The cog is needed to transmit rotation. One turns and rotates another with its teeth. The other will rotate in the opposite direction, and if they are of different sizes, at a different speed. A huge amount of things work thanks to the correct ratio of cogs, so the cog is an excellent standard metaphor for settings, mechanisms, and work in general.
We start drawing the cog with a circle. Around the circle, we'll draw a larger circle with a dashed line. All that's left is to draw neat teeth over the gaps in the dashed line.