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.


2  About the only advanta= 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= understand your suggest about the 5% trim weight.


       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

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