I have just had an article on the early Rolex screw down crown published by the NAWCC, and I thought a short synopsis might be of interest here.
Rolex were not the first company to use a screw down crown. In the 19th century watches (called explorers watches) were fitted with screw caps over the crown to protect against dust and moisture. In 1881 Ezra C. Fitch obtained a US patent on a development of this idea, using the watch crown itself as the cap. The image below shows some details from the patent. You can see that the crown is internally threaded and screws down onto the pendant, the base of the crown making a seal against the shoulder "e" on the pendant.
Fitch specified a left hand thread for the crown and pendant. The reason for this was to use the winding ratchet to enable the crown to be screwed down once the watch was fully wound. The user unscrews the crown in the direction of winding, and then winds the watch. When the watch is fully wound, the crown can no longer be rotated forward, and the left-hand thread allows it then to be screwed down backward onto the pendant. But this presents a problem. If the watch is fully wound and the crown screwed down and then the user notices that the hands need to be set to the correct time, he is in a pickle. The crown can't be unscrewed, because the spring is fully wound! He must wait until the watch has run down some before he can unscrew the crown to set the hands.
Ezra C. Fitch became president of the American Watch Company of Waltham in 1883, and watch cases with screw down crowns to his patent were produced in small numbers by the American Watch Company in the 1880s. The design doesn’t seem to have achieved great commercial or popular success, so perhaps the extra complication wasn’t found to be necessary or worthwhile on a pocket watch by the watch-buying public.
A few months after Fitch registered his patent, another inventor, Almon Twing of Waltham, also registered a patent for a screw down crown. Twing's design was superior to Fitch's, incorporating a clutch into the crown and winding stem assembly so that when the crown being screwed or unscrewed on the pendant, it was free to turn relative to the stem, but when the crown was free of the pendant, the clutch engaged it with the stem. This design doesn't seem to have actually been produced.
The Rolex Screw Down Crown
In early Rolex Oysters, the number 114948 is usually stamped inside the case back. This refers to a Swiss patent registered by Paul Perregaux and Georges Perret of La Chaux-de-Fonds on October 30, 1925, and published on May 17, 1926. Wilsdorf must have thought that this was the breakthrough he had been looking for, because he bought the rights to the patent, and then registered it in Britain as GB 260554, and also in Germany and the United States.
The winding stem 4 and socket 6 are screwed together to form a single item. The crown 8 is connected to this stem and socket assembly by two screws 9 and 10. The ends of these screws slide in the yellow longitudinal slots 11 and 12 in the socket. This permits the crown to move axially with respect to the stem, as shown in the difference between figures 1 and 2, whilst being locked to the stem rotationally. As the crown is unscrewed from the case, the two screws slide up the longitudinal slots until the crown is clear of the pendant.
The screw threads on the pendant and inside the crown are left-handed, for the same reason as in the Fitch patent. When the watch needs winding, the crown is unscrewed clockwise, in the direction of winding. Once the watch is fully wound the crown is screwed back down anticlockwise, against the mainspring winding ratchet. This has the same drawback as the Fitch patent, in that the crown can then not be unscrewed again until the watch has run down.
Another poor feature of this patent is that the waterproof seal is formed by the gasket 16 against the case, which is in a very exposed position, and would not have lasted long given the gasket materials available in the 1920s; leather, cork or felt.
Because of the left handed thread and the problems in winding and hand setting, and the clumsy gasket, patent CH 114948 wasn't suitable to enter production. Wilsdorf got the technicians at Aegler working and by October 1926 they had come up with a much better design. The patent for this was registered on October 18, 1926 as Swiss patent CH 120848.
This design incorporates a clutch, like Twing's earlier patent. It is a dog clutch formed by the yellow square item 12 on the stem engaging/disengaging with the square hole 9 in the base of socket 6. When the crown is screwed down on the pendant the yellow square is free of the red hole and the crown can turn freely without turning the stem. But when the crown is unscrewed from the pendant, the yellow square engages with the red hole and turning the crown then also turns the stem.
The seal is also better. It is formed between the shoulder on socket 6 and the end of the stem, and Rolex used a lead gasket here to get a better seal without using organic gasket materials.
The CH 120848 Mystery
Curiously, patent CH 120848 isn't mentioned in Oyster case backs, and I have not been able to trace versions of it being registered in any other country. I am a bit mystified about this, as 114948 and 260554 appear in every early Oyster I have looked at, whilst having very little to do with the design, whereas the apparently much more significant 120848 doesn't get a mention.
being in Australia I just received my latest magazine yesterday.
I have not had the chance to read it properly yet, but the one part I did read from beginning to end was your contribution!
Thank you for such a well written and meticulously researched article.