What is the role of brake bands in an automatic transmission?

Brake bands in an automatic transmission serve the crucial function of holding parts of the planetary gear set stationary. When the transmission shifts gears, the brake bands are engaged to apply friction against specific components of the gear set, thereby locking them in place. This allows the transmission to select the appropriate ratio for power transfer from the engine to the wheels.

By securing certain gears within the planetary gear assembly, brake bands facilitate the smooth operation and transitions between different driving modes or ratios. Without the ability to hold these components stationary, the transmission would struggle to change gears effectively, leading to poor performance and potential mechanical failures.

While the roles described in the other choices pertain to different aspects of transmission function—like fluid management and lubrication—they do not accurately define the primary responsibility of brake bands, which is focused specifically on controlling the gears' positions during operation.