Ring frame end breakage distribution
Control of end breakage rate is the prime requirement for getting better ring frame performance and for achieving higher spindle speed, an improved method of testing significance of end breakage by any action without being influenced by day-to-day fluctuations.
Control of end breakage rate at ring
frame is the first step for improving ring frame productivity. It not
only leads to ends down loss but also restricts spindle speed. Ends-down
denotes those spindles where end has broken and is waiting for piecer
to mend it. Ends-down loss is given by:
d = e X 0.75 t
Where,
d = ends down %
e = end breakage rate (breaks/100 spdl hrs)
t = patrol time of piecer in hrs
Further patrol time of piecer also
increases with end breakage rate. As a result ends-down loss increases
exponentially with increase in end breakage rate as shown in Figure 1.
In addition if end breakage rate
goes beyond manageable levels, idle spindles will increase. Tenter
(piecer) will make those spindles, where breaks occur repeatedly, as
idle.
Repeated occurrence of end breaks in
a few spindles is often cited as reason for poor ring frame
performance. Ridgy build of bobbin, as shown in Figure 2, will be found
on spindles where repeated end breaks occur.
Roller lapping also increases with
increase in end breakage rate. Since roller lapping involves higher time
for mending, patrol time of the tenter increases and results in more
ends down %. Some studies were therefore undertaken to find out the
distribution followed by end breakages under different conditions and
compare it with theoretical. An improved method for assessing the
significance of improvements in end breakage rate by any action is also
proposed.
End Breakage Distribution
End breakage occurrence being a rare
occurrence, distribution of end breakage rate can be expected to follow
Poisson distribution. In actual practice distribution differs from
Poisson due to variety of reasons like 1. Variability in probability of
breaks from spindle to spindle, 2. Disturbances and defects in spindles
3. Day-to-day variability in mixing 4. disturbances in working of
preparatory 5. Poor maintenance 6. Variability in R.H. temperature. A
study of distribution of end breakage rate will provide useful clues in
regard to the quality of maintenance and process control.
Spindle Speed
Closeness of fit of end breakage
distribution to Poisson depends on conditions of spinning. End breakage
distribution was determined on 80s combed yarn on a ring frame spun on
normal and 20% higher spindle speed. Actual distribution was compared
with Poisson in Figures 3 and 4 under these conditions. Actual
distribution is close to Poisson at normal spindle speed (Figure 3).
When spindle speed is increased not only end breaks increases but also
departure from Poisson is very marked (Figure 4).
Another study was conducted by ‘over
spinning’ the mixing to 100s and determining end breakage distribution
(Figure 5). End breakage distribution again deviates markedly from
Poisson in 100s. This leads to the inference that the differences
between spindles (in regard to probability of breaks) becomes more
pronounced when spinning conditions become critical. This arises from
disturbances in settings, defects in parts and back material variations.
The yarn spun on spindles with repeated occurrence of breaks is found
to be consistently finer in count than that on spindles without breaks.
High occurrence
of end breaks is because of lower
yarn strength at the front nip in these spindles because of finer count.
So high count variation is one of the reasons for repeated end breaks.
End breakage distributions in well
maintained and poorly maintained ring frame sections on the same count
and spindle speed were determined and are given in Figures 6 and 7.
Distribution as per Poisson is also plotted. End breakage distribution
deviates markedly from Poisson in poorly maintained section (Section 1
Figure 7). While no spindles give more than 4 breaks in section 2, as
many as 1.85% spindles give breaks more than 4 in section 1. These
breaks obviously come from disturbances and defects in spindles, rings
and drafting system on these spindles.
This shows that clues to quality of
maintenance can be obtained by comparing actual end breakage
distribution with ideal. Ring expert data system by Uster is a useful
attachment to ring frames as it gives spindle wise distribution of end
breakages. A sensor moves along the whole length of the frame close to
the traveler and detects without contact movement of traveler. When an
end has broken traveler will not be rotating and will be detected by
sensor as an end break. The equipment gives end breakage rate
distribution spindle wise and indicates the ‘rogue’ spindles, which give
repeated end breaks. Similar online systems have been developed by
other manufacturers like Premier.
Common causes for repeated end-breaks on a few spindles are:
1. Ring frame defects and disturbances
2. Preparatory deficiencies
Ring frame defects
1. Disturbed spindle centering is one of the major causes of repeated occurrence of end breaks. This arises because:
1. Spindle centering schedule is not followed strictly
2. Proper gauges and lighting is not available. Painting top of the gauge white and use of a portable
light help to improve accuracy. Electronic spindle gauge can help to reduce subjectivity but requires training.
3. Vibrating spindles and bobbins
4. Worn out rings, spindles and lappets
5. Defective cradle retention spring. Cradle stays in a lifted condition resulting in poor control over fibres.
6. Low top roller pressure, This can arise from worn out hose pipe or plunger or disturbed height setting.
7. Missing bottom apron. Sufficient
number of spare aprons should be kept in each staff to facilitate prompt
replacement of broken apron.
Defects and disturbances in preparatory
Long thin places in roving due to
sliver splittiing in the creel or partial lapping on roller at speed
frame and draw frame. Disturbed working in preparatory like roller
lapping or frequent breaks.
Proposed method to estimate improvement
High day to day and time to time
within a day variability in end breakages comes as a major impediment in
drawing definite conclusions about any actions taken. Proper
methodology to be followed in designing experiments to assess
improvements in end breakages from any action is first discussed. An
improved statistical test which will help to detect differences to a
greater accuracy without being affected by day to day variations is
proposed. Examples are given from actual studies to explain this method
and bring home its usefulness in interpretation of results. Though the
discussion has been restricted to ring frame end breakages, the same
principle will hold for breakages in other processes as well.
Experimental Design
The two sources of variability in end breakages that should be taken into account while designing experiment are:
1. Day to day and shift to shift variations
2. Machine to machine variations.
It is therefore imperative that the
two parameters or materials to compared are allowed to run on a pair of
ring frames “side by side” and simultaneous study of end breakages is
taken to cover all doff positions more than once. Machine difference can
be taken care of by interchanging the variables between the machines.
An even better method for overcoming the machine effect is to carry out
the experiments on more than one pair of machines.
Analysis of results
To facilitate statistical analysis,
the results are divided into units, each of one day or shorter duration.
In the usual method, standard deviation and standard error are
estimated from the unit test results, from which standard error of
difference is calculated. This method has the drawback that this
overestimates the variability in difference of end breakages because of
day-to-day to variations. Day to day fluctuations not only increase
variability of end breakages but also causes the breaks in the two
experimental set ups (normal and modified) to move up and down in
unison. The difference in end breakages on the other hand is not
affected to the same extent by day-to-day variations.
A better method under such
conditions would be to calculate the difference in end breakage rate for
each unit test and estimate the standard deviation and standard error
of the same and check the average difference against this to find the
significance. Even if statistical test is not done, such a method will
show from visual examination the likelihood of the difference being
real. If the difference in unit tests is of the same sign in most of the
tests, the difference is more likely to be a real one and not a
‘chance’ one.
The following examples will help to substantiate the merit of this method:
Ring Cleaning
Ring frame performance gets affected
by deposition of wax, fly, dirt and metallic substances over a period
of working and ring cleaning at periodic intervals is suggested to
overcome this. To assess the improvements from ring cleaning two ring
frames were chosen. Rings on one side of each frame was cleaned while no
action was taken on rings of other side. Simultaneous study of end
breakages was done on cleaned and normal side of ring frames for a
period of 9 days, with 3 hours study each day. The average end breakage
rate for the two sides for the 9 days are given in Table 1 and Figure 8.
In the normal method, SD of breakage
rate for ‘cleaned’ and ‘normal’ sides are calculated separately from
which SE of difference was estimated. In the improved method, difference
in breakage rate between sides for each day is computed from which SD
and SE of difference is calculated. ‘t’ value for each method is
calculated and is given in last row. SE of difference of a lower order
and ‘t’ value of higher order is found with improved method compared to
normal method.
As a result, reduction in end
breakages by ring cleaning comes out to be significant at a higher level
of confidence limit with the improved method. This is because
day-to-day variations in ring frame performance affect ‘cleaned’ and
‘normal’ sides equally. As a result, end breakage rate on both sides
move up and down in unison. On days when ring frame performance is poor,
both sides tend to give a higher breakage rate. This will be amply
clear from Figure 8.
It is well-known that
σ(y-x)2 = σx2 + σy2 - 2 σx σy r
where σy-x = Standard deviation of difference y-x
σx = Standard deviation of x
σy = Standard deviation of y
r = Correlation coefficient between x and y
Since a positive correlation exists between x and y, σ(y-x)2 is lower than (σx2 + σy2). The positive correlation is because day-to-day fluctuations in breakage have equal influence on x
and y. When values of σx, σy and r are substituted in the above equation, a value for σy-x
in agreement with that by improved method is obtained.
Better Carding
In the 2nd example,
carding quality was improved on selected card by increasing cylinder and
lickerin speeds. The material was channelised separately and creeled on
one side of two ring frames with other side working with normal
material. End breakages were compared on the two sides for 10 days with 3
hrs study each day. The results are shown plotted in Figure 9.
Figure 9 shows that not only end breakages are reduced by higher cylinder and lickerin speed but
also that end breakages by the two set-ups go up and down in unison.
Standard deviation (SD), Standard
error (SE) and ‘t’ values for the end breaks with normal and improved
carding are given in Table 3.
Standard error of difference is
lower and ‘t’ value higher by the improved method compared to normal
method. Once again this is because of the high positive correlation
between end breaks for the two carding conditions.
Make of ring frame
Third example compares the end
breakage rates in two makes of high-speed ring frames fed by the same
back material. Comparison of breakages rates over a period of 10 days is
shown in Figure 10.
Standard deviation (SD), Standard error (SE) and ‘t’ values are given for the end breaks between
two groups of ring frames in Table 4.
Figure 10 shows that end breakage rate is lower on ring frame make II on all but one day. Table 4
once again demonstrates the
superiority of improved method in bringing out the significance of
between the two makes, which is of finer order The difference between
the two makes is statistically significant only when improved method is
used.
Possibilities of reducing end breaks in ring spinning
To reduce end breaks, the following aspects should be taken into consideration:
I. Since end breakage in ring spinning is related to slippage of fibres at the spinning triangle as a result of peaks occurring in the spinning tension fibre, the grip at the front drafting rollers should be increased by having a higher top roller pressure. The use of softer cots also enhances the grip at the front rollers. If the total pressure on the rollers cannot be increased, the grip at the front rollersí nip can be improved by reducing the width of the cots.
II. A reduction in friction between ring and traveller could reduce the peak tension during the rotation of the traveller.
III. Measures should be taken to reduce the mass irregularity of yarn straight after carding.
IV. The width of the drafted ribbon at the front roller nip should be reduced
Conclusion
1. Control of end breakages is the
prime requirement for keeping down efficiency losses and for achieving
higher spindle speed. Repeated occurrence of end breaks in a few
spindles is one of the major causes of high end breakage rate. End
breakage distribution is a useful tool for detecting defective/disturbed
spindles, which give high breakages. By taking corrective action on
such spindles, overall ring frame performance can be improved. Extent of
departure of distribution from theoretical
(Poisson) indicates the scope for
process improvement. Online monitoring systems like Uster Ring data are
useful in detecting spindles giving repeated end breaks.
2. Deviation of end breakage
distribution from theoretical (Poisson) becomes more marked when
spinning conditions are critical (high spindle speed or overspinning).
3. An improved method for checking
significance of difference in end breakage rate brought about by any
action is suggested. This method is less affected by day-to-day and
shift to shift to shift fluctuations in breakages rates and brings out
finer order of differences. Examples are given to demonstrate the
superiority of this method.
Courtesy :
N Balasubramanian,
Ex-Director, Bombay Textile Research Association (BTRA).
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