In a message dated 11/21/02 8:01:16 AM GMT Standard Time, 
meredithlamb@............. writes:


> With all the discussion on a horizontal boom/mast pivot, it
> might be alot simpler to use/make another type mentioned in
> the past emails.  Its based on the Sprengnether horizontals
> general design and has been "revised" for use by John Lahr
> on his horizontal; but it essentially is the same type pivot.
> .............I think its one of the best pivot innovations I know of,
> and most ingenious of John to come up with the variation.
> Theres probably about zero long term problems with this
> conpared to other pivot designs.  Its frictionless as
> John describes in his text.

Hi Meridith

You don't have to be an engineer to realize friction in the lower pivot of a 
Lehman will decrease its sensitivity. Lehman I'm sure was aware of that but 
he was designing a simple instrument for amateurs that was good enough to get 
started. 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. 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. 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.

Best regards,
Cap
In a message dated 11/21/02 8:01:16 AM GMT Standard Time, meredithlamb@............. writes:





With all the discussion on a horizontal boom/mast pivot, it

might be alot simpler to use/make another type mentioned in

the past emails.  Its based on the Sprengnether horizontals

general design and has been "revised" for use by John Lahr

on his horizontal; but it essentially is the same type pivot.

..............I think its one of the best pivot innovations I know of,

and most ingenious of John to come up with the variation.

Theres probably about zero long term problems with this

conpared to other pivot designs.  Its frictionless as

John describes in his text.




Hi Meridith



You don't have to be an engineer to realize friction in the lower pivot of a Lehman will decrease its sensitivity. Lehman I'm sure was aware of that but he was designing a simple instrument for amateurs that was good enough to get started. 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. 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. 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.



Best regards,

Cap