Friction Co-efficient of Air

[Al_The_Man]

I know this technology has been around for a while, but it is always a marvel to see 1100lbs (Table & Jig) riding on a cushion of air.
This was a recent integration of a 36" rotary table to a Fanuc 10m Mill.
The turntable has an air bearing that consists of small pockets machined into the rotary mating surfaces, to allow (near) friction free rotation, the table is supplied with 60lbs of air, this lifts the table less than .001 and allows the motor to easily rotate it, six pnuematic clamps then operate to lock in position..
Suprisingly, it can also be used in the vertical position.
* A Galil DMC1501 was used as the motor indexer and 'married' to the Fanuc PMC.*
Al.

[Geof]

Friction co-efficient of air - very small.

It is neat to see something like this working. Another "air slide-bearing" application I have heard about is having really big air pucks carrying very large electric transformers sliding around on a very smooth floor in a B.C Hydro facility.

[JerryFlyGuy]

In the aerodynamic's world a perfect Laminar flow wing could [ in theory] have a Cd of 0.0025.

I know that Cat industrial equipment is moved in the plant via this method. Theres some food for thought a 100,000lb Caterpillar being pushed around the plant ON air.

I love technology....

Jerry

[keebler303]

fluids are nice in that their friction is almost intirely independant of the force on it.

[NEATman]

I am still amazed by air bearings, and I have been designing them for 10 years now. Incredible tolerances can be achieved using air bearings, as the surface roughness is averaged out, as the surfaces do not touch, and there are no rolling elements. At the last company where I worked, we sold a air bearing spindle that had +-0.000001" radial and axial runout. 1 millionth of an inch. Also, they had built a 72" diamater table that could support 30 tons. They stacked every granite surface plate in the building on this table, but the air from the building AC system was enough to cause this huge mass to rotate. The technician working on the system applied a strip of masking tape to prevent the system from rotating! Very cool. In my current job, the majority of the systems that we build are air bearings, powered by linear motors, and non-contacting linear encoders. CMM's are also a great example of air bearings.
NEATman

[ViperTX]

I wonder what the friction is from one molecule of air rubbing against another molecule....but then again it's really the coefficient of a molecule of air rubbing against the bearing hub.....hmmmmmm....anyone measured that??

[JerryFlyGuy]

Originally Posted by ViperTX I wonder what the friction is from one molecule of air rubbing against another molecule....but then again it's really the coefficient of a molecule of air rubbing against the bearing hub.....hmmmmmm....anyone measured that??

If you could knew the density in relation to the pressure its under it wouldn't really be that hard.

Drag=1/2 x V^2 x density in slugs x area^2[ft] [I believe is the formula-- working from memory here]

You'd need to know the pressure at which the table or unit is working at, and then translate that into density in slugs and the rest is pretty much history. The V^2 part would relate to the mass flow of the air in CFM divided by the airgap and area of the system. On a system which was a disk w/ the air induced in the center the pressure would disipate and the flow would slow the farther from the center of the disk. On a plate w/ many many holes your issue becomes much more complex. However for ruff numbers you could just assume an average and get pretty close.

Jerry

[JRoque]

Incredible. According to this document, a 0.00001 coefficient of friction can be achieved with an air bearing.

Say you'd want an air bearing spindle. How would you drive it? Won't attaching anything to the spindle throw it off it's center? Or would air be used to spin it too? Is this practical for typical machining?

JR

[NEATman]

JR-
The typical way we drive our air bearing spindles is with a frameless (no bearings) brushless DC motor. The air bearings are the bearings for the motor. The motor itself comes as a "kit", a rotor with magnets, and a stator with the windings. The encoders vary in design, but are also non-contact. By skewing the laminated steel windings of the motor coil, you can almost eliminate the cogging caused by the magnetic attraction. There are also some elaborate ironless motors out there which have zero cogging, and are non-contacting.

As far as this technology being practical for typical machining, there are several companies out there making machine tools with this technology. I used to work at one, www.precitech.com and they make many types of maching centers, usually custom designed, or for applications where extreme accuracy is required.

NEATman