New Type of Planetary Gearbox with High Gear Ratio

Abstract

In order to achieve extremely slow motion (for instance, in steer mechanisms, strength testing machines), gearboxes with high gear ratio (thousands, dozens and hundreds of thousands) are used. Common solutions imply here the multi-stage reduction based on planetary or worm gears. The paper considers two new planetary mechanisms based on worm and spiroid gears. The mechanisms provide an extremely high reduction (from dozens of thousands up to hundreds of thousands and more in one stage), coaxial arrangement of the input and output shafts, multi-flow power transfer, smoothness of operation, possibility to adjust or eliminate the backlash due to combining many advantages of common multi-stage planetary gears and multi-stage usual (non-planetary single-row) worm and spiroid gears. One of the mechanisms implies simultaneous meshing of the spiroid satellite worm with two sun gearwheels (movable and fixed). As for the second mechanism, each of two spiroid satellite worms (in general, different from each other) is meshing with the sun gearwheels: one spiroid worm with the movable gearwheel (related to the output shaft) and the other spiroid worm with the fixed one. In the second mechanism the drawbacks of the first one are eliminated, namely, the presence of the auxiliary relative arrangement of the spiroid worm and gearwheel (and the consequent reduction of the efficiency and load-carrying capacity), higher thermal loading of the worm, and limitations of varying the gear parameters. Formulas are derived for calculating the gear ratio with its large value provided by small difference in gear ratios for meshing of the spiroid satellite worm (or two spiroid satellite worms) with sun gears. The method of analyzing the forces acting in the worm and spiroid gearing and the efficiency of the mechanism is described. The method is based on equilibrium condition for torques of the pointed forces relatively to axes of the carrier and satellite; and it is implemented in the software that allowed for performing the numerical investigation of the influence of parameters of the mechanism on its gear ratio. In particular, investigation showed that one should aim at greater reduction at the worm stage of the mechanism, at interaxial angles close to 90º, and at low ratios of the gearwheel diameter and the interaxial distance. The proposed mechanisms are comparable with simple stationary-axes multi-stage worm-type mechanisms by the efficiency; and they provide smaller sizes and coaxial alignment of the input and output shafts.