My question is in the last two paragraphs.

Here's the project.
See my big wheel skates.
http://www.flickr.com/photos/27532210@N04/2565924423/
(that's the first prototype)
Now I'm making a push stick to propel me up slight hills while on in-
line skates. The design is simple. In the picture, you'll notice the
axle placement of the big front wheel skate is far forward of the toe.
The bulk of the apparatus will be an aluminum tube like the steering
column of a kick scooter. At the bottom of that aluminum tube will be a
motor positioned crosswise. The motor shaft will be a small diameter
aluminum tube that will run through two bearings, with the bearings and
the motor fixed to the steering column. Down from that will be like a
bicycle fork but much shorter, extending from the motor/bearings down
three inches to the wheel axle. The bottom end of the fork will not be
permanently attached to the axle, the connection point will be like an
upside down U so that it hooks downward on the axle. What keeps it
there will me simply pushing down on the stick. That downward pressure
will do more than keep the push stick in place, it will press the
roller down against the wheel, and it will indirectly press the wheel
down against the pavement. Disengaging the push stick simply requires
pulling up on it.

I've used a roller in various ways before. The following configuration
that didn't require bearings (unless you consider misalignment) was the
most encouraging. The ride felt good and propelled me at almost 18 mph.
I posted about it before.
http://www.flickr.com/photos/27532210@N04/2891204762/
Obviously the problem with that is the motor is too low to the ground.

(An ultrasmall ultralight motor would be wonderful, but that's a
different subject.)

Something I haven't considered in other efforts when using bearings to
hold the roller pressed against the wheel is the rpm of the motor
versus the maximum rpm of the bearings. Ordinary skate wheel bearings
are plentiful here and I like to use them, but apparently they max out
at much less than a typical cordless drill motor that spins at maybe
about 10,000 rpm.

Apparently what I need is something like an ordinary 608 ball bearing,
but able to handle at least 10,000 rpm with maybe 40 pounds of
downward/radial pressure on the bearing. Any bearing less than two
inches in diameter is okay.

Discussion, suggestions, and keywords appreciated.

Thank you.

On Fri, 03 Oct 2008 02:38:21 GMT, John Doe
wrote:



Apparently what I need is something like an ordinary 608 ball bearing,
but able to handle at least 10,000 rpm with maybe 40 pounds of
downward/radial pressure on the bearing. Any bearing less than two
inches in diameter is okay.

From SKF's catalog: a grease lubricated 608 is rated 36,000 RPM,
43,000 RPM with oil. Static load rating is 308 lbs. Those numbers are
under ideal conditions, but unless there's something unusual about
your load or environment, I think you'd get reasonable life at 10KRPM
and 40# load.

NSK has good online tech data if you want more info on the definitions
of the numbers I quoted. And you should find the formulas you need to
calculate life under various conditions, if you want to dig deep
enough to check my SWAG.
http://www.tec.nsk.com/

--
Ned Simmons

Ned Simmons wrote:

wrote:

Apparently what I need is something like an ordinary 608 ball
bearing, but able to handle at least 10,000 rpm with maybe 40
pounds of downward/radial pressure on the bearing. Any bearing
less than two inches in diameter is okay.

From SKF's catalog: a grease lubricated 608 is rated 36,000 RPM,
43,000 RPM with oil. Static load rating is 308 lbs. Those numbers
are under ideal conditions, but unless there's something unusual
about your load or environment, I think you'd get reasonable life
at 10KRPM and 40# load.

That possibility just occurred to me. I've been studying McMaster
information and I was going to ask again, thinking that maybe I was
wrong. Thanks. Earlier today, I started looking at low rpm bearings,
not realizing that the selection of ball bearings is actually larger
in the higher RPM categories.