What gear type has teeth that are cut straight across the inside rim?

The correct answer is the internal spur gear. Internal spur gears are characterized by their teeth that are cut straight across the inside rim, allowing them to mesh with external spur gears or another internal spur gear effectively. This design creates a compact arrangement that can transmit torque and rotational motion efficiently within a confined space.

This type of gear is often used in gearboxes and mechanisms requiring a reduction in size or when a unique layout is needed due to spatial constraints. Its ability to efficiently transfer motion while maintaining a simplified design is a key advantage, especially in applications where weight and size are critical factors.

In contrast, external spur gears, while also having straight teeth, are cut on the outer rim and mesh with other external gears. Kite gears and bevel gears each have distinct configurations and are designed for specific applications involving different orientations of shaft connections or unique motion requirements.