Negative set keyless mechanism

Last updated on August 4, 2024

During some recent research on a particular Tavannes trench watch, I came across a reference to the US patent “U.S. PAT 24 May 1904”. This is stamped on the movement and is a reference to a patent lodged for what is known as a negative set stem winding and setting mechanism. This of course prompted more research and below is what I have managed to uncover. A negative set keyless mechanism in a watch consists of a two-piece stem. One part is located in the movement and the other in the case. The movement stem has a square socket in its outer end, which connects with the square end of the case stem to turn the movement when the crown and case stem are turned.

Winding and setting

The keyless mechanism is spring-biased into hand setting mode, which is the mode the mechanism adopts when the movement is outside of the case. To switch to winding mode, the movement stem is pushed towards the centre of the movement. When in hand-setting mode, the keyless mechanism applies a drag on the wheel train, which can impact timekeeping. To avoid this, the mechanism is usually kept in winding mode with the crown against the case. The spring in the keyless mechanism pushes the case stem and crown outwards, so a detent mechanism is fitted in the case to hold them in place.

Installation

To install a negative set movement in a case, the crown is pulled out, and the movement is inserted. When the crown is pushed in, the case stem engages with the movement stem, and the keyless mechanism switches to winding mode, where it remains unless the crown is pulled out to set the hands or remove the movement from the case. The detent that holds the case stem in one of two positions is formed by grooves in the stem and a split spring steel sleeve. The sleeve is split at its lower end to form four claws that grip the stem, and it is held in place by a brass plug. The sleeve opens when the crown is pulled out to allow the swelling on the stem to pass, and it grips the groove in the stem below the swelling to hold the crown in the hand-setting position.

Tavannes trench watch with a negative set keyless mechanism.

When the crown stem is pulled out into the hand-setting position, the spring in the keyless mechanism pushes it into the hand set position. The mechanism is put into hand set mode by removing something that is stopping it, hence the name “negative set.”

Cyma and Tavannes

Some Cyma and Tavannes watches feature movements with the American negative set keyless mechanism, which is patented under US 760647. The patent was granted to Henri-Frédéric Sandoz on May 24, 1904. This was a US version of the Swiss patent No. 16853, granted to Sandoz on August 26, 1903. Henri-Frédéric Sandoz was a prolific inventor who registered 39 patents for improvements to watches between 1889 and 1907 in his own name, and 11 more were registered by the Tavannes Watch Co. SA under his direction before his death in 1913.’

‘American’ setting

The association of negative setting is often referred to as ‘American’. This was because American pocket watches were sold as movements by watch companies like Elgin and Waltham. The cases were made by watch case companies like the Illinois Watch Case Company. A jeweller or watchmaker would put the two together for the customer. The watch companies made their movements in a small number of standard sizes. This standardisation meant that the movements be put in cases from many different case makers. By making the crown and stem part of the case, a retailer could easily swap any 18-size movement into any 18-size case.

As American watch companies began to sell watches cased at the factory, they switched over to the positive setting. This is where the stem and crown are part of the movement. These are also set by pulling up on the crown, but the ‘default’ position is the winding mode. This is how virtually all modern mechanical wristwatches work.

Summary

I have not had a watch in my collection with a negative set keyless mechanism. However, at some point, I am sure I will. The main thing to remember is that in the default position, the crown is not engaged. You need to pull the crown out to set the time. However, you need to push the crown in to engage the winding position.