Gap Sensing Air Bearings FAQs

Why is it important for air bearings to have gap sensing (GS)?

Improper installation is a leading cause of failure for an air bearing. Measuring the air gap provides the best confirmation of proper installation. For critical applications, knowing the health of the air bearing is paramount to system performance; the gap sensing port allows for efficient and economical real-time monitoring of the bearing load, pressure and air gap.

How does the gap sensing feature work?

Aerostatic air bearings include an inlet hole which permits delivery of the externally pressurized gas to the air film. Bearings with the gap sensing feature incorporate a secondary hole next to the supply inlet allowing access to a special channel leading directly to the air film. The air within this sensing channel will change in pressure as the air gap increases or decreases. By correlating this pressure to bench top measurements, the applied load and air gap are determined 1.

What equipment do I need use this feature?

A pressure gage or sensor with 40-80 psig range and 0.25-0.50 psi resolution is recommended.

How do I correlate the measured GS pressure with air gap?

This may be accomplished by using a Gap Sense Curve graph or mathematical interpolation of the curve function. GS graphs can be found on each product page or in the downloads center. Graphs have been generated for each gap sense air bearing model showing the GS pressure versus lift for three different supply pressures. Simply select the appropriate curve based on the air bearing model and supply pressure. Then locate the intersection of the curve and the GS pressure to find the lift.

My application requires better accuracy of the lift than can be read from the graph. Can you provide the raw data or mathematical function for the curve?

Yes. In addition, we can provide data sets for each air bearing.

If my application uses a supply pressure other than the three pressures shown on the graphs, how do I determine the gap of the air bearing film?

The gap sense pressure is approximately linearly dependent upon the input pressure within the operating limits shown on the graph. The ratio of absolute supply pressure to absolute GS pressure is constant for a fixed air gap:

(Psupply + 14.7 psi) / (GSP + 14.7 psi) = Constant

This ratio can be used to scale any given GSP measurement to an existing supply pressure curve:

For example: you have measured a gap pressure of 60 psig for the LRAP250 air bearing at a 70 psig supply pressure. What is the air gap?
Scale up to the 80 psig curve:

(80 + 14.7 psi) / (70 + 14.7 psi) = 1.12

60 psig X 1.12 = 67 psig

Using the 80 psig curve, intersect at 67 psig GSP to find the equivalent air gap at 170 µinches.

What can be done to control the response time for the GS pressure measurement?

As the air enters the GS channel, an amount of time must pass for the volume to become pressurized. The length of time depends on the size of this volume and the supply pressure. To shorten the response time, adjust the length of the tubing to be as short as possible. If extremely fast response is required, we can provide an electronic pressure sensor integrated within the bearing housing.

How do complex machine tool assemblies benefit from air bearings with gap sensing features?

In many applications, air bearings are installed deep inside the structure of a machine tool and obtaining access to measure the air gap may require substantial time and effort for disassembly. Due to the fact that a conventional, and sometimes bulky, displacement indicator is not used to measure the air gap, the machine tool may require significantly less disassembly.

What should a machine designer keep in mind when planning for the installation of GS air bearings?

To know the air gap of air bearings with the GS feature, one needs only a simple connection to a common pressure gage/sensor via a piece of flexible tubing. The machine designer may include this tubing connected to the bearing leading to a conveniently located terminus. This design would allow for permanent access to the GS port of the air bearing(s). Minimizing tubing length will provide faster response. A step further: the GS bearing may be connected to an inexpensive digitizing pressure transducer which would provide real-time air gap information to the control software of the machine tool. Another step further: an electronic pressure sensor is integrated within the bearing housing.

What is the difference between the meanings of the phrases: “air gap”, “lift” and “flying height”?

Generally, these terms have the same meaning: the amount of distance between the mating surfaces of the air bearing. The jargon will vary depending on the market or industry segment. We use all of them here interchangeably.

How is a GS bearing installed?

After the bearing has been mounted with a ball stem, flexure assembly or adjustable hardware, a pressure gage/sensor is connected to the gap sensing port of the bearing housing. The preload is then adjusted until the pressure gage/sensor reads the correlated gap pressure for the bearing.

How is installation of a gap sensing (GS) bearing easier than conventional air bearings?

The GS feature establishes a novel approach to air bearing installation. When a gap measurement is necessary and use of a displacement indicator is not feasible or desired, the air gap distance can be known by simply connecting a length of tubing and a pressure gage/sensor to the bearing. This pressure reading is then correlated to bench top measurements.

How are other air bearings installed?

Typical mounting and adjusting of air bearings use a few different approaches to determining the proper preload and air gap.

1. Adjust the bearing preload until a desired INLET pressure is achieved. This approach is not applicable in all cases.
2. Adjust preload until a desired flow rate is achieved.
3. Adjust the preload until friction is noticed and then reduce the preload by a small amount (this method is not recommended by Specialty Components.)
4. Adjust the preload until the air gap measured by an appropriate displacement indicator is within desired operating limits.

For all NON-GS bearings, we recommend the fourth option stated above.

Must all GS bearings be installed using the gap sensing port?

No. You may install the bearing in any recommended manner. However, always keep the port plugged with the provided fitting and o-ring when the sensing port is not in use.

Is bearing performance sacrificed by the presence of this feature?

No. GS air bearings are identical in every way to a similar bearing without this feature.

How does gap sensing reduce failure or crashing of an air bearing?

Provides for proper installation
Offers live monitoring of bearing performance

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