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Image Coutesy: Google          We know that in every machine, motion is transmitted from one shaft to another. This is done by using Gears, Coupling etc., To transmit rotory motion a set of gears are used. “Gear train is a combination of gears which are used to transmit rotory motion from one shaft to another shaft”. A gear train is constructed in a frame by mounting gears so that the teeth of gears passing away rom each other (Without slipping); It provides a smooth transition of rotation from one gear to another. –> A gear train is two or a lot of gears operating along by meshing their teeeth and turning each other to produce power and speed.–> It increases Torque and reduces Speed and viceversa.Types of gear train§   Simple Gear Train§   Compound Gear Train§   Epicyclic Gear Train§   Reverted Gear TrainSimple gear train :-          By the name itself we are able to understand that it’s a simple arrangement of gears. When there is just one gear is mounted on each shaft (or its own shaft) then this kind of gear train is termed as “Simple gear train”.         The relative speed of gear train is determined by the two gears circumference. It’s necessary to come to a decision, the driven gear speed will be more, less or same as the drive gear before we tend to implement simple gear train. Simple gear trains are used to transmit motion from input shaft to output shaft wherever they’re massive distance apart. When the gap between the input and output shaft is a smaller amount and up to the sum of centre distances of gears then the two gears 1 and 2 are made to mesh with one another to transmit power from input shaft to output shaft.Image Courtesy: http://nptel.ac.in/courses/          Some times, the gap between the input and output shaft is more than the sum of centre distances of gears then the motion is transmitted by using any number of intermediate gears, these intermediate gears are referred to “Idler Gears”. Idlers gears speed dont have an effect on the speed of driven shaft. When even number of gears are present in gear train, the motion of driven gear is always opposite to the drive gear.          The ratio of gear train is defined as, “it is the ratio of the angular velocity of  driver (Input gear) to the angular velocity of driven gear (Output gear)”. If ‘n’ number of gears are present in a simple gear train then they are numbered from the driving gear to driven gear as 1, 2, 3 …; their rotational speeds as N1, N2, N3…. and their number of teeth as T1, T2, T3 … respectively. Then the gear ratio is represented asCompound gear train :-          Compound gear train is outlined as, when there is more than one gear (atleast 2) is rigidly mounted on same shaft then that kind of gear train is termed as Compound gear train. So that pairs have same speed (rpm).         When there’s a desire of huge changes in speed or power output and there is less space between input and output shafts then this kind of gear trains is employed. Both the direction of drive gear and number of shafts are using in gear train will determine the rotation direction of driven gear shaft.Image Courtesy: Google          Consider a compound gear train with 6 gears and 4 shafts as shown in Fig., Then the gear transmission ratio of compound gear train isSpeed ratio of meshing gears 1 & 2 is similarly speed ratios of (3 & 4) and (5 & 6 ) Where                       N2 = N3 and N4 = N5           (Since they are on same shafts)             Multipley all above equations,We get  Planetary gear train :-          Just like compound gears, planetary gears are used when there is a desire of huge changes in speed or power output across small distances.          When compared to other gear trains the arrangement of compound gear train is more complex planetary gear train is similar to our planet system, that’s the reason it got the name. In our solar system, sun present at the centre and planets revolve around the sun. Gravitational force holds them in their places.          Basically in any planetary gear set consists three main components or gears they are:                .       The sun gear               .       The planet gears                    .       The ring gear          The sun gear is present at the centre and the planet gears revolve around the sun gear. These gears are held together by ring gear. Based on system one or more than one planet gear rotates around the sun gear. Sometimes connecting arm is used in between planetary gears for holding them together. Image Courtesy : Google          Unlike ordinary gear trains, an epicyclic gear trains is used to define more than one input to obtain a specific output i.e., Each of these three gears can be the input, the output or can be held stationary. So analysis is a little harder and more natural. Compared to other gear trains, there are many gear ratios obtained from less space. These four combibations and the resulting speed and power outputs are listed in TableImage Courtesy: GoogleReverted gear train :-          A Reverted gear train is extremely just like a compound gear train. They’re both used when there’s a desire of huge changes in speed or power output but hte space for accommodating several gears is restricted.Image Courtesy : Google          A reverted gear train is also a compound gear train, having the input and output shafts on the same axis (Co-axial). If these two gears are mounted on the same shaft, one of them must be loosely mounted. And also the distance between the centres of the two gears in each pair must be the same.           The gear transmission ratio is calculated similarly like compound gear train.