Hi There,

       I recently moved my Lehman, but I then noticed that the Tilt Stability 
over several days now seemed to be variable to poor as the air temperature 
changed. The mass position had previously been very stable. The period is set to 
about 30 seconds, so it is sensitive to even tiny variations of tilt / ground 
movement. 

       The level set bolts are about 5/16" Al alloy and are threaded through 
the 3/4" thick Al alloy crossbar, so the expansion coefficients should match 
quite well. The threads are quite a good fit. I used a centre drill to make a 
concentric V cone in the ends of the bolts and stuck a 3/16" SS ball bearing in 
the end with two component acrylic adhesive. This gives a smooth hard contact 
point which does not tend to 'wander' in position as it is adjusted. The 
threads are lubricated with MS2 grease. I use three SS support plates, about 3" 
square x 1/8" thick, glued to the concrete floor with pool adhesive. 
 
       I have now fitted each of the three level set bolts with a SS wavy 
spring washer and a SS lock nut on the top of the frame. What seems happen is 
that as the Al crossbar expands / contracts with changes in temperature, this 
causes the 60 deg bolt threads to move radially, enough to upset the cross 
balance trim at a 30 second period. It gives a slip / stick movement to the threads 
and they don't seem to  'recover' to their initial positions during the daily 
thermal cycle.  

       Tightening the lock nuts so that the wavy washers are about half 
compressed dramatically reduced the mass position variations. This ensures that the 
threads remain locked in position and any expansion / slip takes place 
between the SS balls and the horizontal SS ground plates.

       I note that McMasterCarr stock SS 1/4" UNF fully threaded bolts. A 
3/16" SS ball bearing should fit well in the ends of these bolts. SS bolts are 
severely work hardened during manufacture, so I heat the ends bright red hot 
with a propane / air flame to soften them a bit and then allow them to cool, 
before trying to drill them.
       I clamp the bolt vertically in a hand vice, slip on a brass sleeve and 
use a tungsten carbide centre drill in a drill press to make the V cone in 
the end of the bolt. The sleeve is 1.5" long by 0.5" OD brass. The 1" end is 
drilled out to fit the bolt OD and the 1/2" end is drilled out to fit the 1/4" 
centre drill. This seems to align the cone axially within a few thousands of an 
inch. 

       These modifications have produced a dramatic decrease in the observed 
drift of the mass position. 

       Regards,

       Chris Chapman   
Hi Ther=
e,



       I recently moved my Lehman, but I then=20=
noticed that the Tilt Stability over several days now seemed to be variable=20=
to poor as the air temperature changed. The mass position had previously bee=
n very stable. The period is set to about 30 seconds, so it is sensitive to=20=
even tiny variations of tilt / ground movement. 



       The level set bolts are about 5/16" Al=20=
alloy and are threaded through the 3/4" thick Al alloy crossbar, so the expa=
nsion coefficients should match quite well. The threads are quite a good fit=
.. I used a centre drill to make a concentric V cone in the ends of the bolts=
 and stuck a 3/16" SS ball bearing in the end with two component acrylic adh=
esive. This gives a smooth hard contact point which does not tend to 'wander=
' in position as it is adjusted. The threads are lubricated with MS2 grease.=
 I use three SS support plates, about 3" square x 1/8" thick, glued to the c=
oncrete floor with pool adhesive. 

 

       I have now fitted each of the three lev=
el set bolts with a SS wavy spring washer and a SS lock nut on the top of th=
e frame. What seems happen is that as the Al crossbar expands / contracts wi=
th changes in temperature, this causes the 60 deg bolt threads to move radia=
lly, enough to upset the cross balance trim at a 30 second period. It gives=20=
a slip / stick movement to the threads and they don't seem to  'recover=
' to their initial positions during the daily thermal cycle.  



       Tightening the lock nuts so that the wa=
vy washers are about half compressed dramatically reduced the mass position=20=
variations. This ensures that the threads remain locked in position and any=20=
expansion / slip takes place between the SS balls and the horizontal SS grou=
nd plates.



       I note that McMasterCarr stock SS 1/4"=20=
UNF fully threaded bolts. A 3/16" SS ball bearing should fit well in the end=
s of these bolts. SS bolts are severely work hardened during manufacture, so=
 I heat the ends bright red hot with a propane / air flame to soften them a=20=
bit and then allow them to cool, before trying to drill them.

       I clamp the bolt vertically in a hand v=
ice, slip on a brass sleeve and use a tungsten carbide centre drill in a dri=
ll press to make the V cone in the end of the bolt. The sleeve is 1.5" long=20=
by 0.5" OD brass. The 1" end is drilled out to fit the bolt OD and the 1/2"=20=
end is drilled out to fit the 1/4" centre drill. This seems to align the con=
e axially within a few thousands of an inch. 



       These modifications have produced a dra=
matic decrease in the observed drift of the mass position. 



       Regards,



       Chris Chapman