Revisiting the screw-down crown, a key element for improving water resistance

Since most diver's watches and tough field watches use screw-down crowns, it's easy to imagine that this mechanism is a way to improve water resistance and durability. But what exactly are the benefits of screw-down crowns? This time, we'll reconsider the benefits of screw-down crowns and introduce how to use them properly.

The introduction of the revolutionary screw-down crown

　Rolex is the first name that comes to mind as a pioneer in providing waterproofing to wristwatches. According to the company, "Rolex invented the Oyster in 1926, and it was the world's first waterproof wristwatch case, featuring a patented system in which the bezel, case back, and crown were screwed into the middle case." What is noteworthy here is that the screw-down crown had already been invented in 1926, and that it was considered important, along with the screw-back, as a mechanism for improving waterproofing.

　Numerous methods have been proposed to improve the water resistance of the crown, such as Mido's successful mass production of the cork crown sealing system in 30. However, the fact that the screw-down crown, which was put into practical use earlier, is still in use today shows how revolutionary and rational its structure was.

Screw-down crown structure and measures to improve water resistance

　A screw-down crown has a female thread on the inside of the crown and a male thread on the tube extending from the case, and these are used in a mated state. During normal use, the screw is tightened and cannot be operated, but when winding or adjusting the time, the screw must be loosened to release the mating.

　The screw-down crown's threads are designed to prevent water from seeping in. However, Seiko is one brand that realized this was not enough under high water pressure, and so they equipped their diver's watches with a very thick gasket inside the crown, greatly improving the water resistance.

　In order for the gasket (O-ring) to fully demonstrate its waterproof performance, it must be compressed appropriately, and tightening the screw of the screw-down crown is also necessary to compress the O-ring.

　If you have a screw-on lid insulated bottle, take a look at the lid. Many have an O-ring or similar rubber gasket. When you tighten it, you should hear a "squeak" sound and feel the resistance of the rubber. When you do this, the O-ring (or similar gasket) is crushed by the screwing of the lid, ensuring water resistance. While the structure is different from a screw-down crown, the principle is similar.

　If you look at a catalog of industrial O-rings, you will see the appropriate placement method for each shape and the appropriate "compression rate," which indicates how much to compress them. You will understand that how much compression is important for optimal use of the O-ring. With a screw-down crown, the crown retracts as it is tightened, eventually compressing the O-ring appropriately and completing the tightening process. In this state, the O-ring is designed to provide maximum water resistance.

　There are other benefits to using a screw-in crown. In the case of a crown and winding stem that are not screw-in, it is the O-ring that supports the force perpendicular to the winding stem on the case side. Therefore, when the crown is subjected to force, the O-ring is compressed in the direction of the pressure, reducing the adhesion on the opposite side and, strictly speaking, reducing the water resistance (if there is a crown tube, it acts as support when pressed to a certain extent, making it less likely that the water resistance will be significantly reduced).

　On the other hand, with a screw-down crown, the crown is firmly connected to the case through a tube attached to the case. This means that even if force is applied vertically to the crown and winding stem, no force is applied to the winding stem, which reduces deformation of the O-ring and prevents a deterioration in water resistance. From the above, it can be seen that the robustness is significantly improved compared to non-screw-down crowns.

Important points to note when using models with screw-down crowns

　As mentioned earlier, screw-down crowns are designed to ensure water resistance when tightened. Therefore, operating the crown when your hands or the watch are wet can allow water to enter the case, so this should be avoided. Also, you should never use a watch with the crown screwed in an unscrewed position. Not only will this not ensure water resistance, but if the crown is subjected to any kind of impact in this state, there is a very high risk of the winding stem breaking.

　If the O-ring is properly compressed, water resistance will be ensured, so it's best to tighten it firmly. That said, it's difficult to determine how tightly the crown is. If you're worried that you haven't tightened it enough, try tightening it yourself and then have the staff at the store where you purchased it check how tight it is. After that, don't forget to check whether you can loosen it yourself. Even users with sufficient finger strength can loosen the crown after long-term use (due to vibration, contact with the back of the hand, or forgetting to tighten it), so it's a good idea to check how tight it is on a daily basis.

　The most important thing to watch out for with a screw-down crown is jamming the screws. Forcing the male and female threads into the crown when they're not properly aligned can damage the screw surface and cause it to jam. In this state, the screw won't tighten all the way, and the gap in the threads is large, meaning water resistance cannot be ensured. In the worst case scenario, because the screw surface is damaged, the tube and crown will need to be replaced, and you'll need to be prepared to spend a fair amount on repairs.

　To avoid this, we recommend checking the position of the threads before tightening. Before tightening, push the crown straight in (it doesn't need to be very forceful) and turn it in the loosening direction. You will feel the threaded surfaces come into contact with each other and slide. If you continue turning it in this manner, you will hear a "click" as the screws begin to engage and you will feel them sink in. If you start tightening slowly from this point, they will engage smoothly. The appropriate starting point for tightening varies depending on the individual watch, so if there is a mark on the end of the crown, you can use that as a guide to remember it.

　Although it is not a screw-down crown, the Panerai Luminor is a watch that clearly demonstrates that water resistance is ensured by compressing an O-ring. A locking mechanism called a crown protector presses the crown against the O-ring, ensuring water resistance of 300m (although this varies depending on the model).

　This method will also minimize the "failures" that occur when screwing in the parts, as mentioned above.

Finally

　The screw-down crown is a rational solution to improve water resistance and robustness. To ensure its performance, it is important to understand how it works and use it properly.