Choosing a Fan Design That Minimizes Trade-Offs

Jeff Smoot, CUI Inc.


Some cooling applications require a different approach in fan design

Figure 1. Cross section of a sleeve bearing

In the same way that oxygen and water are essential to human life, cooling fans are critical to many of the devices we rely on every day. From home appliances to industrial-grade machinery, and from laptop computers to the large-scale data centers, the need to remove hot air is essential. If the fan fails, the machine it’s meant to be cooling could eventually fail too – or, more likely, be shut down automatically to prevent overheating. Both scenarios are undesirable, meaning that device designers need to select fans that will be up to the task for the duration of their product’s expected life.

The importance of the fan bearing

A key criteria that impacts a fan’s expected life is the type of bearing used in the design. This crucial element ensures the rotor can turn smoothly. Much gets asked of the fan’s bearing over the course of its life: as well as ensuring the rotor can make a significant number of rotations, the bearing may also be required to operate at different orientations, and be sufficiently impact-resistant to withstand being bumped or dropped.

Fan motor bearings are typically of either the sleeve or ball bearing variety. Both have their pros and cons, and designers are often forced into trade-offs when choosing one or the other. Let’s look at some of the characteristics of these two bearing types.

Sleeve bearings: low cost comes at a price

The sleeve bearing is the simpler, and therefore cheaper, of the two traditional bearing types. In this design, the central shaft pin spins inside a cylindrical sleeve, with oil lubricating the bearing. The sleeve is responsible for holding the rotor in the correct position relative to the motor stator, making sure the distance between the two remains constant. As well as costing less than ball bearings, sleeve bearings are typically more impact-resistant. However, the design has its disadvantages. (See Figure 1)

For one thing, you need the gap between the inside of the sleeve and the shaft to be as small as possible, to keep rotor wobble to a minimum. However, the tighter the sleeve, the more friction you have to overcome when you start and spin the rotor. Consequently, sleeve bearings can be slower to start and require more energy to operate.

A further friction-related issue with sleeve bearings is caused, ironically, by the oil rings and Mylar washers at either end of the bearing bore. These retain the lubricant needed to keep the shaft spinning smoothly and quietly, but their very presence adds friction. They also trap some of the gas created by rotational friction. When this gas can’t escape, it solidifies into particles of nitride, which clog the bearing, hamper the shaft’s rotation and ultimately shorten its life.

The other big drawback of basic sleeve bearings is a consequence of the sleeve having full responsibility for holding the rotor in position. In other words, the full weight of the rotor rests on the inside of the bearing sleeve. As the rotor turns, gravity means the shaft will gradually wear away the inside of the sleeve. If your fan is always operating in the same position, your sleeve will develop an oval shape that can cause additional noise and rotor wobble. Alternatively, if your fan is going to be operating at multiple angles, the inside of the bearing will wear in different directions, resulting in an uneven shape that makes these noise and wobble issues worse. In the long term, all of this wear shortens the life of the bearing, and potentially the whole fan unit.

So while the sleeve bearing is both low cost and robust, these inherent drawbacks mean designers often look to fans with alternatives. The most common of these is the ball bearing.

Ball bearings: longer life but costlier

Ball bearings consist of a ring of steel balls around the rotor shaft. When used in fan motors, you’ll typically find a pair of them on the shaft, separated by a ring of springs. When it comes to fans, this approach has a number of advantages over sleeve bearings. (See Figure 2)

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Figure 2. Cross section of a ball bearing

Firstly, ball bearings reduce the amount of friction you must overcome when starting and operating your fan. Secondly, the springs between the two ball bearings help offset any tilt caused by the weight of the rotor. By extension, the reduced bearing wear this results in means your mean time between failures (MTBF) with ball bearings is typically much higher than you’ll achieve with a sleeve bearing.

Despite these pros, ball bearings have their cons. The fact that they can be used at any angle makes them seem more attractive than sleeve bearings for portable devices. However, at the same time, ball bearings are less robust and must be treated with greater care.

Ball bearings are also noisier than sleeves, while their greater complexity and component count means they’re pricier as well.

The need for a third way: enter omniCOOL

Both sleeve bearings and ball bearings have drawbacks that force designers to compromise in some way. This is why CUI has developed a new type of fan that bridges the gap between traditional ball-bearing- and sleeve-bearing-based designs, thereby removing many of the trade-offs we’ve looked at. (See Figure 3)

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Figure 3. Cross section of a fan motor with the omniCOOL system

Known as the ‘omniCOOL’ system, this new fan design uses a magnetic structure to balance the rotor, in combination with an enhanced sleeve bearing.

The rotor in an omniCOOL system operates like a spinning top that never falls over and can operate at any angle.

The tip of the shaft works like the point on the spinning top, held in place by a supporting cap. The magnetic structure sits in front of the rotor and uniformly attracts it around its entire circumference, balancing the rotor regardless of the angle at which the fan is operating. Consequently, the inside of the bearing doesn’t have to support the weight of the rotor – this is instead borne by the magnetic flux and supporting cap.

The omniCOOL system reduces or removes many of the drawbacks you get with traditional sleeve or ball bearings. For example, with the magnetic structure actively balancing the rotor, the tilt and wobble issues common with standard sleeve bearings are minimized.

And because the shaft doesn’t rest against the inside of the bearing, friction between the two is dramatically lower than with a traditional sleeve bearing.

Experienced designers will know that a magnetic structure of this kind could be applied to a traditional sleeve or ball bearing. However, this on its own wouldn’t be enough to overcome all the challenges we looked at earlier. And this is where the other key component of the omniCOOL system comes in: the enhanced sleeve bearing.

The enhanced sleeve bearing

The sleeve used in the omniCOOL system is a specially hardened one, which gives greater resistance against abrasion and heat. This enables operation at up to 90°C, while traditional sleeve bearings can typically only withstand up to 70°C.

The hardened sleeve and reduced abrasion (thanks to the magnetic structure balancing the rotor) also dramatically extend the bearing’s life – test results have shown an omniCOOL system lasting over three times longer than a standard sleeve bearing when operated at 70°C, rising to five-and-a-half times longer at 20°C.

The other key difference between the enhanced bearing used in the omniCOOL systems and standard sleeve bearings is the removal of the oil rings and Mylar washers. The magnetic structure minimizes the chance of the shaft rubbing against the inside of the bearing, meaning that these components are no longer needed. This cuts down on friction even further, and provides clear space at either end of the bearing for any gas produced by rotational friction to escape. It also reduces the cost and complexity of the overall design, thereby speeding up manufacturing and quality assurance, compared to more complex setups.

No more trade-offs for designers

The omniCOOL system addresses the long-standing headaches that equipment designers have faced when choosing between fans with standard sleeve or ball bearings. The blend of the magnetic structure with an enhanced sleeve bearing results in fans that are quiet, strong, durable and can be used at any angle, while being cheaper than equivalents with ball bearings.

Next time you need a fan to cool one of your products and are weighing up whether to go with a sleeve- or ball-bearing-based design, add an omniCOOL-based fan to the options you consider. You might well find it provides the perfect balance of price and capability for your end product.

CUI Devices