Re: [CAD_CAM_EDM_DRO] re:Re: magnetic spindle bearings

Elliot Burke wrote:

Jon Elson wrote:

>>Air bearings in the 1 -2" diameter range routinely get stiffness of 1
>>million Lbs/inch
>>deflection. That means a 1 Lb load deflects the bearing only one
>>micro-inch. This is
>>MUCH stiffer than ball or typical oil-journal bearings. Needless to
>>say, an air
>>bearing has VERY low friction, and it doesn't need micro-inch sensors,
>>power amplifiers
>>servo loops, etc.
>>
>>
>Jon, I'd sure like to know what air bearings have that sort (1Mlb/in) of
>stiffness!
>My search found numbers of 40 klb/in and below.
>And yes, these were carefully designed and made bearings.
>AFAIK, the larger stiffnes numbers are associated with liquid bearings,
>which are limited to lower speeds.
>
>Any references?
>

I think Dover just got bought out, and I haven't been to their web site
recently. But, they had
a line of inspection air bearings and air spindles for disk drive servo
track writers and such that
were spec'ed at 1 mega-Lb/Inch.

>
>I have read an older (1970) book on the subject of hydrostatic bearings, and
>it seemed in agreement with the results of my web search. At that time the
>complexity of air bearing stiffness calculations made the author omit them
>from the book, so I don't know if he included the hydrodynamic effects with
>the hydrostatic effects in his conclusions.
>
>

Ah, this is a major error. As small air bearings get over 50,000 RPM or
so, and larger ones even at slower
speeds, they get a lot stiffer. I suspect that it is VERY difficult to
analyze this, as it has an
enormous amount to do with the fine structure of the material finishes
and how they interact
with the boundary layers in the tiny space between the parts. I did
read a book called "hydrodynamic
and hydrostatic fluid film bearings" which had a great deal of info on
air bearings. It did give
some info supporting that air bearings were actually STIFFER than oil
film or rolling-element
bearings. I think this may have referred only to the rotating state,
where the hydrodynamic
effects are becoming dominant. I know from experience once when a hose
blew off and I
lost hydrostatic air on my Westwind air bearing spindle, that it kept on
working just fine
at 24,000 RPM until it finished the hole it was on. I was kind of
amazed. that was a demonstration
of the effectiveness of the hydrodynamic effect. (Of course it lost all
cooling, too!)

Old Browne & Sharp surface grinders from the 1900's were run with steel
journals in cast iron
bearings - dry! These were clearly running as air bearings. They ran
cooler dry than with
oil in them. There was no air source, so they were hydrodynamic, only.
I'm still amazed
at this, but it has been reported as true by a number of people, and was
apparently in the
operating manual of the machines.

Jon

>The air bearing applications I've seen have been been rather low force so
>the deflections were small.
>
>The single point diamond turning lathes stopped using them a long time ago
>because of the limitation in stiffness.
>
>Maybe there has been some great design improvement in air bearings over the
>last few years?
>
>We don't have to be afraid of servos controlling spindle axis location,
>consider a CD player, which has two axis servo control of the read head to
>sub micron tolerance with very simple and inexpensive apparatus. A
>capacitance bridge and some force coils should do it. It is possible that
>the forcing function could be built into the motor, further reducing
>complexity.
>
>Elliot B.
>
>
>
>Addresses:
>FAQ: http://www.ktmarketing.com/faq.html
>FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
>Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
>Unsubscribe: CAD_CAM_EDM_DRO-unsubscribe@yahoogroups.com
>List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@...
>Moderator: jmelson@... timg@... [Moderator]
>URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
>OFF Topic POSTS: General Machining
>If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
>http://www.metalworking.com/news_servers.html
>
>http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.
>
>NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
>bill
>List Mom
>List Owner
>
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
>