What is the calculation for determining motor shaft RPM using a tachometer?

[Tweaked9107]

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?

 

[cragar]

wouldn't the rpm of the shaft be 1400 rpm unless I am missing something , or are you wanting the angular velocity

 

[Filip Larsen]

If the tachometer is of the type that uses a contact wheel to pick up rotation, then the shaft RMP_s = RPM_t * d_t/d_s, where subscript s is for shaft and t for tachometer. With the values you gave, the 1400 tachometer RPM then corresponds to 3675 shaft RPM.

 

[Tweaked9107]

If the tachometer is of the type that uses a contact wheel to pick up rotation, then the shaft RMP_s = RPM_t * d_t/d_s, where subscript s is for shaft and t for tachometer. With the values you gave, the 1400 tachometer RPM then corresponds to 3675 shaft RPM.

Yeh that's the answer I had as well. I just thought it seemed too simple to be the same principle as gear ratio's. Thanks for your help.

 

[PJC01]

The calculation for determining motor shaft RPM using a tachometer is as follows:

Motor shaft RPM = (Tachometer RPM) * (Tachometer diameter / Motor shaft diameter)

In this case, the calculation would be:

Motor shaft RPM = (1400rpm) * (31.5mm/12mm) = 3666.67rpm

It is important to note that this calculation assumes that the tachometer is accurately measuring the rotational speed of the motor shaft and that the tachometer is properly calibrated. If there is any uncertainty in these factors, the calculated RPM may not be completely accurate. Additionally, it is always recommended to have a center drilled hole for more accurate measurements.