Traction-drive speed reducers offer certain advantages over geared speed reducers. In particular, they generally run quieter than geared units and provide an opportunity for higher efficiency by eliminating sliding motion between contacting elements. In order to generate a sufficiently high output torque, some means must be provided to create a normal force between the rolling elements. This normal force, along with the friction coefficient, enables the device to transmit torque from one rolling member to the next. The speed reducer proposed here is designed so that the configuration of the rolling elements creates the needed normal force in response to the torque exerted back on the system by the downstream loading. Thus the device is self-actuating. Since the normal force is only present when needed, the rolling elements of the device can readily be disengaged, thus eliminating the need for a separate clutch in the drive system. This feature can be exploited to design a transmission with several distinct speed ratios that can be engaged and disengaged in response to changing speed requirements.
In this paper, the pure rolling contact stresses in conical rollers under normal loading are studied. The existing analytical expression for pure rolling contact pressure in uniform cross-section cylinders is modified to determine the contact area, the contact pressure, and its distribution in conical rollers under normal loading. The Hertzian contact stresses are determined for the modified pure rolling contact pressure distribution. The theoretical results are compared with the finite element analysis using ABAQUS, for different vertex angles, materials, and loads. The results of the theoretical model are found to be consistent with the finite element simulations in predicting the contact pressure, the distribution, and the contact stresses in conical rollers.
Fatigue life investigations have been made for cylindrical hollow rollers in pure rolling contact. In addition to normal loading, the rollers have been subjected to tangential loading of 1/3rd the normal load value. Sufficient coefficient of friction has been used to ensure no slipping occurs. Two main models were built with different hollowness percentages to investigate the hollowness percentage that gives the longest fatigue life. The first model consists of two cylindrical rollers of same size, while the second model consists of two rollers of different sizes. Two cases have been studied, when both rollers are hollow and when only one roller is hollow. The stress distribution in the roller body and the resulting deformation has been investigated using the finite element package, ABAQUS. Then the Ioannides-Harris (IH) theory was used to predict the fatigue life of the hollow rollers in pure rolling contact. Investigations have been made for five different materials, CVD 52100, Carburized steel, VIMVAR M50, M50NiL and Induction-hardened steel. It has been found that the optimum hollowness percentage with the longest fatigue life ranges between 50% and 70%. Many factors affect the optimum hollowness percentage, like the kind of the material used for the cylindrical roller, whether the rollers in contact are of the same size or different size and whether the hollow roller is in contact with another hollow roller or in contact with solid roller. At the optimum hollowness percentage, the roller can live hundred times the life of solid roller. So, as the endurance limit of the material increases, as the fatigue life of the rollers increases too. It has been found that cylindrical roller in contact with another identical sized roller has shorter fatigue life than the cylindrical roller in contact with a bigger roller. That might be related to increase the flexibility of the system that acts as a spring mass system and to the increase of the contact surface area. In case of identical sized models, the longest fatigue life achieved was two hollow rollers of 50% percentage of hollowness. When only one roller is hollow, the optimum shifts to 70% percentage of hollowness. For the non identical sized rollers, the optimum is around 50% but when one roller only is hollow, the fatigue life is longer. That might be related to optimum flexibility that gives the longest fatigue life. If the flexibility of the system is very high, the fatigue life of the roller is reduced because of the effect of the bending stresses.
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