On Mon, 31 Mar 2014 09:27:40 -0500, Richard wrote:

On 3/31/2014 8:08 AM, Lloyd E. Sponenburgh wrote:
fired this volley in
:

What happens if you put a bearing within a bearing - the outer race of
the inside one being the inner race of the outside one? Is friction
reduced? Is the maxiumum speed changed? Temp rise?
You increase any inherent 'play' in the system. Also since the 'middle
race' is no longer heat-sunk to a shaft or housing, risk the
possibility of heating it without any way to remove the heat.

OTOH, you've distributed the wear over at least twice as many rolling
elements, and if everything were perfect the balls and races would turn
more slowly than a conventional bearing. IF everything were perfect.

Lloyd

I always thought that would be like two resistors in parallel as far as
friction goes?

Nope. That would be true if the friction in a ball bearing were
proportional to velocity. It's not -- when everything is sitting still
the two shafts act like they're connected with a spring. Then, once
there's enough force to get the balls rolling (as it were) there's a
relatively constant friction force. There is a small element of friction
that rises with velocity, but it's not usually very big.

I know the gory detail on this from stabilizing gimballed platforms that
have to hang from helicopters. One of the biggest paths by which the
aircraft motion gets into the platform is through the bearings, and one
of the biggest tradeoffs (once you're buying the World's Most Expensive
Bearings) ends up in bearing strength vs. bearing friction -- basically,
the more load the bearing can bear, the more friction it has.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com