We specialize in the design, manufacture and testing of custom air bearings for those applications requiring the highest levels of precision. Air bearings are selected for these applications because they are completely non-contact, which allows them to offer many benefits not seen by other bearing types:
- Zero friction motion; allowing nanometer position resolution
- The ultimate in smoothness with extremely low motion errors (straightness less than 250nm)
- No wear, minimal particle generation and no maintenance on bearings
- High speed operation, greater than 1 meter/sec
While air bearings are non-contact they do rely upon a thin film of pressurized air to support the carriage load. The air film thickness or air gap is typically between .0002” – .0008” (5 – 20µm) and its selection has a significant effect on total bearing stiffness, load capacity, manufacturing costs and air flow requirements. A decreasing gap will provide higher stiffness but it is usually at the sake of manufacturing and assembly costs. To assure reliable performance of the air bearing we recommend that the ratio of air gap thickness to manufacturing tolerances be at least 3:1. For example, an air gap of .0005” requires a manufacturing tolerance on the guide-way and carriage plates on the order of .00015”. I’ve seen recommendations of 2:1 but this is typically inviting trouble. The manufactured parts must be treated with the utmost care as one scratch or raised edge could cause problems at assembly or even when it arrives at your facility.
This is the reason that many companies to do not offer air bearings. To produce quality air bearings requires the right expertise at all levels, from design to manufacturing and inspection. To achieve these tolerances the manufacturing of air bearings requires the use of precision grinding machines and processes that can achieve results on the order of ~.0001”/12”. This is quite an achievement considering most air bearing plates are produced from aluminum. With an air gap of .0002”, I would recommend a tolerance of .000050”/12 which is usually reserved for precision lapping. The cost of the guide-way manufacturing can increase by 2X just between grinding and lapping. These are some of the design trade-offs that we can easily help you with.
While simple at first glance, the air bearing is rather complex to design. In addition to the air film thickness there are other design elements that are critical to the performance of the air bearing as shown in Figure 1. These are the supply pressure (Ps), orifice or restriction type, air film pressure (Pgap), and preload method.
The supply pressure, Ps, is usually in the range of 50 – 75psi (.35 – .52 MPa) and provided by a compressed air system. One will often hear that the supply must be “clean dry air”. This aspect of the air bearing design is often overlooked or considered as an after-thought. However, we treat the air supply requirements just as important as any other elements of the design. The air supply system must first be sized to provide the continuous flow of the bearing without causing excessive duty cycle requirements on the compressor (25 – 50% duty cycle is our recommendation). The compressed air system should include an aftercooler to reduce the outlet air temperature, multiple levels of filtration (5 micron down to .01 micron), a water separator to remove gross levels of water and then most importantly an air dryer to provide a dew point temperature down to a minimum of 37°F. This is really the bare minimum as we recommend a desiccant air dryer that will provide a dew point temperature down to -40°F. When the pneumatic system is complete it should be considered “instrument air”. Why is this necessary? Water vapor, oil vapor and solids must be kept to a minimum otherwise they will cause drag on the air bearing or in some cases I’ve seen the bearing stop floating from excessive water vapor. This is the reason you do not want to use shop air to run your air bearing. A compressed air system of instrument air quality has many components so it will require maintenance. However, we can help you select compressed air systems that provide superior performance with long maintenance intervals to provide trouble-free air bearing life.
The orifice or inlet restriction is usually provided by a simple annular orifice, jeweled bearing or porous material. It’s this element of the air bearing that allows air to flow into the gap and provides stiffness to the pressured air film. The number and placement of orifices determine the pressure profile in the air gap. Except for porous materials the pressure profile is not a constant but drops between orifices. Jeweled bearings are often used in conjunction with small pockets which distribute the air. This type of design can sometimes lead to “pneumatic hammer” or static instability in the air film. Its cause is related to poor damping in the air film and an improper ratio of pocket volume to thrust face volume. We can develop a solution that meets your specifications and avoids these kinds of issues, especially when extreme straightness or flatness is a concern.
Our expertise is with linear flat pad bearings which must always be held down by a positive force or “preload”. The form of preload can be gravity (the payload itself), an opposing flat pad (see figure 2 above), magnetic force or vacuum. The preload will reduce the available load capacity but provides an increase in stiffness. We have used all of these preload methods and finally selection depends on the goal of the application and design constraints.
The main carriage plates are usually manufactured from aluminum to keep mass to a minimum and acceleration rates to a maximum. However, the main stationary guideway can be supplied in a number of materials depending on the application. First, aluminum is a good choice because it’s inexpensive and can be manufactured to reasonably tight tolerances. However, when long travels are involved (> 48”) it is best to consider granite. Granite can be produced in lengths greater than 48” while at the same time can be lapped to very tight tolerances. It also has inherently better internal damping than either aluminum or steel. Granite has a coefficient of thermal expansion (CTE) close to steel if bi-metal effects need to be kept to a minimum. However, when minimum deflection is needed for a long unsupported span then ceramic is the best choice. It has the highest stiffness to weight ratio and can be lapped to extremely tight tolerances, even tighter than those of granite. The main drawback is cost; expect to pay 3X what you would in aluminum or granite. Below is a comparison of guideway materials.
Air Bearing Guideway Materials
|6061-T6 Aluminum||23.6||2,700||68.9||167||• Lowest Cost
• High Conductivity
|American Black Granite||5.8||3,087||72.4||3.2||• Low CTE, Very Stable
• Long Travels
• Can be Lapped to Tight Tolerances
|Ceramic (96% Al2O3)||6.1||3,720||303||24.7||• High Stiffness to Weight
• Can be Lapped to Tight Tolerances
• Very Brittle
We have designed custom air bearings in all of the common configurations, single axis, compound axes, single plane, split axes and gantry as shown below: