In a weaving loom, the warp threads, which are parallel to each other and define a warp, are connected to means which permit their selective displacement perpendicularly to the plane of the fabric being woven so that the woof threads may be introduced between the warp threads that are raised and those that are not, thus defining the filling or woof.
The displacement of the warp threads is accomplished by means of control threads, each of which is provided at its bottom with a loop through which a warp thread is passed, its opposite top end being fixed to a hook at the bottom of a driving element which controls the movement of the control thread. At present the machines have a fixed capacity and a number of control hooks limited to 1,344. This number is insufficient for weaving fine threads so that several control threads intended to be raised at the same moment have to be connected to the same control hook.
The set of control threads, or harness, can be in the form of a straight harness 2 as shown in FIG. 1 in the case of the straight-fall mechanism shown in the FIG. 1 drawing, or a helix 3 as shown in FIG. 2 in the case of a lateral fall to the left or right. The connecting control threads 4 between the control hooks 5 and the loops about the warp threads pass through a guide board 6, i.e. a board that has holes 7 for the passage of each control thread 4, thus assuring that the control threads are kept in order and are aligned with the corresponding warp thread. The guide board determines the density of the weave.
Each warp thread is moreover subject to the action of spring or elastic pull-backs which are fixed on a hook beam and situated, with respect to the layer of warp threads, on the opposite side of where the control threads are situated. These pull-backs assure that each warp thread returns to its initial position when the force exerted on it by the control thread ceases to act.
If a modification is introduced to the guide board for the purpose of changing the density of the weave, while maintaining the same harness, it is found as shown in FIG. 3 that the straightness of the warp is changed, making it impossible to weave. FIG. 3 represents a fixed control block 8 of driving elements and two possible arrangements of the control threads 4 resulting from two ways of employingthe guide board 6. As seen from the drawing, the warp threads in the first configuration (shown the the dash-dot line) are in the same plane. However, in the second configuration, represented by a dashed line, the warp threads are disposed along a curved surface, preventing the filling threads from being introduced.
Only a special type of mounting with two or three superimposed guide boards permits deviations in the density of the weave of up to 15 to 20%.
If one wishes to change the density of the weave, it is therefore necessary to completely redo the set-up, which represents a considerable cost per installed control thread; it is further necessary to put the loom out of production for several weeks, which amounts to a serious loss of production.