epicyclic (or planetary) gear train

EPICYCLIC (OR PLANETARY) GEAR TRAIN

• When the axis of rotation of one or more gears is allowed to rotate about another axis, then the gear train is known as epicyclic gear train.

• Epi means upon and cyclic means around. Thus in epicyclic gear train one or more gears move upon and around another gear.

• Difference between epicyclic and other gear trains: 

□ In case of simple, compound and reverted gear trains, the axes on which the gears are mounted are fixed relative to each other.

□ But in epicyclic gear trains, the axes of the shafts on which the gears are mounted may have the relative motion between them.

• Advantage: The advantage of epicyclic gear trains over simple or compound gear trains is that it can achieve high speed reductions within a very limited space.

• Applications: The epicyclic gear trains find many applications in:

□ automobile differentials

□ machine tools

□ hoists

□ pulley blocks

□ wrist watches

□ aircraft propeller reduction drives

□ automatic transmissions, etc.

• Types of epicyclic gear trains: The epicyclic gear trains can be sub-divided into two classes:

1. Simple epicyclic gear trains, and

2. Compound epicyclic gear trains.

1. Simple Epicyclic Gear Trains

• When there is only one gear on each shaft in any epicyclic gear trains, then they are called as simple epicyclic gear trains.

• A simple epicyclic gear trains is shown in Fig.5.8.

• Three main elements of the any epicyclic gear train are:

(i) Sun gear: The gear at the centre, whose axis is fixed, is called the sun gear.

(ii) Planet gear: The gear whose axis moves (which is free to revolve around sun gear) is called planet gear.

(iii) Arm: The arm carries the planet gear and is free to revolve about the fixed axis.

• In epicyclic gear train shown in Fig.5.8:

□ Gear A and arm C rotate about the fixed axis O1.

□ Gear B rotates about the moving axis O2 and also about the arm C.

□ Arm C revolves about fixed axis O1.

• The epicyclic gear trains are also called as planetary gear trains or sun and planet gear trains, because of the fact that gear B rotates around gear A just like a planet moving around the sun.

• In Fig.5.8, if the arm C is fixed, then gear A drives gear B and hence it becomes simple gear train. Instead, if the arm C rotates, then the gear train becomes epicyclic gear train.

2. Compound Epicyclic Gear Train

• When there are more than one gear on a shaft in any epicyclic gear trains, then they are called as compound epicyclic gear trains.

• A typical compound epicyclic gear train is shown in Fig.5.9.

• The epicyclic gear train shown in Fig.5.9 has a sun gear, three planet gears, the arm, and an annulus gear. Annulus gear is nothing but the internal gear.

• The sun gear is free to revolve; whereas the annulus is fixed.

• Shaft A is the input shaft and shaft B is the output or driven shaft.