The most common stopwork in wristwatches is the Maltese cross. It is named after a wheel the form of a cross with several arms. All arms except one have a concave end. The Maltese cross is moved by a wheel with one finger fixed on the axis of the barrel. This finger fits between the arms of the Maltese cross while the arm of the Maltese cross fits into openings in the finger wheel on both sides of that finger. This device will turn until the one longer arm which does not fit into the openings of the finger wheel blocks the mechanism and thus the unwinding of the barrel and prevents overwinding the mainspring.
While this mechanism does work well, it also has its disadvantages. The Maltese cross is switched step by step, thus influencing the power delivered by the barrel. The stopping is effected by metal sliding on metal, a process prone to variations, again influencing the barrel.
By design, the bearing of the Maltese cross is fragile which is critical if the barrel is fully wound up and all the power of winding the watch is transferred onto the Maltese cross when it blocks. The force that comes to bear in this instance is several times greater than the force produced by the barrel. To avoid these problems, Andreas Strehler designed a free running stopwork with only a minimal and continuous friction and without any discernible influence on the precision of the watch. It’s functioning – as with most great inventions – is amazingly simple:
To the barrel-arbor is fixed a toothed pinion which continuously moves a gear ring with inner toothing. This gear ring is embedded but moving freely in an eccentric cut out of the barrel. The pinion has one longer tooth as a blocking finger. This blocking finger is stopped by the gear ring both before the full winding and before the complete unwinding of the mainspring. The gear ring has enlarged tooth spaces allowing the blocking finger to pass freely. The reduced tooth spaces of the gear ring only let the normal teeth of the pinion but not the blocking finger. The axial play of the gear ring is limited by the clearance between barrel and ratchet-wheel.