Gears are used for the transmission of motion through engagement of gear teeth which will either result in an acceleration or reduction of speed. These teeth can be applied in various forms, the most common being the involute gear tooth profile. A gear train is made when a combination of gears in mesh are used to transmit motion. A frequent application of epicyclic gear trains is accomplishing a large speed reduction in a small space.
A planetary or epicyclic gear train is one type of gear train used to transmit motion. Epicyclic gear trains consist of two or more gears mounted so that the center of one gear revolves around the center of the other. Epicyclic gear trains also known as a planetary gear train are gear trains with relative motions of axes. 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. All the planets are mounted to a single rotating member, called a cage, arm, carrier. As the planet carrier turns, it delivers low-speed, high-torque output. In some systems every member rotates, but many have at least one component is not rotating.
Three main configurations exist of planetary gears for various applications:
- Two inputs, one output and no fixed element. This mechanism combines the speed of the two inputs.
- One input, two outputs, and no fixed element. This creates a differential that splits the input torque to the two different outputs.
- One input, one output, and one fixed element. This will reduce the reduce speed of the input.
The specific problems that the planetary gear set solve make the mechanism attractive to engineers in many different industries. Advantages of the use of a planetary gear train are low vibration, high speed reduction ratio, and the low cost for the entire train layout. Some of the common uses for planetary gear transmissions are robotic arms, hybrid vehicle power transmissions and turbine generators. Despite the advantages of epicyclic gear trains such as compact structure, lightweight and high power density, they may have relatively low efficiency compared to simple gear systems. The principle power losses in gear trains are caused by sliding friction between meshing gear tooth surfaces, churning of lubrication oils and friction in shaft support bearings.
Planet gears, for their size, engage a lot of teeth as they circle the sun gear; therefore they can easily accommodate numerous turns of the driver for each output shaft revolution. Simple planetary gears generally offer reductions as high as 10:1. Compound planetary systems, which are far more elaborate than the simple versions, can provide reductions many times higher. This reduction can be contributed to the velocity relationships of the components of the systems.
Since planetary gears mesh with the sun gear and ring gear at several locations, more teeth are engaged to drive the load, compared to a conventional gear and pinion mesh. Therefore, for the same load, planetary gearing requires smaller gears than a standard pinion-to-gear reduction. Likewise, the radial arms of the planet carrier transfer a substantial moment to the output shaft – another illustration of the efficiency of a concentric arrangement. Different types can be used in the application of the epicyclic gear train from spur to spiral bevels in order to change the effect of the torque in the system. Helical gears can be used for load capacity beyond spur gears, given comparable gear sizes and numbers of planets – because helicals are angled, not straight-toothed, even more teeth mesh at once. But with helical planetary gearing there are axial reactions, and these don’t cancel with multiple planets like the tangential and separating gear reactions do, so bearings have to account for the thrust load. Another advantage of multiple gear mesh points is the torque density can be increased. Through these multiple gear mesh points, applied load to the planetary gears is distributed. This also increases the torsional stiffness of the gear train by a factor the same number of the planet gears. This stiffness allows for higher positioning accuracy and repeatability of requirements. Planetary gear trains are
The load taken up by the planets is, in real situations, not perfectly balanced. One planet might by chance end up radially closer or further than the others from the sun axis, or the axis of the carrier rotation might be slightly off. As the precision of manufacture goes down, and the number of planets goes up, the tendency for imbalance increases. Sometimes the effect of an imbalance is small and the operation is able to accept it. Some designs will be sensitive even to the slight imbalances, and may require high-precision components and assemblies; pinpointing the proper locations of planet pins around the axis of the sun gear could be the key.
There are disadvantages in the application of planetary gearboxes. One disadvantage of using this style of gearbox is the loss of lubrication leading to failure when running at high speeds because the lubricant is slung away. This disadvantage can be overcome through the use of pressurized forced lubrication systems. Another solution is the use of grease lubricant for the life of the gearbox. Power losses such as the mechanical loss of friction are increased due to the multiple planet branches is another one of the disadvantages that must be taken under consideration when choosing to implement a planetary gear set. Inevitable assembly and manufacturing errors, which lead to an increase in noise during operation and decreasing the reliability over time, have a much greater effect in planetary gear set than in rack-and-pinion gear set.
Calculation for Planetary Gear Trains:
R: Gear Ratio
N: number of teeth
In order to determine the number of planet gears needed is correct for the application the following constraint must be met:
P: number of planet gears
