Rectangular gears are a fascinating example of non-circular gears designed to intentionally produce a varying output speed from a constant input speed. Unlike standard circular gears that maintain a fixed gear ratio throughout rotation, rectangular gears create a continuously changing gear ratio as they mesh — accelerating and decelerating the driven gear in a predictable, repeating pattern with each revolution. The geometry of the rectangular profile means that as a corner of one gear engages the other, the effective radius changes dramatically, causing the driven gear to speed up and slow down in alternating pulses. Historically, this mechanism was employed in printing presses where type was mounted on a rectangular roller — the variable speed allowed the roller to dwell momentarily during the impression stroke and move faster during the return, improving print quality and efficiency. This movement is a compelling demonstration of how gear geometry can be deliberately shaped to engineer specific motion profiles.
30. Rectangular gears. These produce a rotary motion of the driven gear at a varying speed. They were used on a printing-press, the type of which were placed on a rectangular roller.