Horological Meandering

nickd
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Performance...

I've a couple of comments on "performance".

People often comment on the "feel" or "smoothness" of the chronograph action.  This is largely determined by the distance the pusher has to move and the force that has to be applied to make the columnwheel rotate.  There are several factors:

- the number of teeth on the ratchet wheel determine the angle the columnwheel has to move through for each click.  In general, the greater the angular rotation, the longer the travel of the mechanism that acvitates it.  If the depth the pusher is depressed stays constant for all mechanisms, then the fact that it's a lever means that you have to apply more force for a columnwheel with fewer ratchet wheel teeth. 

- the overall design and layout of the movement is a factor.  Early pocketwatch chronograph mechanisms were often built on top of an existing pocketwatch movement, which severely limited the positioning of the components.  The position of the columnwheel and the position of the pushers can mean that there's a long and complex multi-pivoted lever to connect them, as well as tortuous, spaghetti-like routing of the other levers.  Simplest is best, and some designs are just too complex for their own good.

- the friction of the various levers against the columnwheel surfaces.

- the force that has to be applied to the levers to activate them (make them move).  This is partly determined by the way the levers are arranged and their pivot points (basic physics/mechanics), partly by the friction due to pivots and surfaces in contact, partly by the mass of the levers, partly by the inertia of the components they're moving etc.

- where the levers make contact with the columnwheel.  If all the contact points are on one side of the columnwheel, there's an increase in friction as the columnwheel wheel "leans" against its pivot.  Arranging the contact points symetrically around the columnwheel reduces this friction.

- the length of the levers and the number of intermediate joins and pivots increase the possibility of small errors in tolerances or position being magnified by the effect of the levers.

- the strength of the various retaining springs.

The profiles of the columnwheel pillars are a key factor, but there's a trade-off to be made.  Pusing the chronogrpah button doesn't make "one thing" happen - it activates a sequence of actions that have to take place at very precise intervals for the chronograph to function correctly.  This phasing is crucial, and is largely determined by the movement of the levers as they slide/jump in and out of the columnwheels.  You can make the action smoother and lighter by adjusting the levers' contacting surfaces or the profiles of the pillars, but you mustn't interfere with the phasing of the sub-operations that have to take place at each click.  At a more nit-picking level, depending on the design, the eccentric depthing adjustments sometimes also rotate the levers' contacting surfaces very slightly (the change in depth of engagement of the lever into the columnwheel doesn't take place along a radius but along a line that's at a slight angle to a perpendicular to a tangent), so variations in depthing could potentially slightly modify phasing and the force required to activate the chronograph.

All-in-all, the designer has more than enough to worry about, and really getting it right requires CAD simulation. 

The desire for symetrical placement of the registers on the dial is a constraint, and if the registers could be placed anywhere the design could be simplified.  The same goes for the positions of the pushers.  Putting the majority of the chronograph components under the dial can also allow for a certain simplification.  Minerva built the rattrappante mechanism of their 19-25 into the base plate, so making use of dead space and only increasing the height of the basic chronograph movement by some 0.4mm.  There are some very rare early pocketwatches where the hours/minutes are in a sub-dial at 12:00, there's a chronograph sweep seconds, and the minute register is other than at the visually "balanced"  6:00.  This allows for a certain rationalisation.

These comments are based on spending time understanding (and drawing) old pocketwatch chronographs, and reading old articles and patents.  Perhaps modern wristwatch chronographs have got it all sorted out - I don't know.  Suitbert...?

nick

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