Types of Drafting System
Drafting System Can Be Broadly Classified Under Two Name
· Regular drafting system without apron. And
· Drafting system with apron.
The system with apron can again be classified into three groups –
· Single apron system.
· Double apron system.
3-Over-3 Double Apron Drafting System
· 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.
· 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 rollers to the surface speed of the back rolls and is a product of the break draft and the main draft.
4-Over-4 Double Apron Drafting System:
· 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:
Rieter Double Apron Drafting System with Long Bottom Apron
· In the double apron drafting system the fiber guidance unit consists of two aprons rotating with the middle rollers. The top apron must be pressed against the bottom apron with controlled force in order to provide guidance. For this purpose there must be a gap between the two aprons at the apron delivery end (tensor gap) that is precisely adjusted to the fiber volume. This gap can be adjusted by using different spacer plates, shoes, etc.
· The top aprons, which are often plastic aprons, are always short. However, the bottom aprons can be as short as the top aprons or quite a bit longer and guided accordingly around deflecting elements. The advantage of long bottom aprons over short aprons is that they are easy to replace in the event of damage. They are also less inclined to become clogged with fiber fly.
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