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 2.7 BELT-DRIVE FANS. The general arrangement for a belt-drive fan is shown
in Figure 2-14. Generally, the motor speed is 1,750 rpm and the fan speed
is somewhat lower. The relationship between the fan rpm and the motor
is:
(PD motor/PD fan) x motor rpm
Fan rpm =
the pitch diameter of the pulley on the motor
where:
PD motor =
the pitch diameter of the pulley on the fan
PD fan =
FIGURE 2-14
Typical Belt Drive Fan
Pitch diameter is a term used to describe the diameter dimension
in between the outside diameter of the belt and the inside diameter of
the belt (Figure 2-15).
Example: A 1,750-rpm motor with a 3-inch pitch diameter
pulley needs to drive a fan at 950 rpm. What size fan pulley
should be used?
Solution:
950 rpm =
(3/PD fan) x 1,750
PD fan =
(3) x (1,750/950)
= 5.53 inches
2-12
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