Fan Bearing Types – Weighing the Pros and Cons

Fan Bearing Types – Weighing the Pros and Cons

By Ryan Smoot

For many electronic systems, fans are an important component designed to keep the system within recommended temperatures, ensuring that the electronics work optimally and last their full operational lifetime. There have been attempts to find alternative thermal management techniques, but none have proven to be as efficient and cost effective as the fan.

The fan works by using a rotor that spins on a bearing to displace air. The reliable operation of the bearing is key to the fan design, as the fan can rotate thousands of times per minute and be required to have a lifespan of many years. This process puts the bearing under an enormous amount of stress, so it is essential that it is up to the task.

There are two bearing designs that are widely used, the sleeve bearing and ball bearing, and each has its pros and cons.

Sleeve Bearing Fans

Sleeve bearing fan designs are inexpensive, rugged and simple, which has led to their widespread use in many applications. The rugged design ensures that they are able to work in many harsh environments and their simplicity means that they are less prone to malfunction. Another benefit to sleeve bearing fan designs is that they tend to create less noise while operating, allowing them to be used extensively in quiet areas like offices.

The central shaft of a sleeve bearing fan is encased in a sleeve-like structure, with oil for lubrication to ease rotation. The sleeve offers protection to the shaft and ensures the rotor is kept in the right position, preserving the gap between the rotor and stator.

Diagram of a sleeve bearing
Drawing of a sleeve bearing and its labeled components

There can be a balancing act to get the correct gap size between the shaft and the sleeve. Too little space results in an increase in friction, which makes the fan harder to start and draws more power. If the gap is too big, then the rotor can wobble. The second drawback to sleeve construction is that the sleeve is the only physical medium holding the rotor in place, and over time the shaft will erode the bearing bore. This phenomenon is worse if the rotor always rotates in the same direction, which will eventually lead to the bore taking-on an oval shape, resulting in noisier operation and a shortened operational lifetime. If the fan is moved around or re-orientated, the bearing will be eroded in different places and become uneven – making wobble and noise even worse. In addition, the sleeve-type construction requires oil rings and Mylar washers to prevent the lubricant from leaking, which causes more friction to the shaft and stops gases from escaping. Trapped gas solidifies into nitride particles that impede movement and can shorten the fan’s operational life.

Sleeve bearing fans can be found in many designs, particularly in those that operate in normal temperatures and on static equipment. Applications, such as computer and office equipment, HVAC appliances and industrial cabinets make extensive use of sleeve bearing fan designs.

Ball Bearing Fans

Ball bearing fan designs are intended to combat some of the drawbacks found in sleeve-bearing fans. In general, they are less prone to wear and tear and can operate in any orientation and at higher temperatures. However, ball bearing fans are more complex and expensive than sleeve-bearing designs, as well as being less rugged. As a result, impacts can greatly affect the overall performance of a ball bearing fan. They also tend to create more noise when in use, which can restrict the areas they can be deployed.

Ball bearing fan designs use a ring of balls around the shaft to solve uneven wear and rotor wobble problems. Most fan motor designs have two bearings, one in front of the other, and these bearings are usually separated by springs. The bearings offer reduced friction compared to sleeve designs and the springs can assist with any tilt in the fan that the rotor’s weight may induce. If the springs are placed the whole way around the shaft, the device can be operated at any angle without wear or friction, giving a more reliable design.

Ball bearing fans can also be found in high density computer applications and datacenters, where performance, temperature and MTBF are more important factors than noise. They are also widely used in industrial applications for cooling electronics systems or as blowers for industrial drying applications.

Diagram of a ball bearing
Drawing of a ball bearing and its labeled components

omniCOOL™ System Fan Bearing

Ball and sleeve bearing designs are not the only two options available. There is an alternate option introduced by CUI Devices called the omniCOOL system™, found in our line of advanced sleeve bearing fans. Utilizing either magnetic rotor-balancing, commonly referred to as the magnetic structure, or an enhanced bearing, the omniCOOL system technologies work to provide extended fan life and improved performance.

Present on fans with a “-V” suffix on the part number, the omniCOOL system's magnetic structure makes the rotor function in much the same way as a spinning top – one that can operate at any angle without falling over. The magnetic structure is set at the front of the rotor where its flux runs parallel to the motor shaft’s direction. In that position, the magnetic structure attracts the rotor uniformly irrespective of the rotor's angle.

Diagram of a fan motor with the omniCOOL system’s magnetic structure
Drawing of a fan motor with the omniCOOL system and its labeled components

The tip of the shaft is kept in position through a supporting cap at the front of the bearing bore to form the rotational point of the rotor. This method takes the weight of the rotor away from both the shaft and the bearing sleeve. The magnetic field also pulls the shaft down, lowering its center of gravity, resulting in minimized tilt and wobble. This allows the fan to be used at any angle that is required, as well as minimizing friction.

Alternately, on fans with a “-C” or “-CF” suffix on the part number, the omniCOOL system features an enhanced bearing design for operational improvements. Integrating specialized grooves on the outside of the bearing to promote the circulation of lubricant around the shaft, the enhanced bearing prevents lubricant build-up and minimizes friction, which offers similar benefits of reduced noise, greater efficiency, and extended life expectancy compared to conventional fan designs.

Specialized grooves on outside of the bearing promote an enhanced lubrication cycle
Specialized grooves on outside of the bearing promote an enhanced lubrication cycle

Both omniCOOL system technologies minimize the cost-performance tradeoff present in traditional sleeve and ball bearing designs. However, the magnetic structure of the -V series fans deliver a longer life expectancy, while the enhanced bearing of the -C and -CF series fans are more economical.

Bridging the Gap in Traditional Fan Designs

The omniCOOL system alleviates the negatives of both ball bearing and sleeve bearing fan designs, resulting in a robust fan that is quiet and cost-efficient with a longer operational life. By negating the drawbacks from the other two types of fan construction, the omniCOOL system can be deployed to replace either type of fan. Instead of being forced into a compromise, designers can have access to a fan construction that delivers the best of both worlds.

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Ryan Smoot

Ryan Smoot

Technical Support Engineer

With an extensive knowledge of CUI Devices' products, Ryan Smoot provides customers with a wide range of technical and application support capabilities in the field. His management of CUI Devices' robust CAD model library further offers engineers with an invaluable resource for streamlining their product designs. When he's not assisting customers, Ryan enjoys running, the outdoors, and spending time with his wife and two kids.