is permitted. The pitch can vary two degrees either way without trouble. Substituting a four- blade fan for a three-blade fan adds very little to the volume of air moved. A motor and fan must be properly matched to obtain desired results. Motor speed and direction of rotation are important. The motor shaft must precisely fit the hole in the fan hub to prevent slipping or wobbling. The fan is usually anchored to the motor shaft by one or more setscrews through the hub between the blades and the motor. When purchasing fan blades, the supplier needs to know the direction of rotation, number of blades, degree of pitch, hub hole size, blade type (square or round), and diameter (from blade tip to blade tip). 25.12.2 Centrifugal Blowers Centrifugal (squirrel cage) blade blowers are used in applications where airflow is controlled and directed by ducts, such as furnaces and central air conditioning units. Excess airflow causes air pressure (called static pressure) to build up inside the duct. Static pressure ensures even distribution of air to all vents. As duct pressure increases, the volume of air delivered by the fan is reduced. The centrifugal blower is sized to deliver enough airflow to maintain proper static pressure levels. Centrifugal blowers are measured by blade length and the diameter across the blade wheel. Some blowers are mounted directly on the motor shaft (direct drive), while others are belt-driven, Figure 25‑63. The direction of rotation is determined by the curvature of the blades. Forward-curved blades are designed to cup incoming air and throw it forward by centrifugal force. A B Goodheart-Willcox Publisher Figure 25‑63. Blowers in heating and air conditioning installations may be direct drive or belt-driven. A—A direct drive centrifugal blower. B—A belt-driven cen- trifugal blower. Chapter 25 Motors 519 Copyright Goodheart-Willcox Co., Inc.