Henry Hughes and Son Bubble Sextants Mk IX (all models)
Freeing a stiff Index Mirror Bearing
Many of the Mk IX sextants, along with many
others of this vintage, have got sticky or seized index mirror
bearings. You may be able to feel this as a stiff wheel. A sure
fire method of checking the index bearing itself is to select
a bearing of about 25° and then carefully, with one
finger only, press down on the ten degree selector plate just
on the corner above the letter “D”.
The sector plate should move down with a
click, and more importantly back up under the influence of a spring
with another click. The distance is only about half a mm but if
all is in order with the index mirror bearing the clicks being
present means any stiffness must be elsewhere. This is good news
because stripping the sextant is normally required to clean and
lubricate the bearing. A stiff wheel can be caused by several
other faults but most can be rectified by careful examination
cleaning and lubrication.
With many sextants to free off, I studied
this problem and came up with a solution that has worked with
more than twenty of my instruments so far. You might like to try
it. And I put these photos forward as a guide.
First open the sextant. You might
like to dust off any dirt at this stage, there is usually some
lurking in the recesses, I find a lot of straw particles presumably
part of the original packing.
Next, remove the 4BA alloy split
nut on the index bearing spindle. I use a small screwdriver in
the slot. Take care this is soft alloy!
With the split nut
(1) and square centred alloy washer (2) removed, undo the nut
(3) and remove it and the spring (4) revealed below it.
You can now see an
alloy spindle with a square end. This part is stationary and the
threaded circular boss around it is the index bearing housing.
This boss has a thread on it (the nut 3 came off it). I have made
a simple extractor tool which screws on to the boss and when firmly
seated a screw is done up against the alloy spindle carefully
(alloy is very soft!).
Look carefully at
the extractor in this picture. It fits inside the mirror mount
onto the index mirror bearing. The mirror mount can move up and
down a small amount (say 1mm) with the extractor fitted.
In cases of really
stiff bearings I flood the extractor with oil, apply LIGHT pressure
with the screw and then blow hot air over the whole assembly.
Again take great care and do not use too much force or heat. Eventually
the bearing should break its seal. You can now flush through with
a light oil and work the bearing back and fore many times to polish
up both surfaces.
This is obviously
not as good a method as a complete strip, but will do no harm
other than get oil everywhere to trap dust, but you may decide
not to worry about that and so this may well be all that you need
to get your Sextant working again.
In the second photo
I show the gearbox and the four screws holding on the cover I
have marked with red spots. There are a lot of other parts that
will have to be disconnected before the cover can be fully released.
However, with the four screws removed and the screw in the centre
of the shades knob turned 180 degrees anti clock, you can prise
up and move the cover a couple of mms to the rear, then squirt
some WD40 or other thin oil in. A really cheap and dirty method
I must agree, but again, it will save you a lot of time and will
do no damage apart from, again, getting oil everywhere!
Here is rough sketch
of the extractor tool I made and so far have used with complete
success.
EXTRACTOR LUBRICATOR
Back to Mk IX