LO 11-7 Identify and describe the types, features, and applications of cooling fans

Viscous Fan Clutches

One of several common methods of controlling fan operation is through a temperature-controlled, modulating fan clutch. These fans are coupled to the drive through a highly viscous silicone fluid. The amount of fan clutch slippage is determined by a temperature-sensitive, bimetal control valve that regulates the quantity of silicone fluid in the clutch coupling mechanism. When the clutch is cool, little silicone fluid enters the clutch and the fan freewheels. As the temperature of the air in the fan clutch area increases, more fluid enters the clutch drive mechanism, creating more drag or silicone fluid shear within the coupling mechanism Figure 62. The fan speed is slower than engine speed and is adjusted according to the heat load across the radiator Figure 63. These clutches are quieter because fan speed is kept proportional to heat load. Because their operational speed is proportional to engine temperature, they conserve energy and extract significantly less energy from an engine than a directly coupled fan.

FIGURE 11-62: A variable-speed engine fan connects the inner and outer hub by shearing silicone fluid between discs.

FIGURE 11-63: This silicone cooling fan hub freewheels when the hub is cool. As more heat is absorbed by the hub, internal drag causes the fan speed to increase, moving it closer to the driven speed of the hub. Note the bimetal spring in the center of the hub, which is connected to a valve that controls fluid movement.