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Introduction to Epicyclic Gear Train | Working And Advantages

Introduction to Epicyclic Gear Train | Working And Advantages and Disadvantages Of The Epicyclic Gear Train.

Epicyclic gearing also called as planetary gearing. It is a gear system that consists of one or more outer gear (planet gear) rotating about a central (sun gear). The planet gear are mounted on a moveable arm (carrier) which itself may rotate relative to the sun gear. Epicyclic gearing systems may also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears.

Figure show an example of epicyclic gearing. It is used to increase output speed. The planet gear carrier is driven by an input torque. The sun gear provides the output torque, while the ring gear is fixed.
Parts Of Epicyclic Gear train
Parts Of Epicyclic Gear train 

Gear ratio:
The gear ratio in an epicyclic gearing system can be different by the design of the gear teeth and the ways of input rotation to the gear. The three basic components of the epicyclic gear are:

1. Sun: The central gear
2. Planet carrier: Holds one or more peripheral planet gears, of the same size, meshed with the sun gear
3. Annulus or Ring Gear: An outer ring with inward-facing teeth that mesh with the planet gear or gears

In many epicyclic gearing systems, among the three basic components, one component is held stationary; one component is input, provide power to the system and last component is output, receive the power from the system. The ratio of input rotation to output rotation is dependent upon the number of teeth in each gear, and upon which component is held stationary.

One situation is when the planetary carrier is held stationary, and the sun gear is used as input. In this case, the planetary gears simply rotate about their own axes at a rate determined by the number of teeth in each gear. If the sun gear has S teeth, and each planet gear has P teeth, then the ratio is equal to -S/P. This rotation of the planet gears can inturn drive the annulus, in a corresponding ratio. If the annulus has P teeth, then the annulus will rotate by P/A turns for each turn of the planet gears.

In summarize:
1. One turn of the sun gear results in - S / P turns of the planets
2. One turn of a planet gear results in P / A turns of the annulus
3. One turn of the sun gear results in –S / A turns of the annulus

One situation is when the annulus may also be held fixed, with input provided to the planetary gear carrier; output rotation is then produced from the sun gear. This configuration will produce an increase in gear ratio, equal to 1+A/S.

These are all described by the equation:
(2+n)ωa + 2ωs – 2(1+n) ωc = 0
Where n is the form factor of the planetary gear, defined by: n = Ns/Np

Advantages and Disadvantages of the epicyclic gear system
The planetary gear box offers a set of distinct advantages which makes it an
interesting alternative to traditional gear types such as helical and parallel shaft gear
boxes in applications requiring:
• High reduction ratios
• Compact and lightweight with high torque transmission
• High radial loads on output shaft

Disadvantages of planetary gear systems
• Complexity
• Assembly of gears is limited to specific teeth per gear ratios
• Efficiency calculations are difficult
• Driver and driven equipment must be in line to avoid additional gearing

A good example of the everyday application of a planetary gear system is the automatic transmission of a car.

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