Sun and Planetary Gear Arrangement: What is Epicyclic Gear Train

What is Sun and Planetary Gear or Epicyclic Gear?

The sun and planetary gear arrangement, sometimes also called as Epicyclic gear train, is a gear train, where typically few smaller gears rotate and revolve by meshing around a bigger sun gear. So, that’s how the name comes.

Details of the Sun and Planetary Gear Arrangement

• Various arrangements of the sun and planetary gear are possible and used across the industry. And the complexity increases with the addition of the numbers of planetary gears.

• For the purpose of the discussion, we will use the following simple Epicyclic gear arrangement:

• As you can see from the above picture, the planet gears are connected by a rigid arm in such a way that the rotation axis of the combined planet gears and that of the sun gear lies at the same line.

• As the sun gear rotates, the meshed planet gears also rotates in the opposite direction and as the planet gears rotates it causes rotation to the arm connecting the planet gears.

• The beauty of this kind of gear arrangement is that you will have multiple options for giving input torque and getting output to and from the gear box. How?

• Look at the cross section drawing of the gear above. The axis-1 and axis-3 are the axis of the planetary gears. Axis of the arm is marked as axis-2 and the axis of the sun gear is marked as axis-4.

• You can use the axis-4 as input and axis-2 as output for transmitting the powers. One of the benefits you will get here is both the input as well as the output axis fall in the same line. So whenever you need to transmit power and require reduction as well then you can think of using this configuration.

• Similarly, depending upon your power transmission requirement you can think of using any other axis combinations as your input and output axis.

Applications of Sun Planet Gear Arrangement

• The sun and planetary gear is used in most of the automatic transmission system. In automatic transmission compound sun and planetary gear is used.
• These kinds of gear also find application for converting reciprocal motion to the rotary motion in steam engine.
• Another application of the sun and planetary gear is electric screw driver. You need to achieve a moderate reduction ratio in a limited space.
• This kind of gear trains can provide solution for wide range of transmission problem if you can customize and understand the system suitably.

Conclusion

The sun and planet gear arrangement is a non linear or Epicyclic gear train. As the name suggests, the construction of this type of gear train is similar to that of our solar system.

Most of the automatic transmissions use this kind of gear train. Though we use the basic gearing law for calculating the gear ratio for the sun and planet gear arrangement as well but slightly different way.