Gummed-up AN 5851-1 bubble sextant averager.

9 12 2011

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  ”Refilling Mark V/AN5851 bubble  chambers” ,  ”Overhaul of MkV/An5851 bubble chamber” ,  ”AN5851-1 : jammed shades carrousel” ,  ”A Byrd sextant restored” ,  ”Update on Byrd Aircraft Sextant”, “A nautical sextant bubble horizon” and “Sealing A10 vapour pressure bubble chambers.”

Bob Hauser asks a question in a different category that I think best deserves an answer in the form of a short blog. He asks

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?

Generally, watches and clocks do not respond well to being flooded with lubricant for a variety of reasons: there is very little power at the end of a watch gear train, at the point where the rate at which the machine runs down is regulated by the escapement and balance wheel, so that even the surface tension of oil between the gear teeth or in the coils of the balance spring can bring the mechanism to a halt; the pivots, or bearings about which gears and other parts rotate, are provided with tiny oil wells (“sinks”) and the shafts are shaped at the end to keep the oil where it belongs. If the oil strays on to the plates of the mechanism, the oil that should be confined to the bearing tends to follow it; and  excess oil combines with fine dust and grit so that the bearings and pinions (those gears with relatively few teeth) eventually grind themselves to a halt.

So what is Bob to do? One could say “Sufficient unto the day is the evil thereof,” but ideally, the clockwork mechanism should be removed from the sextant, stripped down, cleaned, re-assembled and oiled in the correct places. This of course needs clock-repairing  knowledge and practice. While the official manual explains how to remove the mechanism, it involves removing substantial bits of the sextant first, with all the risk of introducing new problems or damaging or disturbing the optical system. I suggest that he simply expose the clockwork and try to do a bit of dry cleaning, removing visible Reel-X from the plates where they are accessible and from the gears that he can reach. He should pay special attention to the balance mechanism, removing any lubricant from between the coils of the hairspring and excess oil from the pivots. For the larger parts like the plates and large gears, small pieces of old cotton handkerchiefs applied with fine forceps are ideal, though he should take care not to leave stray threads behind. For smaller, more delicate parts, I suggest scraps of lens paper which, though it is not all that absorbent, it less likely than paper towels or handkerchieves to leave fibres behind.

How to access?


Figure 1 : Remove two screws and nut and bolt.


Figure 2 : Remove two screws.


Figure 3 : Remove one screw.


Figure 4 : Remove one screw. Note washer.

Once the lighting unit is out of the way you can get access to all the twelve round-headed screws that hold the sheet metal cover around three sides of the clockwork mechanism. Remove the screws and cover (Figure 5).

Figure 5 : Remove twelve screws and cover.

This partially exposes the mechanism. Concentrate on getting as much excess oil from the low power end of the gear train. The stars in Figure 6 show the important areas. There is plenty of power further up the gear train, but it will still pay patiently to remove as much oil as you can see and get at.

Figure 6 : Important areas from which to remove excess oil.

Re-assembly is the reverse of dis-assembly. Good luck!




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