Introduction to 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.
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.
Epicyclic Gearbox Parts :
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.
Epicyclic Gearbox Diagram :
Construction of epicyclic Gearbox:
In the epicyclic gearbox, the epicyclic gear train is a very general term. Basically, it involves 3 gears: a sun gear, a planet gear and a ring gear, the underlying concept being many gear ratios can be obtained from a small volume as compared to other types of gear trains which take up more space. Unlike simple gear trains, an epicyclic gear train requires defining more than one input to obtain a specific output, hence making the analysis a little difficult and non-intuitive.
Working of epicyclic gearbox:
The working principle of the epicyclic gearbox is based on the fact the fixing any of the gears i.e. sun gear, planetary gears, and annular gear is done to obtain the required torque or speed output. As fixing any of the above causes the variation in gear ratios from high torque to high speed. So let’s see how these ratios are obtained
First gear ratio:
This provide high torque ratios to the vehicle which helps the vehicle to move from its initial state and is obtained by fixing the annular gear which in turn causes the planet carrier to rotate with the power supplied to the sun gear.
Second gear ratio:
This provides high-speed ratios to the vehicle which helps the vehicle to attain higher speed during a drive, these ratios are obtained by fixing the sun gear which in turn makes the planet carrier the driven member and annular the driving member in order to achieve high-speed ratios.
Reverse gear ratio:
This gear reverses the direction of the output shaft which in turn reverses the direction of the vehicle, this gear is achieved by fixing the planet gear carrier which in turn makes the annular gear the driven member and the sun gear the driver member.
Note- More speed or torque ratios can be achieved by increasing the number of planet and sun gear in epicyclic gearbox.
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.
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
Advantages of Epicyclic Gearbox :
The planetary gearbox offers a set of distinct advantages which makes it an interesting alternative to traditional gear types such as helical and parallel shaft gearboxes in applications requiring:
- High reduction ratios
- Compact and lightweight with high torque transmission
- High radial loads on the output shaft
- It is quieter in operation
- Uniform distribution of load over all gears having greater tooth contact.
- All gears are constantly in mesh, so a change of one gear to another is possible without any loss.
Disadvantages of planetary gear systems
• 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
Application of Epicyclic Gear train :
- A good example of the everyday application of a planetary gear system is the automatic transmission of a car.
- The epicyclic gear trains are used in the back gear of lathe, differential gears of the automobiles, hoists, pulley blocks, wrist watches, etc.
- The epicyclic gear trains are useful for transmitting high-velocity ratios with gears of moderate size in a comparatively lesser space.
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