Which of the following planetary gear train control devices can be used to hold members stationary?

• A. The sprag or roller type one-way clutch
• B. All of the choices can be used as holding devices.
• C. The hydraulic clutch
• D. The servo and band mechanism

Answer: (B)

Holding a member stationary in a planetary gear train is a common way to create different gear ratios by fixing one component while others move. A sprag or roller type one-way clutch acts as a lock when torque tries to drive the member in the opposite direction, letting motion occur freely in the intended direction but preventing rotation in the other direction. This makes it an effective holding device when you need to prevent movement in one scenario.

A hydraulic clutch uses hydraulic pressure to engage friction elements that couple or brake components. When applied as a brake around a drum or to a planet carrier, it can clamp the member and stop it from rotating, thus holding it stationary under load.

A servo and band mechanism provides a band brake that tightens around a drum or ring gear. When the band is tightened, it resists rotation and holds the targeted member in place.

Because each of these can be actuated to prevent rotation of a specific member in a planetary gear set, all of them can serve as holding devices.