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How to Convert FTM to RPM

Calculations involving motors and their operation and operating speed deal with values for RPM and FPM. FPM stands for feet per minute. The FPM and RPM (revolutions per minute) units of measure relate to each other by an equation taking into account the diameter of the drive shaft of a motor. FPM is equal to the RPM multiplied by pi times the diameter. Pi times the diameter of the drive shaft is the geometric calculation for circumference of the shaft expressed in feet. Every rotation of the shaft would traverse a distance equal to the circumference of the shaft.

Instructions

- 1
  Measure the diameter of the motor shaft. For the calculations, express the measurement in terms of feet, not inches.
- 2
  Measure the FTM value for the motor. This distance will indicate the speed of revolutions of the motor shaft.
- 3
  Insert the values measured into the equation, RPM = FPM / pi * diameter. For example, assume the FPM for a given motor is 36.5 feet and the drive shaft of the motor measures 1 inch. Convert the drive shaft to feet by dividing by 12, which equals 0.083 feet. Insert the values into RPM = FPM / pi * diameter = 36.5 / 3.1415 * 0.083 = 36.5 / 0.261 = 139.85. The motor is running at 139.85 RPM.

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