Now that we have seen what can and cannot affect water resistance, the next question is - “How does water resistance really work? Where are all these gaskets and seals go? What case designs are out there and which type is better?”
Crystal sealing -
Figure 1A. Two piece Case Designs
Figure 1B. Snap Back Case With Vertical Seal
Glass crystals (either mineral or sapphire) are normally held in place by a nylon “L” shaped gasket (orange, left, in the Figure 1A), you can see that as the external pressure increases, the glass will press down on the foot of the “L” an improve the seal. For less demanding applications, there is such a thing as an “I” gasket, it just does not have the foot. Seiko is the primary exception to this, using a soft rubber “L” gasket, the principle is the same, it just requires a screw-in or snap-on ring to hold the crystal down, as there is no friction to speak of in this case.
Plastic crystals can be installed in one of three ways, first is the use of adhesive, hardly anyone other than the most cheap watches use this method these days. The second (shown in the center Figure 1A) is the tension ring or armored crystal. Here the skirt of the crystal is expanded by a steel spring (pink) and again, friction holds it in place. The sealing is achieved by the plasticity of the crystal, in cases where extra sealing is required, you can apply a liquid seal on the mating surface prior to crystal installation. This will dry and form a thin rubbery gasket, but generally this is not required unless the surface finish of the case has been damaged. Third is the wedge seal (right, Figure 1A). In this case, the crystal is compressed radially inward and slipped into a conical undercut in the case, when released, the crystal, attempting to expand back to its original shape wedges itself into the case. Because of the angle the crystal will tend to press downward into the case.
The nylon gasket is probably the best being good to hundreds of atmospheres due to the pressure increasing the seal compression, with the tension ring coming in second. It should be noted that increased pressure on a tension ring crystal will also increase sealing as the geometry of the crystal it such that pressure on the crystal tend to flare the shirt, but the lack of a soft seal still relies on a good surface finish to achieve a water-tight seal. The wedge type crystal is very limited, usually not more than 44 psi as the elasticity of the plastic is all there is to achieve sealing, these are rare today, due to this limitation.
Case design -
The screw back case (left, Figure 1A) is most common these days, as it provide a very good seal. The actual designs may vary, placing the o-ring groove in the case-back, or just above the threads, but the principle is the same. The threading of the back into the case compresses the gasket. The snap-back case (center, Figure 1A) works the same way, only the compression is governed by design, not the amount of torque applied the back. Sometimes, but oddly enough less often that the manner shown, you will see the gasket situated so it seals against the vertical face of the case wall (Figure 1B), this is a better as wear on the “snapping” surfaces does not decrease the compression.
Figure 2. Frontloading Case Designs
Frontloaders (Figure 2) - There are two types to frontloaders: monoblocs (left) and assemblies (right). The assembled type is common in US dress watches of the late fifties and early sixties. The case back is a separate piece but permanently sweated or pressed into the bezel. These usually are fitted with the wedge type crystal using the downward wedging affect to hold the dial-movement assembly in place (right). This was done for ease of production and elimination of threading, which will not be that strong in gold cases. On the diver’s watch front loaders, where better water protection was required, more positive crystal retention is employed, either a bayoneted ring or a screw-in ring is used to positively lock the crystal to the case (left).
Monoblocs are just that, the entire case is hogged out of one piece of steel, but other wise are the same asassembled case frontloaders.
The last type of case back closure if the use of multiple small screws, this is, in principle, just a flat plate with screws at the corners.
Figure 3. Two Piece Crown
In the frontloader, the stem becomes a problem. How can one remove the stem from the movement without having access to the back of the movement, where the stem release is located? The two piece stem was the usual answer in the past (Figure 3). This arrangement allows the stem to slide apart when the movement is withdrawn straight out the front, or by pulling on the crown, the stem will snap apart.
Today, special stems required by two piece stems are less common, therefore, modern frontloaders usually have a small screw or plug over the stem release button. To remove the movement the plug or screw is removed, the stem release pushed and the stem withdrawn as normal, then the movement is free to be removed. However, to me, this sort of negates the entire point of a frontloader, namely, having one less opening in the case to water-seal.
The pros and cons of the major case designs are as follows:
- very economic in use of material, it can be made entirely from thin sheet
- very simple to open and close
- very poor water resistance, usually not even dust proof
Snap back -
- inexpensive to manufacture
- capable of reasonable water resistance
- difficult to open without damage to the case
- difficult to close, if high water resistance design
Screw in back-
- more expensive to manufacture
- capable of very good water resistance
- requires special tools to open and close
- more expensive to manufacture than a snap back but easier to manufacture on a small scale than a screw-in back
- capable of very good water resistance, provided the screws are torqued in sequence
- can be opened and closed with a common screwdriver
Frontloaders (both assemblies and monblocs)
- one less opening in the case (if no stem release plug)
- more expensive to manufacture
- more difficult to open and close
- requires special parts for two piece stem (if no stem release plug)
- theoretically a much stronger case
Crown designs -
Figure 4. Swiss Non-screw Down Crown
Figure 4 is a picture of a typical Swiss non-screw down crown with an annular gasket. There are variations with two o-rings but these are in principle the same as shown, the second o-ring will only be for back-up. The limitation is that seal compression is governed by the width of the o-ring and the distance between the wall of the crown and the wall of the case tube. Turning the crown will wear the inside of the o-ring and over time the gasket compression will decrease.
When the o-rings are new, these are easily capable of withstanding pressure around 300-400 psi, this will drop as the o-ring wears.
Note that even when the crown is in the full out position, the o-ring is still in contact with the case tube. This means the crown will continue to be water proof with the crown in the setting position.
Figure 5. Seiko Non-screw Down Crown
Figure 5 is the non-screw down crown design favored by Seiko. It places the o-ring on a stem extension of the crown and seal against the inside of the case tube, but works the same as the Swiss model, and has the same limitations.
Figure 6. Internally Threaded Screw Down Crown
Figure 7. Externally Threaded Screw Down Crown
Figures 6 and 7 are the most common screw down crown designs, both are double sealed with an o-ring similar to those of the non-screw down designs seen above and a compression gasket in the head of the crown. The compression gasket is always the primary seal, the annular seal is a back-up to prevent moisture ingress when the crown in the setting position. Since the upper gasket comes out of contact with the case tube on unscrewing, there is very little wear on the upper gasket, leading to a long service life, with no degradation of pressure resistance over the life, if properly compressed.
Proper compression is achieved through torquing the crown to get about 30% thickness compression of the upper gasket. Over-torquing leads to two types of failure, one, thread damage, ie stripping the threads, and two cutting the gasket, which is basically pinching the gasket until the case tube cuts the gasket.
Figure 8. Early Screw Down Crown
Figure 8 is an early design for screw down crowns, commonly seen in Seiko 6105, 6306 and 6309 diver’s watches. These do not provide additional water resistance or improved wear resistance over their non-screw down brethren, but merely prevent the stem from being pulled out (and possibly moved to reset the time, as the water resistance will remain the same). However, since Seiko used this design on the non hand-windable automatic movements, the wear on the o-ring is reduced greatly.
Figure 9. Compressed O-ring Design
Figure 9 is an example of an annular o-ring, similar to that used in the non-screw down crown shown in Figure 4, but applies pressure to the top of the crown to expand the o-ring to achieve better compression. In this example, when the cap is screwed home, the crown squishes the o-ring and expands it horizontally into the crown and case tube. When the cap is off, the relaxed position of the o-ring reduces the gasket pressure on the case tube, so when winding or setting the pressure is reduced, this equates to reduced wear. Other manufactures have been known to use a lever with a camming surface mounted on a bridge over the crown to achieve the same affect.
Other holes in the case -
Figure 10. Standard Two Piece Pusher
Figure 11. Standard Two Piece Pusher With Screw Collar
Figure 12. Economy Pusher With C-Clip Retention
Pushers- Figures 10, 11 and 12 show typical pusher designs used on chronographs and multi-function watches. Sealing is achieved in a manner similar to that shown in Figure 4. Wear in these is from the up and down movement of the o-ring on the case tube. In Figure 10 the chamber marked “A” is a water trap, when the pusher is submerged this chamber may fill with water and actuation of the pusher will “pump” the water into the o-ring, increasing the water pressure on the gasket. At near the pressure limit this may fail the seal. However, trapped air will tend to preclude water entry, and given the small gap between the pusher cap and case tube the air will have a hard time getting out if immersion is quick.
The pusher with a screw collar is functionally the same as the plain design and offers no improvement in sealing or wear, the collar simply prevents accidental depression of the pusher when the collar is locked down.
Figure 12 shows the typical economy pusher popular today. It uses a “C” clip (purple in the figure) to retain the pusher head instead of a screwed in stem. These pushers use a floating seal to simplify the manufacture of the component parts, which tends to compress the seal on pusher depression. This designs major drawback is that the o-ring is easily pinched when the assembly is installed due to the small size of the o-ring.
Pressure Relief Valves
Figure 13. Doxa/Rolex Patent Pressure Relief Valve
Figure 13 is the Doxa/Rolex patented relief valve design. It is incredibly simple. When the internal pressure exceeds the external pressure plus the spring force, the poppit moves up and unseats the rubber gasket. Just as a point of fact, the internal pressure will never actually equalize, as the spring force must be over come as well as the external pressure.
Figure 14. Manual pressure Relief Valve
Figure 14 is a manual pressure relief valve. Here the cap must be manually unscrewed to unseat the gasket, and manually closed to return the case to the water-tight state.
Figure 15. Automatic Pressure relief Valve with Safety Cap
Figure 15 is an automatic pressure relief valve with a safety cap. In this design the valve is locked shut when the cap is screwed down, but when the cap is unscrewed, the valve actuates automatically as in the Doxa/Rolex design.