When a coordinate measuring machine glides across its travel with no visible contact, no lubricant and no stick slip, it is almost certainly riding on an air bearing, and in most precision machines the surface it rides on is granite. The pairing of lapped granite and pressurized air is the quiet foundation of modern metrology equipment, semiconductor stages and linear motor platforms. This guide explains how a granite air bearing works, why granite is preferred over ceramics in most applications, how load capacity and stiffness are determined, what the air supply requires, and how to specify one with a practical selection checklist.
How Air Bearing Guidance Works
An air bearing replaces solid sliding contact with a thin film of pressurized air, typically a few micrometers to a few tens of micrometers thick. Air is fed through small orifices or a porous region in the bearing pad, spreads into the gap between the pad and the guide surface, and escapes at the edges. The pressure inside the film carries the load, so the carriage floats rather than touches. Because there is no metal to metal contact, there is no static friction to overcome at start of motion, no wear particles, and no lubricant to attract dust or evaporate in a cleanroom.
Two properties of the film matter as much as the load it carries. The first is averaging: the film bridges over microscopic highs and lows on the guide surface, filtering short wavelength errors so the payload sees a much smoother trajectory than the raw surface profile. The second is damping: the air film dissipates vibrational energy, which complements the natural damping of granite itself. Together these effects produce motion with sub micrometer straightness and nanometer scale resolution, which is exactly what CMM air bearing stages and optical inspection systems demand.
Why Granite Instead of Steel or Ceramic
The guide surface is half of the air bearing system, and its quality decides the whole. Granite brings three advantages. It can be lapped to outstanding flatness, routinely at the micrometer level over large areas, which gives the air film a stable and predictable gap. It has high internal damping, so the combined structure does not ring the way a steel way does. And it is dimensionally calm: its thermal expansion is a small fraction of steel, and it does not corrode, so the gap geometry survives years of temperature cycling and humid cleanroom air without re work.
Ceramic guides, typically alumina or silicon carbide, appear in some machines and have genuine strengths. They are harder than granite, tolerate higher contact loads where hybrid ball air systems are used, and can be lighter for the same stiffness in certain geometries. In practice the choice comes down to the application. For most CMM and metrology platforms, granite wins on cost per unit of accuracy, on the size range available, and on repairability, because a worn or damaged granite guide can be re lapped in place, while a ceramic guide usually means replacing the whole component. Ceramics tend to earn their premium where extreme hardness, aggressive environments or very high accelerations dominate.
Load Capacity and Stiffness
Load capacity in an air bearing is simply the integrated pressure of the film over the pad area, derated by the pressure profile between supply and edge. A rough practical guide: with typical supply pressures around 4 to 6 bar and efficient pad designs, an effective film pressure in the range of 1.5 to 3 bar over the pad area is achievable, so a pad of 50 x 100 mm carries on the order of 75 to 150 kg with margin. Multi pad carriages add capacities directly.
Stiffness is the change of gap per unit of load change, and it is what keeps the payload from sagging toward the guide as the load shifts. Film stiffness rises with supply pressure and with a smaller design gap, but too small a gap makes the bearing sensitive to surface flatness errors and to particles. Well designed bearings on lapped granite run in the middle of the range, combining meaningful stiffness with tolerance to real world surface quality. When comparing suppliers, ask for the load versus gap curve and the film stiffness figure at your operating load, not just a maximum load number, because a bearing that floats high at light load and dives at heavy load will show it in your straightness data.
Air Film Gap and Air Supply Requirements
The design gap is chosen to match the flatness of the guide and the cleanliness of the air. Larger gaps forgive surface error and particles but reduce stiffness; precision machines commonly run gaps near 5 to 12 micrometers on lapped granite. This is why guide flatness is specified so tightly: the film can only average errors that are small relative to its own thickness.
The air supply needs the same care. Typical requirements are clean, dry air at 4 to 8 bar, filtered to 0.1 micrometer or better for precision work, with a dew point low enough to prevent condensation in the lines, commonly below 10 degrees C at line pressure. Oil is the enemy: even traces of oil mist eventually migrate into the film and deposit on the guide, so oil free compressors or coalescing and carbon filters are standard. Regulators and reservoirs should be sized so that consumption stays steady, because pressure ripple appears directly as motion ripple on the stage. Consumption depends on pad size and gap, from a few liters per minute for small pads to well over a hundred for large multi pad carriages, so the supply should be designed from the total, with redundancy or an accumulator where an unexpected shutdown would let the machine settle onto its guides.
Where Granite Air Bearings Are Used
The same physics serves several machine families, and the emphasis differs with each one.
| Application | What the air bearing does | Why granite fits |
| CMM and measuring machines | Carries the probe carriage with frictionless, repeatable motion on X and Y rails | Lapped flatness and damping give sub micrometer straightness over long travel |
| Metrology systems | Provides drift free reference motion for optical and tactile sensors | Thermal stability keeps calibration valid between re checks |
| XY stages | Supports payloads on planar multi pad layouts with nanometer resolution drives | High stiffness combined with excellent flatness across large plates |
| Linear motor platforms | Replaces mechanical guideways where contact would add ripple and particles | Non magnetic, clean and compatible with high acceleration profiles |
| Semiconductor inspection equipment | Positions wafers and sensors without generating wear debris | No lubricant, no particles, stable in cleanroom conditions |
A Practical Selection Checklist
Before requesting a quotation, work through six items. They take an hour to prepare and save weeks of iteration.
Load: state the payload mass, its center of gravity offset, and whether the load is constant or changes during the cycle. Preload direction and dynamic forces from motors and cables belong here too.
Travel and envelope: give the required stroke in each axis, the space available, and the position of hard stops and cable management. Guide length sets the number and spacing of bearing pads.
Accuracy requirements: express them as straightness, flatness of the traveled path, pitch, roll and yaw over the full stroke, and repeatability. Numbers here drive the lapping specification of the granite guide.
Air supply: confirm available pressure, dew point and filtration at the machine inlet, and whether the line is shared with other equipment that could cause pressure dips.
Environment: temperature range and stability, humidity, vibration of the floor, and cleanroom class if applicable. These decide material grade, gap sizing and whether isolation mounts are needed.
Mounting interface: provide the drawings of what attaches to the carriage and what supports the guide, including insert positions, so the granite component arrives ready to assemble rather than needing local drilling after delivery.
Installation, Cleaning and Maintenance
An air bearing system is nearly maintenance free, but a few habits protect it. Keep the guide surface covered when idle to prevent debris and accidental contact. Clean lapped granite with mild detergent or isopropyl alcohol and a lint free cloth; avoid aggressive alkaline cleaners that can dull the surface over years. Never slide a carriage on a dry guide with the air off, because even one start stop under load can leave a flat spot on the pad; use the lift or compensation features designed into the machine. Check filters and regulators on a schedule, and watch the gap readings if the machine provides them, since a slowly narrowing gap often indicates filter aging or surface contamination long before it becomes a fault. With those habits, the granite guide can be re lapped after many years of service and the system restored to as new performance, which is one of the quiet economic advantages of the technology.
Start With Your Load and Travel
If you are specifying a CMM air bearing stage, a metrology platform or a custom linear motor machine, send us the checklist above, or simply the load, travel, accuracy targets and interface drawings. We manufacture granite air bearing components and matched lapped guides in house, and our measuring machine granite component line covers the rails, carriages and bases they run on. Our engineers will review your requirements, recommend pad layout, gap class and guide flatness, and return a quotation with lead time, normally within a few working days. Custom manufacturing to your drawing is our standard practice, and the drawing review is free of charge.