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## Main Question or Discussion Point

I am trying to figure out the speed that a motor shaft is running at but i have no reference materials to work with. I had to place a tachometer on the outside of the shaft as there was no centre drilled hole to work from. I got the following results:

Diameter of tachometer: 31.5mm

Diameter of motor shaft: 12mm

RPM of tachometer: 1400rpm

What calculation would i need to work out the rpm of the motor shaft?

Diameter of tachometer: 31.5mm

Diameter of motor shaft: 12mm

RPM of tachometer: 1400rpm

What calculation would i need to work out the rpm of the motor shaft?