# How Do You Calculate Shaft RPM for Different Pulley Sizes?

• Automotive
• jayjay
In summary, the conversation discusses finding the shaft rpm for a project involving a motor and pulley system. The variables involved are motor rpm, motor pulley diameter, Pulley B diameter, and Pulley B rpm (which is equal to shaft rpm). The equation for finding Pulley B rpm is mentioned and it is noted that if Pulley B is fixed to the shaft, it will rotate at the same rate. The conversation ends with the individual expressing gratitude for the help.
jayjay
Hi guy here's an example in the picture for my project.
i need to find the shaft rpm to find what size pulley D will be.
Pulley B attached with a belt that come from a motor.
17.5 hp
3400 rpm
3 inch pulley
if I am right and pulley b is 8.4 inch
rpm 1214
torque 76 ft/lbs i think

the shaft pulley is 5/8

thanks

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jayjay said:
Hi guy here's an example in the picture for my project.
i need to find the shaft rpm to find what size pulley D will be.
Pulley B attached with a belt that come from a motor.
17.5 hp
3400 rpm
3 inch pulley
if I am right and pulley b is 8.4 inch
rpm 1214
torque 76 ft/lbs i think

the shaft pulley is 5/8

thanks
Is this question for schoolwork?

berkeman said:
Is this question for schoolwork?

no for myself why?

jayjay said:
no for myself why?
Because the drawing looks like it came from a textbook problem, and we handle schoolwork questions differently from project questions here at the PF.

Did you do that drawing yourself? It's pretty impressive. What drawing CAD package did you use? Is this for your snowblower project?

berkeman said:
Because the drawing looks like it came from a textbook problem, and we handle schoolwork questions differently from project questions here at the PF.

Did you do that drawing yourself? It's pretty impressive. What drawing CAD package did you use? Is this for your snowblower project?

thats a picture from the internet as an example of the samething I am doing

Consider these four variables...

Motor rpm
Motor pulley diameter
Pulley B diameter
Pulley B rpm (=shaft rpm).

To work out anyone of these you need to know the other three.

Start by writing an equation that relates these four variables an rearrange to give you one for the pulley B rpm.

berkeman
CWatters said:
Consider these four variables...

Motor rpm
Motor pulley diameter
Pulley B diameter
Pulley B rpm (=shaft rpm).

To work out anyone of these you need to know the other three.

Start by writing an equation that relates these four variables an rearrange to give you one for the pulley B rpm.

ok so the rpm for the pulley B will be the same for the saft if i understand

CWatters
Correct. If pulley B is fixed to the shaft then it must rotate at the same rate. Same for pulley C.

CWatters said:
Correct. If pulley B is fixed to the shaft then it must rotate at the same rate. Same for pulley C.
Great that helps me for that part thanks alot

## 1. How do I calculate the shaft RPM?

The formula for calculating the shaft RPM is: RPM = (60 x frequency) / number of poles. Frequency is measured in Hertz (Hz) and the number of poles refers to the number of magnetic poles in the motor.

## 2. What is the importance of knowing the shaft RPM?

Knowing the shaft RPM is important for determining the speed and performance of a motor or machine. It can also help with troubleshooting and identifying any issues with the motor.

## 3. Can I measure the shaft RPM using a tachometer?

Yes, a tachometer is a commonly used tool for measuring the rotational speed of a shaft. However, it is important to ensure that the tachometer is calibrated correctly and that the measurement is taken at a steady state.

## 4. How does the load on the motor affect the shaft RPM?

The load on the motor can affect the shaft RPM as it changes the amount of torque required to rotate the shaft. As the load increases, the RPM may decrease, and vice versa.

## 5. Are there any other factors that can impact the shaft RPM?

Yes, other factors such as the motor's voltage, current, and temperature can also affect the shaft RPM. It is important to consider all these variables when calculating or measuring the RPM of a motor.

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