I recently had a chat with a good friend of mine, Hassan; we talked about the refreshed blog, and my plans for it… he was pretty supportive and loved the idea. He also suggested that the name “WatchProductivity” was quite literal, but not particularly memorable or catchy. I didn’t think it mattered but he urged me to reconsider. Hassan has some recent experience with launching new things, having recently started his company Moodit. Having considered his advice, I had to agree that a catchy name would be useful… it also makes branding a lot easier… and of course, ‘productivity’ is harder to type 🙂

Hence the name “ScrewDownCrown” was born. Figured I would bite the bullet and change it now, instead of waiting for “WatchProductivity” to catch on… the content, of course, remains the same.

So why did I choose this name? Since the underlying theme of this site will be blending watches and productivity, this particular ingenious piece of engineering seems to be as good a choice as any – I found a really informative and fascinating write-up on Vintage Watch Straps and have pasted it below – link to the original article is here.

Hopefully this change is well received. Please share your thoughts on this name – I’d be interested to hear them.

-F

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The Screw Down Crown

The part of a watch case that is most difficult to make waterproof is where the winding stem enters the case. Some early designs of waterproof watches, such as the “explorers watches” produced for the Royal Geographical Society in the late nineteenth century overcame this problem by the simple expedient of a cap that enclosed the crown and screwed down onto the case, totally encapsulating the crown and stem, and the hole in the case where the stem entered. This was a bit of a nuisance because the cap had to be removed whenever the watch needed to be wound or set, and there was always the danger of dropping it. An alternative design that made the crown itself function as the cap was invented and patented in the United States by Ezra Fitch around 1880, but this was not a commercial success. You can read about these and other early designs of waterproof watch on my page about The evolution of the waterproof watch.

On 30 October 1925 in La Chaux de Fonds, Switzerland, Paul Perregaux and Georges Perret, registered Swiss patent No. 114948 for a winding system where the crown could be screwed down on to the case to create a waterproof seal. The patent was granted and published on 17 May 1926. There seems to be very little known about Perregaux and Perret, who are sometimes described as watchmakers and prototype makers.

When Hans Wilsdorf saw this patent, he must have thought he had found the solution he had been searching for. He bought all rights to the patent and had it assigned to him, and then applied for a British patent on 1 September 1926, which was granted as No. 260554 on 21 April 1927. He also patented it in Germany No. 443386, and the United States No. 1,661,232. You can see the Swiss and British patents referenced in the 1929 Oyster case back shown here.

Alongside the British patent number 260554 is stamped the year 1925, the year before the Swiss patent was granted, and before even an application for a British patent was lodged. This is valid because agreements between Britain and Switzerland meant that the application date of the Swiss patent was recognised as the “priority date”.

However, although the Perregaux and Perret patent is often referred to as the patent that made the waterproof Oyster possible, not least by Rolex as can be seen from the Oyster case back pictured, it has some serious practical problems that prevented Wilsdorf from putting it into production.

Referring to the figure from the patent reproduced here, it can be seen that the way the Perregaux and Perret design works is as follows: the stem 4 and socket 6 are screwed together so that they are effectively one piece. The crown 8 is coupled to the stem and socket by the two screws 9 and 10 screwed into the crown. The ends of these two screws can slide in the longitudinal grooves 11 and 12 in the socket that I have highlighted in yellow. This permits the crown to move axially with respect to the stem and socket, screws 9 and 10 sliding up or down in the grooves as shown in the difference between figure 1 and figure 2.

Figure 1 shows the crown screwed down onto the case, the two screws are at the bottom of the yellow slots. Figure 2 shows the crown unscrewed from the case and now the screws are at the top of the yellow slots. The socket attached to the stem has not moved outwards with the crown as it unscrews, the two screws have just slid up the yellow slots. However, the two screws ensure that the stem and socket are locked together rotationally: the stem must follow any rotation of the crown and so while the crown is being unscrewed or screwed back down the stem has to turn.

The crown is threaded internally 15 at its lower end, and this thread engages with the thread on the tube 3 that projects from the case. The thread on the tube and the corresponding thread inside the crown are left-handed.

This is quite clearly stated in the patent: “The present invention relates to improvements in keyless watches and more particularly to improvements in and connected with the winding mechanism of such watches and is concerned with improvements in that type of winder in which the winder is secured in a moisture proof manner to the pendant or equivalent by means of a left hand screw-thread on the winder engaging a left hand screw-thread on the pendant and then screwed down on the pendant compressing packing means. ” (my bold emphasis)

The reason for this is as follows: when the watch needs winding the crown is unscrewed clockwise, in the direction of winding. Once the watch is fully wound, and the hands set if required, then the crown is screwed back down anti-clockwise, which the winding ratchet allows. It can’t screw down in the right hand direction because the spring is fully wound, preventing any further rotation of the crown in that direction. The crown has to be screwed down in the direction allowed by the winding ratchet, which is anti-clockwise, or left-handed, a very unnatural action!

There are some further undesirable consequences of this design. Once the watch is fully wound and the crown screwed down, the crown cannot be unscrewed until the watch has run down somewhat, because the action of unscrewing the crown also winds the watch, and if it is already fully wound it cannot be wound any further without breaking. So if the owner winds the watch fully, screws the crown down, and then realises that the hands need setting, he is stuck for an hour or two!

Another poor feature of this design is that the waterproof seal is formed by the base of the crown compressing 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.

A Better Design: CH 120848

The Perregaux and Perret design was impractical to say the least, requiring a fair amount of education and care on the part of the customer if disaster was to be avoided. Wilsdorf must have soon realised that this design was not suitable to be released to the public. He put on his thinking cap, or more likely got his “technical assistants” working on it, and by October 1926 they had come up with an improved design. The application for a patent for this was registered by Wilsdorf on 18 October 1926 and the patent was granted on 16 June 1927 as Swiss patent CH 120848, a figure from which is reproduced here.

The clever bit of CH 120848 was that a dog clutch was incorporated into the joint between the stem and the crown, so that the crown could rotate freely while being screwed down and unscrewed from the case, but it became rotationally locked to the stem by the dog clutch when it was clear of the threaded tube on the watch case. This meant that the crown could be could be unscrewed at any time to wind the watch or set the hands, and then screwed down onto the case by a right hand thread that would be familiar to any customer.

Referring to the FIG. 1 from the patent, cylinder 6 is fixed into the crown. The base of this cylinder has a square hole 9 in it which I have ringed in red. The plug 10 screws on to the end of the winding stem, and has a circular flange 11 to centre it within the cylinder 6 and support the spring 13, and a square section 12 at its base which I have also ringed in red.

When the crown is unscrewed from the threaded tube 2, which is fixed into the case wall, it is pushed away from the case by the spring. The square section 12 on the stem end plug drops into the square hole 9 in the base of cylinder 6, and the stem and crown are then locked together rotationally as shown in FIG. 2.

As soon as the crown is pushed back towards the case to screw it down, the cylinder is pressed downwards and the square section on the stem pulls free of the square hole in the base of the cylinder. The crown is then free to rotate and can be screwed down on to the case without turning the stem.

A longer version of this history of the development of the Rolex screw down crown was published in the NAWCC Watch & Clock Bulletin in December 2010, “The Rolex Screw Down Crown and its Antecedents”, as shown in the picture above.