In the straight mounted engine - and that's the only one we're talking about here - the cooling fan previously was simply mechanically connected to the crankshaft. However, just then, at high speeds, when the the fan demands a lot from the engine, it is used least, because enough air is blown through the radiator anyway. In contrast, particularly as far as trucks are concerned, when travelling slowly, where their cooling needs are high, the performance of the cooling fan is particularly weak. This problem is very elegantly solved, even without electrics, by the visco-static cooling fan.
One makes good use of the characteristics of the fan, because it is installed directly behind the radiator, in the airstream. Thus, not only can the fan draw the air through the radiator, in addition, it can also check whether it's necessary to also run or not. In it's center, it consists of ribbed aluminium to achieve the largest possible surface area, this then warms up more when the radiator temperature is too high. The broad, bi-metal spring in the center of figure 2 regulates the visco-static liquid, which, similar to the visco-static clutch, connects the fanwheel with the crankshaft. The more the temperature rises, the higher the torque transfer is. 12/10