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

Hydraulic Fan Drive

A hydraulic fan drive system enables variable fan speed to be independent of the engine speed. These fan drive mechanisms are commonly found in off-road equipment and transit buses, where cooling demands can exceed the ability of a conventional fan drive system to move air through a radiator and other heat exchangers. A hydraulic pump and an electronically controlled pressure control valve supply hydraulic pressure to a hydraulic motor that drives the cooling fan. Because airflow across radiators in these applications is not provided by forward vehicle movement due to a rear- or side-of- vehicle location, the fans can actually be driven at much higher speeds than an engine could provide at idle, high-load, or low-speed engine operation. Like electronically controlled, variable-speed fans using air or silicone fluid, cooling speeds can be ramped to avoid shock-loading of the engine or belt drive system. Electronically controlled fan speed yields significant power and fuel savings Figure 65 and Figure 66. The direction of the suction-type fan can even be reversed to blow out debris from the radiator.

FIGURE 11-65: Electronic control of fan speed using a hydraulic-pressurized fan drive system enables fan speeds in excess of engine speed.

FIGURE 11-66: The hydraulic fan drive system of a skid steer loader with a radiator located at the rear of the machine.