In a message dated 17/07/2009, paleoartifact@......... writes:

Yes; the commercial available springs are a problem (unless one is  
satisfied with a 1 to ~ 6 sec period vertical); and/or use a form of  feedback.  
Even with a zero length spring (such as in my old  Sprengnether), temperature 
changes made it tough to keep roughly centered  within a series of 3 ~ 
enclosures with a set period of 15  seconds.

Hi Meredith,
 
    That is why Sprengnethers were often fitted with a  U made from a 
bimetal sandwich and a mass. You could trim out the drift error.  They were 
designed for periods of 30 seconds, but were commonly used at 15  seconds.

Just noted a interesting web article this morning on the Physics Forums  
web site about "Designing and making springs with music wire" (Circa May-June  
2009).  Its at:
 
_http://www.physicsforums.com/showthread.php?t=315723_ 
(http://www.physicsforums.com/showthread.php?t=315723) 
 
Although the wire was twisted prior to spring forming; it  doesn't really 
sound like a "real" zero length spring too  me....but....such might be ~ 
better ~ than a common hardware  spring.  I don't have the expertise to evaluate 
the spring  discussion technical implications to be frank.  

    Thanks.

    You can get a true zero length spring this way. The  problem with 
winding them, is that this is quite a high initial tension. If you  put on too 
much tension, the spring ties itself in knots as you take it off the  mandrel 
! Common hardware springs have a low initial tension, but I don't know  
about the long springs that are used for sliding doors.
 
    Regards,
 
    Chris





In a message dated 17/07/2009, paleoartifact@......... writes:

  
Yes; the commercial available springs are a problem (unless one is=
 
  satisfied with a 1 to ~ 6 sec period vertical); and/or use a form=
 of 
  feedback.  Even with a zero length spring (such as in my old 
  Sprengnether), temperature changes made it tough to keep roughly centere=
d 
  within a series of 3 ~ enclosures with a set period of 15 
  seconds.
Hi Meredith,
 
    That is why Sprengnethers were often fitted=
 with a 
U made from a bimetal sandwich and a mass. You could trim out the drift er=
ror. 
They were designed for periods of 30 seconds, but were commonly used at 15=
 
seconds.

  
Just noted a interesting web article this morning on the Physics Fo=
rums 
  web site about "Designing and making springs with music wire" (Circa May=
-June 
  2009).  Its at:
  
 
  
http://www.physicsforums.com/showthread.php?t=3D315723

  
 
  
Although the wire was twisted prior to spring forming; it 
  doesn't really sound like a "real" zero length spring too 
  me....but....such might be ~ better ~ than a common hardware 
  spring.  I don't have the expertise to evaluate the sprin=
g 
  discussion technical implications to be frank.  
    Thanks.

    You can get a true zero length spring this wa=
y. The 
problem with winding them, is that this is quite a high initial tension.=
 If you 
put on too much tension, the spring ties itself in knots as you take it of=
f the 
mandrel ! Common hardware springs have a low initial tension, but I don't=
 know 
about the long springs that are used for sliding doors.
 
    Regards,
 
    Chris