What is a Compound Gear Train? How are compound gears different from Idler gears? How do you calculate the gear ratio in a compound gear train?

By creating compound gear trains it is possible to produce enormous gear ratios that would be impossible to produce with just a simple gear set made up of two gears. In this video we take a look at compound gear trains. Calculating the gear ratio for a compound gear train is not hard but it is important to recognize that the two gears which share an intermediate shaft share angular quantities not tangential quantities. Even though there are four gears in a compound gear train, there are only two lines of action between the input and output shafts and really only two gear sets.

The subject of Compound Gears often come up in both high school and college physics and engineering classes. They are found in Project Lead the Way (PLTW), Principles of Engineering (POE) and AP Physics.