The band holds the ring gear in the planetary gearset.

In a planetary gearset, the band is a component that is used to hold the ring gear stationary or allow it to rotate, depending on the desired output. Here are the steps to understand how it works:

1. The planetary gearset consists of a central sun gear, planet gears that rotate around the sun gear, and a ring gear that encloses the entire system.

2. The band in the planetary gearset is a simple strap of metal lined with friction material.

3. When the band is tightened around the ring gear, it holds the ring gear from moving. This action changes the output of the planetary gearset, depending on which component (sun gear, planet gears, or ring gear) is used as input, output, or held stationary.

4. If the band is loosened, the ring gear is free to rotate. This again changes the output of the planetary gearset.

5. The tightening and loosening of the band are controlled by hydraulic pressure in an automatic transmission.

6. Therefore, the band plays a crucial role in controlling the operation of the planetary gearset, and thus the gear ratios in an automatic transmission.