Requirements and advantages of air bearings in precision motion

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来源: | 作者:topphotonics | 发布时间: 2022-10-27 | 592 次浏览 | 分享到:

Q1

What are air bearings?

A bearing is the machine element that realizes the constraints of motion as such that only the desired motion remains in the system while reducing friction between moving components. A set of bearings can realize a rotational degree of freedom along an axis of rotation, while preventing undesired motion in other degrees of freedom, such as tilt motion or radial and axial linear motion. Bearings are classified broadly according to the type of operation, the allowed motion, or to the directions of the loads the element can withstand. Well-known type of bearings are the rolling element bearing and the sliding bearings. In this paper we will highlight air bearings as a part of the fluid film bearings.

In an air bearing, a thin film of pressurized air is used to provide an interface layer between surfaces that can transfer loads in a frictionless way. Although called air bearings, the utilized gas is not solely limited to air, e.g., also pure nitrogen is often used. Since the two surfaces are properly separated and contact-free, no wear, particulate generation or excessive friction is present, as opposed to sliding or rolling contact bearings. Due to the low friction in the film, air bearings are excellent in high-speed applications, but also in high-precision applications, where the lack of friction facilitates more precise positioning and a smoother motion.

A further distinction is made between aerodynamic bearings, where the air film is established through the relative movement between the surfaces, and aerostatic bearings, which are externally fed by overpressure to maintain the air film even when there is no relative motion between the surfaces. Aerodynamic (or self-acting) bearings are found in the magnetic read/write heads in disk memory storage devices but also in thrust bearings for electrical generator turbines. Aerostatic bearings have a wide range of applications, such as the linear guideways of coordinate measuring machines or wafer processing machines in the semiconductor industry. In the remainder of this article, we will focus on the precision motion applications where aerostatic bearings are employed, although certain aspects also hold for aerodynamic bearings.

The film in the aerostatic bearing is realized by supplying the gas into the bearing gap via a restriction. This restriction meters the flow of pressurized gas to an optimal level for the designed bearing gap, such that it matches with the flow through the bearing gap, which results into a load carrying capacity and stiffness of the film. The restriction can be realized in various ways, e.g., machined feed holes, glued orifices, porous media etc.

Air bearing components are available on the market in the form of flat bearing pucks, bushings, bearing slides, radial bearing segments etc. These components match with a high-precision counterpart, such as a precision ground shaft for the air bearing bushings, or a precisely lapped granite surface for the air bearing pucks. The designer can develop and build his mechanical design utilizing these off-the-shelf components. A typical example is the Coordinate Measuring Machine, that uses air bearing pucks for the linear guideways, which can easily be interchanged during a maintenance of the machine.

Another approach is to highly integrate the bearing function into the structural elements of the mechanical design, i.e. the air bearings are deeply integrated into mechanical structure, e.g. a slideway with areas that serve as air bearings, while at the same time this monolithic structure also offers the mounting surfaces for motor, encoder, functional area for the application. In OEM applications, one will often see this approach, where the air bearings are incorporated into the linear stages of the machine. Another example is the air spindle, where the feed channeling and restrictors are implemented into the housing of the spindle which also holds space for a.o. encoder feedback, direct drive motor and liquid or air cooling.

▲ Figure 1: Left: an individual air bearing component for a rotary shaft support. Right: a highly integrated air bearing solution containing radial and axial air bearing, motor and encoder feedback.

What are the requirements for a good air bearing system?

Before switching to the advantages of air bearings over traditional mechanical bearing solutions, we first highlight the requirements which are important for a proper operation of the air bearing system.

CDA delivery

Clean Dry Air (CDA) is a critically important requirement for the optimal performance of any externally pressurized air bearing system. An air bearing system for precision applications has a typical air gap of only a few µm up to around 10 µm. A multi-stage filtration that removes particles and oil out of the air supply is required to clean the air coming from a compressor unit. General-purpose filter cartridges help to remove the bulk particles, while a coalescing filter removes oil and water including particles that passed the general-purpose filter. Activated carbon filters remove oil vapors and hydrocarbons to bring the contamination level to even lower levels. Membrane, adsorption or refrigeration dryers are used to bring the dew point of the supplied air down to a level which guarantees no condensation downstream in the air bearing application.

LAB Motion Systems recommends the ISO 8573-1:2010 Class 1.3.1 standard as to the required air quality. The requirements included in this standard are as follows:

Class 1: particulate	In each normal cubic meter of compressed air, the particulate count should not exceed 20.000 particles in the 0,1 – 0,5 micron size range, 400 particles in the 0,5 - 1 micron size range and 10 particles in the 1 - 5 micron size range.
Class 3: moisture content	A pressure dewpoint (PDP) of -20°C or better
Class 1: oil	In each normal cubic meter of compressed air the amount of oil is limited to 0,01 mg. This is a total limit for liquid oil, oil aerosol and oil vapor combined

▲Figure 2:To achieve these stringent air quality levels, a careful approach to component selection is needed. When using an oil-free air compressor and storage tank with draining point, an air filter unit can be used to reach the required air quality level. The following schematic shows a layout of the general system setup.

Besides cleanliness, it is of utmost important to foresee a reliable source of air at constant pressure. An oversized compressor system delivers pressurized air, which is regulated down to a supply pressure for which the air bearing system is designed. When the pressurized air supply fails and a lower pressure is realized, the load carrying capacity of the bearing is reduced which results in lower bearing performance and can ultimately result in a bearing seizure. Costly downtime of the machine and a replacement of the bearing components might be required after such an event. Therefore, in many applications, a pressure guard is foreseen, which monitors the correct air pressure, and triggers the motion control unit not to activate the motion of the system, or to slow down the system to a standstill in order to protect the air bearings. A storage tank in combination with a pressure sensor can help to maintain a safe operating pressure during failure of the air compressor and to allow for a safe deceleration of the system before the supply pressure drops below a safe threshold, making sure that the air supply is reliable at any time during motion.

A clean and dry environment

Aerostatic air bearings exhibit a good resistance to a dusty environment, since the air bearing has the capability to keep the dust out of the bearing gap and clean the raceway. Attention has to be paid to avoid dust build-up in confined areas where the dust cannot be blown or wiped away. Moreover, sticky dust due to moisture or other substances with high affinity to the surfaces, is even more problematic.

Oil or water contamination on the guideways of the air bearing is absolutely to be avoided, since the moisture will creep into the bearing gap when the supply pressure is switched off, creating friction and interruption of the air film.

Overload and temperature load

Typically, air bearings have a lower overload resistance as compared to mechanical bearings. In some applications, this drawback is resolved by using crash-resistant coatings or full porous graphite surfaces which are known for the resistance against seizure.

An overload situation can occur due to mechanical overload (e.g., a spindle crash in a CNC machine tool). Another often overlooked aspect is a proper mounting of an air bearing system. In order to realize the full performance of an air bearing, a low distortion mounting is required. Whether it is an air bearing bushing or a rotation stage, it is required that the interfaces of the air bearing with the environment (e.g., mounting sleeve for a bushing, a machine bed for a rotation stage, or a wafer chuck for a linear stage) distort the air bearing components as little as possible. The precision of the system and its load carrying capacity can only be guaranteed when the mounting instructions are carefully followed. Flatness and cleanliness of the mating surfaces before installation is important, as well as respecting the tightening torques as specified by the manufacturer.

Thermal overload due to temperature gradients in the machine and differential thermal expansion of materials used in the construction1 also result in distortions within the air bearing components and, as a result a lower performance. A good thermal management of both the room in which the machine is located2, as well as internal heat sources within the machine, such as direct drive linear motors or heat originating from the process (e.g. laser welding) is mandatory.

① Although granite is a popularly used material due to its stability and machinability, it has a fairly different coefficient of thermal expansion (CTE) compared to metals such as aluminum and steel.

② Very often, high precision machine equipment is utilized in a climatized environment.

Why would one use air bearings? Advantages of air bearings

Air bearings have some clear advantages over rolling element bearings of which the application can benefit. The main advantages are listed below.

Near zero friction and wear

Since the air film separates the moving and stationary surfaces, no static friction is present in the bearing. This yields very high repeatability in positioning and infinite resolution of the motion. In high-speed air bearing application friction losses become more prominent due to viscous shearing in the air film, although due to the low viscosity of the gas3, the frictional losses remain much lower as compared to oil lubricated systems in similar working conditions.

③ The viscosity of air is of the order of one-thousandth of the viscosity of light machine oil.

The non-contact nature of air bearings also yields a silent and low-vibration operation, which is highly desirable in high precision applications, for instance metrology machines.

Cleanliness

Fully separated moving parts also mean theoretically zero wear, which result in a virtually infinite lifetime of the bearing component and very low particle generation. In cleanroom applications, this is a significant advantage, apart from the fact that grease or oil lubrication of mechanical bearings can be avoided by switching over to air bearings.

▲Figure 3: a high-end air bearing gantry stage XY system with linear motors and optical feedback for semicon inspection purposes.

Dynamic properties

A well-designed air bearing will realize excellent stiffness and damping properties. The supply pressure and surface area of the air bearing film is proportional to the stiffness that can be realized. Nevertheless, the proper dimensioning of the restriction in combination with features in the bearing gap are of the utmost importance to realize a stiff bearing. The film damping in the bearing gap results in excellent dynamic performance. In squeeze film damping, the surfaces that realize the air film, move so that the gap in between expands and contracts. When the gap contracts, the gas film is squeezed between the two surfaces. In slide film damping, the motion of the air bearing parallel to the base, leads to shearing within the gas film. The motion of the system generally results in a mixture of both squeeze and slide film damping, although often one is dominant.

High speed applications

Air bearings are widely used in high-speed applications, where they exhibit lower frictional losses and better dynamic performance as compared to mechanical bearings. In high-speed spindle applications, air bearings can exceed the acceleration of mechanical spindle bearings which are prone to ball slippage above certain acceleration levels.

Conclusions

Although air bearings sometimes have the connotation to be sensitive to failure and difficult to implement, they have significant advantages over traditionally used mechanical bearings. When consequently following the requirements of the air bearings with respect to air supply, environmental conditions, installation guidelines and loading, a trouble-free operation with high performance can be guaranteed for many years.