Thursday, 15 October 2009

The Principles of Double Needle Bar Warp Knitting Machine

To demonstrate the working procedure of double needle bar raschel machine,a simplified version of the knitting elements
is illustrated in Fig.240 showing the two needle bars marked with the letters f (for front) and b (for back),the two guide bars marked with the number 1 and 2,two trick plates and two sinker bars.
The double needle bar machine is almost symmetrical so that the needle bars can be identified as a front and back only by the position of the batching device.The definition of front and back bars as well as the definition of the first and the second guide bars are made fro the sole purpose of establishing the knitting relationships each of guide bars and each of the needle bars in the following descriptions.

The needle bars are independently operated in an up and down movement while the guide bars are swing alternately among the needles of each bar.The simplified knitting cycles illustrated in Fig.241 1-5 can be describe as follows:

  • Stage 1- The guide bars are positioned at the back of the machine,above the back needle bar.The front sinker is placed forward to secure the fabric while the front needle bar ascends to clearing position.The guide bar perform the under-lap shoging movement for the front needle bar and then swing to the hook-side.
  • Stage 2- On the hook side of the front needle bar,the guide bars shog an overlap according to the pattern mechanism and then swing back.Note that not every guide bar needs to form an overlap as further explained in the following paragraphs.
  • Stage 3- The swing back is completed,the yarns are wrapped within the hooks so that the front needle bar can start to descend.The front sinker bar retreats while the back one move forward.
  • Stage 4- Front needle bar descent,the previously formed loops which rested on the needle stems close the latches.The front sinker bar continues to retreat,the back one is now above its needle bar.The guide bars swing for the third time,this time to the front in order to clear the way above the back needle bar.Under-lap shoging movement fro the back needle bar can start.
  • Stage 5- Front needle bar is at knockover position and the needles form new loops.Back needle bar,now with its fabric secured by the sinker bar,ascends to clearing position.
The sequence describe is only one half of the knitting cycle and the movement of the back needle bar is identical to that of the front.Every knitting cycle of this machine type incorporates six swinging movements which are produced by the machine camshaft.

Drafting design for a double needle bar uses different procedure from single needle bar machines.Two rows of dots are necessary to show one knitting cycle of the machine.The first row always represents the knitting course of the front needle bar while the second represents the knitting course of the back needle bar.

The up and down movement of the needles as well as the swinging movement of the guide bars are constantly produced by the driving mechanism.
The different fabric types and effects are produced by the patterning mechanism.

When both guide bars move in the same direction in 1 and 1 lapping movement as illustrated in Fig.242,the same needle are constantly wrapped by the same yarns and unconnected chains are produced.The top view shows the movement of the guide bars between two needle bars and demonstrates this case.By observing the lapping movement one can deduce that the needles on the front needle bar are always wrapped with 2 - 0 movement,while the needle on the back are always wrapped with a 2- 4 movement.
The 1 and 1 lapping movement is not normally uses on the double bar needle machines and in fact any constan reciprocating movement will form a similar effect.

To make continuous fabric,it is customary to double each lapping movement and only then to form an underlap.A double face fabric is fromed according to lapping movement in Fig.243.Both guide bars perform overlaps on both needle bars.

The chain notations are:
Guide bar 1: 2-0-2-0/2-4-2-4
Guide bar 2: 2-4-2-4/2-0-2-0

Note :That four chain link are used for every knitting cycle,two for the first phase (front needle bar sequence) and two fro the second phase.

To produce two separated fabric,one on the front needle bar and the second on the back one,each of the guide bars is knitting on one needle bar only.The swinging movement is not altered but each of the guide bars,by not overlapping a needle bar,does not connect their yarns to these needles.

Fig.244 illustrates this procedure.The first guide bar overlaps the front needle bar during the first phase of the first knitting cycle,swings in and out of the second needle bar without overlapping it and then again overlaps the needles of the front needle bar during the first phase of the second knitting cycle.In the same way,the second guide bar overlaps only the back needle bar.Two separated fabrics,each of them with a construction of 1 - and - 1 are formed.

The chain notations for this case are:
Guide bar 1: 2-0-2-2/2-4-2-2
Guide bar 2: 2-2-2-0/2-2-2-4

If the guide bars switch their lapping movements, bar 1 overlaps the back needle bar while guide bar 2 overlaps the front (Fig.245),two fabrics which are locked together by the underlap are formed.

The chain notation for this fabric type are:
Guide bar 1: 2-2-2-0/2-2-2-4
Guide bar 2: 2-0-2-2/2-4-2-2

A third guide bar can be added between the previous two in order to increase the knitting possibilities.When the third guide bar is only threaded one guide finger on one side,it can be driven to connect the two separated fabrics,produce by the fully threaded guide bars as illustrated in Fig.246 a.The fabric which formed in this way can be twice as wide as the knitting machine.

By threading the middle guide bar through two guide fingers,one on each side,a tubular fabric is formed as demonstrated in Fig.246 b.

A fully threaded middle guide bar is used to produce a sandwich of two fabrics connected by the yarns.The fabric is then cut and separated into two plush fabrics by special equipment.The principle is illustrated in Fig.246 c.

The chain notations for the last three cases are:
Guide bar 1: 2-0-2-2/2-4-2-2
Guide bar 2: 2-0-2-0/2-4-2-4
Guide bar 3: 2-2-2-0/2-2-2-4

With the use of more guide bars,the fabric produced on each of the needle bars can be much more elaborate.

-Literature : Warp Knitting Production by Dr.S.Raz,Melliand Textilberichte GmbH ,1987.