Watch-case assembly resistant to thermal shock

Abstract

A watch-case assembly comprises a bezel made of sintered alumina or tungsten-carbide cermet, a crystal made of sapphire, and a watch-movement casing formed of a nickel steel having a composition by weight of between 35% and 50% nickel. In such an assembly the thermal-expansion coefficient of all materials lies between 6.1 and 7.0 .times. 10.sup..sup.-6 cm/cm.degree.C so that a thermal shock will not damage the assembly.

Description

FIELD OF THE INVENTION

My present invention relates to a watch case. More particularly this invention concerns a watch-case assembly including a watch case of hard material and a crystal of hard material.

BACKGROUND OF THE INVENTION

At present it is common to make a watch case of extremely hard material such as very hard metal or sintered alumina. These cases are used with crystals made of synthetic sapphire. The hard material is not normally employed to make the entire case but is only a bezel or cap which protects a metallic casing made of stainless steel in which the watch movement is mounted. Such extremely hard materials are used for the bezel and crystal as they are scratch resistant. They have the disadvantage that they cannot be plastically deformed but are otherwise ideally suited for such use.

When these materials are used together their different coefficients of thermal expansion can lead to breakage or damage to the steel between the movement casing and the crystal. The expansion coefficient for the stainless steel parts normally used to contain the movement is 20 .times. 10.sup.-.sup.6 cm/cm.degree.C (11.1 .times. 10.sup.-.sup.6 in/in.degree.F). The thermal-expansion coefficients for the sapphire crystal parallel and perpendicular to its optical axis are equal to 7.0 .times. 10.sup.-.sup.6 cm/cm.degree.C (0.9 .times. 10.sup.-.sup.6 in/in.degree.F) and 6.1 .times. 10.sup.-.sup.6 cm/cm.degree.C (3.5 .times. 10.sup.-.sup.6 in/in.degree.F). The sintered-alumina bezel has a thermal-expansion coefficient equal to approximately 6.7 .times. 10.sup.-.sup.6 cm/cm.degree.C (3.7 .times. 10.sup.-.sup.6 in/in.degree.F). The interaction of these various coefficients creates a great possibility that a sudden change in temperature, as for instance when the watch is worn by a swimmer or a skier, will break the crystal or at least rupture the seal between it and the casing.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide an improved watch-case assembly.

Another object is the provision of such an assembly which has a high hardness crystal and a scratch resistant case, but which does not exhibit the above-described fragility to thermal shock.

SUMMARY OF THE INVENTION

These objects are attained according to the present invention in a watch-case assembly comprising three principal elements, a hard-material bezel or cap, a crystal made also of hard material, and a metallic casing for the watch movement. In accordance with the present invention all three of these parts are made from materials having substantially similar thermal-expansion coefficients.

In accordance with the present invention the bezel is made of sintered alumina and the crystal of sapphire. Both of these materials have practically the same thermal-expansion coefficient, since the sintered alumina is merely a polycrystalline form of sapphire. The bezel can also in accordance with this invention be made of tungsten-carbide cermet (WC-Co composite) whose expansion coefficient can be matched with that of the sapphire in an appropriate type of composition. In accordance with the present invention any hard metal whose expansion coefficient can be adjusted to match that of sapphire can be used for the bezel.

In accordance with yet another feature of this invention the container is formed of a ductile, easily machinable metal. Nickel steel having a content of between 35% and 50% by weight of nickel has the desired thermal-expansion coefficient and is ideally suitable for use in the assembly according to the present invention. Such an assembly can be subjected to relatively brusque thermal shock without any fear that the different expansions of the different parts will result in damage of them.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become more readily apparent from the following, reference being made to the accompanying drawing in which FIGS. 1 - 3 are sectional views through details of assemblies in accordance with the present invention.

SPECIFIC DESCRIPTION

As shown in FIG. 1 a cap or bezel 1 of sintered alumina surrounds a casing 3 for the watch movement. The casing is formed with a groove 4 in which is tightly received the edge of the sapphire watch crystal 2. The container 3 is made from annealed 42% nickel steel.

The assembly shown in FIG. 2 has a bezel 1' of sintered alumina, a crystal 2' of sapphire formed on its outer rim with an outwardly projecting lip 8, a casing 3' of the above-described 42% nickel steel, and an L-section circular gasket 7 which receives the edge of the edge 2' and is also formed of 42% annealed nickel steel. In such an arrangement the circumferential lip 8 serves to protect the relatively soft part 7.

Where it is desired to completely hide the soft-metal part 3 and 7, bezel 1" is provided having an inwardly projecting overlapping lip 9 that overlies the parts 7 and 3. The sapphire crystal 2" has an outwardly projecting lip 11 that underlies the lip 9 and is clamped between the L-section gasket 7 and the watch casing 3.

All of the parts of the assemblies described above have a thermal-expansion coefficient equal to between 6.1 .times. 10.sup.-.sup.6 cm/cm.degree.C (3.5 .times. 10.sup.-.sup.6 in/in.degree.F) and 7.0 .times. 10.sup.-.sup.6 cm/cm.degree.C (3.9 .times. 10.sup.-.sup.6 in/in.degree.F).

Claims

I claim:

1. A watch-case assembly comprising:

a cap of hard material;

a crystal of hard material secured in said cap; and

a metallic watch-movement casing secured to said crystal and cap, said crystal, said cap, and said casing having substantially the same coefficient of thermal-expansion.

2. The assembly defined in claim 1 wherein said cap is made of sintered alumina.

3. The assembly defined in claim 1 wherein said coefficient lies between 6.1 .times. 10.sup.-.sup.6 cm/cm.degree.C and 7.0 .times. 10.sup.-.sup.6 cm/cm.degree.C.

4. The assembly defined in claim 2 wherein said cap is made of tungsten carbide cermet.

5. The assembly defined in claim 2 wherein said crystal is made of sapphire.

6. The assembly defined in claim 3 wherein said crystal is formed with an outwardly projecting lip overlying said casing.

7. The assembly defined in claim 3 wherein said cap is formed with an inwardly projecting lip overlying said casing.

8. The assembly defined in claim 3 wherein said container is made of a nickel steel having a composition of between 35% by weight and 50% by weight of nickel.

9. The assembly defined in claim 8 wherein said steel has a composition of about 42% by weight of nickel.

10. The assembly defined in claim 9 wherein said crystal is made of sapphire.