and which extends parallel to the longitudinal axis x x at the same time forming a closed loop by passing over two spools 22 23 of which one 22 is fastened to the element 2a fixed to the component 2 and the other 23 is fastened to the element 3a fixed to the component 3 a traction between the components 2 and 3 results in a traction of the band 21 it may be noted however that the rigidity spring function is ensured essentially for a traction of the band this being explained by the use of glass fibers of which the general direction is parallel to the longitudinal axis x x under compression the rigidity is near to zero but this does not present any special problem particularly in respect of this specific use described hereafter with reference to fig1 as in the preceding exemplary embodiment fig3 8 the two diaphragms 20 ensure the guidance of the movable parts in this case however annular pieces 24 25 are provided so as to continue to ensure the centering of the movable parts in the event of a breakage of the diaphragms 20 it would of course also be possible to consider providing such pieces 24 25 in the first embodiment furthermore to guarantee that the nominal length of the assembly is maintained whilst at the same time preserving a tension in the glass fibers an adjustment of length at one of the ends of the band can be provided moreover the limit stop abutments are ensured by the annular piece 25.