April 10, 2018 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 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.
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 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.
Ball and sleeve bearing designs are not the only two options available. There is a third system introduced by CUI that uses magnetic rotor-balancing, also referred to as magnetic structure, and an improved sleeve bearing to offset the downsides found in other fan designs. This third bearing type is called the omniCOOL™ system.
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.
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.
The magnetic structure can be applied to both traditional sleeve and ball bearing type fans to help alleviate the issues described above, but only to a certain extent. For further improvements, the omniCOOL system uses an advanced bearing design that has been strengthened for additional resistance in the areas where contact occurs. The hardened bearing design is capable of operating at temperatures up to 90°C.
The reduction in rubbing also means that less lubricant is required, allowing the oil rings and Mylar washers to be removed from the design, eliminating another source of friction. This leads to less noise and less resistance at start-up, while reducing the number of parts. Fewer parts mean that the omniCOOL system is easier to manufacture and more reliable than other designs.
The omniCOOL system alleviates the negatives of both ball bearing and sleeve bearing fan designs, resulting in a robust fan that is quiet, cost-efficient and can operate at any angle. By negating the drawbacks from the other two types of fan construction, the omniCOOL system can be deployed to replace either type of fan and bring the benefits of both fan types to any design. Applications that favored sleeve-based designs now have an alternative that is also low-noise, rugged and reliable, while also offering a longer lifetime and the ability to be used in any orientation. Those applications that required higher-performance ball bearing designs can now keep the higher temperature resistance and lower wear and tear while gaining improved impact resistance and a more rugged construction. 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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