Professor of Mechanical Engineering
University of Mining and Metallurgy
Bearings in precision equipment such as computer disk-drive spindles, laser scanners/printers, and gyroscopes, need excellent stiffness and damping qualities to run true at high speeds. Until recently, designers of these applications relied on hydrodynamic oil and gas bearings, or ball bearings.
But sliding bearings that employ magnetic fluid (ferrofluid) for both lubrication and sealing have several advantages over conventional designs. For example, miniature ball bearings run at high rotational speeds tend to vibrate and make noise because of the motion of the balls. High rotational speeds can also change properties of lubricating greases that, in turn, trigger large and rapid torque fluctuations. These fluctuations make it difficult to control the speed of the bearing unit. Operation at elevated temperatures can as well alter lubricating-grease properties and shorten bearing life.
In contrast, ferrofluid bearings that carry loads on a film of oil don't suffer friction fluctuation and are more controllable. Ferrofluid bearings also have extremely low nonrepetitive runout and maintain high rotational accuracy at high speeds. They run quietly and need no mechanical seals as do conventional designs, so they last a long time and without periodic maintenance.
Air bearings as well have negligible nonrepetitive runout. And they operate with low friction. But the low viscosity of air necessitates bearing clearances on the order of a few microns and large bearing diameters. In addition, bearing surfaces must contain highly precise, shallow herringbone grooves to generate sufficient aerodynamic pressure and boost bearing stiffness. These features require a special, high-tolerance manufacturing process that adds costs and is difficult to maintain for mass production. Another concern: The small clearances make the bearings susceptible to dust contamination.
But ferrofluid bearings cost less than air bearings because manufacturing tolerances need not be as tight. Compared with same-sized air bearings, ferrofluid bearings have better damping and carry higher loads. They are also stiffer than both air and ball bearings at rotation speeds above 10,000 rpm. Bearing stiffness in this case scales with rotation speed because of the hydrodynamic effect of the oil film.
Special oil-based ferrofluids are stable colloids comprising a base liquid, ferromagnetic particles, and a stabilizing, dispersing surfactant that suspends the particles in the base liquid. Such magnetic fluids are characterized as having low volatility and viscosity, and high saturation magnetization.
High-saturation magnetization keeps the fluid well within the bearing cavity under centrifugal, shock, and vibration forces so it doesn't contaminate the surrounding environment. Low volatility and evaporation rate ensures long service life and further stops environmental contamination. A low viscosity consumes little power and minimizes temperature rise and thermal expansion of bearing components. A small temperature rise also minimizes losses of oil viscosity and load capacity. Ferrofluid's excellent lubricating properties and colloidal stability lets it perform well under high shear rates and loads. In addition, viscoelasticity of the lubricating oil gives ferrofluid-film bearings exceptional damping properties.
A tubular cavity between the upper and lower radial bearing surfaces acts as a reservoir for magnetic fluid. At the other end of the cavity is a single-stage ferrofluid exclusion seal that retains ferrofluid within the housing during operation. The seal is comprised of an annular permanent magnet and pole piece, which forms a radial gap with the shaft surface. The bearings support high-speed, small-diameter precision shafts such as in computer-disk-drive spindles.
Magnets mount in the casing when using a nonmagnetically permeable shaft. For a magnetically permeable shaft, magnets mount on the rotary shaft itself. A magnetic field retains the ferrofluid in the cylindrical gap between the magnets and rotary shaft.
Two sets of opposed helical (herringbone) grooves pump ferrofluid into the bearing cavity as the shaft rotates in either direction. When the shaft stops, ferrofluid is retained in the bearing cavity by means of magnetic-fluid seals formed by a magnet, pole pieces, and the ferrofluid itself.
A bushing with a spherical-shaped concave thrust surface supports both radial and thrust loads of a rotating pivot. Ferrofluid attracted by magnetic force stays in the small gap between the porous radial bearing bushing, thrust bearing bushing, and pivot, keeping bearing surfaces lubricated.
An octagonal mirror attaches to a radialdrive-type inner rotor. A ferrofluid hydrodynamic bearing supports the rotor in the radial direction. Axial loads react through a magnetic-attractiontype thrust bearing. It consists of ring-shaped permanent magnets attached to both the rotor and bearing housing. The radial bearings mount in the bearing housing — one each at the top and bottom — with compound seals. The seals consist of a magnetic-fluid seal and a viscousscrew seal above the upper bearing. The viscous seal uses an inverse-screw groove that generates inward pressure by rotation. The bearing inner diameter contain three separate oil grooves for generating dynamic pressure, while a triangular-shaped outer diameter promotes circulation of lubricating ferrofluid inside the bearing cavity.
Oil-lubricated bearings in small sizes are stiffer than ball bearings and gas bearings at rotation speeds above 10,000 rpm. Bearing stiffness scales with rotation speed because of the hydrodynamic effect of the oil film. In addition, viscoelasticity of lubricating oil gives such designs excellent damping properties.
Changing properties of lubricating grease in ball bearings causes friction torque to fluctuate at high rotational speeds. In contrast, friction torque in ferrofluid bearings scales linearly with rotational speed.
Ferrofluid fills the space between the spindle and bushing. Pumping action of the herringbone grooves builds stable oil-film pressure on the bearing-sliding surface. A magneticfluid seal atop the bearing prevents ferrofluid from escaping.
Gyro-spin bearings provide a stable and rigid axis of rotation for a spherical gyro rotor that is driven by an induction motor. Ferrofluid sits between the rotor and stator bearing surfaces. Small permanent magnets embedded in the rotor circumference form a fluid bearing between the rotor and stator. A movable caging plate with magnets in its surface magnetic fluid between surfaces of the caging plate and a top surface of the rotor, form a fluid bearing between an under surface of caging plate and an upper surface of rotor.
Comparing characteristics of ferrofluid and ball-bearing spindles
FERROFLUID BEARING SPINDLE
|Repeatable runout, um|
0.125 to 1.25
1.25 to 5.0
|Nonrepeatable runout, um|
0.25 to 1.25
Self-contained exclusion seal
Needs exclusion seal
|Power consumption (bearing alone), W|
2 to 8
2 to 5