Shapes
They need not be circles
Most gears are circular, and then the ratio stays constant. Shape the teeth on noncircular bodies and the ratio can vary.
If the teeth are arranged on noncircular bodies, the speed and torque ratios vary. Britannica Scholars: gear
The Science Museum Group holds a teaching model of two heart wheels in gear, transmitting a variable velocity ratio, made by J. Schröder in 1868. Science Museum Group: heart wheels
The model illustrates the point that gears do not have to be circular, provided that their shapes are correctly matched. Science Museum Group: heart wheels Gears of this type can provide a transmission of varying speed, a useful feature in, for example, some textile machinery. Science Museum Group: heart wheels
The model was purchased from Herr J. Schröder, an educational model maker from the Polytechnic Institute in Darmstadt, Germany. Science Museum Group: heart wheels
Britannica makes the same general point: if the teeth are arranged on noncircular bodies, the speed and torque ratios vary.