Q: Understanding Main Gears Vs. Planetary Gears?

There is often a debate among expert mechanics about what type of transmission system is the most efficient and effective method of applying horsepower to the wheel. Some believe that automatic transmissions are best suited for consumers that commute frequently, while others stand true to supporting the manual transmission. Meanwhile, there are several aspiring mechanics that simply don’t know the difference between the internal components that make up these two different transmissions.

The job of any transmission is to apply power from the engine, as provided by the flywheel, to the drive axle of the vehicle. There are two primary ways of accomplishing this with transmissions that are found on the road today and in past years:

• Using a manual transmission that is comprised of a system of main gears and a manual clutch to apply power gradually through the direction operation of the driver.
• Using an automatic transmission that utilizes a series of planetary or epicycloidal gears that "automatically" shift gears without the input of the driver or having to engage a clutch pedal

Both transmissions are designed to accomplish the same task, but utilize two extremely different gear designs and philosophies to achieve their goals.

What is a manual or main gear set?

In a manual transmission, the gears are designed like most traditional gear sets we see daily. One gear has multiple "teeth" that are conjoined with a corresponding gear that also has a similar design. In the middle of each gear is an input shaft that drives the gear or receives input from the gear and transfers that momentum to a drive axle. In many cases, the primary design of a manual transmission allows individual gears to mesh with other gears to create a different gear ratio that produces a different power supply. This is often referred to as a "main gear" set up.

A manual transmission is operated by the driver as they utilize a clutch and a manually shifted lever to select a desired gear. The gears are designed to work efficiently in sequence, however the driver can change this sequence manually. This is the benefit of a main gear system; it’s adaptable and is reliant upon the direct choice of each driver. There are some manual transmissions that will permit a driver to skip gears as they increase the gear, but will not allow them to "downshift" in this same method. Those are known as sequential manual transmissions.

What is a planetary gear?

Inside of many automatic transmissions is a planetary gear set, which is pretty much as the name describes: set up in a similar style as the planets orbit the sun. A planetary gear is comprised of three primary rings, which include a center ring (known as the center sun gear) and an outer ring gear (also called the annulus). The gears are propelled by a third set of rings, that are connected to the smaller, center ring gear and the outer, larger circumference gear called the planetary gears.

Most automatic transmissions have two planetary gears, three forward gears, and one reverse gear. The planetary gear is used to smoothly transition the independent gears to a higher gear ratio once the internal hydraulic pressure reaches a desired setting. Hydraulic pressure is built up through the transmission fluid as the engine produces revolutions per minute (or RPM). Inside the transmission, the two (or in some modern transmissions – three) planetary gears are connected by a center drive axle that supplies power to the drive shaft, to the drive axle, and then to the wheels.

In general, the main difference between a main gear and planetary gear is their design and use of application. Planetary gears are used in automatic transmissions and main gears are used in manual transmissions.