Theory of Machines

Lesson 19.

19.1.1 SIMPLE GEAR TRAIN

As the name suggests this is the simplest type of gear train in which one gear is mounted on single shaft. Such gear trains are generally used where there is no constraint of space and large centre distance can be maintained between the gear mounting shafts. The gear ratio produced by the train depends on the number of teeth on each gear.  Idler gears have no effect on the speed ratio or train value of gear train. For different gear ratios, different combinations of gears are required. It may be worth mentioning here that meshing gears must have same module.

Fig 4.10 Simple Gear Train

19.1.2 COMPOUND GEAR TRAIN

In Compound gear train more than one gear rotate on single shaft. Such type of gear trains are used where the designer wants a compact layout and when large changes in speed or power output are needed.

19.1.3  Epicyclic Gear Train

Epicyclic means to move upon and around in circular manner. Consider the diagrammatic representation of simple epicyclic gear train in the fig 4.12.  In this gear train gear B meshes with gear A. Considering the arm AB to rotate about axis of gear A, then the gear B is forced to rotate upon and around gear A.

Fig 4.12 Epicyclic Gear Train

Compound epicylic gear trains are also referred to as Planetary gear trains. In such gear trains one or more gears orbit about the central axis of the train.

Fig 4.13 Planetary Gear Train

As shown in the figure 4.13 the sun gear, N₁ engages all three planet gears simultaneously. All three planet gears are attached to a plate (the planet carrier), and they engage the inside of the ring gear. The output shaft is attached to the ring gear, and the planet carrier is held stationary. Different gear ratios can be produced depending on which gear is used as the input, which gear is used as the output, and which one is held stationary. The major applications of Epicylic Gear train is in differential of automobiles, lathe machines etc.