Structure of Ring Frame Yarn Packages
Build of cops
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The cop as
shown in Figure.1 comprises of three visually distinct parts – the
barrel like base A, the cylindrical middle part W, and the conically
convergent tip K. It is built up from bottom to top from many conical
layers as shown in Figure.2, but constant conicity is achieved only after the formation of the base.
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In the base portion itself, winding begins
with an almost cylindrical layer on the cylindrical tube. The initial
layers are conical in shape, thicker at the base and thinner at the tip.
With the deposition of one layer on another of these conical layers,
the conicity gradually increases.
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Each layer
comprises a main layer, also called as winding layer and a cross-layer,
also called as binding layer which are shown in Figure 3.
The main layer is formed during slow raising of the ring rail,
individual coils being laid close to each other or on each other.
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The main layers are the effective cop filling
layers. The cross layers are made up of widely separated steeply
downward-inclined coils of yarn and are formed during rapid lowering of
the ring rail.
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They form the separating layers between the main layers and prevent pulling down of several layers simultaneously, known as slough off
when yarn is drawn off at high speed in back winding machines. In the
absence of such separating layers, individual yarn layers would
inevitably be pressed into each other and layer-wise draw-off of yarn
would be impossible.
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Raising and
lowering of the ring rail is caused by the heart shaped cam and is
transmitted by chains, belts, rollers, etc. to the ring rail. The long
flat part of the cam surface forces the ring rail upward, slowly but
with increasing speed. The short steep portion causes downward movement
that is rapid but with decreasing speed.
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The formation of the base
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The
heart-shaped cam and the delivery roller are coupled together by the
drive gearing. Thus, the length of yarn delivered for each revolution of
the cam is always the same. But, due to the presence of the cam N (Figure-4)
between the tape and the pulley during the initial stages of cop
building, the lift or the height of the layer is shorter to start with.
The position and design of the cam N is selected such that the height of
the layer increases gradually, till it is moved totally away from
getting in contact with the tape. This is attained by winding of the
tape on the Drum T for each double layer formation. Once this stage is
reached, the heights of the further layers do not change till the end.
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Therefore, the volumes of the individual double layers need to be
equal. Deposition of double layers on the tube begins with a small
average layer diameter d1. The average diameter increases gradually with each newly deposited layer.
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With
constant layer volume and increasing height of the layers in the
beginning, this can have only one result, namely a continual reduction
of the layer width from b1 to b2 to b3, and so on till the height reaches fixed level.
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Since the
ring rail is also raised by a constant amount ‘h’ after each deposited
layer, it follows that curve, rather than straight line, arises
automatically in the base portion.
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The formation of the conical layers
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It has
already been mentioned that the ring rail is not moved uniformly. Its
speed increases during upward movement and falls during downward
movement. At the tip of each layer it is higher than at the base of the
layer that is the ring rail does not dwell as long at the tip as it does
at the base – less material is wound, the layer is thinner at the tip.
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If it is
assumed by way of example that the ring rail is moving twice as fast at
the top of its strokes as at the bottom of the stroke, the first layer
would be half as thick at the top as at the bottom, i.e. b1/2instead b1.
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The first layer would correspond to a trapezium with the side b1 at the bottom and the side b1/2
at the top. This is followed by the deposition of the second layer.
Owing to the lifting of the ring rail, the upper portion of the new
layer would again be deposited on the bare tube.
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The average
diameter at the top would be the same as that of the first layer, and
the volume, and hence the thickness, would also be the same, that is b1/2.
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Each newly deposited layer will have this thickness of b1/2 at the top. At the bottom, however, the diameter is increasing continually, the layer thicknesses decline from b1 to b2 to b3 to b4… Accordingly, continually narrowing trapezia are produced.
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At some
stage, the trapezium will become a parallelogram, i.e. the lower side
will be the same size as the upper side: both will be b1/2. Since all other winding conditions now remain the same, no further variation can now arise in the layering.
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One conical layer will be laid upon the other until the cop if full, that is when the cylindrical portion of the cop is formed.
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The gearing
change wheel has little influence on this sequence of events. If too
many teeth are inserted, the final condition of constant conical layers
will be reached too soon and the cop will be too thin. It will be too
thick if the ring rail is lifted too slowly.
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The winding Process
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The winding Principle
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As in the
case of the roving frame, two components with different speeds must be
used in order to enable winding to occur. One assembly is the spindle,
the other is the traveller representing the remnant of the flyer.
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Also, the
speed difference must be equal over time to the delivery length at the
front cylinder. In the roving frame, each assembly has its own regulated
drive. In the ring spinning frame this is true only for the spindle.
The traveller is dragged by the spindle acting through the yarn.
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The speed of
the traveller required to give a predetermined speed difference arises
through more or less strong braking of the traveller on the running
surface of the ring. Influence can be exerted on this process by way of
the mass of the traveller.
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Variation in the speed of the traveller
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In ring
frame winding, diameter of winding changes continually with raising and
lowering of the ring rail, since the winding layers are formed
conically. The traveller must have different speeds at the base and the
tip.
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Assuming for
example a spindle speed of 18,000 rpm, the layer diameters of 46mm at
the base and 25mm at the tip, and a delivery of 25 m/min, the traveller
speed at the base will be,
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Variation in the Yarn Twist
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The equation is generally used to calculate
the number of turns in the yarn. As just established, this is not wholly
accurate since the turns arise from the traveller and not from the
spindle.
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In the given
example, 173 turns per minute are missing at the base of the winding on
the cop (larger diameter), and 318 turns per minute at the tip (smaller
diameter). However, these missing turns are a theoretical rather than a
practical problem, for two reasons.
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Firstly,
the inaccuracy of measurement in estimation of yarn twist in
instruments is greater than this twist variation. Secondly, the yarn
finally receives its full twist in any case. This happens as soon as the
yarn is drawn off the cop over the end, since each rotation of the yarn
about the tube leads to insertion of an additional turn in the yarn.
The compensation of the missing turns can then be explained easily.
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If 318 turns per minute are missing at the top, and 25 m of the yarn to be wound up in this period, the result is
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Drm = 318 /25 = 12.73 turns / m
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During unwinding, each yarn wrap on the cop (one circumference) produces one additional turn. At the tip (cop diameter 25 mm):
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Dra = 1000 mm/min / 25 mm = 12.73 turns /m.
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That is,
exactly the number of turns previously missing. Care must however be
taken that cops are always unwound over end, even during twist tests.
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Friday, August 14, 2015
Structure of Ring Frame Yarn Packages
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