A crown tube is a small insert on the side of the watch case. The winding stem passes through it, and the crown sits on it. It limits the crown’s sideways movement, which lowers the risk of a broken winding stem, and gives the crown’s gasket a surface to seal against.
How is a crown tube fitted?
The tube is set into the case middle. It is fixed in one of two ways. It can be threaded and screwed into the case. Or it can be friction-pressed into the case so that it holds tight. Once in place, it links the crown on the outside to the winding stem inside the movement. When the crown is pulled out to its different positions, it slides along the tube. The tube also restricts the crown’s sideways movement. This lowers the chance of the winding stem breaking.
How does a crown tube keep water out?
A water-resistant crown fits over the tube. The crown has a gasket, and the tube gives that gasket a surface to seal against. For this to work, the tube has to match its crown. The fit must be close enough to keep out water and dust. On diving watches especially, the tube can have a thread or a bayonet cut into it. The crown then locks onto the tube and squeezes the seals. When a watch is serviced, the gasket at the crown tube, an O-ring, is replaced as standard.
Where it comes from
Screwing a crown onto its tube to seal a case is not a new idea. In the 1920s, Rolex applied for a patent on a water-resistant crown that screwed onto its tube. The same principle is found on the screw-down crowns of many dive and sports watches. The crown is turned down onto the tube. This compresses the gasket and makes a watertight seal.
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