A crescent-shaped separator is used on some __________ gear motors.

A crescent-shaped separator is specifically designed for use in internal gear motors. The unique design of the crescent shape helps to separate the meshing gears within the motor, allowing for efficient sealing and improvement of the hydraulic performance. This design is critical in internal gear systems where the rotor and gears are located within a closed casing and must function smoothly while handling the fluid dynamics.

In contrast, external gear motors typically do not utilize a crescent separator because their design focuses on the interaction between two external gears, which does not require the same type of separation for optimal performance. Planetary gear systems revolve around a central gear and its surrounding satellites, which again do not benefit from a crescent separator. Spur gears operate with straight-cut teeth and do not involve the complexity found in internal gear configurations, making a crescent separator unnecessary. Therefore, the crescent-shaped separator is aptly suited for internal gear motors, highlighting the specific engineering aspect of their design and function.