Air Bearings: Porous block vs Colander for Air Feed

[Swamp Thing]

TL;DR

What are the tradeoffs between air bearings built with porous materials, versus built with non-porous perforated parts

This video talks about how air bearings can be made using porous materials like graphite.
Is it possible to get a comparable performance using non-porous materials, with an array of pin holes?

The video talks about how the carbon seat can be easily worked into conforming with the matching part, but what if we are able to achieve the same tolerance by other means? In that case, would there still be any benefits from porous material in terms of the air pressure distribution and air dynamics etc --- apart from the process of shaping the part?

 

[Lnewqban]

I have no experience with those bearings.
If I would have to make a decision about it, I would select the type that requires less machining time (perforating many pin holes at different angles may become costly and time consuming).
The effect of lifting the matching part and keeping certain gap may be equally achieved by the two methods, if enough pin holes uniformly distributed are made.

 

[256bits]

Wiki has a brief on air bearings
https://en.wikipedia.org/wiki/Air_bearing

 

[Baluncore]

Is it possible to get a comparable performance using non-porous materials, with an array of pin holes?

Yes it is.
The advantage of the porous material is that it bleeds air from the entire surface. It would be expensive and unnecessary to drill so many pinholes.

Drilling many deep pin holes in a solid material may be slow and difficult without breaking drills. You might use a laser, but the swarf or exhaust must exit through the same hole you are trying to feed the drill or laser light into. The drill must enter perpendicular to the surface, that may be difficult with a colander having two spherical surfaces.

 

[Ranger Mike]

I have been in the coordinate measurement machine (CMM) business since 1980. Back then the CMMs had precision mechanical bearings or air bearings. The accuracies were very competitive. This was before the geometric errors could be compensated for by “error Mapping”. The machines had to be accurate mechanically. About 1982 one cmm supplier started to use porous media air bearings in lieu of mechanical bearings. The engineers got the idea from the NASA Space Shuttle research using new manufacturing techniques.

There were many advantages with porous media bearings. As the video above shows, air seeps out of the media surface. Older design air bearings were simply around bearing pad with and orifice. But the air bearing required mucho Air PSI to “float” the CMM bridge on the granite plate and permit the Z axis quill to float. We are talking about 0.001 to 0.003” air gap. When you have this big an air requirement you need lots of compressed air.

Compressed air is one of the most overlooked expense in a machine shop operation. It is like electricity. It is always there and considered a necessary evil. When you have wide air gaps the compressor is running all the time the CMM is operating. When you have huge air gaps the air pressure tends to “chill” the guideway which will cause inaccuracy due to thermal gradients. You also have settling time required as you have “shoe lift” of the air bearing during acceleration and deceleration. All of these add up to non-ideal metrology and limit the accuracy desired. One more thing about porous media bearings is they can have the surface contaminated up 80% without impact on accuracy. Very forgiving. All these problems go way with porous media bearings. Less compressor run time means less expense, better accuracy, less maintenance.

 

[Swamp Thing]

Mike, lots of good info there. Thanks.

 

[Ranger Mike]

Thank you.. one fixed cost no onethings about is cost of compressed air. we di calculation of electric cost of air compressor running back in the 1980s. If one of the young turks out there want to do a greennie thing,, recalculate the cost. Industry did introduce an air shut off for the Z axis counterbalance whe nnot in use for 30 seconds. This closed theair supply for the most offending air hog. The Z axis pneumatic counterbalance mechanism.