Planetary gear (also called epicyclic gear) is a kind of a gear arrangement usually used for applications where dealing with high gear ratio and small dimensions is present. Although there are many different types of planetary gear, the three and four wheel types are the most common. Gearboxes that use a three wheel design implement the three planetary gears in two different stages. However, a three wheel design cannot make use of three planetary gears since here, three wheels addresses the number of the distinctive sizes not the number of the planetary wheels. One stage can sometimes come to a ratio of ten and there are certain instances when a higher ratio is required. When one needs to achieve a higher ratio, they should pair two or more stages and this should be done in an enclosure so that a gearbox with variable ratio and axis direction of the rotation is created.
A three wheel planetary gear stage comprises the four following parts: sun gear, planetary gears, planetary carrier and a ring wheel. The sun gear is also called a center and is designated with an S. The planetary gears are in fact three gears that are in constant rotation around the Sun gear. The planetary carrier is what holds the planetary gear in place as to prevent jamming and it is marked with a C. The ring wheel is also called the outer gear rim, it is marked with an S and serves to lock the rotation of the various components that a gear comprises. Planetary gearbox efficiency wise, one can achieve two rotational directions and four ratios through the help of the ring wheel.
The gearbox itself serves primarily to transform the revolving instances. It achieves this by either transforming the revolving moment to a slower and stronger rotation or to a faster and weaker rotation. It depends on the force and speed required. The gearbox minimizes the speed of the engine so that it can create a larger torque.
In order to achieve the planetary gearbox efficiency required, the gearbox features a planetary design named spur gear. This design accounts for the missing teeth of the gear, cut in a straight line that is parallel to the gear's axial line and they rotate around each other. The shape of the teeth of the gear are involutes and this means that the line that rests between the gears is linear. Another important remark to mention is the fact that a gearbox might experience certain mechanical losses, for instance: rolling losses, sliding losses, gear bearing losses and gliding losses.
Sliding losses take place when two surfaces are sliding one against another and the teeth interaction results in power loss. This can be prevented by separation of the surfaces and by applying a lubricant.