Why magnetic bearings for high-speed applications?

Bearing types

There are several types of high-speed bearings. Roughly speaking, high-speed bearings can be listed from the least to the most advanced in the following order:

Standard high-speed ball bearings
Air bearings
Fluid film bearings
Active magnetic bearings

Standard rolling element bearings can withstand moderate rotational speeds. At higher speeds (>10,000 rpm), conventional high-speed bearings require frequent maintenance, being responsible for up to 70% of all machine failures. Achieving high rotational speed requires special types of high-speed bearings. Typically used special high-speed bearings include high-speed ball bearings, air bearings, fluid film bearings, and active magnetic bearings (AMB). This blog specifies some of the technical features and economical aspects of using these high-speed bearing types.

We will take a look at the following high-speed bearings with their advantages and drawbacks:

High-speed ball bearings
Air bearings
Fluid film bearings
Active magnetic bearings

Figure 1: Advantages and drawbacks of different types of bearings

What are high-speed ball bearings?

The high-speed bearing technology that is the closest to the traditional bearing technologies is the high-speed ball bearings. The most effective improvement in ball bearings for increasing the rotational speed is to have a continuous oil circulation that lubricates the bearing actively by injecting or spraying oil or oil-air mixture directly inside of the bearing. The balls and bearing races can apply more advanced materials or coating for mitigating friction. The balls can be ceramic to improve durability and reduce weight of the balls. Also, the size of the balls can be minimized enabling a higher rotational speed of a single ball due to decreased diameter.

The most dramatic drawback of high-speed ball bearing is still the limited rotational speed. High-speed ball bearings are prone to wearing and must be frequently replaced. The total cost of ownership for an OEM for conventional high-speed bearings is high due to costly lifetime operational expenses. Even during scheduled maintenance breaks, replacement of the bearings requires disassembling and reassembling the machine, replacing the bearings, replacing the lubrication oil and oil filters, as well as servicing the oil pump, hoses, and reservoirs. Scheduled maintenance breaks typically take place every 12-18 months, and they cause the machine to be out of service for at least one day.

Figure 2: Schematic view of ball bearing. Ref. [1].

Air bearings

Air bearings generate a pneumatic cushion of the gas medium in between the rotating and stationary parts of the bearing. The increasing rotational speed of the rotor pressurizes the air inside of the sealed bearing. When the pressure is high enough, there is an air cushion in between the rotating and stationary parts and the rotor levitates on the air cushion without contact with the steel parts. When the rotational speed decreases, the pressure drops and there must be supplementary air nozzles to pressurize the air to generate the air cushion. Even the external air nozzles are not enough to create the levitating pressure without high-speed rotation, and the threshold speed to lose the air cushion is at moderately high when going down from nominal operational speeds. Special surface treatment is required to tolerate high temperatures in the air bearings.

When the air cushion is lost, the rotating and stationary parts are in contact resulting in the fast wearing of the contact surfaces. The air bearings are suitable for small applications with lightweight rotors having continuous high-speed operation. These are not suitable high-speed bearings for applications having start-stop cycles or applications where the external loads are changing along with the operation.

Figure 3: Schematic view of gas foil bearing. Ref. [2].

Fluid film bearings

In fluid film bearings, the rotating part is separated from the stationary part by a hydraulic oil or other fluid cushion. The fluid cushion is much stiffer than air cushion and the fluid film bearing can support even very large machines such as multimegawatt multi-stage steam turbines. The bearing type can be a journal bearing where the fluid cushion is created using uneven circle geometry, lobes, or more advanced technologies such as tilting pads, where the stationary part of the bearing has a number of tilting pads.

Fluid film bearings have high damping. Damping is also important with fluid film bearings as the resonance frequencies of the rotor systems supported by fluid film bearings are at critical rotational speeds many times. They suffer from instability due to the hydrodynamic phenomena. Also, the temperature and flow of the fluid must be carefully maintained as the increasing fluid temperature affects the viscosity and may lead to a destructive, too thin fluid cushion.

On the other hand, the viscosity of the fluid causes high energy losses at high speeds. The required auxiliary components and systems include naturally the sealing of the bearing and fluid circulating system including fluid pump, hoses, filters, and reservoirs.

Figure 4: Schematic view of four pads tilting pad bearing. Ref. [3].

Magnetic bearings

Magnetic bearings are the ultimate selection for high-speed applications as they are frictionless bearings. Typically, magnetic bearings are active magnetic bearings (AMB). The AMBs apply electromagnetic forces to levitate the rotating part without contact with the stationary part. The electric supply feeds current to the copper coils generating a magnetomotive force. The active control requires some feedback from the actual position of the rotating part that is usually collected using position sensors of different kinds. The levitation can be achieved at zero rpm, which is not possible in air and fluid film bearings. Therefore, the magnetic bearings are capable of serving within the speed range from zero up to a physical limit of the rotor material strength.

Figure 5: Schematic view of a radial active magnetic bearing. Ref. [4].

Magnetic bearings require a backup system that ensures a smooth landing in case of electrical blackouts. First, backup power systems like uninterrupted power supply or battery can be used. Also, the power can be taken from the DC link of the electrical machine. If the event is more severe, there are auxiliary ball bearings that can tolerate short yet high loads and excessive rotational speeds for a short time period. Typically, the dynamic performance of rotor systems equipped with magnetic bearings is excellent when compared to rotor systems supported by any other bearing type. The total cost of ownership for AMB supported rotor system is much lower than a rotor system with any other bearing type. Magnetic bearings ensure the highest efficiency introducing massive money savings. There is no need for maintenance and the system can be completely oil-free without a single drop of oil in the system. Naturally, the obtained position data from built-in sensors can be collected and analyzed for other purposes as well such as condition monitoring.

Figure 6: Operating principle of active magnetic bearings.

More educational content here: https://spindrive.fi/what-is-active-magnetic-bearing/.

High-Speed bearings applications

Typical applications for high speed bearings are related to transferring or compressing gasses. Higher rotational speeds enable to achieve the same process outcome with lower energy. Typical applications are, for example, air turbocompressors as alternatives for screw or scroll compressors, turbo blowers as alternatives for geared low-speed electric motors, turbocompressors for organic Rankine cycle units or other waste heat recovery applications, flywheels for higher kinetic energy capacity to name a few. The ultimate rotational speed capacity of magnetic bearings is one of the key technologies in hydrogen economy.

Summary

We at SpinDrive are high-speed bearings manufacturer and supplier for different industries. In particular, we manufacture active magnetic bearings. We have the world leading expertise in high speed electric machines and especially in magnetic bearings. We have a flexible business model and the delivery can be adjusted to meet your needs to balance between the costs and ease. We have a very customer-centric approach and we are here to solve your problems and eventually deliver the best magnetic bearings products to elevate your business.

If you have an industrial application where you require high-speed bearings for, please feel free to contact us and request for further information.

References:

Stoev et al., Introducing Concepts and Methodologies of Fault Detection into Electrical Engineering Education: the Induction Machine Example, 2017 IEEE Global Engineering Education Conference (EDUCON)
Barlak et al., Changes of tribological properties of Inconel 600 after ion implantation process, Bulletin of the Polish Academy of Sciences, Technical Sciences 64(4):827-834
Badawi et al., Performance analysis of tilting pad journal bearing using COMSOL Multiphysics and Neural Networks, Alexandria Engineering Journal, 59(2):865-881
Knospe et al., A multitasking DSP implementation of adaptive magnetic bearing control, Control Systems Technology, IEEE Transactions on 5(2):230 – 238.