Drafting irregularities and their causes and remedies,Amount of draft and draft distribution on strand irregularity,Draft distribution,Recommended total draft range,Limitations of apron drafting and the scope for improvement.

1. Drafting
in Spinning Process
B DHARANI
M TECH-TEXTILE

2. Drafting irregularities and their causes
and remedies
Irregularity has three effects:
• Since a length of yarn tends to break at its weakest place, which is nearly always the
thinnest place,and since the more irregular the yarn, the thinner is the thinnest
place, irregularity causes the yarn to be much weaker than a regular of the same
average count would be.
• The yarn, roving or sliver may break at the thin,weak places during the passage
through the machines, so making it necessary to join or ‘piece’ the broken ends
before production can be resumed.
• Irregularity in the yarn leads to irregularity in the cloth, which when excessive may
spoil the appearance of the cloth.

3. Amount of draft and draft distribution on
strand irregularity
• The drafting section of the roving frame may be 3 over 3 or 4 over 4 with or without
apron roller arrangement. Only the 3-over-3 or 4 over 4 roller with double apron
drafting system are still to be found in modern machines offered by manufacturers.
• In general 3/3 drafting system is used, but for higher draft applications 4/4 drafting
system is used. The draft often has limits not only at the upper limit (15 to 20), but
also at lower limit. It is around 5 for cotton and 6 for synthetic fibers. e 3-over-3 or 4 over 4 roller with
double apron drafting system are still to be found in modern machines offered by manufacturers. In general 3/3 drafting system is used, but for higher draft applications 4/4 drafting system is used. The draft
often has limits not only at the upper limit (15 to 20), but also at lower limit. It is around 5 for cotton and 6 for synthetic fibers.
• or without apron roller arrangement. Only the 3-over-3 or 4 over 4 roller with double apron drafting system are still to be found in modern machines offered by manufacturers. In general 3/3 drafting system
is used, but for higher draft applications 4/4 drafting system is used. The draft often has limits not only at the upper limit (15 to 20), but also at lower limit. It is around 5 for cotton and 6 for synthetic fibers.
• e drafting section of the roving frame may be 3 over 3 or 4 over 4 with or without apron roller arrangement. Only the 3-over-3 or 4 over 4 roller with double apron drafting system are still to be found in
modern machines offered by manufacturers. In general 3/3 drafting system is used, but for higher draft applications 4/4 drafting system is used. The draft often has limits not only at the upper limit (15 to 20),
but also at lower limit. It is around 5 for cotton and 6 for synthetic fibers.

4. In production of a roving, a 3-over-3 roller with double apron drafting system is
commonly used to attenuate the sliver.
• In a system of three pairs of rollers, there are two drafting zones. The first or back
zone is designated the “break draft,” while the second or front zone is called the
“main draft”.
• A pair of endless aprons is positioned in the high-draft front zone and made to move
at the surface speed of the middle-roller pair.
• As fibers enter the high-draft front zone, the aprons will hold them and assist in
keeping them moving at the surface speed of the middle rollers, while preventing
the short-fibers being dragged forward by those fibers nipped and accelerated by the
front rollers.
e pairs of rollers, there are two drafting zones. The first or back zone is designated the “break draft,” while the second or front zone is called the
“main draft”.
• ❸ A pair of endless aprons is positioned in the high-draft front zone and made to move at the surface speed of the middle-roller pair.
• ❹ As fibers enter the high-draft front zone, the aprons will hold them and assist in keeping them moving at the surface speed of the middle rollers, while preventing the short-fibers being dragged forward by those fibers nipped and accelerated by the front rollers.
• system of three pairs of rollers, there are two drafting zones. The first or back zone is designated the “break draft,” while the second or front zone is called the “main draft”.
• ❸ A pair of endless aprons is positioned in the high-draft front zone and made to move at the surface speed of the middle-roller pair.
• ❹ As fibers enter the high-draft front zone, the aprons will hold them and assist in keeping them moving at the surface speed of the middle rollers, while preventing the short-fibers being dragged forward by those fibers nipped and accelerated by the front rollers.
• nd 1.5; therefore the front draft (i.e. main draft) is responsible for the major part of the total attenuation desired.
• ❻ The total draft is defined as the ratio of the surface speed of the front rolls to the surface speed of the back rolls and is a product of the break draft and the main draft:

6. • The magnitude of break draft is usually small, varying between 1.1 and 1.5; therefore
the front draft (i.e. main draft) is responsible for the major part of the total
attenuation desired.
• The total draft is defined as the ratio of the surface speed of the front rolls to the
surface speed of the back rolls and is a product of the break draft and the main draft:

7. In 4-over-4 double apron drafting system, there are three drafting zones such as break
draft, middle draft and front draft (or main draft).
• Back draft is apply in back zone which affects roving evenness. Less amount of draft
is apply in middle zone where the condenser is used to condense sheet like sliver.
Maximum amount of draft is apply in front zone where the apron is used to move at
the surface speed of the middle-roller pair.
• The total draft is defined as the ratio of the surface speed of the front rolls to the
surface speed of the back rolls and is a product of the break draft, middle draft and
the main draft:

10. Draft distribution:
• An important consideration of the drafting zone is the draft distribution i.e. how
much draft should be given in the back and front zones .
• In a 3-over-3 drafting system, there are two drafting zones such as break draft, and
main draft. The first or back zone is designated the “break draft,” while the second or
front zone is called the “main draft”. The total draft is the product of break and main
draft. The break draft is in the range of 1.03 to 2.03 and all the remaining draft is
given at the main drafting zone, generally ranging from about 5 to 18.

11. Recommended total draft range
•

12. • If the break draft is increased beyond an optimum value then the evenness of the spun
yarn drastically reduces due to the formation of thick and thin places. It is recommended to
keep the break draft as low as possible. The problems associated with the higher break
draft than recommended are given as follows:
 Requires higher drafting forces that can create vibrations in the back zone of the drafting
system. Break draft may have to be as low as 1.022 to prevent roller vibrations.
Tends to create roving irregularities such as thick and thin places

13. • Table: Fiber and Machine Parameters Influencing Drafting Force

14. Function of apron in roller drafting
Roller drafting
• A double apron drafting arrangement, comprising 3-over-3 rollers with long bottom
aprons and short top aprons is mostly used for short-staple spinning. The rollers are
made of steel and the top rollers are covered with synthetic rubber .
• The bottom rollers are positively driven whereas the top rollers are run by frictional
contacts with the former. The top rollers must be pressed with relatively high force
over bottom rollers by springs (mostly), air pressure or magnetically; different
machinery manufacturers follow different approaches.
• The drafting force (F) needed to draft the fibre strand comes in effect from the
loading of the top rollers (N) as F = μN. Selection of top roller force depends on the
fibre types and volume of fibres present between the top and bottom rollers.

15. • The adjustment of top roller force may be done in steps or continuously, depending
on the systems offered by the machine manufacturers. The purpose of having a
synthetic rubber surface on the top rollers is to improve the contact between the
fibres and top rollers. Top roller forces per roller shaft are within 100 to 300 N.
3over 3 rollers

16. • A force is applied to the top middle roller that presses the fibres to be gripped. As
the fibre strand is progressively thinned down, the number of fibres at the nip of the
middle roller of the ring spinning machine is very much less than that observed in
the drawing machine.
• Under this condition the frictional forces between the fibres held by the middle
rollers drop rapidly towards the front roller. A short fibre whose trailing end is
released from the nip of the middle rollers has poor contact with the other fibres
held by the middle rollers. This short fibre can be easily pulled out by the fibres held
by the front rollers and it accelerates, reaches the front roller earlier than it should
do. This leads to uncontrolled movement of short fibres, resulting in poor irregularity
to the yarn.

17. • In this context, it is essential to extend the grip over the short fibres very close to
the front rollers so that they are guided to move at the velocity of the middle rollers
till they reach very close to the front rollers. For this, two revolving aprons (made of
synthetic materials) kept under tension by cradles/nose bars are driven by the
middle rollers to guide the fibre strand in the front drafting zone.
• In order to exert controlled pressure on the fibres, the top apron is forced by spring
pressure against the lower apron and also constrained with an exit opening close to
the front rollers.

18. • With this arrangement, the forces acting on the fibres due to the external load
applied on the middle top roller do not drop rapidly towards the front rollers .Thus,
the frictional forces between the fibres held by the middle rollers do not drop rapidly
towards the front rollers. This helps in regulating the motion of the short fibres. The
exit opening between the aprons is determined by spacers or distance clips of
different sizes that must conform precisely to the volume of fibre strand; the ‘rule of
thumb’ is smaller sizes correspond to finer yarns, but the correct setting of the gap is
established by practice.

19. • For a given yarn count, the specific gravity, crimp and rigidity of fibres must also be
taken into account as these parameters differ with the type of fibres processed and
accordingly suitable spacers must be selected. Spacers are inserted between the
nose bar of the lower apron and the cradle edge of the top apron at the exit
opening. Fibre guidance length (from the nip of the middle roller to the apron exit)
depends on the length of fibres to be processed and is set by selecting cradles of
appropriate lengths.
• The bottom roller surfaces (except the middle roller) are helically fluted to flex the
fibres between the flutes so as to improve their grip over the fibres. The middle
bottom roller is knurl fluted to improve the static friction between the apron and the
roller so that apron slip is kept under control. Knurled flutes improve the service life
of aprons compared to helical or axial flutes.

20. Limitations of apron drafting and the
scope for improvement
Apron Drafting Roller Drafting
Better control of short fibres movement Poor or no control of short fibres movement
Better yarn quality with a given draft Poor yarn quality for the same draft
Regularity of material is better Regularity of material is poor
Less influenced by changes in staple length and
length distribution.
More influenced by changes in staple length and
length distribution.
Higher range of draft Lower range of draft
Smoother, durable and reliable running. Less smoother, durable and reliable running.
Lower end breakage rate and roller lapping. Higher end breakage rate and roller lapping.
Automatic clearer increases yarn cleanliness. No such clearer and thus poor yarn cleanliness.
Higher production. Lower production.
Higher spindle speed. Lower spindle speed.