Mechanically Speaking: How Clutch & Flex Fans Work

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In the hypothetical world, a fan wouldn't be required if your car was constantly driven at high speeds (definitely an interesting concept, but not too practical).  Airflow from the vehicle's velocity would be sufficient to allow for adequate air movement through the radiator surface, with the result being proper cooling. While this would be an ideal situation, it's seldom possible.  Or realistic.  Because of this, a fan of some sort becomes a necessity. 

While electric fans are by far the most common today (here, an electric motor simply turns the fan on or off, depending upon a pre-determined temperature), many vehicles on the road today – particularly vintage examples -- still rely upon mechanical fans.

There have been a number of mechanical engine-driven fan types used on passenger cars over the years – some fixed, some “flexible”.    A fixed fan rotates constantly with the water pump shaft.  On the other hand, a common thermal clutch fan operates by way of a viscous coupling. The fan clutch is a small fluid coupling bolted to the fan just behind the radiator. It incorporates a thermostatic device that controls the fan speed. This ensures that the fan will rotate at a speed that is sufficient to maintain engine cooling. Simultaneously, the thermostatic devise can disengage the fan when it is no longer needed (engine is cooled).

So how does it work?  The key is the clutch.  It is engineered with a fluid coupling partially filled with a silicone-based fluid designed for just this purpose. Within the clutch, you’ll find several passages (cavities), separated with a valve of sorts.  On the nose of the clutch, you’ll find either a coiled bimetal spring or a flat bimetal strip.  As the temperature of the air passing over the radiator rises, the bimetal spring simply expands (actually, the coils unwind).  In the case of the bimetal strip, it bends in or out depending upon the temperature.  As the coils in the spring unwind (or in the case of a strip, bends or flexes), more silicone fluid is allowed to enter the fluid coupling.  With more silicone fluid in place, the slip within the clutch is reduced.  That means the fan is allowed to slowly rotate.  As the temperature continues to rise, the coil will fully unwind (expand).  Eventually, the clutch on the nose of the fan will become almost completely locked, which in turn allows the fan to draw the maximum amount of air through the radiator.  As the air passing through the radiator cools (for example, driving at uninterrupted highway speeds), then the fan coupling decreases.  Eventually, the fan slows and freewheels.  This reduces noise, saves engine power and of course, improves fuel economy. Most fan clutches allow the fluid to drain into the [fluid] cavity when the engine is shut down.  As a result, the clutch (and the fan) becomes engaged.  When you start an engine, even from cold, it will take a minute or two for the clutch to slow down (hence the extra noise when the vehicle is first started – the fan is locked “on”). It’s not hard to see that the fan clutch is a simple but actually brilliant device.

Simple flex fans have been in use for a long period of time too.  They reduce their pitch as engine RPM increases (or more correctly, as pulley speed increases).  These are even simpler devices than clutch fans. Here, the blades (stainless steel, aluminum, fiberglass or sometimes composites) rely upon centrifugal force to perform the work. As pointed out above, the pitch of the respective blades flatten as the engine speed increases. At low engine speed, the fan draws the maximum amount of air through the radiator as possible, but as the engine increases (and theoretically, the vehicle speed increases), the fan flattens.  This draws in little air and consumes less power to turn.  The flex fan is an extremely simple device.  More in the accompanying photos:

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The actual fan used in conjunction with most clutch setups doesn’t look much different than any other fan. Essentially, it’s sandwiched between the water pump and the fan clutch, however the fan not hooked up directly to the water pump pulley (the fan clutch is).

Here’s a look at a common replacement fan clutch. Behind the gold colored face is a series of cavities separated by a valve device. A special silicone fluid is used to make the coupling inside the clutch. The fan clutch is positioned directly behind the radiator.

The pointer shows the coiled bi-metal spring fitted to this particular fan clutch. As the temperature increases, the spring “uncoils”, which in turn allows the silicone coupling fluid to move within the clutch. The text offers more information.

Flex fans are even simpler devices than clutch fans. Here, the fan is a one-piece assembly engineered with a series of blades that can physically flatten (more in the next image). This example is manufactured from stainless steel, however you can come across flex fans manufactured from aluminum, fiberglass and composite materials.

In a flex fan application, the fan blade (shown here) physically flattens due to centrifugal force. As the engine speed increases, the blade is allowed to flatten, which in turn, frees horsepower in the engine and improves fuel economy. As the engine speed slows, then the blade returns to the normal pitch, effectively drawing more air through the radiator when it is most needed.