Providing of Sliding Bearings Reliability of Transmissions Gear Wheels of Transport Cars by Optimization of Assembly Tolerances
In the design of gearboxes and transfer boxes of heavy-duty vehicle`s transmissions, sliding bearings are often used as supports for gear wheels.Analysis of the results of statistical processing of gearbox parts failures and transfer boxes of wheeled tractors with a pulling force of 30 kN indicates the need to improve the reliability of the sliding bearings of the gear wheels. Such plain bearings ensure free rotation of the gear wheels in case of the torque transmission absence, and when locked by a clutch, the radial load of the gear wheel is sensed, while operating in the slipping rolling mode. Such units are poorly understood and in the technical literature sources there are no recommendations for their engineering, which often leads to errors when selecting operation gaps and press fitted in gear wheels.The manufacturers of the transmissions assemblies replaced the plain bearings, which in this case are made of metal-ceramic bushes, for roller bearings. Also, the application of the bearing unit with a pair of friction "steel-steel" was introduced. Such changes did not lead to an increase in reliability, and in the first case led to the early appearance of the brinelling on the bearing race. To control the reliability of gear wheel bearings, it is necessary to minimize the maximum stresses in the bearing material that depend on the gap in the bearing and operation conditions, the physical and mechanical properties of the contacting materials, the amount of interference in the "gear-bushing" coupling, the geometry of the gearing and the structural features of the assembly. The cause of bearings failure is plastic deformation of bushes material, that can lead to weakening of the press fitted gear and bearing rotation in the gear. In the future, it can lead to violation of the friction regime, bearing overheating and adhesive wearing. This often causes spontaneous inclusion of the transmission and movement of the vehicle.With the using of solid-state models of a plain bearing, a shaft and a gear wheel, numerical calculations of their stress-strain state are carried out. The results of bench tests of stress distribution in the bearing material under the influence of various operational factors are presented. The analysis of the obtained results has shown the possibility of increasing the reliability of the bearing assembly by justifying the tolerances for assembling the "bushing-gear" and "shaft-bushing" assemblies. An improved technique for calculating the limiting value of the clearance in a plain bearing is presented, based on the strength conditions and the preservation of the accuracy of the gearing. As a result, recommendations were offered for grouping of the production assembly tolerance for coupling "shaft-bushing" to dimensional groups, which make it possible not to introduce changes in the design of a plain bearing or gear wheel and at the same time ensure the normative values of their failure-free operation.