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What is an aerodynamic air bearing?

wallpapers Industry 2021-03-15
In the case of aerodynamic bearings, also known as self-acting bearings, special features may be manufactured into the mating surfaces which act like a small pump.  At rest, the mating surfaces may be in direct contact.  Relative motion forces the air molecules from atmosphere into the gap where they begin to accumulate with increasing pressure. As the speed increases, a velocity induced pressure gradient forms across the clearance. The increased pressure between the surfaces creates the load carrying effect. The load capacity is dependent on the speed at which the surface moves and therefore at zero speed, the air bearing supports no load.
 
In general, aerodynamic bearings suffer from decreased load carrying capacity. In addition, the zero-load at zero speed effect causes starting and stopping friction and may result in some wearing of the air bearing surfaces. Notwithstanding some of the disadvantages, self-acting bearings have found widespread industrial use.  The most ubiquitous example may be the magnetic read/write heads of a disk-based hard drive memory storage device.  In this case, a flat thrust-load carrying air film is created between the disk surface and the head.  An armature positions and lightly preloads the flat head against the surface while a motor provides relative motion by spinning the magnetic disk.  As pressure builds, the head elevates above the disk surface.  As hard drive technology has improved, the distance at which the air bearing head flies has been reduced to as small as 3 nanometers.  This gap is about 30,000 times thinner than the average thickness of a sheet of notebook paper.  Small air gaps present fabrication difficulties which are discussed in the Manufacturing Challenges section below.
 
However, large air gaps can be designed for if the relative velocity of the mating surfaces is sufficiently high. As the velocity increases so does the pressure.  With this increase in pressure, the separation between the mating surfaces can be increased.  In fact, hypersonic relative velocities of flat aerodynamic pads have successfully been built supporting thousands of pounds. This type of bearing could find use in the proposed Hyperloop concept.
 
Again, the aerodynamic air bearing's principle advantage is its ability to act without an external pressure source. The load capacity is limited by the area of the bearing and the relative velocity of the mating surface.  Therefore, aerodynamic air bearings can be applied in limited cases:
 
Where the application requires lack of an external pressure source
 
Where the application can provide enough relative velocity to generate lift for a given size of the bearing

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