Planetary movement was first used by early Greek astronomers in a model which shows orbital movements of planets in the space. Planetary gear systems consist of planet gears mounted around a central gear called sun gear and a movable arm or carrier which connects the planetary gears and rotates relatively to the sun gear. In planetary gear system axis of one or more planet gears makes a circle around the central sun gear. The sun gear is mounted on a shaft. Also planet gear carrier is mounted on another shaft which is in the same axis. Planet gear carrier is like a beam which consists of one or more arms at right angle to the shaft. Each arm has a rod parallel to the shaft and each rod has a free rotating planet gear. Generally three planet gears are used for load sharing. However these gears are same to each other and increasing the number of gears does not affect operation of the system. Planetary gears are at equal distance from the shaft and each other, planet gears are meshed with the sun gear and rotate relatively around it. Annulus is the outer ring with inward-facing teeth that mesh with the planet gears so the planet gears rotates around the sun gear while rotating inward the outer ring. Various gear ratios. There are several configurations for planetary systems. Typical configurations require that out of the 3 components: one is the input, one is the output, and one is stationary so various gear ratios can be carried. For example, one configuration is where the sun gear is the input, the annulus is stationary, and the planet carrier is output. In this configuration planet gear carrier will move in same direction with the sun gear but relatively slower. If planet gear carrier is stationary, the outer ring will move in same speed with the sun gear but in opposite direction. If planet gear carrier is the input, and the sun gear is stationary, the outer ring will move in same direction but faster. In another configuration all three components may be stationary and then torque will be transferred directly as a shaft. There are many kinds of planetary gear systems. In some rotational transfer systems there may be a lot of planetary gear mechanisms mounted side by side but also in some systems sun gears in various dimensions assembled with appropriate planet gears and outer rings may be mounted in a single planetary gear mechanism. Every single gear has various numbers of teeth and each of them may be locked on a rotary shaft or on a clutch mechanism. The biggest advantage of the planetary gear mechanisms is that high rotational ratios can be produced by using small number of relatively small dimensioned gear systems. Also the input and output shafts are coaxial so the mechanism is extremely well assembled. Other advantages are eliminated radial loads, working silently and the facility of using planets in steps. In recent years two new improvements assisted in the using planetary gear systems in high powered transmission systems. The first one is that load can be shared equally between stepped planets fulfilled by adopting floating sun gears and planet gears. The second one is the improvements in threading machinery and techniques that made producing the inner gears as sensitive as the outer gears possible. On demand planetary gears can be used without any stationary components. The main advantage of planetary gear systems is supplying demanded gear ratios in an interval with small space and without the need of clutching or detaching gears to each other.