You might consider a marble slab for the base of the instrument. That is
what I used. You can generally find scrap pieces at a stone dealer. Mine
is 18"x36"x1 1/2" white marble. It was easy to work with. You can grill it
with regular masonry bits and it has good mass. Best of all it was not
expensive -- about $30.
http://24.116.175.108/idahopsn1.htm then click on "My site for pictures"
Jan Marshall
jandmarshall@............
http://myweb.cableone.net/jandmarshall/
Nampa, ID
-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@.................
Behalf Of ChrisAtUpw@.......
Sent: Saturday, March 08, 2003 3:22 PM
To: psn-l@..............
Subject: Re: Reinforced concrete base for Lehman instrument
In a message dated 08/03/03, beezaur@.......... writes:
> temperature changes. I would be most surprised if 1:1 sand and cement
> was not strong enough in practice, . . .
> Have you considered making a T base frame out of say 1.5" black
> steel angle? . . .
> Regards,
> Chris Chapman
Chris,
Thank you.
I think you are right about the grout mix being strong on its own. It
isn't a trick to get regular concrete to 6000 psi compressive strength
(garden variety being 2000-3000 psi), and in fact the stone is what usually
breaks first in the really high strength stuff. I still worry about
brittleness - this device will have to survive a couple of moves in the next
few years. How much water is typically used?
Unless you drop it or hit it with a hammer, I would not expect
brittleness to be a serious problem. I don't know about the water. It needs
to be saturated. I used an eccentric dowel on an electric drill to agitate
the mix and get the air out. It has a surface which is just liquid.
Since rebar is a problem in concrete for the base, I assume I don't have
to ask about pouring the pier over 8 ft R/C pilings to better couple with
more stable ground at depth. It sounds to me like the soil/piling interface
would have the same noise problems as rebar/concrete. But please correct me
if that technique has proven reliable.
I do not know. The moisture variations could be considerable, but
the temperature variations are likely to be small. You also have tilt to
consider. This may be a limiting factor in the period you can use. I would
place a plinth on the earth / subsoil clear of any piles.
Steel or aluminum is "plan B" for the base of the instrument. The
support for the boom will be a metal tripod, bolted to the base. I would
like to keep the natural (resonant) frequency of that structure very high,
kHz if possible. I want to put my pickup and damping hardware on the same
base as the boom, hence the 40 in length. I hadn't expected a "T" to get me
there, but the design you describe may well be stiff enough, especially with
channel or square structural tubing.
I suggest that you stick to steel. It is much more rigid. A
completely rectangular design is the easiest to construct. A simple design
with the least problems is more likely to get built! You can fit 6 to 8"
lengths of flat plate between the horizontal and vertical parallels to
greatly increase the rigidity and provide a pan for mounting sensors or
damping devices. You could use 1" L to brace the top of the U hoop to the
end of the T arm. Could one of the drive on steel ramps sold for car / truck
maintenance be converted - they come ready welded? Just a wild idea!
Another thought: has anyone considered using carbon arrow shafts or
kevlar / spectra bowstring for boom components? They are pretty cheap now,
and their strength-to-weight ratios give them very good vibration
characteristics, and some bowstring absolutely does not creep.
Carbon could be OK, but how do you plan to make the fittings? Metal
is easier to fabricate and fit. You can easily get nickel plated piano wire
down to 8 thou. from a music shop, it does not have any 'bending' memory and
it works just fine. See http://www.daddario.com/ Suggest that you forget
kevlar or other plastics. A ball bearing in the end of the arm gives an
excellent bottom suspension up against a stainless steel razor blade glued
flat onto the crossbar.
Regards,
Chris Chapman
You=20
might consider a marble slab for the base of the instrument. That =
is what=20
I used. You can generally find scrap pieces at a stone =
dealer. Mine=20
is 18"x36"x1 1/2" white marble. It was easy to work with. =
You can=20
grill it with regular masonry bits and it has good mass. Best of =
all it=20
was not expensive -- about $30.
http://24.116.175.108/idahopsn1.htm then click=20
on "My site for pictures"
Jan Marshall
jandmarshall@............
http://myweb.cableone.net=
/jandmarshall/
Nampa, ID
-----Original Message-----
From: =
psn-l-request@.................
[mailto:psn-l-request@...............On Behalf Of=20
ChrisAtUpw@.......
Sent: Saturday, March 08, 2003 3:22=20
PM
To: psn-l@..............
Subject: Re: =
Reinforced=20
concrete base for Lehman instrument
In a message dated 08/03/03,=20
beezaur@.......... writes:
> temperature changes. I would be most surprised if =
1:1 sand=20
and cement
> was not strong enough in practice, . . . =
>=20
Have you considered making a T =
base=20
frame out of say 1.5" black
> steel angle? . . .
>=20
Regards,
>=20
Chris Chapman
Chris,=20
Thank you.
I think you are right about the grout mix =
being=20
strong on its own. It isn't a trick to get regular concrete to 6000 =
psi=20
compressive strength (garden variety being 2000-3000 psi), and in =
fact the=20
stone is what usually breaks first in the really high strength =
stuff.=20
I still worry about brittleness - this device will have to =
survive a=20
couple of moves in the next few years. How much water is typically=20
used?
Unless =
you drop=20
it or hit it with a hammer, I would not expect brittleness to be a =
serious=20
problem. I don't know about the water. It needs to be saturated. I =
used an=20
eccentric dowel on an electric drill to agitate the mix and get the =
air out.=20
It has a surface which is just liquid.
Since rebar is a problem in concrete for the base, I =
assume I=20
don't have to ask about pouring the pier over 8 ft R/C pilings to =
better=20
couple with more stable ground at depth. It sounds to me like =
the=20
soil/piling interface would have the same noise problems as =
rebar/concrete.=20
But please correct me if that technique has proven=20
reliable.
I do =
not=20
know. The moisture variations could be considerable, but the =
temperature=20
variations are likely to be small. You also have tilt to consider. =
This may be=20
a limiting factor in the period you can use. I would place a plinth on =
the=20
earth / subsoil clear of any piles.
Steel or aluminum is "plan B" for the base of the =
instrument.=20
The support for the boom will be a metal tripod, bolted to the base. =
I would=20
like to keep the natural (resonant) frequency of that structure very =
high,=20
kHz if possible. I want to put my pickup and damping hardware on the =
same=20
base as the boom, hence the 40 in length. I hadn't expected a "T" to =
get me=20
there, but the design you describe may well be stiff enough, =
especially with=20
channel or square structural=20
tubing.
I =
suggest that=20
you stick to steel. It is much more rigid. A completely rectangular=20
design is the easiest to construct. A simple design with the =
least=20
problems is more likely to get built! You can fit 6 to 8" lengths of =
flat=20
plate between the horizontal and vertical parallels to greatly =
increase the=20
rigidity and provide a pan for mounting sensors or damping devices. =
You could=20
use 1" L to brace the top of the U hoop to the end of the T arm. Could =
one of=20
the drive on steel ramps sold for car / truck maintenance be converted =
- they=20
come ready welded? Just a wild idea!
Another thought: has anyone considered using carbon =
arrow shafts=20
or kevlar / spectra bowstring for boom components? They are pretty =
cheap=20
now, and their strength-to-weight ratios give them very good =
vibration=20
characteristics, and some bowstring absolutely does not =
creep.
Carbon =
could be OK,=20
but how do you plan to make the fittings? Metal is easier to fabricate =
and=20
fit. You can easily get nickel plated piano wire down to 8 thou. from =
a music=20
shop, it does not have any 'bending' memory and it works just fine. =
See=20
http://www.daddario.com/ Suggest that you forget kevlar or other =
plastics. A=20
ball bearing in the end of the arm gives an excellent bottom =
suspension up=20
against a stainless steel razor blade glued flat onto the crossbar.=20
Regards,=20
Chris Chapman=20