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engine cooling fans off highway engine with fanengine cooling fans off highway engine with fan

What to know when specifying engine fans for construction, agriculture, mining, power generation or other off-highway applications

September 3, 2020

Putting a fan on an engine to keep it from overheating seems simple enough. But it's actually a fairly intricate process dependent upon a number of factors.

Construction OEMs usually focus on noise and productivity. Mining's all about reliability. Agriculture and power generation have their own priorities.

No matter the goal, its outcome is generally impacted by the following variables:

  • Type: Fan type, the environment the fan is working in, and what type of airflow configuration is selected.
  • Fit: Diameter, potential obstructions, mounting configuration, and shroud and tip clearance.
  • Performance: Fan speed, air density, static pressure, airflow and sound.

Common axial fan types

There are three common types of engine fans for off-highway equipment: metal, fully-molded and modular.

Metal fans are the most durable and flexible in terms of design, but they're heavy and can weigh down the entire engine system. Fully-molded fans are highly efficient at providing maximum airflow with minimal noise, but they predictably aren't very flexible.

Modular fans, which feature various blade customizations that can be bolted to a center disk, are sometimes the best of both worlds, providing highly effective airflow, efficiency and noise reduction.

engine cooling fan types
From L to R: Examples of metal, molded and modular fans.

Suckers and blowers

Spend some time around a diesel engine application engineer, and you'll probably hear the terms "sucker" and "blower" from time to time. No, they aren't talking about panfish or lawn equipment -- there's a big difference in whether an engine fan pushes, or blows, warm air away from the engine vs. "sucking" in RAM air and moving it across the engine.

This has a significant impact on both airflow and performance. Blower configurations can help to keep radiators clean, but they aren't often as efficient as sucker fans.

Two types of mounts

There are usually two ways to mount a fan to an off-highway engine: either directly to the shaft or to a fan drive, or fan clutch, that controls fan speed.

Shaft-mounted fans use a hub and pushing and can often be found in electric or hydraulic motors. This is also a popular setup in engines with a gearbox assembly (which is especially common in the European Union).

But to optimize fan speed, a fan drive is usually necessary. When this is used, a pilot and bolt circle are used to place the fan directly on the drive. The drive is then connected to the shaft via a pulley.

This setup is also useful with hydraulic motors and in water pumps, among other examples.

Finding the right fit

So you know what you're trying to optimize for. You know what type of fan you want to do it.

Now how do you make sure it fits properly into the engine design?

This is where testing can be a great ally, but the following table lists general rules of thumb for fan fits as they relate to the radiator and the shroud. (Hint: To determine a fan's pitch width, place the fan on a flat surface and measure its maximum height from the bottom of said surface.)

Radiator Side Opposite Side Blade Tip
Recommended clearance (sucker) 1 * pitch width 3% fan diameter 1.25-3% fan diameter
Recommended clearance (blower) 1.5 * pitch width 3% fan diameter 1.25-3% fan diameter
Potential obstructions Radiator and accompanying brackets, misc. hardware FEAD pulleys, crankshaft dampeners, crankshaft pulleys, finger guard Shroud, radiator and accompanying brackets, finger guard

Managing stress

Any solid object spinning at thousands of RPMs is subject to considerable stress. In-vehicle strain testing can determine how fast a fan can spin without going into resonance.

Different fan designs have different maximum tip speeds. Resonance is often caused by airflow obstructions that cause 1-4 pulses per revolution. 

In general, a larger-diameter fan rotating slower is usually the best choice.

[RELATED: More a more in-depth discussion of fan application, check out this recording of Horton's recent off-highway fans webinar.]

By the numbers

Application engineers an also run what are called fan curves to find the sweet spot between output power, static pressure and airflow for a given application. There are even formulas called fan laws that display how physical input factors will impact performance.

At a high level, power varies as a cube of speed. For example, a 10-percent RPM increase would mean a 33-percent increase in fan power.

engine cooling fan laws
Fan laws show how different input factors affect fan performance.

Immerse yourself in immersion

Fan immersion is the percentage of a fan's pitch width buried within the shroud. It's measured in the direction of the airflow.

The optimal position for the fan is usually at 65-70 percent immersion within the shroud. "Optimal immersion tests" can be run in any AMCA-certified wind tunnel for any shroud-fan configuration.