NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Backlash
From: Clive Sutherland
Date: 2005 Nov 21, 20:42 -0000
From: Clive Sutherland
Date: 2005 Nov 21, 20:42 -0000
Some years ago when I was servicing a sextant for a friend I noticed an excessive and variable (i.e. dependant on the part of the arc being used) backlash problem.My surmise was that at sometime during its life the index micrometer worm had been clumsily grated across the gear arc, and back lash was occurring wherever the worm did not now properly engage. This only occurred only over certain parts of the arc, mostly near the parking position, as the sextant was put away in its box. This could occur on a sextant if the clamping lever had been insufficiently gripped as the index arm was rapidly moved and the teeth on the gear (which was made of a hard metal) come into contact with the softer arc and damage the tops of some the teeth. If you were to look at the shape of the damaged teeth in profile their normal triangular shape would be turned into that of a breaking wave. My thoughts are that If you find backlash occurring on a old or second hand sextant or anytime on a plastic one, look carefully at the shape of the gear teeth. If any damage is not very bad It may be carefully repaired. But! Beware, always look very carefully if you are buying second hand. Even a new plastic sextant is not immune, especially if "shop soiled!" This, in my opinion, is the greatest risk with plastic sextants. They do need Extra Tender Loving Care! CLIVE
