Chris,

Thank you kindly for your advice.  Do you have pictures of your set up 
pivot and top suspension?

Rob

ChrisAtUpw@....... wrote:
> In a message dated 09/04/2009, rog@.......... writes:
>
>     Is a scalpel blade against a steel ball bearing the current best
>     consensus for a Lehman pivot?
>     rob
>
> Hi Rob,
>     Crossed cylinders can be a bit better. The surfaces need to highly 
> polished. The SEP uses 1/8" OD polished Tungsten Carbide needle 
> rollers, but these may be rather expensive / difficult to obtain 
> depending on where you live. See 
> http://www.bgs.ac.uk/education/school_seismology/seismometer.html You 
> mount both the vertical rollers on the frame.
>     However, a polished SS plane rolling on a SS ball bearing works 
> perfectly well and it may cost only 50 c! See http://www.smallparts.com/
>     I use a 1/2" SS bearing and a large scalpel blade 'trimmed' oval 
> with a carbide disk and glued to the arm with two part acrylic 
> adhesive. You can use bearings down to 1/4" OK, but mount them on the 
> vertical frame, NOT on the arm! My top suspension is an 8 thou music 
> wire. I can set the period to 60 seconds OK and it works for some 
> days, but it has a significant tilt drift. My 'normal' set period is 
> 20 seconds. I measured +/-2mm drift over a fortnight with this, in a 
> max drift range of +/-10mm.
>     The two 'suspensions' which are very likey to give trouble are the 
> knife edge on a plane and a point in a cup. The materials in contact 
> are loaded close to or over the maximum allowed stress. I advise 
> against trying to use either of these types.
>     It may be beneficial to attach the top support ~3/4 along the arm 
> as shown above. The balance between the mass and the weight of the arm 
> itself can be set to minimise the vertical load on the bottom bearing. 
> This improves the stability.
>     Regards,
>     Chris Chapman
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