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

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Discussion Overview

The discussion revolves around calculating the RPM of a motor shaft using a tachometer, particularly when the tachometer is mounted externally due to the absence of a center-drilled hole in the shaft. Participants explore the relationship between the tachometer's RPM and the motor shaft's RPM, considering the diameters of both components.

Discussion Character

  • Technical explanation, Mathematical reasoning, Debate/contested

Main Points Raised

  • One participant seeks to determine the motor shaft's RPM based on the tachometer's RPM and the diameters of both the tachometer and the motor shaft.
  • Another participant suggests that the motor shaft's RPM might simply be 1400 RPM, questioning if the inquiry is about angular velocity instead.
  • A third participant proposes a formula for calculating the shaft RPM based on the tachometer RPM and the diameters, indicating that the shaft RPM corresponds to 3675 RPM given the provided measurements.
  • A later reply agrees with the calculated RPM of 3675, expressing surprise at the simplicity of the calculation, likening it to gear ratios.

Areas of Agreement / Disagreement

There is no consensus on the correct RPM of the motor shaft, as participants present differing views on whether the tachometer RPM directly reflects the shaft RPM or if a calculation is necessary.

Contextual Notes

The discussion does not clarify the assumptions regarding the type of tachometer used or the specific relationship between the diameters and RPMs, leaving some mathematical steps unresolved.

Tweaked9107
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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?
 
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wouldn't the rpm of the shaft be 1400 rpm unless I am missing something , or are you wanting the angular velocity
 
If the tachometer is of the type that uses a contact wheel to pick up rotation, then the shaft RMPs = RPMt * dt/ds, 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.
 
Filip Larsen said:
If the tachometer is of the type that uses a contact wheel to pick up rotation, then the shaft RMPs = RPMt * dt/ds, 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.
 

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