A gear train comprises several gears that work together to get the rotational direction and necessary speed ratio. Depending on their configuration, gear trains are categorised as simple, compound, epicyclic and reversed gear trains. 

Compound gear trains are one of the four gear train types that can transmit power over short or limited distances. 

A compound gear arrangement is a mechanism that employs multiple gears on a single shaft or compound gears to obtain a higher gear ratio. Sometimes, in this gear train type, compound gears are also placed on more than one intermediate shaft. A compound gear has two or more gears of different sizes fixed together to spin at the same speed.

The main aim of this gear train type is to transfer power over short central distances. Therefore, this gear train has idler components that help to reduce the distance between the driving and driven gears.

A compound gear train has four components that help in the functioning of this arrangement. These are as follows:

• Input Gear: An input gear is a primary component that receives power from the input shaft in the arrangement.

• Output Gear: As the output gears sit on the output shaft in the arrangement, this is where the power is transmitted efficiently.

• Compound Gear: Compound gear means using one or more components on a similar shaft to allow them to spin in the same direction.

• Idler Gears: Idler gears are a significant part of compound gear train arrangement. These components help to transmit power over a certain centre distance or obtain a certain rotational direction at the output shaft between the output and input shafts.

The components of compound gear trains will give you a basic idea of its working principle. Here is the working of a compound gear train explained in detail:

• Firstly, the first gear sits on an input shaft where the input power is transferred. In this arrangement, the second gear sits on the same shaft on which the third gear is placed. It is to comply with the short distance available for power transmission. So, this second and third gear on the same gear is compound.

• A compound gear might be more than two. It all depends on its final application or the requirements of a manufacturer. It continues until the number of gears is adequate for the necessary gear ratio output. However, idler gears can also be compound gears like other components. 

• Lastly, the power through these gears comes from the sixth gear. It is important to note that modern gear transmission arrangements might have more than ten gears in a gear assembly.

A compound gear train comes with multiple advantages. Here they are below:

• One of the significant benefits of this arrangement is you easily get a high gear ratio.

• This arrangement uses smaller sizes of gears. This makes it more compact than the simple gear train.

• Compound gear trains allow higher speed reduction with less centre distance between output and input shafts.

• As there is an absence of slip, there is higher transmission efficiency.

The following are the simple steps to find a gear ratio of a compound gear train:

Step 1: To find the gear ratio of a compound gear arrangement, first, identify the role of each component in the gear assembly. 

Step 2: You must use driving, driven and idler gears to find a gear ratio. A gear that drives another component or helps to spin another component is a driving gear. Similarly, a gear driven by another or spinning with another gear is a driven or follower gear. There are single gears sitting on intermediate shafts that are idler gears.

Step 3: Then, use the below formulas to determine the gear ratio of compound gear trains. Below are the formulas:

• Depending on several teeth, the equation is - i equal product of the number of teeth on follower gears divided by the product of the number of teeth on drivers.

• Depending on the pitch circle diameter: the equation is - i equal the product of the pitch circle diameter of follower gears divided by the product of the pitch circle diameter of driver gears.

Step 4: Finally, you will receive the gear ratio of a compound gear train.

Compound gear trains have numerous applications in various streams. A few applications of a compound gear train are as follows:

• Clocks and Watches: Clocks and watches use this gear arrangement to receive various speed ratios to spin the different hands of a clock at an appropriate speed.

• Milling Machine: In a milling machine compound gear assembly controls the bed movement efficiently.

• Radial Drill Machine: In this mechanism, compound gear arrangement increases the torque output at the spindle and reduces the speed.

• Lathe Machine: A lathe machine uses a compound gear assembly to receive different speed ratios to spin the spindle.

• Timing gear in a vehicle: A compound gear train is used in a valve timing mechanism to enhance the exhaust and inlet valves of an engine.

So, here is all about a compound gear train and its functioning. If you want to understand the mechanism of a compound gear train, you can watch videos of the same. The main reason manufacturers use this arrangement is due to its efficiency on a shorter distance without the use of any additional shafts.