Mainspring

Abstract

A timepiece mainspring including, in the manufactured state, an eye and a portion formed of coils with an outer coil of radius R, the eye and the portion formed of coils being connected by a neck portion having substantially zero curvature, the timepiece mainspring wherein the neck portion has a length L.sub.C comprised between 1.5 and 10 times, and preferably between 2 and 8 times, the radius R. The mainspring having this specific geometry reduces the risk of premature breakage during use, typically for an application with a k factor lower than 10.

Claims

1. A timepiece mainspring, comprising, in an initial state at the end of manufacturing before assembly in a barrel drum: an eye; a portion formed of coils with an outer coil having a radius; and a neck portion configured to connect the eye and the portion formed of coils, wherein the neck portion has substantially zero curvature, and the neck portion has a length comprised between 1.5 and 10 times the radius.

2. The mainspring according to claim 1, wherein the length comprised between 2 and 8 times the radius.

3. The mainspring according to claim 1, wherein the radius is comprised between 2 and 10 mm.

4. The mainspring according to claim 1, wherein the portion is formed of coils that touch each other.

5. The mainspring according to claim 1, wherein the portion is formed of coils that are remote from each other.

6. The mainspring according to claim 1, wherein the mainspring is intended for applications with a k factor higher than or equal to 5 and lower than 10.

7. The mainspring according to claim 1, wherein, after assembly inside the barrel drum, an efficiency between a torque supplied during unwinding of the mainspring and a torque required for winding is higher than or equal to 80%.

8. The mainspring according to claim 1, wherein the mainspring is made from an alloy of cobalt-nickel-chrome, containing from 44 to 46% wt. % of cobalt, from 20 to 22 wt. % of nickel, from 17 to 19 wt. % of chromium, from 4 to 6 wt. % of iron, from 3 to 5 wt. % of tungsten, from 3 to 5 wt. % of molybdenum, from 0 to 2 wt. % of titanium, and from 0 to 1 wt. % of beryllium.

9. The mainspring according to claim 1, wherein the mainspring is made from an austenitic stainless steel.

10. A timepiece, comprising: a mainspring including, in an initial state at the end of manufacturing before assembly in a barrel drum; an eye; a portion formed of coils with an outer coil having a radius; and a neck portion configured to connect the eye and the portion formed of coils, wherein the neck portion has substantially zero curvature, and the neck portion has a length comprised between 1.5 and 10 times the radius.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 represents a plan view of the mainspring according to the invention having an increased neck length Lc.

(2) FIG. 2 is a diagram showing the winding curve (upper curve) and unwinding curve (lower curve) of the mainspring according to the invention.

DETAILED DESCRIPTION

(3) The present invention relates to a mainspring 1 represented in FIG. 1 in the manufactured state. The ‘manufactured state’ means the initial state at the end of manufacturing, before assembly inside the barrel drum. The mainspring according to the invention is more specifically adapted to applications with a k factor (ratio of the barrel core diameter to the spring thickness) higher than or equal to 5 and lower than 10. It includes, in a conventional manner, an eye 2 and a portion 3 formed of coils with an outer coil of radius R. The coils can touch each other as represented in FIG. 1 or be remote from each other (not represented). Eye 2 is connected to portion 3 by a neck portion 4 having substantially zero curvature forming an area of inflection between eye 2 and portion 3 of reverse curvature to that of the eye. Eye 2 and portion 3 formed of coils are manufactured in a known manner, for example by hammering and calendering respectively.

(4) According to the invention, the neck portion has the characteristic of having an increased length L.sub.C compared to prior art springs which typically have lengths L.sub.C smaller than or equal to radius R of the outer coil. More precisely, in the manufactured state, this length L.sub.C is greater than the external radius R of portion 3, with values comprised between 1.5 and 10 times radius R and preferably between 2 and 8 times radius R. Typically, external radius R is comprised between 2 and 10 mm. By way of example, the mainspring according to the invention has a radius R of 5 mm and a length L.sub.C of 40 mm. Its other dimensions are as follows: a total deployed length of 500 mm, a thickness of 90 μm and an eye diameter adjusted for a core diameter of 1.5 mm, i.e. typically comprised between 1 mm and 1.5 mm.

(5) As a result of increased length L.sub.C, the difference in curvature between the wound state and the manufactured state is reduced at the beginning of the calendered area. Hence, the plastic deformation experienced by the spring is lower when it is first wound, which limits the risk of premature breakage.

(6) The mainspring according to the invention thus enjoys optimised geometry, which reduces its fragility during use. Further, torque measurements during winding and unwinding demonstrated that this spring geometry ensures good performance of the spring, with an efficiency greater than or equal to 80% between the torque provided during unwinding and that required for winding. By way of example, FIG. 2 represents the winding curve (upper curve) and unwinding curve (lower curve) for measurements made after a half-turn of unwinding. For this example, efficiency of 84% was obtained.

(7) The mainspring according to the invention can, for example, be made from an austenitic stainless steel or a cobalt-nickel-chromium Nivaflex® alloy containing from 44 to 46 wt. % of cobalt, from 20 to 22 wt. % of nickel, from 17 to 19 wt. % of chromium, from 4 to 6 wt. % of iron, from 3 to 5 wt. % of tungsten, from 3 to 5 wt. % of molybdenum, from 0 to 2 wt. % of titanium, from 0 to 1 wt. % of beryllium.