Bearing trade-offs for cooling fans

The life of most fans is set by the shaft bearings. Simple comparisons show why different bearing technologies can exhibit widely varying MTBF figures.

Jeff Smoot, CUI Inc.
Cooling fans are critical to many of the devices we rely on every day. A key criterion that affects fan life is the shaft bearing. This crucial element ensures the rotor can turn smoothly. 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 able to withstand bumps and drops.

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.

The sleeve bearing is the simpler and cheaper of the two. 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, ensuring 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.

For one thing, the gap between the inside of the sleeve and the shaft must be as small as possible to keep rotor wobble to a minimum. However, the tighter the sleeve, the more friction to be overcome when the rotor starts spinning. Consequently, sleeve bearings can be slower to start and need 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 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: The full weight of the rotor rests on the inside of the bearing sleeve. As the rotor turns, gravity makes the shaft 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 will operate 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 this wear shortens the life of the bearing and potentially that of the whole fan unit.

So, while the sleeve bearing is both inexpensive and robust, these inherent drawbacks mean designers often look for alternatives. The most common of these is the ball bearing.

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 several advantages over sleeve bearings.

First, ball bearings reduce the amount of friction to be overcome when the fan starts and operates. Second, the springs between the two ball bearings help offset any tilt due to the weight of the rotor. By extension, the resulting reduced bearing wear typically brings a much higher mean time between failures (MTBF) than available 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, 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.

Because both sleeve and ball bearings force compromises, CUI has developed a new type of fan that bridges the gap between traditional ball- and sleeve-bearing-based designs. 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 operates. Consequently, the inside of the bearing needn’t 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 of traditional sleeve or ball bearings. For example, the magnetic structure actively balancing the rotor minimizes the tilt and wobble issues common with standard sleeve bearings. 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 measure on its own wouldn’t be enough to overcome all the challenges mentioned earlier. And this is where the enhanced sleeve bearing of the omniCOOL system comes in.

The sleeve used in the omniCOOL system is specially hardened to resist 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 bearing 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.

Another advantage of omniCOOL systems is they need no oil rings and Mylar washers. The magnetic structure minimizes the chance of the shaft rubbing against the inside of the bearing, so oil rings and washers are no longer needed. This further reduces friction and provides clear space at either end of the bearing for the escape of any gas produced by rotational friction. It also reduces the cost and complexity of the overall design to speed manufacturing and quality assurance, compared to more complex setups.