In all cases the reciprocating weft way motion of the
rapiers, in and out of the shed, is such that are kept out of the shed, is such
that they are kept out of the shed (or, if they are mounted on the loom frame,
out of the sweep of the reed) sufficiently for it to be changed and the weft
beaten up. The rapiers are usually out of the shed for about a third of the
cycle, and there are two main cases to be distinguished - that of continuous
rapier motion throughout the cycle, and that of intermittent motion with a
rapier dwell outside the shed. In the former case the rapiers have to move
further out beyond the selvedges than is necessary for weft insertion, to
reconcile their continuous motion with the need to keep them clear of the
working width until the weft insertion stage of the cycle recurs. This has the
disadvantage that the rapier reaches the point at which the weft is to be
picked up having already attained a substantial fraction of its maximum velocity.
On being picked up by such a rapier the weft velocity picked up by such rapier
the weft velocity picked up by such rapier the weft velocity is abruptly
increased from zero to a high value. Against this, the rapier drive can be a
simple robust and relatively inexpensive crank actuated mechanism.
A rapier displacement timing curve for a rapier whose path
length is confined to that needed for weft insertion. This displacement has
therefore to be confined to that needed for weft insertion. This displacement
has therefore to be confined to the appropriate part of the loom cycle too, and
such motion is readily obtained from a cam actuated mechanism. During rapier
displacement the forms of the two curves are the same, and it is clear that the
velocity at which the weft is picked up is lower for the intermittent motion.
The pick up velocity may still be appreciable though, as rapier velocity can
increase very rapidly near the beginning of the displacement. During a
sinusoidal displacement for example, a rapier would reach 0.4 of it maximum
velocity with in the first 5% of its range of motion.
An interesting intermediate case arise when the rapier drive
generates 'a continuous motion and the rapiers are mounted on tr1c sley. The
rapier motion then depends on the combined effects of the drive and the motion
of the sley, which can result in the rapiers having a quasi-dwell when they are
withdrawn from the shed, as occurs on the Picanol PGW loom.
Transfer
Velocity
It is the simplest forms of weft transfer the displacement
-timing curves for the two rapiers are in phase, and the rapiers will be
considered to meet and transfer the weft when they are furthest in to the shed
and reversing their direction of motion. This for continuous rapier motion and
the corresponding curve for the weft is shown by the broken line*. Thus
although the transfer system enables the weft to be inserted during both the
rapier insertion into the shed and withdrawal from it, the weft-insertion
velocity drops to zero at the moment of transfer. Only a limited interval is
available for weft insertion over the required distance and this fall in
velocity is not in itself desirable.
Zero transfer. Velocity can be avoided by arranging for the
two rapiers to be moving in the same direction during transfer, as shown if
Fig. 5b for example. However, this entails other changes, which are not in
themselves advantageous. The rapiers have to travel further in to the shed for
instance, and may also be in it for a smaller fraction of the cycle.
Shows only one of the types of position timing curve by
which nonzero transfer velocity could be attained, but their relative merits,
and the balance of advantage and disadvantage in anyone case, have still to be fully
analyzed. Detailed studies of the extent to which such possibilities are
exploited in practice are also lacking.
Overall
Width and Working Width
Flexible rapier looms are often made in working widths
ranging from about 11/2 m to 4m. When the rapiers are
with drawn from the shed each is either wrapped round the circumference of a
drum or, more often, enters a C-shaped guide, the lower part of which may pass
under the loom. In either case, in practice, the space needed for that part of
the loom lying outside the working width is generally constant for a given
model of loom, regardless of the value of the working width, W.
The overall width (A) can therefore be written as A = W + Kf
(1) and the constant Kf is about 3m.
