FUNCTIONAL MECHANICAL PART

Abstract

A mechanical part (2, 3) including a functional surface intended to come into frictional contact with another functional surface, wherein the functional surface of the mechanical part (2, 3) is made of polyethylene furanoate.

Claims

1. A mechanical part of a horological movement, which part comprises a functional surface configured to come into frictional contact with another functional surface, wherein said functional surface of the mechanical part is made of polyethylene furanoate.

2. The mechanical part according to claim 1, wherein the entire mechanical part is made of polyethylene furanoate.

3. The mechanical part according to claim 1, wherein the mechanical part is a pallet, an escape wheel, an arbor of a wheel set, a bearing, a mainspring or toothings of gear wheels.

4. A functional assembly comprising the mechanical part according to claim 1 and another mechanical part comprising the other functional surface to be in frictional contact with the functional surface of said mechanical part, wherein at least the other functional surface of the other mechanical part is made of a material selected from polyethylene furanoate, ruby and steel.

5. The functional assembly according to claim 1, wherein the frictional contact is lubricated.

6. The functional assembly according to claim 1, wherein the frictional contact is lubricated using an oil.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0017] FIG. 1 shows the curves of the average dynamic coefficient of friction as a function of load for the PEF/PEF pair and the POM/POM pair.

[0018] FIG. 2 shows the curves of the average dynamic coefficient of friction as a function of load for the PEF/PEF pair, the PEF/ruby pair and the PEF/steel pair.

[0019] FIG. 3 partially shows a functional assembly comprising two parts, specifically an escape wheel and a pallet of a pallet-lever with a wheel made of PEF according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention relates to a mechanical part which, on one or more of its so-called functional or contact surfaces, is subject to friction with one or more functional surfaces of another part or of the same part. The mechanical part can be used in any system where friction is a concern. Applications can include automotive or electronic parts, etc. More specifically, it could be a part used in the horological field, and could in particular be a part of the movement. Examples of parts include a pallet, an escape wheel, an arbor of a wheel set, a bearing, a mainspring and toothings of gear wheels. Said part can be in contact with another part. By way of example, in the horological field, the functional assembly 1 shown in FIG. 3 can include a first part 2, which is a pallet 4 of a pallet-lever 5, and a second part 3, which is an escape wheel 6. More specifically, the pallet 4 has a rest plane A and an impulse plane B which cooperate with the rest plane C and impulse plane D of the tooth 7 of the escape wheel 6. These planes A, B, C and D are highly-stressed functional surfaces subject to high levels of friction and/or contact, which requires special materials to be used in order to reduce friction. Typically, the wheel at least is made of PEF to improve tribological performance. Alternatively, one functional surface of a part can be in contact with another functional surface of the same part. For example, this can be a mainspring formed by a blade with one face of the spring intended to be in contact with another face of the spring.

[0021] According to the invention, the mechanical part is at least in part made of polyethylene furanoate (PEF). In this way, at least the one or more functional surfaces are made of PEF. Preferably, the part is made as a whole from PEF. Comparative tribological tests with POM have been carried out and the results are shown in FIG. 1. The tests were carried out with a contact lubricated using watchmaker's oil using a ball-on-flat tribometer in alternating linear mode with a 2 mm diameter ball, the ball and the flat being made of PEF for one pair and of POM for the other pair. The tests were carried out with a maximum sine wave speed of 10 mm/s, an amplitude of 4 mm, and a normal force of 1, 5, 10, 25, 50 and 100 mN over a distance of 15 m. The loads were swept over a wide range to represent the different possible contacts in the movement and in particular the evolution of the escapement contact during the different release and impulse phases.

[0022] The coefficient of friction (CoF) of the PEF/PEF pair is seen to be at least equivalent if not lower than that of the POM reference. As soon as the load is sufficient, the oil reduces the CoF to the extremely low values typically found for hydrodynamic friction. Use of PEF allows the CoF to be reduced even further and obtains values below 0.05. Moreover, with an improved coefficient of friction, no deterioration in wear was observed.

[0023] Friction with other antagonists that are typically used in the horological industry, specifically ruby and steel, was assessed under the same testing conditions, with the ball being made of these materials during the tests. The results are shown in FIG. 2, with the PEF/PEF pair given for comparison. In general, the CoF is even lower because the elastic deformations of the antagonist are limited, thus limiting frictional forces. The values achieved are well below 0.05 over a fairly wide range of loads. The hardness of these antagonists and the use of a high load do not deteriorate the PEF and no wear can be seen.

[0024] In conclusion, the results of the tribological tests carried out on parts made of PEF are at least as good as those carried out on parts made of POM.