In a message dated 22/11/2002, CapAAVSO@....... writes:

> The point-in-a -dimple bottom pivot will always have some stick-slip 
> friction which will interfere with its sensitivity to small earthquake 
> movements. If you replace the point with a ball its rolling friction is 
> less but nevertheless there will still be small residual stick-slip 
> friction. 

Hi there Cap,

     Sorry, but there should be no residual stick slip friction in a rolling 
contact on a perfectly flat surface. Obviously, if you try to roll a ball on 
a rough surface there would be variable resistance, but a surface can be 
polished to optical precision and any resistance reduced below observable 
limits. The axis of rotation with a sphere on a flat is very precisely 
defined. 
  
John Lahr's completely frictionless Sprengnether type wire-under-tension 
pivot is 
> much better than ball bearings and just as easy, if not easier to make, and 
> it stays put rather than slithering around like ball bearings. I use John 
> Lahr's system with 0.008" music wire, obtainable from any music store, and 
> it shows the finest detail in the microseisms if I turn up the gain. 

     Flexing wires and foils have very low friction, but it is not 
identically zero due to changing stresses, metal flow and relaxation over 
time. Single flexing wires and foils do not have an entirely stable hinge 
point with time and temperature. The crossed foils and wires are better in 
this respect. You want the rotation axis to be extremely stable.

However, and this is important, even with frictionless pivots my Lehman is 
unstable 
> over time unless I set its period for about 10-15 seconds. I doubt anyone 
> can do much better with ball bearing pivots unless they are on bed rock and 
> even then I don't see how they could possibly get month's-long stability at 
> 30-40 second periods as has been claimed in recent postings. My guess is 
> what these people actually have are strong-motion detectors rather than the 
> Lehmans they think they have.

     OK, your system is not completely stable over time. Can you tell us why 
this is? If nothing changes, it should be perfectly stable! Do temperature 
changes, or large rates of change of temperature, effect it? Does it react to 
rain? Wind? Frost? Time of day? Do the offsets build up gradually, are they 
sudden or random? Do the mounting screws rest on flat plates glued to the 
base plinth?

     There are many factors which can effect the stability. Look at the Tmax 
design. What do you notice? I noticed that the base was made out of steel 
which is more rigid than Aluminum and more importantly, that it was much 
wider than 'traditional' Lehman designs. Ideally, you should use an 
equilateral triangle, like Sprengnether. The support screws will be steel in 
steel, not steel or brass in Al, so differential expansion movements should 
be less of a problem. 

     If someone is getting better results than me, I ask myself what I am 
doing or not doing differently and what are the limitiations of my equipment 
and my site.
     
     Regards,

     Chris Chapman
In a message dated 22/11/2002, CapAAVSO@....... writes:



The point-in-a -dimple bottom pivot will always have some stick-slip friction which will interfere with its sensitivity to small earthquake movements. If you replace the point with a ball its rolling friction is less but nevertheless there will still be small residual stick-slip friction. 




Hi there Cap,



     Sorry, but there should be no residual stick slip friction in a rolling contact on a perfectly flat surface. Obviously, if you try to roll a ball on a rough surface there would be variable resistance, but a surface can be polished to optical precision and any resistance reduced below observable limits. The axis of rotation with a sphere on a flat is very precisely defined. 

  

John Lahr's completely frictionless Sprengnether type wire-under-tension pivot is 

much better than ball bearings and just as easy, if not easier to make, and it stays put rather than slithering around like ball bearings. I use John Lahr's system with 0.008" music wire, obtainable from any music store, and it shows the finest detail in the microseisms if I turn up the gain. 




     Flexing wires and foils have very low friction, but it is not identically zero due to changing stresses, metal flow and relaxation over time. Single flexing wires and foils do not have an entirely stable hinge point with time and temperature. The crossed foils and wires are better in this respect. You want the rotation axis to be extremely stable.



However, and this is important, even with frictionless pivots my Lehman is unstable 

over time unless I set its period for about 10-15 seconds. I doubt anyone can do much better with ball bearing pivots unless they are on bed rock and even then I don't see how they could possibly get month's-long stability at 30-40 second periods as has been claimed in recent postings. My guess is what these people actually have are strong-motion detectors rather than the Lehmans they think they have.




     OK, your system is not completely stable over time. Can you tell us why this is? If nothing changes, it should be perfectly stable! Do temperature changes, or large rates of change of temperature, effect it? Does it react to rain? Wind? Frost? Time of day? Do the offsets build up gradually, are they sudden or random? Do the mounting screws rest on flat plates glued to the base plinth?



     There are many factors which can effect the stability. Look at the Tmax design. What do you notice? I noticed that the base was made out of steel which is more rigid than Aluminum and more importantly, that it was much wider than 'traditional' Lehman designs. Ideally, you should use an equilateral triangle, like Sprengnether. The support screws will be steel in steel, not steel or brass in Al, so differential expansion movements should be less of a problem. 



     If someone is getting better results than me, I ask myself what I am doing or not doing differently and what are the limitiations of my equipment and my site.

     

     Regards,



     Chris Chapman