It means his mental gears are spinning at a million rpm.

When the trailer's wheels turn, the differential gears spin and power the blades.

In a mechanical system, gas turbines or diesel engines turn gears and a long drive shaft to spin the propeller.

Machinery hummed, and gears spun in frictionless silence.

I asked her to look under the sinker plate, to see whether all the brushes and plastic gear were spinning freely.

Jonathan could almost see the gears whirring and spinning inside his head.

The teeth of spur gears clatter together when the gears spin, generating a characteristic whine.

It's amusing to watch the tiny little gear madly spin while the fat gear doesn't budge.

The gears spun in the head, and the lenses swiveled to cover the arm.

The gears were spinning in his head as he tried to work out what I was doing.