As is known, in a conventional weaving loom, the warp yarns are unwound from a beam and then pass over a back-rest roller. Downstream of the latter, depending on the selected weave, the various parallel warp yarns are drawn alternately upwards and downwards in order to form the shed. The various warp yarns subsequently meet again at the fell point, where the comb beats up the cloth after each weft insertion.
Within the shed, the yarns are drawn upwards and downwards by means of healds. It will easily be appreciated that these warp yarns drawn in this way undergo mechanical stresses whenever the shed opens. These stresses lay the yarns very firmly onto their successive guides (yarn guides, shield eyes, comb, etc.), thus causing fraying and bringing about yarn breaks which bring the weaving loom to a stop, thus slowing the actual production speed and impairing the uniformity of the cloth obtained.
Some designers of weaving looms attempt to reduce the stresses imparted to the yarns by giving the back-rest roller a reciprocating movement in synchronism with the opening of the shed. Unfortunately, the considerable weight of the back-rest roller and, consequently, its high inertia reduces mobility, the more so because the beating-up frequency is high.
It will be appreciated that the mechanical stresses which the yarns undergo are the greater the higher the beating-up speeds. Thus, it was found that, for high-speed weaving looms, that is to say mainly those in which the weft yarn is driven by water jet or air jet, it is difficult to exceed the speed of 1000 to 1200 beats per minute, without seeing signs of serious damage to the warp yarns and a large number of breaks, the more so because the warp yarns have a high modulus of elasticity.
This phenomenon, which is the more pronounced, the more rigid the warp yarns are, gives rise, in particular, to more frequent malfunctions during the weaving of polyester than during the weaving of polyarnide.
The invention therefore attempts to solve the problem of warp yarn breaks following high mechanical stresses experienced by the yarns at the moment the shed opens and, as a consequence, to increase the beating-up speed of the looms, whilst at the same time maintaining the same weaving quality.