Recommendations for Resolution of the Problem of
Loose CFEB Connector Screws.
Statement of Problem:
The 2-56 X 1/4" brass screws used for strain relief of the two 50-pin SCSI PCB
connectors (for Skewclear cables) on the CFEB's have been installed with insufficient
torque and without a locking mechanism. This has lead to a number of screws either
partially or completely backing out at some point prior to the chambers arriving at
CERN.
Resolution of Problem:
General Procedures
1) Any future installations of these screws will include the use of Loctite
Threadlocker 262 (permanent strength) on the treads.
2) For CFEB's that have had the screws already installed but have not yet
left OSU for FAST sites, each screw will be checked for tightness and
a drop of Loctite Threadlocker 290 Wicking Grade (for pre-assembled
parts) will be applied to the exposed threads on the top side of the
CFEB. Through capillary action the Loctite will wick its way into the
threads were it will cure and prevent any unintentional backing out of
the screw.
3) For CFEB's already shipped to FAST sites but not yet installed on a
chamber, the screws should be checked for tightness and then wicking
grade Loctite applied to the exposed threads on the top side of the
CFEB. This should be the same procedure described in 2 above. OSU
will ship the required wicking grade Loctite to each FAST site. Please
see the attached photos for the location of the screws and where to
apply the Loctite.
4) For CFEB's already installed on a chamber at FAST sites and CERN,
the following steps should be taken:
a) Remove the CFEB cover plate.
b) When fully seated the end of the screw should protrude out
the topside of the connector by 0.050” or 1.3 mm (see
photos). Using a dental tool or small-tipped screwdriver,
probe the screws to check for looseness. (If the screw is
loose, it will wiggle when probed).
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c) If the screw is secure, then apply a drop of the wicking
grade Loctite to the exposed threads on the topside of the
CFEB.
d) If the screw is not secure, remove the CFEB, tighten the
screw, then apply the Loctite and reinstall the CFEB. (It
may be possible to tighten the screws without completely
removing the CFEB by leaving the input cables attached
and lifting the back end of the CFEB until there is
sufficient room for a small screwdriver, then tighten the
screws. It is up to the discretion of the technician to choose
which method is easier.)
Procedural Notes
Due to the low surface tension of the wicking grade Loctite, it is difficult to apply
a small drop directly from the bottle without flooding the area with the Loctite solution
and creating a mess. Therefore it is necessary to apply the Loctite using a syringe with a
fine guage needle (27 ga.). The Loctite freely flows out of the needle when brought into
contact with the screw threads. Only the slightest pressure is needed on the syringe
plunger.
This Loctite product is designed to cure in the absence of air (anaerobic) when
confined between close fitting metal parts (0.05 to 0.15 mm). See the technical data sheet
(TDS) for more details. Because of the curing characteristics of this product, the needle
of the applicator syringe will become plugged if it is allowed to sit for more than about
30 minutes. Therefore it is suggested that Loctite is applied to as many screws as
possible during a short interval of time, and that the syringe is purged of Loctite when not
in use. Extra syringes will be provided in the “repair kit”.
Once applied, the wicking process happens quickly. However there will be
excess solution remaining around the screw that will not cure. This excess solution must
be removed with a tissue.
Safety Notes
The Loctite solution contains toxic ingredients so care should be taken when
handling. In particular, avoid prolonged contact with skin, and eyes. See the Material
Safety Data Sheet (MSDS) for more details.
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Specific Instructions
The connectors with
potentially loose screws
Photo 1. A CFEB showing the connectors in question.
The four brass screws to be checked
Photo 2. Detail indicating the screws that need to be checked.
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Try to wiggle the screw with a probe.
Photo 3. Probe the screws to check for looseness.
The screws have Phillips heads (cross-head).
Photo 4. If a loose screw is found, tighten it from the bottom side of the
board with a small Phillips screwdriver.
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Photo 5. A Packaged syringe and component parts.
Photo 6. Snip a small opening in the new Loctite bottle, remove the
plunger from the syringe and fill with Loctite.
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Photo 7. Insert the plunger and purge the air from the syringe using a
tissue to absorb any ejected Loctite.
Photo 8. Apply the Loctite to the exposed threads. Apply only just
enough for the liquid to surround the screw. Almost no pressure is
needed on the plunger.
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Photo 9. Apply Loctite to a large group of screws, then remove the
excess with a tissue.
Photo 10. If the syringe applicator will not be used for more than 5 to
10 minutes, empty the syringe back into the Loctite bottle.
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Photo 11. Clear the needle of fluid by running the plunger in and out.
Finish by removing the plunger and drying the excess liquid with a
tissue. Set the syringe aside to be used later. If later you find the needle
to be plugged, use a new syringe.
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