Hydraulic machines and tools use fluid power to do extreme work. In heavy equipment for example, hydraulic fluid is transmitted throughout the machine to various hydraulic motors and hydraulic cylinders and is pressurized according to the resistance present. The fluid is controlled directly or automatically by control valves and distributed through hoses and tubes. The popularity of hydraulic machinery is due to the large amount of power that can be transferred through small tubes and flexible hoses, and the high power density and wide array of actuators that can make use of this power. Examples of main hydraulics applications in an excavator are boom cylinders, swing-drive, cooler fan and track-drive.