PSN-L Email List Message

Subject: Re: boom angle of horiz seis
From: "steve hammond" shammon1@.............
Date: Tue, 15 Feb 2000 20:36:49 -0800

Jack, Sean, Dewayne, Ted,
I have to agree with Jack however in reading between the lines, Sean, I
think you said the same thing I'm about to say. This is how I view this
problem. Let the upper pivot point be point A, the lower pivot point be B,
and the point that the upper guide wire intersects the boom be point C.
Align A-B to be vertical at 0 degrees. Adjust the length of A-C so the boom
is at a 90 degree angel to A-B. This is the infinite critical point and the
boom wanders. If I were to swing the boom left to right for 150-degrees the
boom would remain at 90 degrees to A-B because A-C is constant and the
length remains unchanged. To create the pendulum effect,  I must increase
the angel between A and B, let's say  by tilting the base 1-degree. One
result is that point C is fixed in relation to points A and B and has
nothing to do with the period. Points A and B control the period. If I swing
the boom 150-degrees again, the relationship of A-B and B-C remain constant.
The period is then measured in the time it take to make one full cycle. The
pendulum effect is created by the angel between A-B not the dip in C.  I can
prove this to you. If I use a bubble level and a turnbuckle between points
A-C and level my boom in relation to the ground, my angel in relation to A-B
is now 89-degrees to compensate for the 1-degree tilt however the period
remains constant and is unchanged regardless of where I position point C.
So you say,  why does it work better for some systems to adjust the boom
slightly lower as in Ted's case. Well, I happen to know Ted, and his system
(the last time I checked) uses a knife edge on his boom. It is the critical
contact point and if the stop block is perfectly aligned with the boom then
the angel between A-B and B-C is 90-degrees making the knife edge to block
align at 0/0.  I use a point and a cup in my system and have found that mine
works better when the boom is set level using a bubble level.
My 2C worth.
Regards, Steve Hammond PSN Aptos, California
-----Original Message-----
From: Jack Sandgathe 
To: PSN-L Mailing List 
Date: Tuesday, February 15, 2000 4:57 PM
Subject: Re: boom angle of horiz seis

>Sean Thomas:
>        For me to offer a correction to you sir, is like trying to correst
>Isaac Newton or someone, but here goes.  I think you meant to refer to the
>hinges being vertical rather than the boom being horizontal.
>        respectfully, Jack Sandgathe
>At 01:59 PM 2/15/00 -0600, you wrote:
>>........IF the boom is exactly horizontal, it will experience NO
>gravitational restoring
>>force, so it will swing back and forth aimlessly. If the mass end is
>lowered slightly, it will >swing in a shallow curve in the gravitational
>field, with the minimum being at the bottom or center >of the swing.
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