squirrel cage

noun3

1. •
   A circular cage for a squirrel or other small animal, which rotates vertically as the animal runs at the bottom.
2. •
   (figurative) A tedious, repetitive, unfulfilling activity or situation, especially one in which no progress is achieved.

   • The poor stuggle for riches, success, power; but those who achieve them die out. Life is, then a sort of squirrel cage or treadmill. — 1944
3. •
   Any other device, such as a fan or rotor, that resembles a squirrel cage in form.

   • A blower, in the shape of a drum fan (also called a "squirrel cage"), pulls the warm humid air from the room across the coils. — 1996