Here are some pointers on removing the upper shim stack and/or shim
assembly:
1. Check to see whether the shims are bolted to the underside (OVC
plate) of the magnet before pulling out anything. On our magnet, there is a
clamp which holds the shims in place below the magnet, but this may not be
the standard configuration. If there is nothing holding the shims to the
underside of the magnet, put a box under the magnet to prevent the shims
from falling down when you release the upper stack.
2. Before pulling out the upper stack, scratch some alignment marks
into it and the top magnet flange so the stack can be re-inserted with the
same height and radial orientation. This is critical because the height and
radial position of the upper stack sets the height and radial position of
the shims.
3. Loosen the long screws at the top of the stack, then gently pull
the stack upwards. Inspect the two O-rings at the bottom of the stack to
see if they are damaged or unseated. If necessary replace them, using a
thin film of epoxy cement or grease to hold them in place as you re-insert
the stack.
4. Check the fiber optic spin rate sensor at the end of the stack.
Make sure there isn't any dirt on the surface of the transducer.
5. If all is well with the upper stack, remove the shims, being
very careful not to stress the cable carrying the shim current leads.
Inspect the spin turbine at the top of the shims. If the pin holes in the
turbine are blocked, you can remove a screw and pull out the turbine for
sonication. Note that all air connections for spinning and lift pass
through the upper stack; no air connections are made through the shims.
6. To reassemble everything, first re-insert the upper stack and
set it to the correct height and radial orientation. When the upper stack
is locked down, crawl under the magnet and push the shims into the bottom
of the magnet, rotating the shims until they engage the spin rate
transducer on the bottom of the upper stack. Push the shims up as far as
they will go, then have a second person tighten the screws on the top of
the upper stack while you hold the shims firmly in place.
Good luck, and let me know if I can provide any further information.
Best regards,
Ken Fishbein
Facility Manager,
NMR Unit
NIH/NIA/GRC
Ph. (410) 558-8512
FAX (410) 558-8323 or 8173
E-mail: fishbein@vax.grc.nia.nih.gov
Address: Ken Fishbein
NIH/NIA/GRC
4940 Eastern Avenue
Baltimore, MD 21224 USA