May 10, 2017
Calculating Output Speed Using Pulley Diameters and Input Speed
By Michael Keohane
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 1778 rpm and the fan speed is 944 rpm the multiplication factor would be:
944/1778 = .5309
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.
RPM Input/RPM Output = Diameter Out/Diameter In
Fan sheave diameter = 11.5 inches Motor sheave diameter = 6.5 inches
Output RPM = ? Input RPM = 1773
RPM Input/RPM Output = Diameter out/Diameter In
1773/RPM Output = 11.5/6.5
(RPM Output) (11.5) = (1773)(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 = 1002.13/1773 = .5652
Input .5652 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