Hi all,
I've been using the pin-in-cup design for some time now and must change the 
pin about 2 times each year. Over time, the pin has a tendency to get 
mashed a bit and the performance drops way off. A large local event can 
damage it beyond use. I perform maintenance by changing the pin and upper 
piano wire about every three months. I clean the spiders out of the box 
because it is outside on a cement pad, change the wire, and level 
everything in about one hour. I live near the sea and if I don't change the 
wire, the wire rusts and eventually will snap under tension. Usually in 
less than 6 months.  If you would like to see this site here is the URL.
http://pw2.netcom.com/~shammon1/AptosStn.htm

When the pin is nice and sharp, I can set the period as long as 28+ 
seconds. Based on the formula provided by the late Sean-Thomas Morrissey, 
the angle of the 30-cm boom would be between .05 and .1 degree at this 
setting. The real issue when running at this length period is stability. 
Any change in the site will cause the boom to go all the way to the stop 
over night. A good example of this is the first rain following summer. Rain 
water will also dramatically change the site response as well as the 
leveling of the device. Site response has a  major impact on the 
sensitivity of the device. The background site response noise level rises 
as much as 50% when rain water is present. I have seen this on all three 
sites I've maintained for more than 2 years. For example, in San Jose,  I 
had a light-rail train run at the end of my block for several years. Each 
year I took sample of the site frequency response from the passing train. 
The train had 8-sets of wheels and the length of the train was 168 feet 
with two cars. The train rain past the end of my block at 40 - 45 MPH.  At 
the end of summer, an FFT of the sample data yielded scattered resulting 
frequencies showing several dominate frequencies below 12Hrz. Following the 
first rain, the only dominate frequency was 11-12hrz. The water in the 
ground made a dramatic difference. I still have several example files and 
was planning on publishing the results of my study on my website. My 
hypothesis was the rain would increase site response therefor the site 
would have differing results in summer and winter.  While I don't have a 
light-rail train where I live in Aptos, I do have a real Southern Pacific 
train pass twice a day less than a block away at speeds less than 5-10 MPH. 
Now that it has rained for the first time, I will be looking at past and 
current data to comparing it to see if I can show the same results with the 
SP train as I saw with the light-rail.  Both of my Lehman's went to the 
stops following the first rain in the Bay area about a week ago as they did 
in the three other locations.

Regards, Steve Hammond  PSN San Jose,  Aptos California

-----Original Message-----
From:	Frank Cooper [SMTP:fxc@........
Sent:	Friday, November 22, 2002 4:52 PM
To:	psn-l@..............
Subject:	Re: Another horizontal boom/mast pivot to consider

Hello Cap,
You write, "the wild claims you guys have made that are not in keeping with 
what others have said over the years on the PSN."  I am not aware of 
codified PSN doctrine.  Should we heretics kneel in humble submission and 
admit our sins and ask for forgiveness?  I vote for the scientific method.

And you write, "You say, My only suggestion is to try it --- you might like 
it and become "one of these people". Well, I would, Frank, if I thought it 
would work but I already have a frictionless pivot that is better than John 
Cole's and no way can I set my Lehman's period at >20 seconds and have it 
stay there. I am not convinced the ball bearings will do what you say they 
will. I am not one to buy snake oil :-)"   I thought your attitude died 
with the middle ages.  Most of us accept evidence over "I am convinced."

And you write, "But good as they are, they are not as good as John Lahr's 
modified Sprengnether pivot that has no friction at all and can't slither 
and is simpler and easy to make. "  I do not have any experience with John 
Lahr's modified Sprengnether pivot and therefore withhold judgment. I have 
a lot of respect for John Lahr and I think he would have been the first to 
withhold judgment about the ball bearing method until he had all the facts 
which you lack.

Regards,
Frank


  ----- Original Message -----
  From: CapAAVSO@.......
  To: psn-l@..............
  Sent: Friday, November 22, 2002 3:54 PM
  Subject: Re: Another horizontal boom/mast pivot to consider


  In a message dated 11/22/02 7:49:11 PM GMT Standard Time, fxc@....... 
writes:



    OK Cap as a member of the "these people" or the "they" ball bearing 
party, I'll bite,
    I'll respond even though I think your purpose is to have a little fun 
and stir the pot a bit.


  Well, yes, Frank. As a matter of fact I did mean to maybe "have some fun" 
as you say, stirring the pot a bit and hopefully get some discussion going 
concerning some the wild claims you guys have made that are not in keeping 
with what others have said over the years on the PSN. First of all I'm sure 
most will agree John Cole's ball bearings have less friction than a simple 
point-in-a-dimple pivot. But good as they are, they are not as good as John 
Lahr's modified Sprengnether pivot that has no friction at all and can't 
slither and is simpler and easy to make. And what does pivot friction have 
to do with long term stability anyway? Recent postings have been about the 
pier and the soil it's on and how soil moisture and temperature and nearby 
traffic affect stability of the period. Nobody even mentioned the pivots 
until you guys came along claiming to solve all these problems with ball 
bearings. It amazes me that you critize a method with which you have no 
experience? What are the mechanical principles and engineering evidence 
that pivot friction is the reason Lehmans are unstable? You say: "My only 
suggestion is to try it --- you might like it and become "one of these 
people". Well, I would, Frank, if I thought it would work but I already 
have a frictionless pivot that is better than John Cole's and no way can I 
set my Lehman's period at >20 seconds and have it stay there. I am not 
conviced the ball bearings will do what you say they will. I am not one to 
buy snake oil :-)

  Best regards,
  Cap

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