The spiral spring , with non-contiguous and therefore frictionless spirals, is composed of a ribbon of rectangular section embedded at one end B and secured at the other end O to an axis perpendicular to the winding plane.
In watchmaking , the spiral spring is a small spring coiled in a spiral and constituting with the balance the regulating member of the mechanical watch . It returns the pendulum to its starting position at the end of each alternation.
We will assume that the axis is movable without friction and, less obviously, that it does not tend to move radially when rotated under the effect of a torque C.
Under these conditions, any torque perpendicular to the plane of winding, applied at O, is transmitted integrally at B, which would not be the case if at this point the end was not embedded but simply hooked, as will be Probably the case for this industrial realization:
The spiral spring, mainly used in precision equipment ( watches , electrical appliances, etc.), is the subject of a very special manufacturing process, whose operating procedures and tests are codified by the Technical Center for the Watch Industry. The obtaining of non-joining and equidistant turns initially requires a special conformation of the ribbon. The latter, if it were straight at the beginning, would naturally be arranged in the form of a roller with contiguous turns of very different behavior because of the frictions. Such an arrangement is found in the case of springs reminiscent of ribbon meters in their casing.
When the ends of the spiral spring are normally connected, that is to say embedded, any variation of the bending moment at one of the ends is transmitted totally at the other end: this means that in all cases the moment of Bending along the ribbon is constant.
At a given point, the curvature of the blade is defined as the inverse of the local radius of curvature. Now, since the section of the blade is constant, every variation of the bending moment is accompanied by a proportional variation of the curvature:
We will agree to say that all along the spring wound under load, the curvature is positive. The figure at the top shows a spiral spring wound up and below it, empty. At the inner end A and the point M, the curvature of the blank is positive and the remainder after winding. The point B corresponds to a point of inflection of the preform, where the curvature is zero. At point N and at the outer end C, the curvatures of the preform are negative and become positive once the spring is wound.
The shaping of the vacuum spring “S” is a complex operation that requires a lot of know-how and experience!
For other types of spiral springs, such as those used in clockwork, the vacuum form is a spiral of Archimedes , the curvatures of which increase or decrease with the oscillations. By adjusting the length of the flexible tape by a suitable device, the spring stiffness and therefore the oscillation frequency of the balance are varied, thus reducing the advance or the delay of the watch or the pendulum as much as possible.
More than any other, the spiral spring is a matter of specialists!
We assumed that the end of the spring was embedded and asserted that under these conditions the torque transmitted by the blade was identical at all points of the latter. If, on the other hand, the end of the spring is simply hooked in C, the moment is zero at this point and variable throughout the ribbon, with a maximum reaching almost 2C !
The method of attachment therefore greatly affects the maximum stress.
In most mechanical clock movements, the two ends of the balance spring are inserted into a pierced piece and are immobilized with a conical pin manually force-fitted to the clamp: a slight rotation of the balance spring can occur, which is detrimental to The precision of the clockwork movement. This defect can be eliminated by careful assembly. To avoid this risk, a patent was filed in the 1960s by the company LIP using a method of sticking the spiral with a special calibrated grain of adhesive, adhesive solid at ambient temperature, but melting under the action of heat.
The spiral spring was invented since antiquity , probably by chance after soaking a wire or bronze wound in spiral or double spiral inverted. This form of spring has, for example, been used for the closure of the fibulae which played the role of safety pin . It is found in the locks of the Middle Ages, then later to animate the mechanisms of watches with spring spiral.
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