Ferrofluids provide hermetic sealing when mechanical seals won't do.
Dept. of Mechanical Engineering
Univ. of Mining and Metallurgy
Magnetic fluids, also called ferrofluids, are advanced materials that can solve complex engineering problems in particular sealing rotary shafts. They offer a cost-effective solution to environmental and hazardous-gas sealing in a wide variety of industrial rotation equipment. Ferrofluid seals generally fall into two categories: those used on high-vacuum-processing equipment, and low-friction, hermetic seals that exclude particulates and gases.
In vacuum-manufacturing processes, magnetic fluids seal feedthroughs transmitting rotary motion into the vacuum chamber with minimal torque losses. Many industrial applications use relatively simple magnetic-fluid exclusion seals to control volatile emissions and protect sensitive environments. For instance, in textile machinery they protect motor bearings from fibrous contaminants. Machine tools use the seals to prevent the cutting fluid containing machined particulate debris from contaminating the lubricating oil circulating through the bearings, thus extending bearing life. They are also successfully used in face and centrifugal-seal applications.
In clean rooms, seals on rotating joints keep oils and particulates inside semiconductor-fabrication robots. And high-speed computer disk drives use ferrofluid exclusion seals to contain harmful dust particles and other impurities that cause data-reading heads to crash into the disks.
Major advantages of magneticfluid seals include:
- Simple design and easy mounting.
- Hermetic sealing with zero leakage.
- High-speed capability.
- Low frictional power losses.
- Smooth rotary motion with maximum torque transmission.
- Maintenance-free operation.
- Long life and high reliability.
Mating ferrofluid seals with conventional seals, such as a mechanical face or labyrinth seal, combines the advantages of both and can further increase effectiveness.
Magnetic fluids are stable colloidal suspensions of small magnetic particles such as magnetite (Fe304). The particles, about 10 nm in diameter, are dispersed in nonmagnetic carrier liquids that include water, hydrocarbons, fluorocarbons, esters, diesters, and polyphenyl ethers. Stabilizing dispersing agents (surfactants) such as oleic acid or polymers coat the particle surfaces to keep them separated and evenly dispersed within the carrier liquid. Surfactants overcome the attractive Van der Waals and magnetic forces between particles and prevent agglomeration and sedimentation.
In the absence of a magnetic field, the particles' magnetic moments are randomly distributed. Applying a magnetic field orients the particles along field lines almost instantly. Ferrofluids respond immediately to changes in applied magnetic field, and removing the field quickly randomizes the moments. In a gradient field the fluid responds as a homogeneous liquid and moves to the region of highest flux. This permits precise positioning and control of the ferrofluid by an external magnetic field. Forces holding the magnetic fluid in place are proportional to the gradient and strength of the magnetic field. Changing the fluid magnetization properties or magnetic field intensity lets users adjust the ferrofluid retention force.
Selecting the proper ferrofluid depends on factors such as environment and operating conditions. Many different combinations of saturation magnetization and viscosity yield ferrofluids suitable for a wide range of applications. Commercial ferrofluids have saturationmagnetization values ranging from 100 to 600 gauss and viscosities from 1,000 to 7,500 cP.
Operating seal life depends on ferrofluid volatility. Products needing long lives must have low evaporation rates. Also, seals operating at high vacuums require low-vapor-pressure ferrofluids.
Simple magnetic-fluid seals consist of an annular, axially polarized permanent magnet in contact with two stationary pole pieces, the magnetic fluid, and a magnetically permeable shaft. A single ring-shaped permanent magnet, or several magnets spaced along the bore of a nonmagnetic retainer, generates magnetic flux. Standard applications use AlNiCo permanent magnets. Special applications may require more-powerful, rare-earth magnetic materials such as samarium-cobalt and neodymium-iron-boron.
The assembly creates a closed magnetic circuit with magnetic force holding ferrofluid in the gaps, forming a liquid "sealing ring" that adheres to the pole pieces and shaft surface. When the shaft rotates, the fluid shears but remains in place, thus creating a dynamic seal. A typical radial gap between pole pieces and shaft is 0.05 to 0.125 mm, although gaps to 0.25 mm can accommodate eccentric shaft motions.
A pressure differential across the seal deforms the liquid ring. Should the pressure differential exceed the "burst pressure," fluid expels from the sealing gap and the seal breaks down.
Pole pieces usually have a focusing structure of toothlike projections which generates high magnetic fields. Shafts made of nonmagnetically permeable materials require a magnetic sleeve to complete the magnetic circuit. Also, if the seal housing is magnetic, a nonmagnetic sleeve must be fitted between the housing and seal to prevent a magnetic short circuit.
Liquid-ring seals offer a useful combination of characteristics. Ferrofluids perfectly conform to the shaft and pole pieces, accommodating small anomalies in the shaft surface and any minor rotation errors. Leaks are virtually zero in both static and dynamic conditions essential when working with hazardous gases. The seals also keep out gas, vapor, and other contaminants critical for vacuum applications.
Because the sealing medium is liquid, friction and wear are negligible. Thus, they do not produce wear particles that could contaminate sensitive processes. Minimal viscous friction also means the seals consume little power and permit rotation speeds exceeding 10,000 rpm. And viscous drag is independent of the pressure differential across the seal, so ferrofluids offer extremely smooth operation even when pressures fluctuate.
Evaporation rate of the carrier liquid determines seal stability. So operating temperatures exceeding 100°C for instance at shaft peripheral speeds >10 m/sec may make water cooling of the seal necessary. This removes heat generated by viscous shear and prevents excessive evaporation.
Most applications use multistage magnetic-fluid seals. Each stage supports a pressure differential proportional to the magnetic-field strength below the projection and the ferrofluid's magnetization saturation value. Typically, a single stage handles a pressure differential of 10 to 25 kPa. The entire seal's pressure capacity is approximately the sum of the individual stages' pressure capacities.
The seals operate in low-pressure-differential applications (to several bar) and in high-vacuum systems exceeding 10- 9 mbar with leak rates <10- 10 standard cc helium/sec.
In many applications, ferrofluid seals operate for several years without maintenance. Seal life depends on the application, but many ferrofluid seals have operated for over ten years without maintenance. They are also suitable for chemically reactive and radioactive environments.
BASIC SEAL DESIGNS
Magnetic fluids offer reliable and cost-effective solutions to many difficult sealing problems, especially in lowdifferentialpressure and high-vacuum applications. The wide acceptance of magnetic-fluid seals is based on distinctive features such as design flexibility, hermetic sealing with zero leakage, low viscous drag, maintenance-free service, and adaptability to hostile environments. They can be engineered for large diameters, large runouts, face and centrifugal seal applications, and other systems where more-conventional seals cannot provide zero-leakage capability. In many cases, magneticfluid seals retrofit into systems originally designed with conventional seals.
Designs come in a wide range of configurations and styles. For instance, vacuum-rotary feedthroughs combine magnetic-fluid seals with precision bearings, and include water-cooled versions for high-speed and high temperature applications. They feature annular channels in the pole pieces for coolant circulation.
Compact seals for rolling bearings feature an exclusion magneticfluid seal and a greased labyrinth seal for additional protection. In spherical journal bearings, a ferrofluid layer between the journal and bushing lubricates the bearing. Magnetic-fluid seals at the ends of the bearing keep out contaminants.
Multistage magnetic-fluid seals combined with the primary mechanical face seal on rotary-pump shafts can reduce volatile pollutant emissions to virtually zero ppm. They are simple, low-cost alternatives to complex, multiple-seal configurations which usually require costly seal-support systems.
Ferrofluids for sealing applications
Low-viscosity hydrocarbon based: