Drawing
1.What are the tasks of draw frame? Discuss them.
Tasks of draw frame:
i) Equalizing
ii) Paralyzing
iii) Blending and
iv) Dust removal
2.Describe the operating principle of a draw frame.
A drawing machine mainly consists of creel, drafting arrangement and delivery system. Drafting arrangement is the heart of drawing machine .
Four to eight card slivers are fed to the drafting zone 3 through creel rollers 2(figure).
The creel rollers are positively driven. After drafting in drafting zone the sliver is reduced to a web which lacks sufficient cohesion. So in order to avoiddisintegration of the web , it is condensed into sliver by passing through a trumpet 4 and the calendar rollers 5. It is then guided to the coiler tube 6 and then to the can 8.
Drafting
a. What is drafting of sliver in draw frame? How does drafting occur?
b. Write a formula for roller drafting of fiber strands (sliver etc.)
c. Show how mass per unit length of a sliver is reduced during drafting.
a) Drafting is attenuation of a fiber strand (sliver etc.)by which mass per unit length of the strand is decreased. In draw frame drafting is performed by passing the fiber strand through pairs of successive rollers. The surface speeds of preceding rollers are greater than the surface speeds of the rollers that follow them (figure)
Conditions for drafting are:
V1<V2<V3<V4and
S1>S2>S3
where V1 V2 V3 and V4 are surface speed of rollers and S1, S2 and S3 are roller settings
b) Formula for roller drafting (d) is given by
d= ( surface speed of front roller)/(surface speed of back roller)
dFR=diameter of front roller
nFR=rpm of front roller
dBR= diameter of back roller
nBR=rpm of back roller
c) Let us consider a case of perfect drafting. the horizontal lines represent the fibers which are assumed straight, parallel and all of the same length (a perfect sliver)
A sectional view of a set of drafting rollers is shown at the top of the figures. The rest of the figures are representing different stages during drafting. The numbers of fibers which cross the vertical lines represent the thickness of the sliver. Suppose the distance between two nips is equal to fiber length. Also suppose that the front roller speed is twice that of the back rollers.
In diagram (1) there are five fibers and fibers A has just been released by the back nip and is about to get the speed of front nip.
Diagram (2) shows the position moment later when fiber B has moved one unit has just been released by the back nip and has reached the front nip. In the meantime, fiber A which has been moving
at twice the speed of B, has moved two units and consequently the leading ends of the fibers A and B are two unit apart, instead of one unit as they were in the original sliver. Since they are now both moving at the same speed they retain this spacing.
In last diagram (4) we see that as a result of drafting the space between fibers after front nip has been increased twice compared to the space between the fibers in the original sliver. Thus the thickness of sliver has been reduced.
Drawing-2
Roller Setting
The distance between the nips of two pairs of rollers is known as the roller setting. In draw frame it is necessary to space the rollers a certain distance apart.
Factors determining roller setting:
1. The staple length of cotton.
For longer staple wider setting is maintained.
2. The bulk of the material being fed.
If the bulk of the material is higher then a wider setting is given.
3. The frictional properties of the fiber
For harsh or rough type of cotton wider setting is selected.
4. Draft used
For higher draft setting should be closer.
5. Type of rollers
For metallic roller wider setting is used than rubber coated roller.
Stop motion in draw frame
Stop motions in drawframe may be classified broadly into two types (i) Mechanical Stop Motion and (ii) electrical stop motion. Mechanical stop motions are now a days out of use. In modern high speed drawframes, stop motions with low voltage electric circuit are used.
The stop motion halts the drawframe in the following cases:
1. Sliver runs out from the creeled can.
2. Sliver breaks in the creel
3. Lap-up in the drawing rollers.
4. Choke in the sliver collecting plate and funnel.
5. Choke in trumpet above the calendar rollers.
6. Opening of the head and doors.
7. Full can contains a per-determined amount of sliver etc.
Working procedure of electrical stop motion:
When any of the mention point occurs the low voltage circuit is completed due to action of the stop motion. The current energizes the main switch, which stop the motor and activated the electric brake.
Use of different colored lights
Colored signal lights on the frame tell the operator whether the machine is running and indicate the types and location of stoppage.
1. The red warning lamp shows that the machine is stopped because of full can.
2. Yellow warning lamp indicate that machine has stopped due to empty can and sliver breakage at the break of the machine.
3. Blue warning lamp shows that the machine has stopped due to the following reason:
i) Front roller lap or ii) Sliver breakage at the front side or iii) calendar roller lap.
Autoleveler
The object of an autoleveler is to measure the sliver variation and then continuously alter the draft. More draft is applied to thick places and less draft to thin places. As a result the deliver sliver is less irregular than input sliver.
There are three types of autoleveler in the industry. They are i) open loop auto levelers ii) closed loop auto levelers and iii) combined loop auto levelers.
i. open loop autoleveler: The open loop autoleveler is used for the correction of short term variations. Fig.1 represents an open loop auto leveler where the solid lines indicate the flow of fibers through the machine and the broken lines represent the flow of information in the autoleveler unit.
The control unit compares the measurement unit signal with the reference signal. The reference signal represents the main output required. The control unit accordingly increases, leaves unaltered or decrease the output of regulator unit. The regulator unit in turn provide a variable speed to the back or front rollers of the drafting zone to give the required draft. (When material has reached the point at which draft is applied). There is no check on the deliver material by the control unit in open loop autoleveler.
ii. Closed loop autoleveler: Closed loop autoleveler are used for the correction of long term and medium term variations. Fig-2 represents a closed loop auto leveler.
Here the measurement unit is located after drafting zone. The measurement signal is compared with the reference signal by the control unit which than determines the output of the regulator. The regulator provides variable speed to the process to give require draft. Here measurement always take place on the material after the point where corrective action is applied.
iii. Combined loop autoleveler: A combined loop autoleveler is designed to correct long, medium and short term variations.
