The Watch Site banner

1 - 20 of 21 Posts

·
Registered
Joined
·
64 Posts
Discussion Starter #1
Sorry if this has been asked before but:-

Seen loads of post here and elsewhere stressing that the screwing down
of the crown imparts no water resistance of its own, it's all in the gaskets.
So I got to thinking about this whilst opening a bottle of a "popular
carbonated beverage".

The cap on this screws down a maximum. 2 full revolutions ( I counted).
No gaskets, no silicone grease, just the plastic cap.
And it's got liquid contents under what I assume is some
considerable pressure, being carbonated.
But it doesn't leak any gas never mind liquid. Even if you
squeeze it really hard (I tried).

So does the screw down crown impart no protection of its
own at shallow depths? Obviously not talking about
even 50m never mind the full 200 or whatever.
Am I missing something obvious here - any thoughts? :undecided:

One for the physics/engineering experts perhaps......

Feel free to shoot me down in flames if this is a "dumb noob"
question. :)
 

·
Registered
Joined
·
2,110 Posts
From an answer given on TZ-UK about this very subject.

http://forum.tz-uk.com/showthread.php?244319-Where-is-WR-failure-most-likely-to-occur&p=2538585&viewfull=1#post2538585

Reproduced below

Yes, and no...
This type was favored by Seiko in the 6105 and 6309 series diver's watches. These do not offer any degree of water protection above the standard non screw down crown.

This is the non-screw down crown design favored by Seiko, only one gasket and it is internal to the case tube.

This is the standard non-screw down crown favored by the rest of the watch world, the gasket is external to the case tube. Note that in all cases of the simple single gasket crown, the gasket still seals the watch even in the extended position. This is true of all designs. So, leaving the crown out, theoretically, does not alter the water resistance.

These next two are screw down crowns that have a compression gasket above the case tube. The difference is which gasket is the primary seal. With the external thread (below) the compression gasket is the primary and the tube gasket is the secondary. The Vostok design illustrated above by Huertecilla, is the externally threaded design but the secondary gasket is omitted.

With the internally threaded screw down crown (below), the primary seal is the tube gasket.

The Rolex Triple-lock crown is a combination of the internal and external screw down crown


There are advantages to each of the various designs, so choosing one over is not just on a whim.... The external thread design is the easiest to manufacture, but the secondary seal will always wear out first, so in the event of primary seal failure there can be total failure, however this ia a very rare thing to happen*. This is the reason Vostok opted to discard the secondary seal, it really doesn't do much, unless you are in the habit of leaving the crown unscrewed...
The advantage of the internal thread is the primary seal is larger and can be designed to last longer than a smaller gasket, also it is the one that will wear out first, leaving the secondary to hold the line until the next service, the disadvantage is the threads are smaller and more easily stripped.
There is one other WR seal design that is rarely used, the cap. The cap in this case actually pushes the crown down slightly and this compresses the gasket as well as spreads it into firm contact with the case tube and crown. The advantage here is during turning of the crown, there is little compression on the gasket so wear is reduced.
 

·
Registered
Joined
·
216 Posts
In this case I would say that the plastic lid itself acts as the gasket. As you screw down the cap, the plastic deforms under pressure, closing off gaps and making a good seal.
In the case of a watch, the gasket deforms to fill in the gaps between the metal surfaces not only from expansion and contraction but also the small imperfections in the metal surfaces.
 

·
Super Moderator
Joined
·
2,960 Posts
I think your premise is faulty- most soda bottle caps do have a flat, compressible rubber-like ring or disc inside that seats against the lip of the bottle top, acting as a barrier against liquid escaping. Unscrew the cap 1/2 turn and try holding it upside down or squeezing it again (but not while I'm nearby!).
 

·
Registered
Joined
·
5,710 Posts
I think your premise is faulty- most soda bottle caps do have a flat, compressible rubber-like ring or disc inside that seats against the lip of the bottle top, acting as a barrier against liquid escaping. Unscrew the cap 1/2 turn and try holding it upside down or squeezing it again (but not while I'm nearby!).
Bingo. The waxy lid bottom is the sealing surface.

We can calculate pressure drop at flow rate across connected threads. Using close tolerances and many threads per inch, threads without any elastomer seals cannot do better than about 95% of a pure vacuum. Such threads and total sealing length cannot be used in a watch crown or crown tube. Therefore to seal to 1 atmosphere the watch design simply must include an o-ring of a flat gasket.
 

·
Administrator
Joined
·
29,170 Posts
I think your premise is faulty- most soda bottle caps do have a flat, compressible rubber-like ring or disc inside that seats against the lip of the bottle top, acting as a barrier against liquid escaping. Unscrew the cap 1/2 turn and try holding it upside down or squeezing it again (but not while I'm nearby!).
Thats what i was thinking.

I like the pictures Duncan :)
 

·
Registered
Joined
·
400 Posts
No matter how precise a tread is, it will never seal 100% against moisture/humidity/water. Even if you have a plastic cap on the bottle, that will not seal properly unless there is a flat rubber/styrofoam pad inside the cap that seals directly onto the lip of the bottle. And that can be compared to the caseback/push-pull crown of any water resistant watch. However, there are cases when a watch has a snap on case and push/pull crown and still has a 200m water resistance rating...but not for diving.
 

·
Registered
Joined
·
2,100 Posts
Sorry if this has been asked before but:-

Seen loads of post here and elsewhere stressing that the screwing down
of the crown imparts no water resistance of its own, it's all in the gaskets.
So I got to thinking about this whilst opening a bottle of a "popular
carbonated beverage".

The cap on this screws down a maximum. 2 full revolutions ( I counted).
No gaskets, no silicone grease, just the plastic cap.
And it's got liquid contents under what I assume is some
considerable pressure, being carbonated.
But it doesn't leak any gas never mind liquid. Even if you
squeeze it really hard (I tried).

So does the screw down crown impart no protection of its
own at shallow depths? Obviously not talking about
even 50m never mind the full 200 or whatever.
Am I missing something obvious here - any thoughts? :undecided:

One for the physics/engineering experts perhaps......

Feel free to shoot me down in flames if this is a "dumb noob"
question. :)
There is a gasket in the cap of a carbonated beverage bottle. If you look carefully at the underside of the cap, there's an EVA insert that seals against the top of the bottle.

It's not the threads that keep the carbon dioxide for escaping, it's the EVA seal.

Myles
 

·
Registered
Joined
·
2,100 Posts
This is the crown seal design used on the 7S26-0020 (e.g. SKX007) and 8L35-0010 (Marine Master) models. The crown screws down, and the sole seal is an o-ring on the crown "shank" that seals against the inside of the case tube.



Unless the case tube tapers, screwing the crown down has no effect on the seal. Rather, it only serves to immobilize the crown and help protect it from impacts. If the case tube does in fact taper internally, then screwing the crown down compresses the o-ring to create a tighter seal. Does anyone know if the case tube is tapered?

Best wishes,
Myles
 

·
Registered
Joined
·
2,470 Posts
I always thought the purpose of a single gasket screw down crown was to prevent the crown from opening inadvertently while under water.
 

·
Administrator
Joined
·
29,170 Posts
I always understood the purpose of a screw down crown was to prevent the crown from getting ripped out by being caught on an some underwater obstacle, a reef, a rock or a wreck.

That is/was my understanding, a watch maker I know has repaired many Rolly Subs he was telling me that guys had set the time and forgot to screw the crown back down before going in the sea/pool.
 

·
Registered
Joined
·
13 Posts
That is/was my understanding, a watch maker I know has repaired many Rolly Subs he was telling me that guys had set the time and forgot to screw the crown back down before going in the sea/pool.

It´s no problem not screwing down the crown, the sealing still holds, but if you have the crown pulled out to set time/date the watch is vunerable.
 

·
Registered
Joined
·
64 Posts
Discussion Starter #18
I think your premise is faulty- most soda bottle caps do have a flat, compressible rubber-like ring or disc inside that seats against the lip of the bottle top, acting as a barrier against liquid escaping. Unscrew the cap 1/2 turn and try holding it upside down or squeezing it again (but not while I'm nearby!).


I must admit that was my first thought as well - I remember waxy/squishy
discs (or cork at one time I think) inside the crown type caps on beer
bottles. However this one has no insert at all and is a one piece moulding of non-compressible plastic as far as I can see - not sure how well the
attached scan shows this.
I've just filled the now empty bottle with water and yes, once the cap
is unscrewed more than 1/2 a turn something will leak out when squeezed
hard. Doesn't leak at all when fully closed though.

Bit of a frivolous question to start with I suppose, I was just
wondering what was going on here. I'm certainly not
going to repeat the experiment on a watch with the crown gasket
removed! :eek:hmy:

Thanks for all your thoughts though, interesting. :)
 

Attachments

·
Registered
Joined
·
549 Posts
HPN, isnt that a ring of softer grayer plastic inside the cap? it looks it from the photo... With the longitudinal grooves in the threads its even clearer that the threads alone cant maintain a pressure differential, but they do supply the compression of the seal.
 

·
Registered
Joined
·
64 Posts
Discussion Starter #20
HPN, isnt that a ring of softer grayer plastic inside the cap? it looks it from the photo... With the longitudinal grooves in the threads its even clearer that the threads alone cant maintain a pressure differential, but they do supply the compression of the seal.

Yes, I can see it looks that way, but it's just the way the light's
caught it in the scan. The cap is all the same colour and hardish plastic material - can't compress anything by hand at all. What looks like greyer
plastic is just a shoulder( if that's the right word), part of the same
one piece moulding and sits inside the bottle opening. :confused:
 
1 - 20 of 21 Posts
Top