wrote:
> Since the VolksMeter uses tungsten carbide to establish the axis
> (extracted from
> ball-point pens), I should have thought of the following a long time ago.
> A key to reducing rolling friction is to work with hard surfaces.
> Another key
> to reducing friction in general (if possible) is to reduce the normal
> force. Both
> are achievable by hanging a pendulum from a rare earth magnet, using the
> ferrous
> property of the tungsten carbide.
> In a brief experiment this morning I stuck a 1/2 in cylindrical rare
> earth
> magnet to the top of a steel door frame and then hung a ball point pen
> from the
> magnet. Discovered that the tungsten carbide tip of the pen could support
> about
> 100 grams of weight. Of course this arrangement is unsatisfactory for a
> seismometer because the physical pendulum that results (swinging pen)
> moves as a
> spherical pendulum.
> To get the required planar motion I took the refills of two pens and
> glued
> them together. The pair of pen points can support about 200 grams of an
> inertial
> mass while constrained to motion in a plane.
> The quality factor of this oscillator proved to be really high, with
> the unit
> swinging in observable free decay for many hundreds of cycles. It is
> clear then,
> that the friction is very small indeed, by (i) taking advantage of the
> hardness of
> both the magnet and the small tungsten carbide balls; and (ii) because the
> field
> gradient of the magnet provides support for much of the mass of the
> pendulum, so
> that the normal force is reduced as compared to most other configurations.
> For you folks who have played with various axis types, what do you
> think
> about this?
> Randall
>
Dr. Peters and all,
Am only adding some further development thoughts here. Some ball point pens
could likely be emptied of their contents; a replacement ball bearing could be
glued in place, and the empty plastic could contain, say; whatever lead shot the
axis ball bearing pivot could handle weight wise. Of course, one could possibly
retain the original ball bearing perhaps by cutting off the ink well tube and glueing.
Of course the plastic is both the "boom" and the "mass" weight holder in one.
One could "entertain" dual pens also for the vertical sensing approach with a
suitable spring....the dual ball bearing pivot/axis points should make it quite stable.
I suppose its also possible to electrically isolate the contact points of the
axis points, and run wires to a bulb somewhere at the base of one of the pens;
for light sensing variation output. Of course there is other ways of suing light
sensing without using a bulb in one of the pens.
Anyway....have tried only one working ball point here. Its a big size, "Gelwriter Rx", I
got from a Costco store. It has a screw on head. Even the pocket clip can
be easily pulled out of the assembly. The only magnetic item therein was the
spring. The clip and the ID ring seem to be stainless. The hand diameter
size is noteably larger than the average ball point pens on the U.S. market.
Some of the plastic is colored to look like metal.
I tried another ball point....but, its ball bearing didn't appear to be even magnetic.
Quite some time back, Chris Chapman checked out a variety of ball point
pens, and found the ball bearing diameters can vary.
Meredith Lamb
On Nov 8, 2007 9:57 AM, Randall Peters <
PETERS_RD@..........> wrote:
Since the VolksMeter uses tungsten carbide to establish the axis (extracted from
ball-point pens), I should have thought of the following a long time ago.
A key to reducing rolling friction is to work with hard surfaces. Another key
to reducing friction in general (if possible) is to reduce the normal force. Both
are achievable by hanging a pendulum from a rare earth magnet, using the ferrous
property of the tungsten carbide.
In a brief experiment this morning I stuck a 1/2 in cylindrical rare earth
magnet to the top of a steel door frame and then hung a ball point pen from the
magnet. Discovered that the tungsten carbide tip of the pen could support about
100 grams of weight. Of course this arrangement is unsatisfactory for a
seismometer because the physical pendulum that results (swinging pen) moves as a
spherical pendulum.
To get the required planar motion I took the refills of two pens and glued
them together. The pair of pen points can support about 200 grams of an inertial
mass while constrained to motion in a plane.
The quality factor of this oscillator proved to be really high, with the unit
swinging in observable free decay for many hundreds of cycles. It is clear then,
that the friction is very small indeed, by (i) taking advantage of the hardness of
both the magnet and the small tungsten carbide balls; and (ii) because the field
gradient of the magnet provides support for much of the mass of the pendulum, so
that the normal force is reduced as compared to most other configurations.
For you folks who have played with various axis types, what do you think
about this?
Randall