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small.planes
10-02-2012, 04:36 PM
Evening all,
Somewhere on the net I found a table of speeds you could run ball bearings at.
Of course now I need to look at it I can't find it.
Any clues?
I need to run a small diameter shaft ( around 5mm) somewhere between 30000 and 300000 rpm, minimal loading and minimal side thrust.
Alternative options?

Dave

willmac
10-02-2012, 05:01 PM
Depends on what bearing you have or can afford. Try the SKF website for data applicable to their bearings as a starting point. 300,000 rpm will be challenging for standard bearings though.:)

rythmnbls
10-02-2012, 06:28 PM
Full compliment hybrid ceramics would fill the bill, often used in miniature gas turbines. Although most commercial GT bearings are 8mm shaft size. Boca Bearings might have what you need.

Steve.

ikdor
10-02-2012, 06:52 PM
Ouch, that's extreme territory. There are other bearing manufacturers (not SKF) that specialise in small high speed bearings for things like dentist drills. Barden, Timken, New Hampshire Bearing.

If you just want to try once, you could grab a 618/5 catalog bearing and see what happens.
If you call a general bearing manufacturer and say you need a 3 million ndm bearing, most will send you to the R&D department.

But I'm going to hazard a guess anyway that you might want to shrink your shaft diameter.

Igor

Evan
10-02-2012, 06:59 PM
That's a bit high for bearings to fit 5mm. It's nothing for really small bearings. The limits are imposed by the deformation of the outer race caused by the centrifugal force of the balls. If you can make the shaft a little smaller to fit a smaller bearing and the smaller bearing will withstand the load then it will make it a lot easier and cheaper to find a solution.

Mcgyver
10-02-2012, 07:33 PM
look into dental handpiece bearings. new ones go to 500k and are common at .125ID - not too much smaller than 5mm. more looking might find metric series, here's just the first i found

http://www.pacamor.com/product_offerings/ultra-high-speed-dental-bearings.php

sch
10-02-2012, 07:57 PM
Koyo steel bearing catalog suggests rpm limits for steel bearings in the range of 50-65k rpm at a 5mm shaft size. Full ceramic, equivalent quality wound go up to ~150k rpm. Air bearings might be a
consideration. Here is a fairly cheap all ceramic zirconia 5mm ID: http://www.vxb.com/page/bearings/PROD/Kit8438 You would have to call to find out rpm rating as it is not discussed.
One paper I saw suggested rotational limits as a product of ID (mm) and RPM ie DxR for steel ~1,000,000 and for ceramic ~2,000,000. For 5mm ID that would work out to 400k rpm. Doesn't
help much as micron size smoothness of ball and race discussed only generally and likely to get to 400K you would need bearings with 10-50 micron smoothness.

Evan
10-02-2012, 10:15 PM
The primary reason that ceramic bearings have higher limits is that the balls weigh much less and exert less centrifugal force. Smoothness determines ultimate wear life.

ikdor
10-03-2012, 03:56 AM
The other reason is that ceramic balls slip less because of their weight. The precession of the rotational axis of the balls takes quite a lot of effort at those speeds and heavier balls start slipping earlier. Slip is a quick way to damage the raceway.
The cage design and lubrication for high speed bearings are also quite different from a standard catalog bearing.

Igor

EVguru
10-03-2012, 06:01 AM
The maximum bearing speed is specified based on a standard design life. If you can accept a shorter life, then you can exceed that speed.

I was talking to the Gas Engine Builder's chap at one show and they use 40 hours as the expected bearing life (lubrication is only by mist in the fuel).

small.planes
10-03-2012, 06:42 AM
Cheers for the links :)
I can make the shaft smaller, its currently in the design and trial stage. 1/8" would probably work.
Im all for adapting my design to the available bits, as otherwise I can make it....

Dave