If the axis of at least one gear in gear train moves relative to fixed axis or frame, such type of gear train is known as epicyclic gear train. In this gear train, one gear rotates over pitch circle of other gear as shown in Figure 15.27. Consider two gear wheels P and Q, the axes of which are connected by an arm ‘a’ is fixed. The wheels P and Q constitute a simple gear train.
Figure 15.27 Epicyclic Gear Train
However, if the wheel Q is fixed so that the arm can rotate about the axes of Q, the wheel P would also move around Q. Thus, it is epicyclic gear train.
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