Ring frame end breakage distribution

  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 = σ_x² + σ_y² - 2 σ_x σ_y r

where σ_y-x = Standard deviation of difference y-x

σ_x = Standard deviation of x

σ_y = 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 (σ_x² + σ_y²). The positive correlation is because day-to-day fluctuations in breakage have equal influence on x

and y. When values of σ_x, σ_y and r are substituted in the above equation, a value for σ_y-x

in agreement with that by improved method is obtained.

Better Carding

In the 2^nd 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.

<sup>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</sup>

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).

Textile mills need to give priority to compressed air savings

Textile mills are aware that of the total electricity units consumed per day towards compressed air, they are losing more than 30 per cent to compressed air leakage.

Energy savings are a major issue in the functioning cycle of a textile mill. In the total electricity units consumed per day (UPD), the mill can reduce only around five to 10 per cent of UPD after energy audit and implementation. Compressed air leakage is a regular hidden cost in a mill. If not identified right away, it aggravates losses.

When a mill goes for modernisation, automated production demands more compressed air. So, instead of arresting existing air leakage, mills often buy more compressors to satisfy production demands. Ironically, leakage increases.

Why are there more losses in the compressed air system now?

The root cause is accumulation of water and dirt in compressed air pipe lines. This makes pneumatic fittings weak and leaky. A pneumatic fitting and component has a life. Beyond its life, leaks start. Soon, a drizzle turns into a shower.

The compressor post air cooler, refrigerated dryer and the zero air loss drain valves fixed in the air receivers at the compressor house, tail end and feed end receiver (Fig 1) are three sub-systems that have to work perfectly so that no water is trapped in the distribution and the load ends. More importantly, water ingress in the air actuation elements and solenoid valves in the machine will spoil their health.

Where is the loss happening in compressed air generation?

Compressed air generation is gets compromised if a compressor takes 5 to 10oC above the surrounding ambient temperature through choked air intake filters. This is two per cent KWH loss. Cool, dry compressors deliver more air output. So, try to give cool, dry air to the compressor air inlet as shown in Fig 5. Keep the load/unload settings to the minimum bar pressure just 0.7 bar, or 10 per cent above minimum required pressure demand from equipments.

Fix one pressure gauge just after the compressor and another gauge at the compressor house air delivery line/receiver. Fix a temperature gauge on the air receiver and keep its temperature 5oC above the ambient, which is a sign of unhealthy compressor package.

Fig 1. Invest in air receiver load feed/tail end model

Many mills have more pressure drop around 1 to 2 Bar in the compressor house. First, bring down the pressure drop to less than 0.7 Bar. We provide an air receiver in the compressor house to arrest the compressed air pulsation and fluctuation due to frequent loading and unloading of compressor.

Fig 2. Pressure sensor located inside existing compressor hood

Fig 3. Fluctuations in KW/KVA in every minute trend graph

The above images show that the location of pressure sensor (Fig 2) matters to us since this aggravates the cyclic fluctuation of load and unload (Fig 3). This can be solved by shifting the compressed air pressure sensor from the hood to the receiver. Many industries achieved savings and safety after shifting this pressure detection and control sensor to air receiver. Other encouraging option is to expand the compressor hot air exhaust so that the air and oil sub systems within the hood work comfortably.

How do you account for the compressed air losses in distribution?

Analogous to the other Transmission & Distribution (T&D) losses, compressed air lost in transmission is less up to the sub-header, compared to the air distribution loss that is more in the machine. The Flanges and O-ring gaskets, fixed in the main header pipe lines, are prone to leak more. To study this and correct the same, provide intermediate or feed air receiver to each of the air consuming department. This will help the mill measure separately, the air losses in the transmission (compressor and header) and the losses in the distribution (sub-header and equipment in the department.)

The air receiver ought to have pressure gauge and isolation ball valve, preferably solenoid operated, and receiver water drain valve system. The Mill Project team always reduces its budget to invest in these sub-systems, but the mill's compressed air running cost goes up because of the absence of these field sub-systems useful to condition and monitor the mill, air daily demand and supply air flow at rated pressure with minimum allowable pressure drops. 

Are we seeing compressed air losses in machine usage?

Many mills operate at 7 to 8 Bar band in load and unload settings. However, only 6 to 6.5 Bar is enough for the load and unload pressure settings. Invest in air bottles to each of the compressed air consuming machines. Many mills fix a digital air pressure gauge (it is the machine OEM initiative) and give only minimum pressure required for the machine.

Say, the autoconers can be retrofitted with three inch PPR pipe under the total machine length and this will give 200 litres buffer tank. All the control and working pressure tapping can be taken from this retrofit pipe. Mills think of pressure boosters to satisfy higher pressure demands of machine OEMs. Discuss with OEM how to reduce the machine operating pressure with spare air bottle as buffer.

How to prioritise compressed air losses in leakage

Losses are happening in the compressed air generation, distribution in the usage. In the usage part, the leakage in the machine is more than the machine usage now. Hence the question of arresting the leakage is becoming a priority now. What is the leakage in the machine during OFF time? Measure this first to arrest next.

This is similar to our domestic 1000 litre overhead tank where, 100 litre /hr or (%) water enters the from the top of the tank. 50 litre /hr or (%) is drained silently through the bottom drain plug. And the balance 50 litre /hr (%) only is used for the house. Like that, some mills record more than 50 per cent compressed air leakage.

By arresting leakage losses to the minimum, mills stand to gain now. Returns on investment are around six months only. But keep in mind, no mill can achieve zero leakage. So, the aim should be to contain air leakage to the minimum possible per cent.

How the air consuming machine oem & compressor oems can help now?

The machine OEMS in the other segments have realised the preciousness of compressed air. Please try to keep OUT, the air sub header, FRL, etc input components visible to the user daily. Do not keep the inlet air header under closed door but it must stand out of the machine for easy and timely attendance of air input problems.

Provide a buffer bottle or pipe line so that air supply to the machine individual pneumatic actuation mechanism is not restricted or starved of air. Always give open access to the worker so that he can daily remove the water collected at the machine end water drain valve at the bottom most point. If possible, provide digital air pressure gauge to the machine inlet. Some mills are already planning to put Rotameter type or digital type air flow instant metering.

Provide air intake Delta Pressure gauge outside the compressor hood so that each mill can around 2 per cent KWH on condition monitoring the same. Kindly educate the air consumer on how to keep compressed air useful to the pneumatic machine health.

When installing the machine, give the consumer the best installation possible. The compressor and compressed air inefficiency starts from compressor house. Allow no Tee but only Y pipe connections in the house.

Why is the installation of the compressed air system not professional?

Fig 4. Do's & Don'ts of compressor hot air exhaust ducting

During my energy audit of more than a few hundred industries, I noticed that eight of 10 mills have fixed same size duct to match mouth size, but of longer lengths (Fig 4). The damage is done to the machine right from the installation! They ought to provide the duct double the mouth size so that hot air from the compressor hood may go out. That one-time expense enables the compressor breathe better forever. 

Mills buy the latest and efficient compressors or machines. They do not plan to mark a tiny fraction of budget cost towards installation and never give time to commission the same professionally.

I request users to revisit their compressor installation manuals and commissioning reports and check for non-conformance. The mill must have a commissioning report of what kw/cfm has ben achieved as reported by the OEM. Then, the mill must do an FAD test twice a year using the pump-up capacity method.

Many mills are forced to change their compressor air end elements - which cost half of what the compressor costs -- within their life period prematurely because of poor installation and because the post air cooler is getting choked. The latter is bound to happen due to textile micro fluff floating. This situation can only improve if the machine HX suction is oriented towards the outer ambient. If the HX sucks only the machine's hot air, then it functions poorly.

Fig 5 Pre-filter to existing air intake filter and its extension pipe to fresh air entry

Does compressed air metering help?

As energy professionals, we are aware that efficiency of compressed air generation is poor. So, first install a multi-function meter to know daily kWh of compressor during loading, unloading and total units. The relative condition monitoring of the machine input KWH will show the relative compressor consumption deviations.

It will be more accurate to provide compressed air flow meter in the compressed air main header. This will prompt the user to know what each compressor gives per kWh input power, say 3 to 6 cfm. When the OEM says it is 6 cfm/kW, the mill user finds it only around 3 to 5 cfm/kW.

This input kW/output cfm ratio will encourage the mill user to find out healthy parameters in each compressor. The user can also close and open the intermediate air receiver valves in each department to know consumption in each individual department. 

So, header flow metering will give a macro view to the management about the efficacy and per cent compressor costing in the mill power break up in units/day. The portable digital flow meter can be used in each machine to routinely measure machine air consumption at the time of switching on and off.

What are zero- and low-cost ways to reduce compressed air consumption?

Give priority to arresting leakage. Outsourced today, leakage detection and correction exercise will give results in three months. It would be a wise decision.

Many mills have stopped one of three running compressors after arresting leakage. When you can hear the compressed air leak in any compressed air pipe line or machine, it means one third, or about 33 per cent of air flow in that airline is leaking. First, arrest audible leaks. Then use an Ultrasonic Leakage Detector to arrest minute leaks.

BEE says, any machine above 10 HP, operated for more than 6,000 hours a year i.e. two shifts a day, needs to fix a kWh meter to assess the machine's productive health. Here we can measure both compressor's air power and tare power now.

Investing in an air receiver can stall other retrofits. These include comforting the compressor and zero air loss compressed air drain valves with pre-fixed Y-type strainer cleanable daily on-line to save the main drain valve. They also include adding the daily cleanable pre-filter to the existing air intake filter, positive cross ventilation inside the compressor hood, load and unload pressure sensor shifting to air receiver and many finer points of daily monitoring that bring down compressed air energy consumption by one third. 

Courtesy : 
S. Ashok, BEE Accredited Energy Auditor