Which lubricant?

16 07 2011

A couple of people have e-mailed me to ask which lubricants to use in their sextants, so my book must have been deficient in that respect. I seem to have given the how and why, but not the what.

Nearly all bearings in the nautical sextant move slowly, are lightly loaded and are somewhat inaccessible, so these need grease, which is essentially soap and oil. The mixture is thixotropic, which is to say it is thick and sticky until sheared, when the oil is released and lubricates the bearing. There is some concern, perhaps largely theoretical, that additives of phosphorous and sulphur compounds may attack bronze and brass, so I suggest a waterproof lithium-based marine grease for everything except the rack and worm.

Grease here would mix with grit and dust to form a moderately effective grinding paste, so all manufacturers who took the trouble to mention it recommended brushing of the rack followed by the application of a few drops of oil. This is then distributed by winding the worm from one end of the rack to the other, followed by brushing off the excess.

While clock oil is sometimes recommended, perhaps because it does not evaporate to form gums, my experience is that it seems to be too thin, and gives an impression of metal to metal contact when rotating the worm. If the oil is too thick, the oil film may vary in thickness, depending on loading and speed of rotation. After some experimentation, I have settled for a monograde SAE 30 “spindle oil”, the sort of stuff that is sold for use on rotating agricutural machinery and in lathe bearings. This gives a silky smooth feeling, with none of the fine crepitus a sensitive hand may detect when there is no lubrication or the lubricant is too thin.

This advice may well have to be modified if the sextant is to be used at low temperatures. I have no experience of this, so your own experience with low temperature lubricants will have to be your guide






2 responses

9 12 2011
Robert Hauser

Recently acquired Bendix AN 5851 had stalled averager that could be wound up to stop as per directions but would then simply remain in that state when the release lever (“no. 3”) was pressed—-for lack of any better solvent/lubricant, I lavished Reel-X on the bull gears and down under those gears into the inner chronometer mechanism and gently turned the pawl driver clockwise by hand and repeated this 4 or 5 X manually until the averager ran on its own for the required 2 minutes …yes, it worked but for how long with that stuff in there before it gums up even worse? Reel-X is a solvent/lubricant that has about the viscosity of sewing machine oil and may wind up being the worst thing to admit in the chronometer like that…can you advise?

9 12 2011

Bob, I have replied to this under the “Aircraft Bubble Sextant” category.

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