> I would like to see a run down pendulum test on this to check it.
Hi Chris and all,
I think I can now mentally see the very likelyhood of increased friction with two rods and a ball over that of the
true 3 crossed rods pivot. Being as the ball is set tightly in a straight linear "groove" (inbetween the two rods), and
the horizontal pendulum/boom/pivot ball swings in a slight up-down arc there would logically be some ball
arching response rotational rubbing friction of the ball on the two rods trying to accomodate this motion of the
Of course; if there was a straight linear motion of the pivot/boom/mass in strict alignment with the two rods
and the ball pivot, the ball would simply rotate in a happier more frictionless harmony.
Judging from all this rotaton arching nature of the traditonal Lehman; I think it would logically be much better to
switch too, or use, the 3 crossed rod pivot. By itself the 3 crossed rod pivot is indeed a excellent choice for
a pivot with its ultra-low frictional loss. This is only my choice; but I've not tried them all.
Of course there is other pivots used in a Lehman which the builder has a choice of using. Conjecture on the
"best" of these will likely go on a long time. Perhaps to ease this gap; it could be possible to have the PSN
members simply do a undampened time duration of their announced variety of seismometer pivot, by say a set amount
of offset (~ like 1/16" one side of the mass), and simply denote the amount of time till the oscillations obviously
disappear on the traces from the normal background noise...this is simply a indication of friction in the pivot. Over time,
the "best" pivot variety would likely become more obvious.
I've mentioned it before; but I'll mention it again...The Sprengnether "offset wire" pivot they used back in the 1950's,
for their horizontal seismometers has been rather a "lost" use pivot. It falls in the torsion aspect as the music wire
slightly bends with boom/mass rotation. Theres NO rods, steel/carbide balls or mirror surface etc., required. Probably
the best visual basic understanding demonstration of it is in John Lahrs website; specifically for the solid ring and
wire shown. If build rigidly, they can literally last "forever". Personally I've never seen one break. The science of
these isn't set in stone; so individual experimentation comes into play....which can be fun to do.
Take care, Meredith Lamb
On Wed, Jun 18, 2008 at 6:39 PM, <ChrisAtUpw@.......
I would like to see a run down pendulum test on this to check it.