Magnetic Bearing Design

magnetic bearing spindle

(Also see our MBC500 Magnetic Bearing Controller Engineering Lab Experiment.)

Magnetic bearings use magnetic fields to levitate spinning rotors and other components. Consequently there is no contact, essentially zero friction or drag, and no wear. LaunchPoint Technologies has used magnetic bearings in energy storage flywheels to enable ultra high-speed operation in a vacuum, in blood pumps to enhance reliability and biocompatibility, and in machine tool spindles for research and in micro-positioning.

LaunchPoint designs magnetic bearings using electromagnets and permanent magnets. Electromagnet-based bearings are referred to as Active Magnetic Bearings and typically have the form depicted in Figures 1 & 2. Coil currents are feedback controlled in order to accomplish stable levitation. This feedback control enables active damping of rotor vibration and very stiff support at the cost of sensors and the electronic control system. The maximum load of active magnetic bearings is limited by the saturation flux density of the iron used. A common rule of thumb for load is 100 psi times the cross sectional journal area in square inches. Thus for a 2 inch diameter bearing occupying 2 inches axially on a rotor, the peak load is approximately 400 lbs. The load can be increased significantly through design optimization and the use of cobalt-based ferrous materials.

Permanent magnet bearings are comprised of cylindrical arrays of permanent magnets as typified in Figure 3. Permanent magnet bearings are compact and inexpensive, but they have roughly half the load capability (50 psi times the cross sectional journal area in square inches) as compared with active magnetic bearings. At additional cost, Halbach arrays can be used in permanent magnet bearings to increase the load capability to about 90 psi. Permanent magnet bearings have little inherent damping, are lower in stiffness than active bearings, and are unstable in the axial direction. The axial instability is usually controlled with an active magnetic bearing as in LaunchPoint’s design for blood pumps.1 LaunchPoint engineers have extensive experience with permanent magnet bearing and have published on the topic. 2

References:

[1] Chen Chen, Brad Paden, James Antaki, Jed Ludlow, Dave Paden, Randolph Crowson, and Gill Bearnson, “A Magnetic Suspension Theory and Its Application to the HeartQuest Ventricular Assist Device,” Artificial Organs, 26(11):947-951, 2002.

[2] B.E. Paden, N. Groom, and J.F. Antaki, "Design formulas for permanent magnet bearings," Journal of Mechanical Design, 125(4):734-738, December 2003.