[RASMB] Replies to "Sticky" fit of cells in a AN-60Ti rotor

[jim.bloom.b at bayer.com]

Hi again everyone!!
     I received 8 replies so far to my request for help and am really
overwhelmed by the generosity and helpfulness of everyone (as some people
replied privately I will respect their privacy and not list names)!!   I have
done a considerable amount of thinking and 'playing' with the cells and rotor as
a result and have come to one conclusion:  my problem is cell related and not
due to size differences in the rotor holes.  In fact the following email from
Michael Morris describes my situation to a 'T':

"Do you load the cell into the rotor when it is warm (e.g., room temperature)
and then
do the run cold? If so, that could explain what you get because the housing
shrinks a
little.

You can:
(i) Cool the cell before you put it into the rotor.
(ii) Make sure the brass screws are not overtightened. They just have to be snug
to
help seal the gasket into place properly, provided the gasket is properly seated
and
has a clean edge. Any more than snug and you will distort the housing enough to
make
the fit difficult. With overtightening you also run the considerable risk of
stripping
the very soft thread on the screw and then having to drill it out. Not pretty."

     I do indeed tighten the brass screws as hard as I can...I am not strong
enough to strip the threads but I have been known to have the screwdriver slip
in the screw slot and 'remove' the slot!  I will stop doing this!  The lab temp
where I store the rotor is about 23 to 24 C and I run the centrifuge at 20 C.
     Another comment regarding cell distortion:

"Regarding your problem - I have had the opposite problem with a cell fitting
too loosely in a rotor hole and the suggestion by Beckman technical was to
tighten the end fitting a bit more (I assume to cause a bit more
distortion). If you decrease the pounds of pressure that you apply to the
torque wrench in securing he windows (even below that recommended) does the
cell slide into the rotor more easily? If so, that may suggest distortion"

     I asked about using spinkote to lubricate the rotor hole and received the
following comments:

"what you can try on the housings and the rotor holes is to clean them with
a mild detergent, just like the ones you would use to clean your dishes
(sorry, don't know the proper name in english), or that kind of liquid
soap you're using for washing your hands, just put some on a cloth and
wipe the rotor holes, and then wipe again with a dry cloth (you can also
do this with the shaft and the hole of the rotor where it actually sits
in, when you feel that the rotor "sticks" a little to the shaft of the
centrifuge; sometimes this happens also to preparative centrifuges, and
there it's the same procedure). Point is, there has to be indeed a thin
layer of lubricant on the rotor holes; however, of all
cases DO NOT USE SPINCOTE for lubricating the rotor holes, or the
centrifuge shaft, as this stuff will get sticky with time and really glue
in your housings (also true for preparative centrifuges), and then it's a
mess, since it is MoS (no clue on the stochiometry), therefore difficult
to remove with detergent. It's better to use a little of the vacuum oil
that should have come with the XL-A, in case you bought a new instrument,
or a similar high-quality oil, and just whipe that on the rotor holes,
better not on the housings, otherwise they will also slip in the
instrument used to assemble the cells.

"I have had similar problems on the old Model E rotors, but never with the
XLA rotors, anyway, I used to help myself in that situation by using some
emory cloth sandpaper and gently polishing the inside of the hole. Make sure
you don't have any residue in there (spilled sucrose, for example). If the
same cell housing works well in the other holes, you are probably just
having a sticky inside surface on the hole. I think it would be OK to use some
lubricant if that helps to slide the cells in more easily. I have done that
before as well. try the emory cloth first. Of course, always make sure not
too overtorque the cell and the brass screws, to make sure the housing isn't
getting distorted."

"Yes, you can use the spinkote to lubricate the holes - I have been doing it
for the last 40+ years with no ill effects and with good results..
However, use a LIGHT coating - otherwise the centrifugal field will move
the excess to the far radius of each hole.  Needless to say, I have
observed the same fit phenomena that you have and like you, I have no
really good idea why this happens except to surmise that the hole boring
mills that Beckman uses are not precisely identical."

"Spinkote can be used, and should be used from time to time, on the inside of
the rotor holes.  Just be careful not to but too much on or else it can find its
way on to the cell windows, the inside of the chamber etc etc"

"You may certainly lubricate the rotor hole lightly with Spinkote. You don't
want too much there, or else the grease goes flying around the chamber, on
the windows, etc. The Spinkote will help a lot."

     When all else fails I guess we may be left with brute force and the
following comments are pertinent:

"If a cell gets stuck in the rotor, I put it upside down on a wooden block
with a little hole in the middle, so that the rotor can stand on its own,
with some space between rotor holes for the housing and the wood, and then
push, push harder, pray. As long the force goes parallel to the rotor
holes, it should be OK."

"As far as using force
is concerned, I strongly recommend that you buy an arbor press - they cost
about $100-150 and operate like a drill press.  They give great control
over applied force and beat using a rubber mallet or any other type of
hammer.  You will have to make a wooden or plastic retainer to prevent the
rotor from sliding because the top and bottom of the rotor slopes - a small
test of your ingenuity.  This is also good for getting cells out of the
rotor if they are tight and for getting tight centerpieces out of barrels,
etc.  You will also have to make some plastic push rods of various lengths
and diameters.  Remember - if it sticks, force it - if it breaks, it needed
replacement.  Seriously, force can be used, but it must be used with due
care and restraint."

"The bottom line that everyone will tell you is that you should NEVER force a
cell into the rotor.  This could do serious damage to the cell housing causing
leaking and ultimately failure during a run.  Damage can also be done to the
rotor that would void any warranty that you have on it."

     Again, I am grateful to everyone who replied and if you don't recognize
your suggestion from the above please don't take offense...I was just trying to
organize/summarize everything into a coherent message.
     Jim