We’ve looked at a lot of cool stuff in recent articles, including fractal vises, engine pistons and wire EDM. But today’s ‘stuff’ is arguably even cooler (and also kinda dangerous…). Not only does it let components float, but one, combined with some MetMo creativity, came close to taking a finger off a team member*. So, let’s dive in and explore the frictionless, silent finger stealers: aerodynamic bearings.
*All is revealed later.
What is an aerodynamic bearing?
They’re fun, frictionless and f-ery f-efficient. But let’s get serious. What actually is an aerodynamic bearing?
Albert Kingsbury’s 1917 Thrust Air Bearing | Source: Google Patents
Aerodynamic bearings are a type of air bearing, of which, there are two main types: aerodynamic (self-acting) and aerostatic (externally supplied air). They both work in a similar way and utilise a thin film of pressurised air (or gas), which acts like a lubricant, creating a small gap between components and letting them move frictionlessly about each other. The only difference is that an aerodynamic bearing uses surrounding air to work. I'll explain how very soon.
A very important air bearing application
Types of air bearings
So, in principle, aerodynamic and aerostatic bearings are similar. In fact, they’re a lot like my weekend dance moves. But rather than a drink or two, these bearings use air to facilitate smooth moving components…
How they do this varies. So, there are several different types of air bearings. Although it’s worth noting, they’re not always interchangeable.
If we group them by function, there are:
- Linear motion air bearings that support components along one or two directions. (e.g. Coordinate Measuring Machines (CMMs))
- Planar air bearings that support components on a flat surface (e.g. positioning systems)
- Rotary air bearings that support rotational movement between parts (e.g. journal or thrust air bearings - these can be both aerodynamic and aerostatic, too!)
- ‘Combined’ air bearings that support both radial and axial loads (e.g. fancy gyroscopes or turbines)
You can also group them by their means of air introduction/film maintenance:
- Orifice air bearings introduce air into the bearing gap through small holes. These are easier to make because they can be made from one material.
- Foil air bearings have ridged foil around the inside of the bearing, making them more stable.
- Porous air bearings use porous materials like carbon or bronze to distribute air across the bearing surface so it’s more stable and stiff.
- Spiral air bearings have curved grooves to stabilise the air film and work better at high speeds.
- Tilting pad air bearings have individual pads that move to adapt to the shaft’s position and load.
Of course, there are lots of different variations and combinations of these too (including roller, tapered, and modular air bearings). The list could go on. Different designs offer different load, speed and stability capabilities. So you can choose the type that best suits your application.
Aerodynamic bearings vs traditional bearings
Now we know what they are, you might be questioning why you might choose them over traditional bearings. The answer is pretty simple. Air bearings are, for the most part, like my meal deal purchases: contactless. No contact means no friction, and no friction means no (or far fewer) inefficiencies.
Traditional bearings, however, do make contact. They have ball bearings. And that friction not only roughens their movement, but it also means they need replacing and maintaining.
At the best of times, this is a pain. But in small, fiddly and hard-to-reach applications, keeping on top of your bearings is an even bigger pain in the aardvark. Aerodynamic bearings prevent it.
Now, let's answer the question on all our lips:
How do aerodynamic bearings work?
Picture this: You’re driving on the M6. It’s cold. Grey. And like a typical summer’s day in the north of England, it starts to rain. Hard. Before long, the M6 is covered in water. And as you’re driving, you feel your car begin to slide. You’re aquaplaning, right?
If you replace water with air, not only would you be levitating, but you’d also be replicating how an aerodynamic bearing works.
We now know that these bearings are self-acting, meaning they generate their own pressurised air and hence air film. But how does this happen?
Let’s use a spinning shaft as an example. There are three stages.
Stage 1: Rest
At rest, the shaft probably touches the bearing surface (if not, there’s a tiny gap). Clearance, now, is asymmetrical. As the shaft starts to turn, its movement draws air into the gap between the shaft and the bearing surface.
Stage 2: Wiggle
As air gets sucked in, pressure changes. This generates the lubricating thin film of air and lifts the shaft up. (Reminder: different designs adjust how airflow and pressure distribute across the bearing surface)
Stage 3: Float
Faster rotational speeds mean the pressurised air becomes err… more pressurised and lets the bearing support heavier loads. The bearing is self-stabilising, so it creates a stability ‘buffer’. If the shaft moves towards the bearing surface and the distance shrinks, the air pressure increases and returns the shaft to its neutral position.
Side note: Steve Mould explains this well, here.
The benefits of aerodynamic bearings
These three stages gift us some pretty cool benefits. For example:
There’s minimal friction. The shaft floats. So there are smoother movements, less heat generation (better thermal stability) and higher efficiency.
They last longer. Because there’s no contact, there’s limited wear, so they require less maintenance and become more reliable.
They’re fast. Again, no contact means they can spin really fast without overheating or breaking.
They’re clean. Air is the lubricant. Not oil or grease. So, for applications where cleanliness really matters, they’re great.
They’re precise. The thin air film allows for tight tolerances and stable positioning.
They’re quiet. No (or very little) contact means they make very little noise.
They’re self-stabilising. The air film adjusts when load or speed changes, so air bearings work well under changing conditions.
It’s not all silent, frictionless fun
Aerodynamic bearings are cool. But they’re not perfect.
Yes, rotational speed and resultant load capacity increase in harmony. But versus traditional bearings, aerodynamic bearings are no match. Traditional bearings can bear heavier loads.
Second, at low speeds, the air film might not fully develop. If there is contact between the shaft and bearing, damage will occur. If you’re continuously starting and stopping, then you might damage the components further.
Thirdly and finally, precision is important. Tight tolerances are importanter. To maintain the small air gap, the gap between the bearing and shaft must be right. This requires skill and high-precision manufacturing.
The history of air bearings
We know what they are, how they work and how they differ from other bearings. But where did they come from?
Holey moley
Well, it all started in the early 19th Century, in 1812. Robert Wills explored how small holes could control airflow to support loads. It was very early days, but this was, in essence, the beginning of the orifice air bearing.
Liquid did gas a solid
Later that century, Osbourne Reynolds, a British physicist, developed what’s now known as Reynold’s Equation. This described the pressure distribution in a fluid film and became the mathematical bearing blocks for understanding liquid lubricated bearings (and later, gas ones). Google it if you dare… it’s a mammoth partial differential. *Gulps*
Developmental friction
In 1897, Albert Kingsbury began experimenting with externally pressurised air journal bearings. Whilst the work was important, Kingsbury exposed just how precise the components needed to be and how manufacturing of the time could not consistently do it.
You try matching a bore and shaft to a consistent 0.0005” gap with 1800s equipment.
Lift off
As we move into the 20th Century, in 1904, George Westinghouse patented a thrust bearing, using air for a vertical steam turbine. This is arguably the first practical air bearing-esque design in industrial machinery.
Runny babbitts
One year later, Anthony George Malden (AGM) Michell developed the Reynold Equation and found a way to account for side leakage. Previously, the equation only applied to plane surfaces - where behaviour was between two infinite two-dimensional pressure fields.
One of Michell’s bearing patents (US1315735A) | Source: Google Patents
Michell’s developments accounted for real-world constraints and ultimately made it more useful. These developments likely helped Michell patent the Michell Bearing, which had a run (or series) of pivoting babbitts to help oil films carry load.
Same same, but different
In 1907 Albert Kingsbury returned. Manufacturing had improved and he could develop his now well-known thrust bearing (a hydrodynamic oil-lubricated bearing).
Like Michell’s, Kingsbury’s bearing also had a series of arcs to support load. So, when he filed for a patent, it was rejected because of Michell’s work. Once he proved his work started first, however, his patent was granted.
Kingsbury’s 1910 patent: US947242 | Source: Google Patents
Soon after, in 1912, Kingsbury’s bearing found its first application – at Holtwood Generating Station. It’s apparently still in full use!
It’s groovy baby
Post-Second World War, technology, materials and manufacturing abilities had improved. Newfound precision gave bearings the space they needed to breathe – and flourish. Research kept coming. And air bearings kept improving, including new theories on how groove design could improve air film stability.
US National Laboratories research helped implement air bearings in new applications, including ultra-precision machine tools. They also developed some of the first porous air bearings.
In fact, in 1965, Check Mate, the first coordinate measuring machine (CMM) to use air bearings hit the market. This used porous air bearings, where the porous material was carbon. Some of the Check Mate machines are allegedly still in use today.
Arguably the biggest success, though, came from IBM. In the '60s, they used air bearings spindles to make their first gen of hard disk drives.
Keeping the world rolling
As we move closer to today, companies started developing their own air bearings, with several focusing on specific industries and applications (e.g. Sheffield Corporation developed their own porous air bearing for CMMs in the 80s). Others began standardising air bearings, making them readily available for folks like you and me.
Finite Element Analysis (FEA) and other computational developments helped model air film behaviour, improving coatings and mechanical properties so air bearings could do and withstand more.
Air bearing uses
It will probably come as no surprise that the use of aerodynamic and aerostatic bearings has continued. We find these frictionless fascinations in more industries than ever. For example:
Aerospace
Today, auxiliary power units (APUs) (the small gas turbine that provides power when the big engines aren’t running), gyroscopes and turbochargers use them. Their contactless selves allow them to work for prolonged periods of time, and provide precise information (like the angle of flight) – and with turbines, work at high speeds.
Electronics
Air bearings' accuracy, reliability and low noise mean they’re often used in hard disk and optical drives. They can reach high rotational speeds for prolonged periods and require comparably less energy to do so!
Turbomachinery
Compressors, turbines and turboexpanders use a range of air bearings so they, too, can work at high speeds with minimal friction – and not need regular replacing. Traditional bearings struggle with wear, friction and regular lubrication. Air bearings do not, making them a great choice.
Machinery
As we’ve seen, air bearings feature in coordinate measuring machines. They also feature in other inspection equipment like rotary and spindle inspectors. Because they’re contactless, debris doesn’t fly off, keeping them suitable for cleanroom environments. So things like wafer (not the delicious biscuit snack) can be produced for semiconductors.
Two different CMMs - air bearings help move the arm
Energy
Unsurprisingly, high efficiency is also desirable in the energy sector. Microturbines and fuel cell systems provide more bang for their buck and reduce environmental impact because air bearings are frictionless.
Frictionless, fidgety fun
Now, not only are aerodynamic bearings very useful, but they’re also very fun to play with. Because once upon a time, we made something quite dangerous. It wasn’t sharp like a scalpel. And it didn’t jump out at you when you pressed it down…
It definitely wasn’t this
So, to save fingers and eyes, we instead opted for something more fun, fidgety and family-friendly: The MetMo Piston.
In all its glory!
It’s like clicking a pen. Only it makes no noise, spins, bounces, and, of course, launches. This one isn’t on our Kickstarter and is available to buy from our store now. Although, runs are limited. If you’d like to get one, take a look and see if they’re in stock here.
So there you have it, the smooth operators of our wonderful world of engineering. I hope you enjoyed reading.