Hi all,
 Chris Chapman recently stated in a private email that he thought a crossed
rod hinge
suspension would have less friction/dampening than a ball bearing
hinge...and he is
so very right!
 Too visualize a crossed rod hinge: Imagine two spaced rods running up and
down this
page. Now; you introduce a horizontal rod centered across the other two
rods, and this
is the inge suspension rod that you hook up your boom/wire too. The crossed
rod hinge
suspension looks like the capitalized letter "H". You can visually rotate
the assembly
to make the hinge center rod workable for your desired horizontal or
vertical instrument.
Of course, for the hanging pendulum (S-G), the rod assembly is simply place=
d
flat atop
a mast, and the center rod oscillates/rotates atop the two outer rods. For
most horizontal
or vertical seismometers use, the assemblys two rods are placed against the
mast, (you
might need acouple here) and the center rod presses against these two rods
via a boom.
Its possible to use the same hinge for angled horizontal or vertical top of
the mast pivots
also. There is NO gouged or filed slot/s in any rods to hold the position o=
f
the crossed
rod; as that would ruin its lesser friction hinge or suspension purpose.
 In my new view...I'd even go so far as to say; ball bearings
hinges/suspensions are
less ideal for most seismometers as they do have a noted problem with highe=
r
friction
for very small rotational displacements, whereas, crossed rod
hinges/suspensions have
less friction in this critical displacement area. Other hinge designs like
razors, points,
cardans (typical S-G hinge), shims etc., are so bad for contact friction,
torque/material
self dampening, that I'll not even consider or recommend their use again.
 I've ran many hanging pendulum (S-G like), table top/edge tests with a
variety of ball
bearings on various contact surfaces in the last few weeks. The purpose of
the tests
was to estimate the friction of the various test models; via offsetting the
pendulum a
set distance, and simply timing how long the pendulum will continue
oscillating till it
visually quits moving. Ball bearings were visually observed to be rather
consistently
prone to stop in a shorter time where the displacement of the mass got down
to ~1//16"
deflection (from zero) oscillations...and usually stopped in a hour or two.
The
oscillations times with ball bearings ranged from 5 to 6 hours.
 In the last few days, I tried acouple different rod materials in a crossed
rod hinge
with the same general table top/edge test platform. The first model, used 3
rods of what
I believe is grade 304 stainless steel 1/4" diameter rods. That oscillation=
s
test ran on
for ~8 hours. The second model used two drill rod shanks (clean round end),
with the
same stainless steel rod hinge across them, and that ran for ~7.5 hours.
Small mass
displacements decay oscillations on the descending order of 1/16", can go o=
n
for
several hours thereafter till the mass stops. While there is alot of
different material
that could be tested; I think its very obvious from just these 2 tests, thi=
s
this specific
type hinge is very much the better choice.
 There is another hinge suspension, that has yielded longer oscillations
decay times
that I've worked with, and that is the Zero Torque Suspension. Those models
on the
same table top/edge tests, gave a range of 10-11 hours. However, I think
amateurs
will find that crossed rod hinge/suspensions will be easier to work with,
and its more
adoptable for *all* hinge situations like on a typical horizontal or
vertical seismometer,
whereas a zero torque suspension might reasonably only be good for a hangin=
g
pendulum (S-G).
 Credit is given to Chris Chapman for suggesting trials of these various
suspensions
and guidance!
 Take care, Meredith Lamb
Hi all,
 
Chris Chapman recently stated in a private email that he thought a cro=
ssed rod hinge
suspension would have less friction/dampening than a ball bearing hing=
e...and he is
so very right!
 
Too visualize a crossed rod hinge:  Imagine two spaced rods runni=
ng up and down this
page.  Now; you introduce a horizontal rod centered across t=
he other two rods, and this
is the inge suspension rod that you hook up your boom/wire too.  =
The crossed rod hinge
suspension looks like the capitalized letter "H".  You =
can visually rotate the assembly
to make the hinge center rod workable for your desired horizontal or v=
ertical instrument.
Of course, for the hanging pendulum (S-G), the rod assembly is simply =
placed flat atop
a mast, and the center rod oscillates/rotates atop the two outer =
rods.  For most horizontal
or vertical seismometers use, the assemblys two rods are placed agains=
t the mast, (you
might need acouple here) and the center rod presses against these two =
rods via a boom.
Its possible to use the same hinge for angled horizontal or vertical t=
op of the mast pivots
also.  There is NO gouged or filed slot/s in any rods to hold the=
 position of the crossed
rod; as that would ruin its lesser friction hinge or suspension p=
urpose.
 
In my new view...I'd even go so far as to say; ball bearings hinges/su=
spensions are
less ideal for most seismometers as they do have a noted problem =
with higher friction
for very small rotational displacements, whereas, crossed rod hinges/s=
uspensions have
less friction in this critical displacement area.  Other hin=
ge designs like razors, points,
cardans (typical S-G hinge), shims etc., are so bad for contact fricti=
on, torque/material
self dampening, that I'll not even consider or recommend their use aga=
in.
 
I've ran many hanging pendulum (S-G like), table top/edge tests with a=
 variety of ball
bearings on various contact surfaces in the last few weeks. =
 The purpose of the tests
was to estimate the friction of the various test models; via offsettin=
g the pendulum a
set distance, and simply timing how long the pendulum will continue os=
cillating till it
visually quits moving.  Ball bearings were visually observed to b=
e rather consistently
prone to stop in a shorter time where the displacement of the mass got=
 down to ~1//16"
deflection (from zero) oscillations...and usually stopped in a hour or=
 two. The
oscillations times with ball bearings ranged from 5 to 6 hours.
 
In the last few days, I tried acouple different rod materials in a cro=
ssed rod hinge
with the same general table top/edge test platform.  The first mo=
del, used 3 rods of what
I believe is grade 304 stainless steel 1/4" diameter rods.  =
That oscillations test ran on
for ~8 hours.  The second model used two drill rod shanks (clean =
round end), with the
same stainless steel rod hinge across them, and that ran for ~7.5 hour=
s.  Small mass
displacements decay oscillations on the descending order of 1/16"=
, can go on for
several hours thereafter till the mass stops.  While there is alo=
t of different material
that could be tested; I think its very obvious from just these 2 tests=
, this this specific
type hinge is very much the better choice.
 
There is another hinge suspension, that has yielded longer oscillation=
s decay times
that I've worked with, and that is the Zero Torque Suspension.  T=
hose models on the
same table top/edge tests, gave a range of 10-11 hours.  However,=
 I think amateurs
will find that crossed rod hinge/suspensions will be easier to work wi=
th, and its more
adoptable for all hinge situations like on a typical =
horizontal or vertical seismometer,
whereas a zero torque suspension might reasonably only be good for a h=
anging
pendulum (S-G).
 
Credit is given to Chris Chapman for suggesting trials of these variou=
s suspensions
and guidance!
 
Take care, Meredith Lamb