In the mid-1950s, my company installed an air-conditioning system in a downtown corporate headquarters building in El Paso, Texas. The system was topped by a large cooling tower on the roof. The tower had a large multibladed axial fan, perhaps 8 to 10 ft in diameter. The fan was designed to run at 600 rpm, driven through a speed-reducing belt drive by an 1,800-rpm motor.
As it happened, the motor drive shaft and the fan drive shaft were the same diameter, which made it possible that the two pulleys could be interchanged. As we all know, if it can happen, it will, and it did. The result of the pulley swap was that the theoretical fan speed was 5,400 rpm instead of the 600 rpm for which it was designed. The electrician contributed his part by wiring the motor to run backward. The backward-running fan would pull upward on it's supports instead of the normal downward force.
Upon initial startup, the large fan dropped itself out of its mooring and lifted up, spinning like a helicopter; flew over the busy street; and descended. I held my breath as the spinning blade dropped toward the street, missing all traffic, pedestrians, and telephone wires.
The event was an example of Murphy's Law, which says that if anything can go wrong, it will. If the two pulley shafts were not of the same diameter, the pulleys could not have been interchanged, and this event would not have occurred. If the motor were not wired in reverse, the fan would have pushed downward into the cooling tower and would not have pulled free.
Come to think of it, this event occureed during the same period when there were numerous reports of UFOs in nearby Roswell, N.M. The large flying rotoating fan wheel could easily have been taken for a space ship. To my knowledge, no such reports were made.
Bertram Waller, PE