CAP AHU, Installation And Maintenance Instructions Manual

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Changing Fan Speed With An Adjustable Sheave
Adjustable sheaves are commonly used to change fan speed in light commercial air balancing
applications. But how is it done? That is what this section is about. This section will also try and
explain how to change the dimensions of a sheave. If you can follow along and become a pro, you can
figure out ahead of time what the change in fan speed will be.
Keep in mind that this article appears somewhere else, need to give the proper credit. Here is the link to
the original article on how to use an adjustable sheave to change fan speed.
Adjustable sheaves are more expensive than fixed ones, and motor sheaves are smaller than fan
sheaves, adjustable sheaves are generally used on the motor shaft and not on the fan shaft.
How They Work
The two halves of an adjustable sheave
As the two sections of an adjustable sheave are turned closer to each other, the belt is forced to the
outside of the sheave. This requires the belt to have to travel a greater distance around the sheave.
This increases the speed of the fan, and then the opposite would be true too yeah? If we unscrewed the
sheave sections, the belt would need to travel less around the groove…
Sheave Measurement
In order to measure the pitch diameter, where exactly the belt rides on a sheave, you may or may not
have to remove the sheave from the shaft. Using calipers, measure the outside diameter of the belt,
while it’s
wrapped around the sheave. The diameter of the sheave is the actual outside dimension of
the sheave. These are usually two different diameters or values.
Screw in or out the sheave in order to increase or decrease the diameter to the calculated size. Now
install the belt around the sheave again. Measure the diameter to verify that the right pitch has been
dialed in.