в контексте описание коробки передач-gear box

Spur gears are the simplest and most common type of gear. Their general form is a cylinder or disk. The teeth project radially, and with these "straight-cut gears", the leading edges of the teeth are aligned parallel to the axis of rotation. These gears can only mesh correctly if they are fitted to parallel axles.[3]

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Возможно, речь идет о редукторе, где применяются оба типа передач.