The term used for the distance along which two gears or threads can fully engage is?

The distance along which two gears or threads can fully engage is best described as the "Length of engagement." This term refers to the span over which the mating surfaces of two gears or threaded components mesh together effectively, ensuring proper transfer of motion and force. In gearing systems, sufficient length of engagement is crucial as it helps to distribute the load evenly across the teeth, reducing the risk of wear or failure.

The concept of length of engagement encompasses factors relating to how the teeth interlock and the duration over which they maintain contact during rotation. It directly impacts the efficiency, noise levels, and durability of the gear system.

In contrast, other terms may refer to different aspects of gear and thread interaction. "Arc of contact," for instance, relates to the angular extent over which two gears are in contact, rather than the linear distance. "Depth of engagement" typically denotes the vertical distance that tooth profiles extend into each other, while "Axis of contact" generally refers to the theoretical line along which the contact occurs, which is less about the actual engagement distance.

Thus, the correct understanding of the term "Length of engagement" is essential for evaluating gear design and ensuring optimal performance in machine elements.