Bearings play a critical role in any piece of rotating machinery. They not only support the load of the rotating shaft assembly, but they also mitigate the friction generated between the moving and stationary parts. Common types of bearings include traditional ball bearings, roller bearings and tapered roller bearings. These bearings are acceptable for equipment operating at normal speeds but are limited from operating at very high speeds due to the amount of heat generated. To operate rotating machinery at 10,000+ rotations per minute (RPM), a different solution is needed to prevent premature failure.
Enter noncontact bearings. As the name suggests, these bearings allow for the rotating assembly to be supported without contacting any other surface. Noncontact bearings offer the greatest efficiency by eliminating typical bearing losses due to friction between the rolling elements. There are two main types of noncontact bearings: fluid bearings and magnetic bearings.
Fluid bearings are further broken down into hyrdronamic or hydrostactic bearings. In both types, a fluid such as oil, water, or air is used as the support medium. Static type bearings utilize externally pressurized fluid from a pump to support the shaft. Dynamic bearings rely on the high speed rotating assembly to create a pressurized wedge between the shaft and bearing surface. For this article we will focus on the air foil type fluid bearings and magnetic bearings as options for high speed turbo compressors. Let’s take a closer look at how these bearings work and the advantages they offer.
Air Foil Bearings
Air foil bearings work in part due to the no-slip condition of fluids. As a rotating shaft increases in speed, air molecules stick to the shaft and are eventually pulled underneath to form a supporting air cushion. This cushion separates the shaft and surrounding components and allows the shaft to rotate freely. The other component required for the air bearing is the air foil and the supporting spring structure that gives the bearing its load capacity.
Air foil bearings have the advantage of needing no external pump or compressor to pressurize the air. The foils consist of thin pieces of high temperature steel that are formed into the proper foil structure. This means that the bearings offer the advantages of no contact during operation and decent load capacity at a very low manufacturing cost.
One disadvantage of air foil bearings is a lower load capacity than traditional roller type bearings. This means the supported shaft assemblies must be smaller than those supported by roller bearings. For this reason, machines using this type of bearing are limited to a few hundred horsepower. While air bearings do not have contact during normal operation, they do have contact during the start and stop periods of machinery operation. This period of sliding friction is often alleviated by the application of a non-stick coating, such as Teflon, to the top foil surface. However, the coating will eventually wear down until the friction during starts and stops is too high, and the bearings fail.
Magnetic Bearings
Magnetic bearings work by using electromagnets to levitate the rotor during operation. Current is injected into an assembly of coils around the rotor that attract the shaft to each coil when active. In addition to general levitation, the injected current is used to keep the rotor spinning about the center of its axis to prevent contact with the stationary parts. The high rotational speed means that the shaft position must be analyzed at a very high rate to inject current to the proper coils and keep the shaft rotating in place. This requires an advanced bearing controller.
Magnetic bearings have several advantages. First, there is no mechanical wear even during start and stop operations. Therefore, there is no practical fatigue life for this type of bearing. Additionally, turbomachinery that experience surge can be protected from damage during surge. Magnetic bearings can actively detect surge forces and inject current to keep the shaft rotating in its axis. The disadvantage comes from the extra cost of the magnetic coil assemblies and active bearing controller. In summary, the added longevity and reliability of the magnetic bearings comes at a higher price for the bearing assembly.
Blowers that utilize noncontact bearings can be more expensive, but they deliver the greatest efficiency and ROI in the form of energy savings by eliminating traditional bearing friction losses. Reach out to one of our experts to see how much you could save by installing an energy efficient machine.
