Lecture 18 Aerostatic Bearings 18-1 Aerostatic Bearings Aerostatic bearings utilize a thin film of high-pressure air to support a load. Since air has a very low viscosity, bearings gaps need to be small, on the order of 1-10 μm. There are five basic types of aerostatic bearing geometries: single pad, opposed pad, journal, rotary thrust, and conical journal/thrust bearings similar to those configurations shown for hydrostatic bearings in Figure 9.2.1. All operate on the principle of supporting a load on a thin film of high-pressure air (typically 690 kpa) which flows continuously out of the bearing and into the atmosphere. 18-2
The choice of liquid or gas lubrication depends upon the type of application for which the bearings are intended. For example, moderate loads and moderate stiffness at high speed will favor gas bearings, while the requirement for high load and high stiffness at moderate speeds favors hydrostatic bearings. Figure 9.3.1 compares liquid and gas film bearings. 18-3 Compares Liquid and Gas Film Bearings 18-4
Hydrostatic bearings exhibit greater film stiffness than aerostatic bearings, usually by a factor of about 5, due mainly to the higher pressures of the lubricating medium. Important differences lie in the performance under dynamic loading. Hydrostatic bearings have superior damping characteristics compared to aerostatic bearings, which may be a critical feature in some applications. The major differences in performance relate to the effects of compressibility of the air film and the viscosity of the two lubricant mediums. 18-5 To ensure that flow rates of aerostatic bearings are kept to realistic levels, bearing clearances must be smaller than those for hydrostatic bearings; hence the quality of manufacture must be higher and tolerances on size must be rigorously controlled, thereby increasing manufacturing costs. In addition, running costs tend to be higher for gas-lubricated bearings as more power is expended in compressing a gas than in raising an incompressible fluid to the same pressure. 18-6
Applications of Aerostatic Bearings More and more machines are using air bearings. Modular air bearings used on a CMMs are made in the form of a simply detachable shoe, held in place by a screwed ball which enables the air bearing pad to take up an alignment parallel to the location face. Air bearings are now commonly used to support LVDT cores to improve reliability and reduce gauging forces. An air bearing live center has been developed recently for improving accuracy for gear checking. Other applications include profile projection equipment employing air bearing slides, rotary measuring tables and machine tool lead screw measuring heads. 18-7 Speed and Acceleration Limits Aerostatic bearings have only viscous friction associated with the air film layer being sheared during motion of the bearing. When using high-speed spindles (surface speeds greater than 10 m/s), the bearing gap should be large enough to ensure that the friction power is less than twice the pumping power. In such situations the temperature rise due to friction within the bearing gap is offset by the refrigeration effects of the gas film as it expands in the gap after leaving the orifice. 18-8
Range of Motion Linear motion aerostatic bearings can have as long a range of motion as it is possible to machine the slideways they rest on, which can be tens of meters. Angular motion aerostatic bearings are not rotation limited. 18-9 Applied Loads Because aerostatic bearings distribute the load over such a large area, large loads can typically be supported. A hydrostatic bearing (3.5 MPa operation pressure) for the same-size pad area can typically support five times the load of an aerostatic bearing. High-pressure hydrostatic bearings (20 MPa) can support even larger loads but may suffer from significant heat generation. An estimate of an air bearing's load capacity can be found by multiplying the effective (projected) area by the entry pressure of the gas as it enters the bearing clearance. 18-10
Applied Loads The effective projected area may be approximated as the area contained between the inlet orifices plus half the area outside the plane of orifices toward the edges of the bearing. The entry pressure is typically one-half the supply pressure. An estimate of the bearing stiffness is obtained by dividing the value above by half the no-load bearing gap. The load capacity of journal bearings is estimated in a similar manner to that described for flat pad bearings. 18-11 18-12
Accuracy Overall accuracy of motion (e.g., straightness) of an aerostatic bearing depends on the accuracy of the components. An aerostatic bearing averages out local irregularities to make them perhaps the smoothest running of all bearings. Maximum peak-to-valley surface roughness of air bearing components, however, should not be greater than one-fourth of the bearing gap. There is no wear-in period associated with aerostatic bearings and accuracy therefore depends on keeping the fluid flow restrictors clean and the pressure source free from pulsations. Aerostatic linear motion bearings have been built with submicron/meter accuracy. 18-13 Repeatability Repeatability depends on the stability of the fluid supply system, including the pump and the devices that regulate the flow of air into the bearings (i.e., flow restrictors). If pneumatic hammer instability, pressure surges, and temperature changes can be avoided, aerostatic bearings can achieve submicron (perhaps soon to be nanometer) repeatability. 18-14
Resolution Since there is no stiction with aerostatic bearings, motion resolution of an object supported by them is virtually unlimited. However, the design of the actuator and control systems is not trivial since aerostatic bearings are not well damped. 18-15 Preload Aerostatic bearings need to be preloaded in order to give them bidirectional stiffness. If a single aerostatic bearing is used to support a load, then as the load accelerates or forces act to increase the bearing gap, the bearing will have very little stiffness. Hence the opposed pad configuration is the most common one used in machine tools. It is also possible to preload a single pad aerostatic bearing with a vacuum pad which usually surrounds the pressurized pad region. The problem with this technique is that it is difficult to achieve less than a negative pressure of 10.5 atm, so the vacuum pad area has to be an order of magnitude larger than the area of the pressurized pad. 18-16
Stiffness Aerostatic bearing stiffness can easily be in the 100 N/μm range. An estimate of the stiffness can be obtained by dividing the estimated load capacity by one-half of the nominal bearing gap. Aerostatic bearing stiffness is also not difficult to calculate accurately, and aerostatic bearings do not have the problem of loss of contact that sliding or rolling contact bearings can experience. Thus designing with aerostatic bearings is usually more deterministic than designing with contact bearings. 18-17 Vibration and Shock Resistance Aerostatic bearings have very good shock and vibration resistance because there is no mechanical contact between moving parts. The air film collapses very quickly in the event of loss of pressure; hence aerostatic bearings should have a large backup reservoir and a control interlock to shut the machine down in the event of pump failure. In this manner touchdown and catastrophic failure can be avoided. In addition, if the bearing is not properly designed, the bearing itself can resonate as the air film alternately compresses and expands. This condition is known as pneumatic hammer and methods to avoid it are discussed later. 18-18
Damping Capability The thin, low-viscosity air film in the bearing gap gives aerostatic bearings moderate to low damping capabilities in the normal and tangential bearing directions respectively. Friction Aerostatic bearings have absolutely zero static friction. Dynamic friction forces are negligible at low speeds (less than 2 m/s). The dynamic friction force on a aerostatic bearing is independent of the loads applied insofar as they do not change the bearing gap. 18-19 Thermal Performance The viscosity of air is very low, so aerostatic spindle bearings are in bearing very tolerant of small changes clearance caused by viscous heating. Most hydrostatic bearings, on the other hand, are significantly less tolerant, and great emphasis must be given to optimizing hydrostatic spindle bearings in terms of friction power and pumping power for surface speeds greater than about 2 m/s. It is important to realize that air cools as it expands, and thus for a precision machine it is important to minimize flow and the resultant refrigeration effect. 18-20
Environmental Sensitivity Because air is always flowing out of the bearing, aerostatic bearings are self-cleaning. The escaping air is not generally collected, so it is not necessary to keep chips and cutting fluid out of the bearing area, although it is still preferable to use bellows or sliding way covers to keep the bearings surfaces clean and prevent them from being damaged. In addition, unlike bearings with oil lubrication, there is no mess associated with air bearings. 18-21 Seal-Ability Aerostatic linear bearings generally ride on rectangular or dovetail rails, so it is not difficult to seal them if required. Rotary motion aerostatic bearings usually do not need to be sealed. Size and Configuration Aerostatic bearings take up very little space themselves, but the plumbing requirements may be significant. It is generally desirable to have only one hose coming to a bearing, so the bearing itself may look like a block of Swiss cheese after all the drilling is done in order to get the air to all the different bearing pads. Kinematic pad arrangements are not necessary because the bearings act like constant force springs that fill whatever gap exists. As the equilibrium gap changes, the stiffness changes but stiffness will always be finite; thus aerostatic bearings are more forgiving of rail and carriage misalignments, as long as they do not cause the gap to change too much, which could cause loss of preload with sliding or 18-22 rolling bearings.
Weight Aerostatic bearings have moderate-to-high performanceto-weight ratios. Support Equipment The biggest drawback of aerostatic bearings is that they require a pump or connection to an air supply system. The system must be kept extremely clean to prevent foreign contamination from clogging the flow regulation devices, which typically are small orifices or slots. Typically, the air is filtered to 1 μm and driedwith a desiccant to minimize condensation within the bearings as the air expands and cools. 18-23 Maintenance Requirements Air cleanliness must be monitored and filters changed according to a fixed maintenance schedule. The air supply system should be inspected periodically for signs of contamination and the bearing rails for signs of wear that would result if a bearing pad's flow restrictor becomes clogged and the pad starved for air. Properly maintained and serviced, an air bearing should never experience any wear. There are many examples which show no wear after 10 years of continuous operation. 18-24
Material Compatibility Aerostatic bearings are compatible with virtually all materials, and the presence of a small bearing gap usually leaves ample room for differential thermal expansion between components; however, one needs to determine the order of this gap change and make sure that it does not alter the bearing's performance too much. If the gap opens up too much, then the bearing will be starved for air and a loss of stiffness will occur. If the gap decreases, little change in stiffness would be seen, but eventually, further reductions in gap would cause the stiffness to deteriorate because the inlet flow restrictors would be improperly sized for the prevailing gap. 18-25 Required Life Aerostatic bearings, whose air supply systems are maintained, can have essentially infinite life. Availability, Designability and Manufacturability Aerostatic bearing spindles and linear bearings are available as off-the-shelf items. It is not difficult to design and build custom aerostatic bearings if basic design rules are followed and one has some experience. The critical parameter in manufacturing aerostatic bearings is maintaining proper orifice and clearance dimensions. For orifices, one can use watchmaker's jeweled bearings. The orifice length should not be greater than four times the diameter, and the edges of the orifice should be as sharp as possible. 18-26
There are four basic types of gas bearings: aerodynamic, squeeze film, aerostatic, and hybrid. The aerodynamic bearing is often called self-acting because it generates its pressure within the gas film by the mechanism of velocity-induced viscous shearing in a converging film, a process similar to that found in hydrodynamic bearings. Unfortunately, the film pressures generated are relatively low. The advantage of this type of bearing is that it is entirely self-contained and is independent of any external source for gas supply. The squeeze film bearing is also independent of an external supply source, but this type of bearing, due to the poor squeeze properties of gas films, has not been found to be generally practical as a solution to a wide variety of engineering problems, although laboratory tests have demonstrated experimental feasibility. 18-27 There are four basic types of gas bearings: aerodynamic, squeeze film, aerostatic, and hybrid. The aerostatic bearing, or the externally pressurized gas bearing as it is often called, has its pressure in the gas film generated from an external source (compressor). A hybrid bearing combines both the aerostatic and the aerodynamic contributions to load. In practice, the combined performance obtained does not greatly enhance the purely aerostatic component of load for speeds used in most applications. 18-28
The most widely used bearing types are journal bearings for rotating shafts, and thrust bearings, either rectangular or circular form, often found in machine tool slideways, or of an annular type found in precision spindle assemblies. 18-29 Cost The principal costs associated with aerostatic bearings are those of machining all the air supply passages and machining long straight rails or very round bores with close tolerances. The cost of maintaining the air supply system should also be considered. An air filter dryer unit, which can provide air for dozens of air bearings, can cost on the order of $650. 18-30
General Operating Characteristics 18-31 General Operating Characteristics 18-32
General Operating Characteristics 18-33 General Operating Characteristics 18-34
General Operating Characteristics 18-35 Analysis of Orifice Compensated Bearings 18-36
Analysis of Orifice Compensated Bearings 18-37 Aerostatic Bearing Spindle Typical working spindles on ultra-precision machines are designed with air bearing technology. They are stiff and hold a radial run-out error of less than 25.4 nanometer (1 microinch) as well as handle radial loads of about 300 pounds. Hydraulic working spindles were found to have better dampening characteristics than pneumatic working spindles, so their presence became eminent in ultra-precision technology. 18-38
Aerostatic Bearing HydroBushing http://pergatory.mit.edu/perg/research/archive/kotilainen/hydrobushing.htm 18-39 Aerostatic Bearing HydroBushing 18-40
Aerostatic Bearing Spindle http://www.nelsonair.com/na_p rods_spindle.htm www.ntn.co.jp/english/product/lineup/ menu3-3.html 18-41 Aerostatic Driving Spindle 18-42