oil in the “dead space” between the meshing gear teeth of the unloaded gear pairs in the no-contact phase. The results of a series of experiments conducted on an appropriate test rig for helical gear pairs, designed and realized at the Dime laboratories, have been then reported and compared with the theoretical expectations, under the condition of “idle gear rattle” varying the lubrication conditions.

Due to the irregularity of the I.C. engine torque, the impacts between the tooth of the unloaded gear pairs of a gear box produce vibrations originating an acoustic phenomenon, known as “gear rattle noise”. The paper presents a theoretical study on the dynamic behaviour of automotive transmissions focusing on the presence of oil between the impacting tooth of the meshing gears. This rattle phenomenon is particularly audible at low regimes and regards all the gear pairs different from the gear inserted to transmit the power. Another operating condition that manifests the gear rattle phenomenon is typically the “idle condition”, when all the gear pairs are meshing but without loading. The presence of oil between the impacting tooth of the gear pairs can play a positive role to decrease the vibrations and than the rattle noise level. In the paper some driveline complete models comprising many components as flywheel, clutch, primary and secondary shafts, are described accounting for the presence of oil lubricant between the teeth of the unloaded gear pairs. The described models consider an oil film lubrication between the tooth of the gear pairs, starting from the Reynolds equation and adopting some hypotheses regarding in particular the oil squeeze effect and also hypotheses about the teeth shape. Moreover some dissipative terms as friction and hysteresis in the clutch springs together with the oil drag torque in the gear box elements have been considered and modelled in the article.

The paper describes a test rig for the experimental validation of theoretical models to analyze the automotive drive-line dynamic behaviour accounting for the presence of oil between the impacting teeth of the gear pairs. The rig is constituted by a single gear pair, driven by an electric motor controlled in speed by an inverter. The peculiarity of the rig is its versatility in order to compare various gear pairs with different tooth parameters and different lubrication conditions. In particular the experimental study regards the acquisition of vibrations, originated during the impacts of an unloaded gear pair, responsible of the “gear rattle noise” phenomenon. At the present the automotive manufacturers investigate experimentally the gear rattle phenomenon generally comparing different cases by the measures of the noise level using microphones. The study aims to measure the gears relative angular displacements of an unloaded gear pair by two encoders, fitted on the primary and secondary shaft, and successively reconstructing the relative motion of the teeth along the contact line. Some results of the first experimental tests, conducted in dry conditions, are reported. The results confirm the test rig capability in detecting the relative motion between the impacting tooth.

The paper describes a wide analysis performed on the most actual methods for prediction of the reciprocating engine bearings behaviour. In the modern IC engines design, it is of great importance the availability of advanced simulation tools that allow to consider a number of physical effects parameters grown important due to the increasing in the specific power. In particular for the crank mechanism bearings, i.e. crankshaft main, conrod and gudgeon bearings, an analysis based on Thermo-Elasto-Hydroynamic Lubrication (TEHD), is need that keeps into account the structural and dynamic aspects of the whole engine, is needed. This paper gives an analysis on the most actual methods for prediction of the reciprocating engine bearings behaviour and shows some tests performed by AVL-EXCITE, a commercial code of interest for ELASIS.

18-20 september, Vietri sul Mare Salerno Italy

proceeding on CD

Atti disponibili solo su cd