In order to understand why the resilience of porous media is important, some context may be useful.
Air bearings are part of the fluid film family of bearings. This means they require some type of fluid to serve as the pressurized medium in the gap between the bearing surface and the journal. This fluid can be oil, water or – in the case of air bearings – air.
This fluid film family group is further subdivided into dynamic and static bearings.
Hydrodynamic bearings (aka fluid dynamic bearings) require motion to get the process up and ‘rolling.’ At rest, there is contact. It is only when there is motion that the bearing gap is created. As the journal begins to spin, it acts like a pump, pressurizing the fluid in a constant flow between the bearing faces.
How Static Bearings Work
Hydrostatic and aerostatic bearings require an external pressure source. Essentially, this source provides a consistent flow to the bearing, creating a pressure differential proportional to the displacement that must be maintained.
For aerostatic bearings, this air pressure is plumbed to the bearing component through a tube. The advantage to this is that the fly height is not dependent on motion, so even at zero velocity, the gap is the same as at 50m/sec.
Controlling Air Flow
There are a number of different ways to control the flow of air into the air gap of an aerostatic bearing. Some air bearing manufacturers use orifices; some use step compensation; New Way Air Bearings focuses on porous media compensation.
A Closer Look at Porous Media Air Bearings
When you look at the surface of a porous media air bearing with the naked eye, it appears smooth and pristine, but on closer inspection, you’ll find that it has millions of submicron-sized pores evenly distributed across the entire bearing face.
A quick way to demonstrate this is by placing a plumbed air bearing under water, where you will see how evenly the air issues from the face of the bearing. This is the secret to the resilience of porous media.
The Limitations of Orifice Air Bearings
In stark contrast to porous media, the pressure is supplied to the surface of an orifice air bearing through a small number of precisely sized holes.
A typical product might have three jeweled orifices on the bearing surface connected by a series of grooves. Even under normal operation, an orifice bearing loses pressure and creates pressure gradients in the air gap as the air expands away from the orifice or groove.
If the bearing face becomes scratched across a groove or near an orifice, the volume of air that escapes may be more than the orifice can supply, causing the bearing to crash. The implications of such a grounding can be catastrophic, causing damage to both the orifice air bearing itself and the granite or steel guide surface. Guide surfaces are usually the most difficult and expensive part of an assembly to repair or replace, not to mention the downtime.
Demonstrating the Resilience of Porous Media with a Freaking Hacksaw
Porous media air bearings are extremely durable, and superior, to the orifice air bearing. With a porous media air bearing, you can literally take a hacksaw and score a deep groove right down the center, and it will still carry a very large percentage of its rated load. This is because air is still bleeding out of the surface area over the balance of the bearing face, and not just a few orifices.
If there happens to be an air supply failure, the restricted nature of the air flow bleeds it out slowly, setting the bearing down gently on the guide surface. And because the carbon face of the bearing is relatively soft, it provides an inherently good plain bearing properties which are less likely to damage that surface. Damage and downtime are far less likely.
A ‘Hack’ for Your Application
If you’re ready to inflict some serious damage over a precision application you may be working on, try something different. Download our ‘What Porous Media Air Bearings Can Do For You’ Infographic. This simple, visual tool provides a clear, concise rationale for air bearing consideration in your next application. Or, if you need this level of resilience right away, contact us directly today for your complimentary consultation!