## PSN-L Email List Message

Subject: Re: folded pendulum
From: ChrisAtUpw@.......
Date: Sat, 30 Jun 2007 14:36:42 EDT

```In a message dated 2007/06/30, tchannel1@............ writes:

> Chris,    This may be hard to answer. In this case, the mass would only be
> an amount required to do the job.
> I have read that some people use very little mass, others a lot. On a
> vertical and on a horz one gets the idea of how much to use. This a bit different,
> and here is where I could use some direction.
>
> 1  If I added no mass, simply the connecting arm, made of the same material
> as the pendulums, and the same length or maybe longer. This arm is the mass,
> and as such is not much of a stress on the SS? shims used as hinges.
> My mock up was made like this. Then I added a large mass, everything seemed
> to work the same. But using a large mass would  put a lot of strain on the
> hinges.

Hi Ted,

If you had a very light central arm, the period of the end pendulums
would be reduced to about 1/4 - the CofG of a vertical bar is half way along
it. Actually, it is the radius of gyration about the end of the bar which is the
critical factor.
You need a mass over 10x the mass of the vertical arms. I would design
for something between 1 and 5 Lbs, depending on the vertical weights. This
hinge shims should cope with this quite easily.

> 2  About the only advantage I can see with a large mass is that air
> currents would be less likely to move it.

A large mass is less effected by air currents, but you will still see
them. You will need an enclosure. This is another reason why I was suggesting
sheet Al side plates. They would tend to conduct heat quite well and would
minimise any internal temperature variations.

>  3 It would be easy to construct it either way,  Perhaps no addition mass
> is too little, and an amount which stressed the hinges is too much?  Is 1/2
> that amount proper? I understand your suggest about the 5% trim weight.

1 Lbf/sq in = 6895 Pa. The strength of chromenickel steel is listed as
1000 to 1500 Mega Pa. Taking the lower figure and assuming that you are using
2 thou x 1" total width hinges gives a max load of about 290 lbs, so a 29 lbs
working load should be OK. You are unlikely to have a load problem.

Before WWII, seismometers often had to move high ratio mechanical pen
arms, with gains up to over 100, so they needed a large mass to overcome
friction. This heavy thinking sort of 'carried over' into amateur equipment.
Some of the early Lehman equipment used a solid steel arm, so you
needed an end mass which was large compared to this to get a long period. A 1/2"
steel bar weighs ~0.665 Lb per foot. 3ft weighs ~2 Lbs, so an end mass of 10
lbs is not excessive.
I use 1/2" nominal = 15mm OD SS water pipe, which weighs 0.143 Lb per
foot, so a 2.2 lb end mass is fine. The large pipe diameter makes it
sufficiently rigid.

Regards,

Chris Chapman
In a me=
ssage dated 2007/06/30, tchannel1@............ writes:

Chris,    This m=
ay be hard to answer. In this case, the mass would only be an amount require=
d to do the job.
I have read that some people use very little mass, others a lot. On a verti=
cal and on a horz one gets the idea of how much to use. This a bit different=
, and here is where I could use some direction.

1  If I added no mass, simply the connecting arm, made of the same mat=
erial as the pendulums, and the same length or maybe longer. This arm is the=
mass, and as such is not much of a stress on the SS? shims used as hinges.<=
/FONT>=

My mock up was made like this. Then I added a large mass, everything seemed=
to work the same. But using a large mass would  put a lot of strain on=
the hinges.

Hi Ted,

If you had a very light central arm, th=
e period of the end pendulums would be reduced to about 1/4 - the CofG of a=20=
vertical bar is half way along it. Actually, it is the radius of gyration ab=
out the end of the bar which is the critical factor.
You need a mass over 10x the mass of th=
e vertical arms. I would design for something between 1 and 5 Lbs, depending=
on the vertical weights. This hinge shims should cope with this quite easil=
y.

ge I can see with a large mass is that air currents would be less likely to=20=
move it.

A large mass is less effected by air c=
urrents, but you will still see them. You will need an enclosure. This is an=
other reason why I was suggesting sheet Al side plates. They would tend to c=
onduct heat quite well and would minimise any internal temperature variation=
s.

3 It would be easy to constru=
ct it either way,  Perhaps no addition mass is too little, and an amoun=
t which stressed the hinges is too much?  Is 1/2 that amount proper? I=20=

1 Lbf/sq in =3D 6895 Pa. The strength o=
f chromenickel steel is listed as 1000 to 1500 Mega Pa. Taking the lower fig=
ure and assuming that you are using 2 thou x 1" total width hinges gives a m=
ax load of about 290 lbs, so a 29 lbs working load should be OK. You are unl=
ikely to have a load problem.

Before WWII, seismometers often had to=20=
move high ratio mechanical pen arms, with gains up to over 100, so they need=
ed a large mass to overcome friction. This heavy thinking sort of 'carried o=
ver' into amateur equipment.
Some of the early Lehman equipment used=
a solid steel arm, so you needed an end mass which was large compared to th=
is to get a long period. A 1/2" steel bar weighs ~0.665 Lb per foot. 3ft wei=
ghs ~2 Lbs, so an end mass of 10 lbs is not excessive.
I use 1/2" nominal =3D 15mm OD SS water=
pipe, which weighs 0.143 Lb per foot, so a 2.2 lb end mass is fine. The lar=
ge pipe diameter makes it sufficiently rigid.

Regards,

Chris Chapman
```