Abstract: Planetary Gear Trains are extensively used for the power transmission and are the most critical component. Planetary gearboxes are used frequently to match the inertias, lower the motor speed, boost the torque, and at the same time provide a sturdy mechanical interface for pulleys, cams, drums and other mechanical components. This paper presents the advantages of planetary gear systems over other, consideration to be given while determining the reduction ratios of gear box, minimum and maximum reductions per stage of planetary gear pairs. Also the case study to determine the reduction ratio of two stage gear box is discussed. The comparison of planetary gear system over helical gears systems is done on the basis of volume, weight and torque density. An epicyclic gear train consists of three gears mounted so that the centre of one gear revolves around the centre of the other. A carrier connects the centers of the two gears and rotates to carry one gear, called the planet gear, around the other, called the sun gear. The planet and sun gears mesh so that their pitch circles roll without slip. A point on the pitch circle of the planet gear traces an epicycloid curve. In this simplified case, the sun gear is fixed and the planetary gear(s) roll around the sun gear. An epicyclic gear train can be assembled so the planet gear rolls on the inside of the pitch circle of a fixed, outer gear ring, which is called an annular gear. In this case, the curve traced by a point on the pitch circle of the planet gear is a hypocycloid. The combination of epicycle gear trains with a planet gear engaging both a sun gear and an annular gear is called a planetary gear train. In this case, the annular gear is usually fixed and the sun gear is driven.
Keywords: Epicyclic Gear Box, Planetary Gear Trains, planet gear, epicycloid curve.