In a message dated 2008/04/02, kwyatt@............. writes:

> If you look at the three leveling screws on the AS-1 seismometer at 
> http://www.amateurseismologist.com/, you see that there are two screws on the end 
> with the vertical support and one screw on the end near the coil.  It is 
> obvious why only three screws are used.  But is their a reason for putting the 
> single screw near the coil and not near the vertical support.  Also, wouldn't 
> there be greater stability if the single screw was actually a fixed pin, leaving 
> the other two to be adjustable screws?  
>  
> Just wondering before I drill the base in my next vertical seismometer.

Hi Kay,

       It shouldn't matter, but it is generally easier to put the lateral 
adjustment screws either side of the vertical support column and the longitudinal 
adjustment at the end of the arm. You have 'free' space here. 
       I suggest the you make all three mounting screws of identical 
construction. If you have one peg leg you will have the problem of matching it's 
expansion with the other two. This is not too important for vertical instruments, 
but it is very important on a Lehman.
       Also, you generally get presented with a site of unknown slope 
direction. Being able to adjust all three screws is an advantage.
       I use a fairly thick baseplate and stainless steel bolts, nuts and 
washers. I drill the hole the same nominal OD as the bolt. I stick a SS nut on 
the underside of the basplate with two component acrylic glue eg Devcon Plastic 
Welder S-220 22045.
       You can also buy 'nut-serts' which you press into suitable holes with 
a vice or a press.
       I centre drill the end of the bolts and stick a suitable OD SS ball 
bearing in the V. Alternatively, you can file the end of the bolt to a cone with 
a small domed end. This helps prevent the seismometer position wandering as 
you make adjustments.
       I thread an ordinary nut onto the bolt, add a wavy washer and screw 
this into the top of the frame. After adjusting the levels I tighten compress 
the wavy washer with the free top nut. This largely prevents any creep due to 
differential expansion.
       I also use three 2" square x 1/8" thick SS ground plates.
       What you MUST NOT do is to use bolts and nuts / threaded holes of 
DIFFERENT materials, with different expansion coefficients. This will ~guarantee 
creep / drift problems.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/04/02, kwyatt@............. writes:



If you look at the three leveli=
ng screws on the AS-1 seismometer at http://www.amateurseismologist.com/, you see that there are two=
 screws on the end with the vertical support and one screw on the end near t=
he coil.  It is obvious why only three screws are used.  But is th=
eir a reason for putting the single screw near the coil and not near the ver=
tical support.  Also, wouldn't there be greater stability if the single=
 screw was actually a fixed pin, leaving the other two to be adjustable scre=
ws?  

 

Just wondering before I drill the base in my next vertical seismometer.



Hi Kay,



       It shouldn't matter, but it is generall=
y easier to put the lateral adjustment screws either side of the vertical su=
pport column and the longitudinal adjustment at the end of the arm. You have=
 'free' space here. 

       I suggest the you make all three mounti=
ng screws of identical construction. If you have one peg leg you will have t=
he problem of matching it's expansion with the other two. This is not too im=
portant for vertical instruments, but it is very important on a Lehman.

       Also, you generally get presented with=20=
a site of unknown slope direction. Being able to adjust all three screws is=20=
an advantage.

       I use a fairly thick baseplate and stai=
nless steel bolts, nuts and washers. I drill the hole the same nominal OD as=
 the bolt. I stick a SS nut on the underside of the basplate with two compon=
ent acrylic glue eg Devcon Plastic Welder S-220 22045.

       You can also buy 'nut-serts' which you=20=
press into suitable holes with a vice or a press.

       I centre drill the end of the bolts and=
 stick a suitable OD SS ball bearing in the V. Alternatively, you can file t=
he end of the bolt to a cone with a small domed end. This helps prevent the=20=
seismometer position wandering as you make adjustments.

       I thread an ordinary nut onto the bolt,=
 add a wavy washer and screw this into the top of the frame. After adjusting=
 the levels I tighten compress the wavy washer with the free top nut. This l=
argely prevents any creep due to differential expansion.

       I also use three 2" square x 1/8" thick=
 SS ground plates.

       What you MUST NOT do is to use bolts an=
d nuts / threaded holes of DIFFERENT materials, with different expansion coe=
fficients. This will ~guarantee creep / drift problems.



       Regards,



       Chris Chapman