I don't think there's much worry about the counterweights going supersonic. If we use a diameter of 8", then the circumference that the tip of a weight describes is 25". At 10,000 rpm's, the tip travels 250,000"/min. This equals 20,833 feet per minute, 3.945 miles per minute and 236 miles per hour. Since the speed of sound is 761.2 mph @ 15* Celcius @ sea level, the same crank would have to spin to 32,250 rpm's to reach mach 1.