PSN-L Email List Message

Subject: Re: Pivots Re: Increasing the period of a Lehman seismometer ?
From: "Robert O. Green" rog@..........
Date: Wed, 13 May 2009 20:34:32 -0700


Thank you.  This does help.  You are very kind for sharing your 
expertise and experience.

I will be trying to follow in you foot steps for my Lehman.


ChrisAtUpw@....... wrote:
> In a message dated 13/05/2009, rog@.......... writes:
>     Chris,
>     Do you have any close up pictures of your upper and lower pivots?
>     Rob
> Hi Rob,
>     Sorry, but I don't have any other photos.
>     The bottom bearing is a 1/2" OD SS ball bearing crimped in a 1/2" 
> hole drilled into the upright bar. The arm is 1/2" HT30 Aluminum 
> alloy, turned flat on the end. A rectangular SS faceplate about 10mm x 
> 8mm and 16 thou thick is stuck onto this using 2 component acrylic 
> adhesive Holdtite ST3295. The faceplate was cut from a Swann Morton 
> medical scalpel blade using a 1" carbide disk. Other brands of 2 
> component acrylic adhesive are available. Unlike the brittle epoxies, 
> acrylic adhesives are slightly flexible, very tough and stick the 
> softer metals quite well.
>     The top suspension is a short 8 thou OD nickel plated piano wire 
> clamped between two bronze disks with an eccentric centre hole. The 
> inside faces of these disks are both recessed about 15 thou so that 
> only the outside 1.5mm rings grip the wire. The wire is threaded 
> through a hole in the centre bolt. Both eccentric disks can be rotated 
> to vary the position of the wire clamp.
>     Behind the disks is the 'wire wind on' SS bolt for trimming the 
> position of the arm. It is held in position by friction at the base 
> using a spring washer and a SS nut. The wire wraps into the bolt 
> thread, so that the wind-on position is constant. The bolt is drilled 
> near the top. The wire is threaded through this hole and is clamped by 
> the top nut. I used a taper reamer to put a small flat on the tip of 
> the first thread in this nut, so that the wire is clamped between this 
> flat and the V of the bolt thread. To trim the position of the mass, 
> you slack off the bolt clamping the bronze washers and adjust the 
> 'wind on' bolt. Then you clamp the bronze washers again.
>     The V suspension is 30lb 7 core SS fishing trace with crimped end 
> loops fitting in V section rings on the 1/2" OD extension rods bolted 
> to the square mounting plate. The top fitting is a 1" and a 1.5" OD SS 
> mudguard washers stuck together with acrylic adhesive. The 1.5" disk 
> has a V section edge to hold the trace wire. I turned this groove, but 
> you can use a triangular needle file. The washer is 1/16" thick. The 4 
> mm SS wire clamp bolt is suitably drilled, the wire is threaded 
> through and clamped between two flat SS washers. This V suspension 
> prevents the mass from rotating about it's long axis due to any off 
> centre forces from the damping blade.
>     The mass should be prevented from rotating / oscillating around 
> the long axis of the arm. This motion may not be damped on amateur 
> seismometers and can give rise to serious resonances at a few Hz, 
> particularly with single wire top suspensions. Unless the axis of the 
> damping force intersects the perpendicular from the centre of mass to 
> the swing axis, any earth motion will try to rotate the mass as well 
> as deflecting it from side to side. This null is difficult to achieve 
> in practice. You can use a V wire or a twin tube suspension. You can 
> also design the magnetic damping to control both rotational and 
> translational motions of the mass - in two directions at right angles.
>     You need to make the arm and the mass from non magnetic materials. 
> Never mount a magnet on the arm. Never use a ball rolling on a plane - 
> they slip too easily. Choose the position of the top support so that 
> the bottom 'bearing' has an almost zero vertical loading.
>     Designing out problems / unwanted responses is something of an art!
>     I hope that this helps?
>     Regards,
>     Chris Chapman
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