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 SandgatheTo: 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: >>Dewayne, >>........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. >> > > >_____________________________________________________________________ > >Public Seismic Network Mailing List (PSN-L) > >To leave this list email listserver@.............. with the body of the >message: leave PSN-L _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>