Planetary Drives
Planetary Drive is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet drives are mounted on a movable arm or carrier which itself may rotate relative to the sun gear. Planet driving systems also incorporate the use of an outer ring gear or annulus, which meshes with the planet drives. Planetary drives are typically classified as simple and compound planetary drives. Simple planetary drives have one sun, one ring, one carrier, and one planet set. Compound planetary drives involve one or more of the following three types of structures: meshed-planet , stepped-planet , and multi-stage structures . Compared to simple planetary drives, compound planetary drives have the advantages of larger reduction ratio, higher torque-to-weight ratio, and more flexible configurations.
The axes of all drives are usually parallel, but for special cases like pencil sharpeners they can be placed at an angle, introducing elements of bevel gear . Further, the sun, planet carrier and annulus axes are usually coaxial.
Advantages and disadvantages
Advantages of planetary drives over parallel axis gears include high power density, large reduction in a small volume, multiple kinematic combinations, pure torsional reactions, and coaxial shafting. Disadvantages include high bearing loads, inaccessibility, and design complexity.The planetary drivebox arrangement is an engineering design that offers many advantages over traditional drivebox arrangements. One advantage is its unique combination of both compactness and outstanding power transmission efficiencies. A typical efficiency loss in a planetary drivebox arrangement is only 3% per stage. This type of efficiency ensures that a high proportion of the energy being input is transmitted through the drivebox, rather than being wasted on mechanical losses inside the gearbox.
Another advantage of the planetary drivebox arrangement is load distribution. Because the load being transmitted is shared between multiple planets, torque capability is greatly increased. The more planets in the system, the greater load ability and the higher the torque density. The planetary drivebox arrangement also creates greater stability due to the even distribution of mass and increased rotational stiffness.