PSN-L Email List Message

Subject: Re: My sticking Lehman?
From: tchannel1@............
Date: Fri, 30 May 2008 17:07:26 -0600


Thanks, Chris,
  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Friday, May 30, 2008 4:53 PM
  Subject: Re: My sticking Lehman?


  In a message dated 2008/05/30, tchannel1@............ writes:


    The second problem:  At 20 seconds, I was missing all but large =
earthquakes, those were coming through just fine. Chris suggest the arm =
might be sticking.  I could see nothing which resembled sticking.


  Hi Ted,

         1) what top bearing / flexure are you using?    (The top hinge =
is a SS wire, don't know the dia.  but very thin.)
         2) what bottom bearing / flexure are you using?   (The bottom =
is a ball bearing on sidebyside drill bit shanks.)(this I think could be =
my weakest point, this arrangement may not be good enough)  (I may =
rework this)
         3) have you checked both the coil and the damping magnets for =
whiskers? They tend to grow with time and can cause the beam to stick. I =
use one of the pen lasers to check that the gaps are clear.   Not sure =
what whiskers are............but it is a new machine.


    I don't know if this was the cause and effect, but I fixed it:

    First this is not the best Lehman I have ever built.  Its okay, and =
with little effort I got the 20 seconds, but I am not sure I could get =
22.  However at 20 something was sticking. =20
    I think at 20 it was so near infinity, it was almost flat. At =
infinity it would never move to center, the arm would just stay where =
you placed it. At 20 seconds it would move to center, if the event was =
large enough to overcome the inertia, but smaller earthquakes would not =
do this.


         This is typical of a problem with the suspension system. I =
don't know what you expected to 'see', but you have just given a good =
description of what is observed. I suspect that you will not have fixed =
the problem by shortening the period, just reduced it a bit. The =
stiction, from whatever cause, will still be there.


    I reduce the period to 15 seconds, and it is working great.  I =
record 6.2M Iceland, and 4.1M California.   I could, now increase the =
period to 16,17,18 etc and retest.   I will see how it performs at 15 =
for a while.


         Regards,








Thanks, Chris,
----- Original Message -----
From:=20 ChrisAtUpw@.......
To: psn-l@..............
Sent: Friday, May 30, 2008 4:53 = PM
Subject: Re: My sticking = Lehman?

In a=20 message dated 2008/05/30, tchannel1@............=20 writes:

The second problem:  At 20 seconds, I was missing = all but=20 large earthquakes, those were coming through just fine. Chris = suggest the=20 arm might be sticking.  I could see nothing which resembled=20 sticking.


Hi=20 Ted,

       1) what top bearing / = flexure=20 are you using?    (The top hinge is a SS wire, don't = know the=20 dia.  but very = thin.)
       2)=20 what bottom bearing / flexure are you using?   (The bottom = is a ball=20 bearing on sidebyside drill bit shanks.)(this I think could be my = weakest=20 point, this arrangement may not be good enough)  (I may rework=20 this)
       3) have you checked both = the=20 coil and the damping magnets for whiskers? They tend to grow with time = and can=20 cause the beam to stick. I use one of the pen lasers to check that the = gaps=20 are clear.   Not sure what whiskers are............but it is = a new=20 machine.


I don't know if this was the cause and effect, but I = fixed=20 it:

First this is not the best Lehman I have ever = built. =20 Its okay, and with little effort I got the 20 seconds, but I am not = sure I=20 could get 22.  However at 20 something was sticking.  =
I think at 20 it = was so near=20 infinity, it was almost flat. At infinity it would never move to = center, the=20 arm would just stay where you placed it. At 20 seconds it would move = to=20 center, if the event was large enough to overcome the inertia, but = smaller=20 earthquakes would not do this.

       This is=20 typical of a problem with the suspension system. I don't know what you = expected to 'see', but you have just given a good description of what = is=20 observed. I suspect that you will not have fixed the problem by = shortening the=20 period, just reduced it a bit. The stiction, from whatever cause, will = still=20 be there.

I reduce the period to 15 seconds, and it is working=20 great.  I record 6.2M Iceland, and 4.1M California.   = I=20 could, now increase the period to 16,17,18 etc and = retest.   I=20 will see how it performs at 15 for a while.

      =20 Regards,

       Chris = Chapman=20

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