PSN-L Email List Message
Subject: RE: Another horizontal boom/mast pivot to consider
From: steve hammond shammon1@.............
Date: Sat, 23 Nov 2002 11:05:12 -0800
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.
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
From: Frank Cooper [SMTP:fxc@........
Sent: Friday, November 22, 2002 4:52 PM
Subject: Re: Another horizontal boom/mast pivot to consider
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.
----- Original Message -----
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@.......
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 :-)
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