Differences Between Orifice and Porous Surface Inlet Air Bearings

Specialty Components designs and manufactures both porous surface and orifice inlet air bearings. It is important to recognize that both methods have advantages and disadvantages for a particular application.

Orifice

Advantages:

  • Precise flow rate control resulting in accurate bearing-to-bearing air gaps
  • Dynamic response can be “tuned” with multi-orifice configurations
  • Better control of lifting tilt on flat air bearings
  • Better suited for use in clean room applications, fewer than five 0.1 micron particles per minute
  • Very high input pressures allowable, (exceeding 300 psig, not all designs)
  • High temperature operating range

Disadvantages:

  • Slight damage will cause performance loss
  • Damage is not easily repaired
  • May not perform as well operating on mating surfaces with gaps or cavities
  • Unfiltered orifice may clog and limit performance (all SCI bearings are inherently filtered)

Porous Surface (carbon or metal)

Advantages:

  • Large grooves in either the porous material or mating surface do not greatly affect performance
  • Good performance at high tilt (the air film becomes tapered)
  • Accidental rough handling may not greatly affect performance
  • Accidental touchdown during operation will generally not damage the mating surface in the case of carbon air bearings.

Disadvantages:

  • Mechanical strength of porous surface and bonding epoxy limits maximum input pressure
  • Accidental exposure of oil on the bearing face is difficult to extract from the porous material
  • Exposure to some solvents will damage the bearing
  • Limited to near room temperature operation without active cooling

Mythbusting

Here are some more things you should know about orifice and porous type air bearings:

Myth: The hole(s) of orifice bearings clog easily making porous carbon bearings superior.

When we design an orifice inlet bearing, we include an internal “catch-all” filter to prevent accidental contamination. This makes the orifice bearing on-par with porous carbon bearings regarding obstruction of the restrictor. Actually, both bearings are sensitive to particles in the air supply, but it depends on the size of the particle. Orifice bearings allow very small particles to pass through while porous carbon bearings do not. This makes porous carbon bearings more sensitive contaminants like oil or liquid vapors.

Myth: Porous surface air bearings tend to provide significantly higher stiffness than orifice air bearings.

Properly designed orifice air bearings with multiple orifice and/or lift pockets can have equal stiffness to similarly sized porous surface air bearings.

Myth: Orifice type air bearings will exhibit unstable vibration under certain conditions as compared to porous carbon bearings.

Dynamic response of the air film is an important consideration for both types of inlet methods. It is important to properly design the air bearing feature sizes and positions to minimize this effect. Orifice air bearings may be designed to have very specific frequency response characteristics while porous inlet bearings have less flexibility in this regard.

Myth: The surface of a porous carbon bearing is very soft and this allows it to become scratched easily thereby reducing performance over time.

Although it is soft and may scratch easily, thousands of miniature air inlets within the porous surface will continue to emit air regardless of the presence of a scratch. In addition, many applications will feature the mating surface as a rail or guideway and often, this is an expensive and difficult to replace component of the machine. When utilizing porous carbon air bearings, if accidental wearing contact of the surfaces occurs, it will be the carbon that becomes worn and not the expensive rail or guideway.

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