Hey Guys/Gals,A question. Does the cap jewel on the upper and lower balance serve any purpose other than cosmetic,or does the tip of the pivot rub on it to keep the shoulder of the staff from rubbing on the jewel?

Hi Cecil,

Always an interesting question when it comes to cap jewels. I know personally of half a dozen good repairmen on this site and I certainly am not one of them

Now most all the balances have capped jewels so you would think that there is a definite answer. I have oft heard that the logic is to prevent airborne contaminants from invading the oil reserves and turning to sludge over a period of time with the rotating pivots. Also some have said to preserve the oil from drying out over a period of time. The tests I have read show no significant time savings between "oil changes" if you will

All of our mechanical watch movements are nothing more than miniature motors that use the mainspring as an energy supply and must be kept oiled either from an actual oil supply and from the mineral effects of the jewels themselves which keep metal from metal contact which we know would wear and erode if it were not for the oil and the jewels

I am certainly no repairman and look forward to hearing the repairman's side of this debate To answer your question a step further, I think most would say the capped balance jewels are there for a slight, if any protective advantage over a non capped jewel.

regards,
bb

All jewels in proper watches are for functional purposes (although some are very pretty!) The cap jewels are placed to provide a low friction contact area for the pivot. Almost all watches have cap jewels on the balance wheel. On other wheels, pivots and shoulders are shaped differently for holes that have cap jewels vs. those that don't.
Mike

Balance Wheel cap and pivot jeweling is what sets "Jeweled" watches apart from all the rest. As Mike explained the design of the pivots for Cap Jewelled staffs are such that they have a Polished end and a straight bearing section carefully fitted to pivot and cap jewels that assure a low friction and (most important) free endplay of the wheel to minimize friction and distortions of the Hairspring in all the different possible positions. Page 73 of the price guide shows an excellent cross-sectional view of this structure and I inserted a very good illustration of the entire relationship from the Joseph Bulova School of Watchmaking Training Manual (a still in print and seminal document for all "would-be watch repair" people).

There is little or no difference in this structure from the "basic" 7 jewel movements" to the 23 + + + overjeweled cost behemoths produced to meet the high jewel count demand of the market.

A Cap Jewel is a very poor "dust cover" but is good at controlling end play with minimum frictional losses and also retaining lubrication for the capped pivots.

With everything else buttoned down, you can actually shake the watch in the face up-down direction and hear the "click-click of the correct end play as the balance wheel taps from one cap to the other