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_ (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/_ (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 ChapmanIn a message dated 09/04/2009, rog@.......... writes:Is a scalpel blade against a steel ball bearing the current best consensus fo= r a Lehman pivot?
robHi Rob,Crossed cylinders can be a bit bett= er. The surfaces need to highly polished. The SEP uses 1/8" OD polished Tungst= en 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&nbs= p;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 rang= e 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 allo= wed stress. I advise against trying to use either of these types.It may be beneficial to attach the top suppor= t ~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 bea= ring. This improves the stability.Regards,Chris Chapman