Westwind air bearings have been proven successful in the harshest of manufacturing environments around the world, providing significant advantages over conventional bearing systems for greater productivity and higher quality. This paper highlights the most significant benefits that air bearings can offer.
Air bearings provide extreme radial and axial rotational precision. Since there is no mechanical contact, wear is minimal, ensuring accuracy remains constant over time.
Air bearing spindles, by their manufacturing construction, are inherently accurate in rotation. Special manufacturing techniques enhance this accuracy to give extreme rotational and axial precision. Air bearing spindles have been designed to achieve rotational accuracy of less than 0.1 microns TIR in both axial and radial directions. As there is no mechanical contact between the rotating shaft and the static support, there is nothing to wear out, ensuring the accuracy remains constant over time – an important feature for manufacturers using statistical process control.
Fig 1 – Typical synchronous radial run-out values: < 10 microns (PCB drilling spindle, high speed).
Fig 2 – Typical asynchronous radial run-out values: < 0.025 microns (Disk test spindle, low speed).
Low shear forces within the air bearing allow extremely high rotational speeds with minimal loss of power and very low heat generation. Speeds can exceed 300,000 rpm.
Air bearings have low fractional drag, permitting high speed whilst retaining very low vibration levels. The frictional resistance to rotation of an air bearing is very small and, subsequently, the loss of power and heat generation is also very small. This allows the shaft to be run at very high surface speeds. In some spindles the higher rotational speeds will result in increased bearing stiffness caused by the properties of aerodynamic and gyroscopic stiffening.
Fig 3 – Current maximum speeds of Westwind air bearing spindles in different market sectors.
The use of air bearings means tool life can be greatly extended.
The lack of vibration and high rotational accuracy means drills, cutters, grinding wheels and boring tools have a much longer life – reducing maintenance and running costs. In particular, in the PCB drilling industry where drill diameters as small as 50 microns are now used, only air bearing spindles are capable of running at the required speeds to ensure acceptable tool life.
Fig 4 – PCB drill tool life versus speed.
Typical increase in grinding wheel life: 1.5 to 4 times dependent on application & wheel type.
The accurate, repeatable motion given by air bearing spindles gives a superior surface finish.
For applications, such as semiconductor processing, air bearing spindles provide a smooth, accurate, repeatable motion – resulting in a better surface finish. Unlike ball bearing spindles, air bearings provide constant bearing stiffness, ensuring minimal subsurface fracture in machining of hard materials. As the stiffness is produced by the uniform flow of air through the bearing, the reaction forces experienced by the shaft from an external load is constant at all points of its revolution. This property is particularly relevant to the production of good surface finish when grinding.
Surface Grinding: < 0.05 micron (2 micro-inch) CLA.
Diamond Turning or Fly Cutting Acrylics and Soft Metals: < 0.012 microns (0.5 micro inch) CLA, allowing consistent optical quality.
Axial: up to 250 N/micron (1,400,000 lb/in) .
Radial: up to 580 N/micron (3,300,000 lb/in).
Fig 5 – Comparison of rolling element and air bearing stiffness .
With no mechanical contact and a clean air supply, free from oil and water, bearing life is dramatically increased.
The absence of any metal-to-metal contact within the bearings ensures practically unlimited life, provided the air supply is clean and free from oil and water. In addition, due to the nature of operation, air bearings provide a constant air purge out from the ends of the bearings, creating a natural barrier to the ingress of harmful external contaminants such as material debris or cutting fluids. This increases machine utilisation and minimises down-time, resulting in an increase in overall efficiency.
Fig 6 – Comparison between bearing life of a ball bearing and an air bearing .
Low friction, constant air flow and efficient power transmission result in minimal thermal growth.
Due to many factors (such as low friction, constant air flow and efficient power transmission) the heating effect in a spindle shaft is small. Additionally, the selection of special materials and construction methods, together with internal liquid cooling channels, can almost completely eliminate thermal growth, so no warm-up period is required.
Fig 7 – Thermal image of water-cooled PCB drilling spindle running at 200,000 rpm.
Only the very minimum of maintenance is required. A regular check of air supply and coolant systems is all that is necessary to ensure complete reliability.
Normally only a regular check of the air supply is necessary to ensure complete bearing reliability. Providing that the spindle is operating within the design specification limits, the spindle should have a long working life. Normal maintenance usually consists of ensuring that air and water supplies are kept clean and to the correct specifications.
Typical air filtration requirement: 0.1 micron
Note: Where a collet or other holding device is fitted to the spindle, specific maintenance regimes must be adhered to.
Fig 8 – Maintenance instructions for a Westwind spindle fitted with a bonded collet .
Air bearings can support heavy loads, allowing them to be applied to many industrial machine tool applications.
Air bearing design can incorporate heavy load-carrying capacity, high stiffness capability, or a combination of the two. In many air bearing applications where spindle speeds are relatively low, large diameter radial and axial bearings can be incorporated.
Radial bearing loads up to 500kg.
Axial bearing loads up to 1000 kg.
Fig 9 – The graph below shows the load capacity with increasing spindle shaft diameter.
Only minimal levels of vibration and audible noise are produced when running an air bearing spindle.
As a result of the high balance standards obtained and the lack of mechanical contact, a Westwind air bearing spindle should produce minimal levels of vibration and audible noise.
Typical balance standard: G0.4 or better.
Typical vibration level: <0.2 mm/sec (low speed spindles). <1.0 mm/sec (high speed spindles).
Typical noise levels: 70 to 80 dBa.
Fig 10– Graph of radial vibration against speed.
Air is the only lubrication used; therefore air bearing technology is ideal where there must be no contamination of the work piece or working environment.
With the elimination of grease, oil and oil mist from the working environment, conditions remain cleaner when using air bearing spindles. Air bearings have no adverse effect on the environment, and are therefore ideal for use in cleanroom applications, such as in magnetic disk drive manufacture. Indeed, it is possible to operate specifically designed Westwind air bearing spindles in high vacuum conditions, as experienced in semiconductor silicon wafer manufacturing.
Typical clean room standard in which spindles can be operated in: Class 100.