Editorial Technical Reference

Air bearing pad orifice plate: feed-hole size, grooves and pockets on an orifice-compensated aerostatic pad

The feed-hole face of an orifice-type aerostatic pad · orifice diameter and count, outlet grooves and pockets, film thickness, supersonic flow and microvibration

In an orifice-compensated air bearing pad, the plate that faces the guide surface does the metering: compressed air enters through a few small feed orifices, spreads along shallow grooves or pockets cut into the face, and leaves through a film a few micrometres thick. The diameter and number of those orifices, the depth and shape of the grooves around them, and the flatness and finish of the face decide how stiff the pad is, how much air it uses and whether the film stays quiet or starts to vibrate.

The orifice face of flat aerostatic pads: feed orifices and orifice inserts, outlet grooves and pockets, face finish, and how their geometry affects film flow, stiffness and microvibration. Choosing between orifice and porous air bearings, air supply and mounting are covered on the Aerostatic air bearings page. Orifice inserts in adhesive dispensing valves belong to the Precision dispensing nozzle page. Porous-carbon and porous-ceramic bearing faces, rolling-element precision bearings and elastomeric bridge bearing pads are outside this page.

CNFX editorial research · Sources checked October 12, 2026 · 3 sources

Set orifice size and groove layout Keep the film flow stable

Stainless-steel plate drilled with a grid of small air holes lying on a dark granite base, with a rectangular metal carriage block above it fed by a clear air hose, in a precision machine enclosure — Multi-hole stainless air plate on granite under an air-fed carriage block

Start with your task

Set orifice size and groove layout0.25–0.4 mm orifices, 15 µm grooves. Keep the film flow stableFilm thickness, Mach number, vibration.

Orifice diameter, orifice inserts and the groove pattern around them

Two published pad designs show the scale of the parts involved: feed orifices of a few tenths of a millimetre, sitting at the ends or crossings of grooves only micrometres deep.

The Strathclyde authors describe the orifice-plus-groove layout as a compound restrictor, combining an inlet restrictor (the orifice) with an outlet restrictor (the groove), and state that the outlet groove structure regulates the pressure distribution in the film and so determines static and dynamic performance. [1] The Pluritec patent forms each orifice in a separate insert rather than drilling it into the pad body, so the metering hole is a replaceable precision part. [2]

Editorial calculation: a 0.4 mm orifice has a flow area of π × 0.2² ≈ 0.126 mm², about 2.6 times the 0.049 mm² of a 0.25 mm orifice, so the two published designs differ more in area than the 0.15 mm difference in diameter suggests. Orifice size has to be read together with the supply pressure and the groove depth, not on its own. [2] [1]

Published pad	Feed orifices	Grooves or recesses on the face	Supply pressure
Pluritec air pad for a machine-tool work table (US 5,326,177)	Four supporting orifices, diameter preferably about 0.25 mm, each formed in a cylindrical insert seated in the pad body [2]	Four shallow grooves parallel to the sides, forming a rectangle that stops short of the corners; one orifice at the midpoint of each side [2]	4 bar in the patent's load test [2]
Strathclyde compound-restrictor test bearing, 40 × 58 mm	Orifice diameter 0.4 mm [1]	Groove depth 15 µm, groove width 0.4 mm; diverging groove width 0.2 mm at a 45° diverging angle [1]	0.5–0.6 MPa absolute in the simulations and tests [1]

Specify orifice diameter and tolerance, number and positions of orifices, whether each orifice is drilled or carried in an insert, groove or pocket depth and width, and the supply pressure the pad is designed for.

Film thickness, supersonic flow at the orifice and nanometre microvibration

The Strathclyde study traces microvibration of aerostatic pads to the air leaving the orifice: at 0.5 MPa supply, no unstable flow appeared with an 8 µm film, while a 20 µm film produced supersonic flow (maximum Mach number above 1) and clearly unstable airflow. [1]

For the conventional linear groove, raising the film from 18 to 20 µm lifted the maximum Mach number from 1.143 to 1.440, with pressure depression near the orifice inlet and air vortices downstream. In a first simulation, narrowing the outlet groove by half cut the maximum velocity from 209.62 m/s to 170.86 m/s but left zero-velocity zones that hindered the air flow, so the authors moved to a diverging, Laval-nozzle-shaped groove and settled on an orifice-to-groove distance of 4.7 mm and a 0.2 mm corner radius. Measured stage vibration stayed below 10 nm. The authors also report that the diverging groove performed less well than the linear one under subsonic conditions, so the groove shape suits pads that run in the supersonic regime. [1]

Pluritec's patent addresses the other end of the load range. With 4 bar supply and no compensation, the gap of its pad changes little for loads above 10 kg but changes rapidly below 10 kg; adding four compensating micropads on the opposite face, fed from the same air supply and giving a preload of roughly 10 kg, keeps the gap nearly constant even as the load approaches zero during sharp acceleration and deceleration. [2]

Ask the supplier for the design film thickness, supply pressure, stiffness and air consumption at your load, and whether the groove geometry has been checked for supersonic flow at the largest film the pad will see.

Machining a multi-orifice plate: internal feed passages, pocket depth and face finish

A Jiangsu University of Science and Technology team building an air-film conveyor plate described each machining step of a face carrying 128 pockets fed from side-drilled passages, which shows the tolerances such plates are made to. [3]

Editorial note: the conveyor plate floats light planar objects rather than carrying a machine axis, so its 150 µm pockets are far deeper than the 15 µm grooves of the Strathclyde bearing; what carries over is the sequence of drilling internal feed passages from the side, connecting them to face ports, and leaving the final depth and roughness to a finishing pass. [3] [1]

Step	Published value
Plate blank	204 × 228 × 20.15 mm [3]
Internal feed passages	32 perforations of 4 mm diameter drilled from the side at 7 mm intervals [3]
Pockets	16 × 8 array milled to a temporary depth of 300 µm, finished to 150 µm [3]
Cell layout	10 mm square pocket with a 4 mm dam; two pairs of 1 mm ports joined by 1 mm × 1 mm grooves [3]
Face roughness	Ra 6.3 µm after rough polishing, Ra 0.8 µm after finish machining [3]
Air supply in tests	500 kPa supply, 100 L/min choked flow set by an orifice [3]

Describe the pad face and the film it must hold

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Pad: [size mm; material; flat or with insert seats]
Orifices: [diameter mm; count; positions; drilled or insert]
Grooves or pockets: [depth µm; width mm; layout]
Face: [flatness; roughness Ra]
Air: [supply pressure MPa; flow L/min; filtration]
Operation: [load N or kg; design film thickness µm; stiffness; preload method]

See manufacturer records Send an enquiry through CNFX

Buyer questions

How large are the feed orifices in an air bearing pad?

Published examples are a few tenths of a millimetre: about 0.25 mm in a Pluritec patent and 0.4 mm in a University of Strathclyde test bearing. [2] [1]

Why do orifice air bearings vibrate?

The Strathclyde study links microvibration to supersonic flow leaving the orifice: at 0.5 MPa, an 8 µm film was stable while a 20 µm film produced Mach numbers above 1 and air vortices. [1]

How deep are the grooves on an orifice pad face?

The Strathclyde bearing used 15 µm-deep, 0.4 mm-wide grooves; an air-film conveyor plate from Jiangsu used 150 µm pockets. [1] [3]

How can an air pad stay stable at very light load?

Pluritec's patent adds compensating micropads on the opposite face that preload the pad by roughly 10 kg from the same air supply. [2]

Sources and what still needs a direct check

Sources checked 2026-10-12. The University of Strathclyde study (published open access in Nanomanufacturing and Metrology) and the Jiangsu University of Science and Technology paper (Micromachines) report measured and simulated pad geometry; Pluritec's US patent describes an orifice pad used on a machine-tool table.

Still to check yourself

Fix orifice diameter, count and positions.
Specify groove or pocket depth and width.
Give the design film thickness and supply pressure.
Check flow stability at the largest film.
Specify face flatness and roughness.