An essential component of manually operated vehicle transmissions is the synchronizer. Synchronizers have the task of minimizing the speed difference between the shifted gearwheel and the shaft by means of frictional torque before engaging the gear. Proper operation requires a sufficiently high coefficient of friction. It is common practice to investigate the friction and wear behaviour under various loading conditions on test rigs or in vehicle tests. An optimised design of the system with regard to appropriate function and durability on the one hand as well as low cost, low mass and compact over-all dimensions on the other hand requires extensive testing. According to the present state of knowledge, derived from numerous experimental investigations, temperature can be attributed the most significant influence on the tribology of synchronizing systems. Therefore, the influence of various loading conditions on contact temperature was investigated; a relation between temperature and tribological performance was established. The Finite Element Method was applied to simulate the thermal behaviour of a synchronizing system depending on different operating conditions. A result of this simulation is the spatial and time-dependent temperature distribution in the area of contact. Characteristics of the tribological performance of a molybdenum coated synchromesh ring in contact with a steel cone were derived from extensive experimental investigations. Significantly different friction and wear patterns can be distinguished. At heavy loading conditions the coefficient of friction is quite high and continuously severe wear occurs; light operating conditions result in a low friction coefficient, whilst no more wear is observed. Between those two extremes an indifferent regime exists, in which both patterns of tribological behaviour occur. A reason for this characteristic behaviour of the system described here was found by means of the Finite Element simulation. Apparently, the friction and wear pattern depends on the temperature in the contact area; for mild wear and low friction coefficients the contact temperature must not exceed a critical value in order to avoid severe wear. Within limits the predicted tribological behaviour and the test results are in good agreement. The calculation of the temperature in the contact area provides a basis for a classification of the load conditions in terms of their thermal and tribological effect, a practically applicable estimation of service life and a design procedure based on numerical simulation rather than on testing. Thus, a reduction of development efforts will be possible.
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