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