If a motor drives a fan with a 6-inch drive pulley and a 4-inch driven pulley at 1720 RPM, what is the resulting fan speed?

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Multiple Choice

If a motor drives a fan with a 6-inch drive pulley and a 4-inch driven pulley at 1720 RPM, what is the resulting fan speed?

When you have a belt drive, the belt moves at the same linear speed on both pulleys, so the rotational speeds scale with the pulley diameters. The driven pulley’s RPM is the drive pulley RPM multiplied by the ratio of the drive pulley diameter to the driven pulley diameter: RPM_driven = RPM_drive × (D_drive / D_driven). Here, the drive pulley is 6 inches and the driven pulley is 4 inches, so the driven speed is 1720 × (6/4) = 1720 × 1.5 = 2580 RPM. That’s faster because the larger drive pulley turns the smaller driven pulley more quickly. If the driven pulley were larger than the drive, the fan would slow down accordingly.

When you have a belt drive, the belt moves at the same linear speed on both pulleys, so the rotational speeds scale with the pulley diameters. The driven pulley’s RPM is the drive pulley RPM multiplied by the ratio of the drive pulley diameter to the driven pulley diameter: RPM_driven = RPM_drive × (D_drive / D_driven).

Here, the drive pulley is 6 inches and the driven pulley is 4 inches, so the driven speed is 1720 × (6/4) = 1720 × 1.5 = 2580 RPM. That’s faster because the larger drive pulley turns the smaller driven pulley more quickly. If the driven pulley were larger than the drive, the fan would slow down accordingly.

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