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

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  Tweaked

• Start date

  Mar 31,

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  Calculation Shaft Speed

In summary, the conversation discusses how to determine the RPM of a motor shaft using a tachometer. The tachometer was placed on the outside of the shaft and the results showed a RPM reading. The calculation to determine the shaft RPM is RPMt * dt/ds, with the given values resulting in shaft RPM. The simplicity of this calculation is noted and appreciated by the participants in the conversation.

• Mar 31,

• #1

Tweaked

23

0

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: rpm

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

 

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• Mar 31,

• #2

cragar

2,552

3

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

 

• Mar 31,

• #3

Filip Larsen

Gold Member

1,876

774

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 tachometer RPM then corresponds to shaft RPM.

 

• Mar 31,

• #4

Tweaked

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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 tachometer RPM then corresponds to 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.

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.

 

• Apr 7,

• #5

PJC01

1,503

0

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 = (rpm) * (31.5mm/12mm) = .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.

 

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

What is shaft speed calculation?

Shaft speed calculation is the process of determining the rotational speed of a shaft, which is the rate at which it rotates in revolutions per minute (RPM).

Why is shaft speed calculation important?

Shaft speed calculation is important because it helps engineers and scientists design and analyze machines that use rotating shafts, such as engines, turbines, and pumps. It also allows them to determine the appropriate speed for efficient and safe operation.

What factors are involved in shaft speed calculation?

The factors involved in shaft speed calculation include the diameter and length of the shaft, the power input, the type of bearing used, and the load that the shaft must support.

How is shaft speed calculated?

Shaft speed can be calculated using the formula RPM = (60 x Frequency) / Number of Poles, where frequency is the speed at which the motor rotates and the number of poles refers to the number of magnetic poles in the motor.

What is the unit of measurement for shaft speed?

Shaft speed is typically measured in revolutions per minute (RPM) or rotations per minute (RPS). In some cases, it may also be measured in radians per second (rad/s) or hertz (Hz).

Knowing the correct shaft speed of both shafts on a belt-driven machine is important when performing machinery diagnostics. Ideally you would do this by first identifying the input and output speeds using a strobe light, photo tach or laser tach. Once you know the accurate speed of both components, use this formula to determine the multiplication factor:

RPM Output/RPM Input = Multiplication Factor 

On a belt-driven fan the two primary speeds required are the RPM of the Motor and the RPM of the fan. For example, if you know the motor speed is rpm and the fan speed is 944 rpm the multiplication factor would be:

944/ = .

The multiplication factor is input into the Nest when building a machine.

If you can&#;t get to the fan shaft to strobe it but you do know the pulley diameters here is how you can determine the output speed and the multiplication factor.

Formula

 RPM Input/RPM Output = Diameter Out/Diameter In

Fan sheave diameter = 11.5 inches                     Motor sheave diameter = 6.5 inches

Output RPM = ?                                                      Input RPM =

Formula

RPM Input/RPM Output = Diameter out/Diameter In

/RPM Output = 11.5/6.5

(RPM Output) (11.5) = ()(6.5)

(RPM Output) (11.5) = 11,524.5

RPM Out = 1,002.13

This number does make sense since the fan will be running slower since it has the bigger pulley.

Multiplication Factor = RPM Out/RPM In = .13/ = .

Input . as the multiplication factor in the Nest.

Here are some video&#;s that discuss the importance of 1X and how to enter a multiplication factor into the Nest when analyzing belt driven machinery.

The importance of finding 1X: https://www.youtube.com/watch?v=ZWxjWiuQpv0

Entering Multiplication Factors into the Nest: https://www.youtube.com/watch?v=ugYyfdvwmf4

 

For more information, please visit propeller power equation.