The formation of the spinning triangle
The turns of twist in a yarn are generated
at the traveler and move contrary to the direction of yarn movement toward the drafting
system. Twist should run back as far as possible toward the nip line of the
rollers, but it never reaches as far as the nip because, after leaving the
rollers, the fibers first have to be diverted inwards and wrapped around each
other. The twist moves up until angle κ (which is
the angle of the fiber arrangement in the yarn) is equal to angle η of the spinning triangle
(Fig. 84). There is therefore always a triangular bundle of fibers without
twist, the socalled spinning triangle, at the exit from the rollers. By far the
most end breaks originate at this weak point, because the yarn tension in the
balloon can be transmitted almost without obstruction as far as the drafting
system, whereas twist in the spinning triangle is zero.
The
dimensions of the spinning triangle (width and length)
(see also: W. Klein, Spinning Geometry and
its Significance, International Textile Bulletin, Zurich, 1993)
The dimensions of the triangle and their
influence on spinning are derived hereafter by some statements in an
uncomplicated scheme, starting with the width of the triangle.
With a given outlet width of W, length (L) of the spinning triangle determines in turn the spinning width (WS), which – unfortunately – is always smaller than W. Due to the difference between W and WS, the edge fibers leaving the drafting system are not caught by the spinning triangle and therefore not incorporated into the yarn. These fibers are lost by forming fly and fluff or they are attached to the outside of the yarn already formed in an uncontrolled manner, thus increasing hairiness. The greater the difference between W and WS, the higher the loss of fibers, the greater the hairiness, and also the adverse impact on yarn structure. Width WS should therefore be as close as possible to W. On the other hand, the length of the spinning triangle depends mainly on the twist according to the following correlation: since twist always rises to a state where tie-in angle η at tie-in point E and fiber disposition angle κ in the yarn are equal, high yarn twist results in a short (L1), but low yarn twist in a longer spinning triangle (L2). This means that the greater length (L2) increases the size of the “spinning triangle” weak point and thus the ends down rate. To keep the ends down rate at the same level as for high-twist yarns, the yarn manufacturer is forced to reduce yarn tension by lowering spindle speed (e.g. when spinning knitting yarns).
With a given outlet width of W, length (L) of the spinning triangle determines in turn the spinning width (WS), which – unfortunately – is always smaller than W. Due to the difference between W and WS, the edge fibers leaving the drafting system are not caught by the spinning triangle and therefore not incorporated into the yarn. These fibers are lost by forming fly and fluff or they are attached to the outside of the yarn already formed in an uncontrolled manner, thus increasing hairiness. The greater the difference between W and WS, the higher the loss of fibers, the greater the hairiness, and also the adverse impact on yarn structure. Width WS should therefore be as close as possible to W. On the other hand, the length of the spinning triangle depends mainly on the twist according to the following correlation: since twist always rises to a state where tie-in angle η at tie-in point E and fiber disposition angle κ in the yarn are equal, high yarn twist results in a short (L1), but low yarn twist in a longer spinning triangle (L2). This means that the greater length (L2) increases the size of the “spinning triangle” weak point and thus the ends down rate. To keep the ends down rate at the same level as for high-twist yarns, the yarn manufacturer is forced to reduce yarn tension by lowering spindle speed (e.g. when spinning knitting yarns).
Not only yarn twist but also machine design
affects the length of the spinning triangle through wrapping angle λ (Fig. 85) of the fiber
strand at the front roller. The wider this angle, the longer the spinning
triangle with all its advantages and disadvantages. However, two additional
advantages of the deflection at the front roller are worth mentioning; firstly
the extra guidance of the fibers by supporting the fibers without clamping
them, and secondly the reduction of the abrupt bending-off of the edge fibers.
Being supported over a small surface area at the front roller up to lift-off
line H, the fibers are
gathered-in curving from the edge and tied in firmly and regularly. Fiber loss
is also reduced (Fig. 86).
Of course, when discussing the advantages of
a longer spinning triangle it is always assumed that most of the fibers in the
fiber strand are longer than the spinning triangle. This is mostly the case, as
the length of the spinning triangle varies according to the machine design
(inclination of the drafting system α,
height IG between lappet
F and front
roller etc.) of different manufacturers in a range between 2.5 and 7 mm
only (Fig. 82).
Fig. 82
Captions for Fig. 82:
D
|
Drafting system
|
C
|
Cylinder support
|
E
|
Spinning path
|
F
|
Yarn guide eyelet
|
B
|
Balloon checking ring
|
R
|
Ring rail
|
S
|
Spindle
|
α
|
Angle of drafting system relative to horizontal
|
β
|
Angle of drafting system relative to spinning path axis
|
γ
|
Angle of the thread on the spinning path relative to the vertical
|
δ
|
Angle of the leg of the thread balloon relative to the spindle axis
|
lB
|
Balloon height (variable)
|
IG
|
Distance between drafting system and thread guide eyelet (variable)
|
IF
|
Distance between thread guide eyelet and top of spindle or tube
(variable)
|
IS
|
Spindle height
|
IK
|
Tube height
|
H
|
Traverse height of the ring rail (winding height)
|
R
|
Distance between ring and balloon checking ring (variable)
|
dH
|
Outside diameter of the tube at the to
|
V
|
Overhang of the top front roller relative to the bottom roller
|
Influence
on the ends down rate
his reasoning is based on a comparison of a
short triangle (Fig. 87, left) and a longer one (Fig. 87, right), and
on the behavior of two fibers (F in the
middle and f at the edge of the triangle).
Both fibers are longer than the spinning triangle (distance K/N to G). Whereas fiber F undergoes no change in direction of movement during its passage through the spinning triangle, fiber f is bent to a greater or lesser extent at N (angle Φ), increasing distance N-G. Consequently, the tension forces from the yarn cause an elongation of fiber f. So if bending angle Φ is large (for short spinning triangles), the elongation of fiber f is very high. That is why the tension forces (Fig. 87, FS) of the yarn during the formation of the yarn pass mainly into edge fibers f (in zone ZS, Fig. 88, left). Fibers F in the core remain almost free of elongation and hence of tension.
Both fibers are longer than the spinning triangle (distance K/N to G). Whereas fiber F undergoes no change in direction of movement during its passage through the spinning triangle, fiber f is bent to a greater or lesser extent at N (angle Φ), increasing distance N-G. Consequently, the tension forces from the yarn cause an elongation of fiber f. So if bending angle Φ is large (for short spinning triangles), the elongation of fiber f is very high. That is why the tension forces (Fig. 87, FS) of the yarn during the formation of the yarn pass mainly into edge fibers f (in zone ZS, Fig. 88, left). Fibers F in the core remain almost free of elongation and hence of tension.
Fig.
87 – Length of the spinning triangle
Therefore almost the entire tension force
of the yarn in the balloon acts only on a certain part of the fibers in the
spinning triangle, i.e. on the edge fibers. As a result, when tension peaks due
to shocks or uneven running from traveler or balloon act on the spinning
triangle, these few fibers cannot bear the full load; they break or the fiber
strand slips apart, causing an end break. That is why end breaks normally occur
within the spinning triangle from outside (edge) to inside (core). This danger
is always present with a short spinning triangle. Owing to the large angle φ, the tensile forces are
distributed very unevenly; high on the edge fibers (zone ZS) and much less on
the central fibers (zone ZO).
Distribution is much better (zones ZL)
with a long triangle. As a result it can be stated that spinning conditions are
improved by reducing angle φ. A long
spinning triangle therefore shows a more uniform distribution of forces (ZL). Since tension
is distributed over the entire fiber mass in these conditions, fewer end breaks
are the obvious result.
Fig.
88 – Spinning triangle – forces acting on the fibers: 1 short staple triangle;
2 long staple triangle
Influence
on the yarn structure
Yarn formation
takes place in the spinning triangle. If the yarn is to have high strength,
high elongation and regularity combined with low neppiness and hairiness, the
fibers in the yarn must be:
·
well
oriented
·
evenly
distributed in length and cross-section
·
wound
spirally around the axis, and
·
all fibers
must be tied in under tension.
Of all the spinning
systems available or known, these requirements are best satisfied by ring
spinning, especially with regard to the last, very important item. However,
this holds true only in conjunction with good spinning geometry, i.e. with an
optimal spinning triangle. If it is too short, core fibers will be tied in
without tension. They can then absorb tensile forces in the axial direction
only to a limited extent, or only after the fibers in the outer layer have been
broken. Since the distribution of tension forces in the final yarn is similarly
uneven to that in the spinning triangle, the yarn shows the same effect. When
stress is applied to the yarn, the edge fibers undergo so much elongation from
the very beginning that the forces acting on them either cause the fibers to
break, or in some cases to slide apart before the loading forces can act on the
neighboring fibers inside the yarn. Fiber breaks proceed successively from
outside to inside. The yarn has low strength. Since the twist inserted in the
yarn is insufficient due to the uneven distribution of tension (the edge fibers
are ultimately wrapped around the core fibers), the negative effect is
reinforced. The yarn structure falls short of the optimum, and most of the yarn
quality parameters suffer more or less.
Concluding
remarks on the spinning triangle
One further remark is required when
summarizing the entire reasoning behind the spinning triangle. Experts
generally discuss what happens at the spinning triangle by concentrating on the
length of the triangle, although the main influencing factors are in fact the
angles, as is shown by this investigation. However, since these angles mainly
depend on the length and vice versa, this simplification is admissible and is
used here, too. Using length as the criterion, it can be stated that long as
well as short spinning triangles have their advantages and disadvantages. Long
spinning triangles might increase the ends down rate by enlarging the “spinning
triangle” weak point and increase hairiness, since the hairiness of the yarn
also depends to a great extent on the area of the spinning triangle. On the
other hand, a short spinning triangle also results in an increase in hairiness
and fly accumulation as well as a reduction in yarn strength due to the
difficulties in tying in the edge fibers, and due to irregular distribution of
fiber tension in the yarn structure. The latter is also responsible for an
increase in the ends down rate. As is nearly always the case in spinning, the
problem is to find the optimum balance. It is therefore evident that as long as
a significant spinning triangle exists, perfect yarn structure with excellent
spinning performance cannot be achieved. In order to improve yarn quality and
spinning performance significantly, it is necessary to find ways to reduce the
size of the spinning triangle drastically, and above all to reduce the width of
the fiber flow exactly to the width of the remaining spinning triangle. This
has been achieved by so-called compact spinning systems. [RIKIPEDIA]
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