Here at New Way® Air Bearings, we remain committed to providing you with the fullest understanding for our products, and how we can properly contextualize them against other motion offerings. Today we want to discuss another popular noncontact motion solution: magnetics bearings. Read on to learn how these bearings operate, the pros and cons, and how they stack up to New Way’s Porous Media™ air bearings.
What is a Magnetic Bearing?
Magnetic bearings provide another avenue to noncontact motion through electromagnetic levitation. As we all learned in elementary school science class, the like poles of a magnet repel while the opposite poles attract. This concept can be extended to electromagnets, wherein Faraday’s Law describes the relationship between a magnetic field and an electric circuit to produce an electromotive force.
Simple electric motors use the basic structure of a stator, rotor, and commutator. Feeding electric current into the motor generates motion, while manually turning the rotor generates a corresponding electrical current. This actually turns the motor into a dynamo, and is the principle behind virtually all power generation, including coal, steam, nuclear, and wind.
Magnetic bearings use all of these principles to levitate their ferromagnetic article—say, a rotating shaft—along the interior diameter of the magnetic bearing. The interior of the shaft is lined with electromagnets to suspend the rotating article. Once in suspension, a complex system of active feedback controllers and sensors continually make micro-adjustments to the magnetic field strength, as magnetic bearings cannot naturally produce an equilibrium state, per Earnshaw’s theorem. A magnetic bearing system is also supported by auxiliary contact bearings, which support the article during power on and power off.
What is an Air Bearing?
Air bearings—and more specifically Porous Media air bearings—operate by generating a nanometers-thick film of air between the porous graphite bearing and the guide surface or rotating article. Whereas traditional orifice bearings use a stainless steel bearing face with drilled holes, Porous Media bearings distribute pressure evenly across the bearing, creating a resilient, frictionless motion system.
The Benefits of Porous Media
Simplicity of Operation
The magnetic bearing’s claim to fame is in providing noncontact motion. However, this comes at an exorbitant cost, both in terms of financial outlay and system complexity. Porous Media bearings need nothing more than the bearings themselves, and the air pressure system which filters and regulates their air supply. By comparison, magnetic bearings require supporting contact bearings, a ferromagnetic metal guide surface or article, and an active control system built into the bearings.
As stated above, electromagnetic levitation will not reduce to a stable configuration. The control system associated with magnetic bearings is computationally complex, requiring nonlinear controls and transfer functions simply to ensure continued operations.
By contrast, New Way Air Bearings are self-stabilizing owing to the squeeze film effect, producing a positive correlation between stiffness and damping. For machine operation, this means that a rotating shaft suspended on air bearings naturally finds its center of rotation, making them ideal for turbomachinery and shaft balancing operations.
Safety is one of the most important considerations for any new project, and it’s our priority at New Way as well. Traditional contact bearings don’t require power, and so a loss of electricity does not affect them. Loss of power in a magnetic bearing system is fatal, as the article will crash down onto the electromagnets. While air bearings still require power to provide pressurized air to the bearing, the loss of air supply is far more controlled.
As air pressure drops off, the bearing itself depressurizes, slowly setting the rotating article or stage down to rest. Because the air bearing fluid layer is only the order of 5 micrometer thick, there is little time for gravity to build up catastrophic speeds. Even if a crash occurs at high speeds, the soft graphite acts as an ablative layer, protecting the guide stage. Even in this situation, because air pressure is distributed across the whole bearing surface, a scratched bearing is still fully operational.
Noncontact motion does not even guarantee smooth, prolonged operation for magnetic bearings, as they are susceptible to age-related failures from sand, dirt, arcing and damaged coils. Air bearings feature zero dynamic components and no supporting electronics beyond an air pump, making them a far simpler solution.
Ease of Retrofit
For many engineering teams, it is far more likely you will look at ways to retrofit an existing system than develop a whole new design. With the infrastructure needed for magnetic bearings to function properly, this may not even be feasible from the get go. On the other hand, New Way air bearings have been used to retrofit contact bearing systems for decades.
We already have existing product lines of drop-in replacement bearings for Sheffield coordinate measuring machines and Canon lithography steppers, ready for installation by authorized rebuilders. Even if your application requires custom engineering, New Way is up to the task, with an inhouse design team ready to get started on custom products.
Supercharge Your Precision Process with New Way
Still want to know more about the outstanding benefits of Porous Media? Learn the ins and outs of air bearings with our free infographic, “Mythbusters: The Truth About Air Bearings”. Download your copy today, and get the facts you need. For any other questions about our products and services, contact us anytime!