The present invention concerns bearings for timepieces and more specifically of the shock absorber type. Designers of mechanical watches have for a long time devised numerous devices for absorbing the energy resulting from a shock, particularly a lateral shock, by the abutment of the arbour against a wall of the hole in the base block through which the arbour passes, while allowing a temporary movement of the pivot-shank before it is returned to its rest position under the action of a spring.
FIGS. 1 and 2 illustrate a device, called a double inverted cone device, which is currently used in timepieces found on the market.
A support 1, the base of which comprises a hole 2 for the balance staff 3 ending in pivot-shank 3a, allows a setting 20 to be positioned, in which a pierced stone 4, traversed by pivot-shank 3a, and an endstone 5 are fixedly secured. Setting 20 is held in a recess 6 of support 1 by a spring 10 which, in this example, includes radial extensions 9 compressing endstone 5. Recess 6 includes two shoulders 7, 7a in the form of inverted cones on which complementary shoulders 8, 8a of setting 20 rest. Said shoulders must be made with a high level of precision. In the event of an axial shock, pierced jewel 4, endstone 5 and the balance staff move and spring 10 acts alone to return balance staff 3 to its initial position. Spring 10 is sized to have a maximum limit of movement so that, beyond the maximum limit, the balance staff comes into contact with stop members allowing said staff to absorb the shock, which the pivot-shanks of the staff cannot do without breaking. In the event of a lateral shock, i.e. when the end of the pivot-shank unbalances setting 20 out of its lest plane, spring 10 cooperates with the complementary inclined planes 7, 7a; 8, 8a to recentre setting 20. These bearings have been sold for example under the trademark Incabloc®. These springs may be made of phynox or brass and are manufactured by conventional cutting means.
Shock absorber bearings in which the spring, the pierced jewel and the endstone form a unit are also known. The advantage of these shock absorber bearings is that they are less expensive.
Thus, U.S. Pat. No. 3,942,848 discloses a shock absorber bearing comprising an annular body intended to be driven into a bridge or plate. A spring, shaped to form a conical recess, is secured to the body. This recess forms a cup bearing inside which a conical balance pivot is engaged. In this design, the pivoting conditions are not very favourable, since the pivoting of metal on metal causes significant friction. Further, a cup bearing according to U.S. Pat. No. 3,942,848 cooperating with a conical pivot is ill-suited for use in a high quality timepiece, since the positioning of the balance is not precise.
Moreover, the springs used in these shock absorber bearings are made of crystalline metal. The use of crystalline metals for these springs may cause certain problems. Indeed, crystalline metals are characterized by weak mechanical properties such as limited elastic deformation which can lead to plastic deformation if the shocks are too great. This is exacerbated by the fact that the springs currently used cannot be devised with complex shapes and, consequently, the elastic deformation of current springs is very close to the limit of elasticity.
Thus, if too great a shock is applied to the timepiece, the movement of the jewels and the balance may be of large amplitude and consequently plastic i.e. permanent deformation of the spring may occur. The spring becomes less efficient at absorbing shocks and re-centring the balance staff in its rest position since it no longer returns to its original shape and therefore loses elasticity.
This permanent deformation may also occur when said springs are handled and set in place, when they are removed for lubrication or during finishing or after sales operations.
Shock absorber bearings in which the spring, the pierced jewel and the endstone form a unit are also known. The advantage of these shock absorber bearings is that they are less expensive.
Thus, U.S. Pat. No. 3,942,848 discloses a shock absorber bearing comprising an annular body intended to be driven into a bridge or plate. A spring, shaped to form a conical recess, is secured to the body. This recess forms a cup bearing inside which a conical balance pivot is engaged. In this design, the pivoting conditions are not very favourable, since the pivoting of metal on metal causes significant friction. Further, a cup bearing according to U.S. Pat. No. 3,942,848 cooperating with a conical pivot is ill-suited for use in a high quality timepiece, since the positioning of the balance is not precise.
Moreover, the fact of using a spring shaped to form a conical recess has the drawback of having a radial play which depends on the axial play or movement. Indeed, the conical shape of the spring allows the wheel arbour to be held properly in normal conditions. However, when the springs are deformed, the spring moves axially and radially. When the spring moves axially, the conical shape of the spring involves the presence of a radial movement as well. It is then noted that the greater the axial movement the greater the radial movement will be.