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
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
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.
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