Air Bearing Benefits 

The Challenge

In many advanced industries, machine builders are constantly pushing the requirements of spindle technology in the quest for higher productivity, greater accuracy, and a longer lifetime. Conventional bearing systems, usually ball bearings, cannot always deliver the very high speeds demanded by these industries whilst still maintaining high rotational precision, thermal stability, and long service life. This is primarily due to the mechanical contact between the balls and the races, which can result in rapid wear and excessive heat generation at high rpm’s, leading to a loss in accuracy and even premature failure of the bearing.

In addition, some of these industries require oil-free bearing systems to avoid workpiece or environmental contamination, particularly in a cleanroom application.

The Solution

For many high technology OEMs, an air bearing spindle is the perfect solution. Westwind air bearing spindles have proved 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.

The Benefits of Air Bearing Spindles

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).

High 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.

Fig 3 – Current maximum speeds of Westwind air bearing spindles in different market sectors.

In some spindles, the higher rotational speeds will result in increased bearing stiffness caused by the properties of aerodynamic and gyroscopic stiffening.

 Increased Tool Life

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. 

 Improved Surface Finish

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. 

 Typical Surface Finish

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.  

 Typical Air Bearing Stiffness Values

Depending on the size of the spindle, air bearing stiffness can achieve values of:

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 .

 Long Bearing Life

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 Thermal Growth

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.

 Lack of Maintenance

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 .

 Large Load Capacity

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 above shows the load capacity with increasing spindle shaft diameter.

Reduced Vibration

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.

Cleanliness

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 a cleanroom, or in automotive rotary atomizer spraying applications.

Typical clean room standard in which spindles can be operated in: Class 100.  

Key Product Lines using Westwind Air Bearing technology:

• PCB Drilling and Routing Spindles
• Coating Atomizer Spindles
• Light Industrial – Precision Machining Spindles

Further Reading

What is an Air Bearing?

Last Updated: 5 July, 2021