On dial-cylinder circular knitting machines the cylinder is considered the front needle-bed while the dial is considered the rear needle-bed. Both needle-beds feature grooves guiding the needle movements. The needles used for dial-cylinder circular knitting machines are mainly latch needles; some manufacturers however use compound needles.
Large-diameter dial-cylinder knitting machines can be divided into different categories according to the knitted good they manufacture:
The dial-cylinder circular knitting machines can be built in such a way that the cams rotate together with the spool rack. In this case it is possible to modify the position of the cams at each machine cycle and at each passage in front of the machine head. Dial-cylinder circular knitting machines can also be built with the cams and the spool rack standing still while the dial and the cylinder rotate with the fabric take-down system. The fixed cam system with rotating needle-beds is mainly used for the production of continuous fabrics while the mobile cams system with fixed needle-beds is mostly used for the production of cloths.
On large-diameter circular knitting machines, cams are subdivided into sections - corresponding to a group of cams that can be removed from the machine at a time - and can incorporate one or more feed systems, depending of the machine model.
In addition, the cams have different profiles according to the type of machine and to the type of fabric to be produced.
On circular knitting machines with fixed diameter - or with variable needle-beds for the production of cloths - there are several needle drive options to carry out different operations: needles can work in knit position (raising cam and tucking cam in a knit position) or in tuck position (raising cam in knit position and tucking cam in non-knit position). To prevent cams from hitting the needle butts and breaking, the control area is normally free from needles; as a consequence, the fabric emerging from the machine will have a 2-3 cm non-knitted area.
The stitch length is adjusted by means of micrometric screws on each feed system. To modify the length of the stitch, the machine includes different pre-set positions that can be retrieved from the head according to the design pattern to be carried out.
The stitch formation cycle on double-bed dial-cylinder machines is similar to that of flat knitting machines. The needles start rising from their lowest position; the previous stitch slips along the needle stem and opens the latch; when the needle reaches its highest position the previous course is on the stem, beyond the open latch.
The needle starts lowering and the thread guide feeds the thread for the new stitch which is seized by the hook; at the same time the previous one slips forward on the stem and closes the latch. Once the previous course has been knocked over on the new course, the cycle is completed. The same movements are carried out by the needles in the dial. Here, however, the dial needles move on a horizontal plane, so instead of raising and lowering movements, we will have forward and backward needle movements.
Picture 110 - The various steps of stitch formation on dial-cylinder knitting machines
Several machine models allow the variation of the stitch knocking over in order to have either a simultaneous or a differentiated knocking-over operation.
In the former case, the needles of the cylinder and the needles of the dial form the stitch simultaneously; in the latter case, by varying some controls or by racking the dial by five of six needles with respect to the cylinder, it is possible to knock over first the needles of the cylinder and then the needles of the dial. With the simultaneous knocking-over technique, the resulting fabric will be more consistent, soft and stretchable since the two series of needles can take up the quantity of thread necessary to form the stitch. On the contrary, with the differentiated knocking-over technique, in order to take up the quantity of thread necessary for the stitch formation, the needles in the dial have to make the thread slip with respect to the needles of the cylinder that have already been lowered. In fact, it is easier to take up part of the thread from the stitches already formed on the cylinder. In this way, the fabric formation will require less thread, resulting in a denser and less stretchable construction.
On dial-cylinder circular knitting machines it is also possible to transfer the stitches from a needle-bed to the opposite one to create complicated design patterns. The stitch transfer is carried out usually from the cylinder to the dial to exploit the wider possibilities for selection
Picture 110 - The various steps of stitch formation on dial-cylinder knitting machines offered by the cylinder needle bed. Obviously, the stitch transfer in the opposite direction is also possible.
To carry out the stitch transfer on mechanically controlled machines, it is necessary to replace some of the knitting feed systems with special feed systems equipped with transfer cams on the cylinder and receiving cams on the dial. In general, there is one transfer system for every two knitting feed systems. Obviously, the replacement of these knitting feed systems with transfer systems causes a reduction of output rates. Besides the transfer cams, it is also necessary to provide the machine with special needles with opening spring, like that of flat-bed knitting machines.
Rib-stitch machines constitute the most typical category of double-bed machines. The dial needles of rib-stitch machines are arranged in staggered position with respect to the cylinder needles.
These machines are mainly used for manufacturing continuous tubular fabric with rib-stitch or derived patterns.
Manufacturers offer rib-stitch machines in a wide range of models with diameters up to 40 inches; the most common models are however the 30, 34 and 36 inches, with gauge from E 10
The models with multi-track selection feature up to 5 tracks on the cylinder and up to 2 tracks on the dial.
Interlock machines are dial-cylinder machines of special design. In fact, the cylinder needles and the dial needles are arranged in front of each other.
Obviously, in order to achieve different needle motions, the needles themselves must be of different types: on interlock machines, needles are generally arranged in such a way that a short needle is alternated with a long one in the cylinder, and a long needle is alternated with a short one in the dial. To drive short and long needles, two cam tracks are necessary on both the cylinder and the dial.
On one feed system, the short needles of a needle-bed and the long needles of the opposite one operate alternately and form a half-course of rib stitches; in the next feed system, the needles operate inversely and form a second half-course of rib stitches interknitted with the previous one. In interlock fabrics, a knit stitch course is therefore made up by two interknitted half-courses of rib stitches.
Interlock machines, built mainly in the 30-inch diameter version, and E 18 to E 32 gauges, feature a huge number of feed systems (up to 108) and are designed mainly for the production of cloths with interlock patterns.
Picture 111 - Arrangement of needles and tracks on interlock machines cylinder
The main difference between fixed and variable needle-bed machines is due to the fact that in fixed needle-bed machines the needles occupy the whole bed and must all and always be used to manufacture a tubular fabric. In variable needle-bed machines the needles do not cover the whole bed; needles are incorporated only in a specific area of the machine bed. In practice, the dial and the cylinder have a section of 50-60° without needles: this empty area accommodates a control unit which transmits the mechanic and electrical commands to all the motions operating during the stitch formation.
When reaching this control area, the knitting process is interrupted. The thread is cut and retained until, at the end of the control area, the knitting process is resumed. The gripper starts operating before the first needle and retains the thread at the beginning of the work process. Another tool (a cutter) is placed after the last operating needle; the cutter cuts the thread at the end of the course. To increase the number of non-knitting needles, it is sufficient to move the cutters towards the grippers; in this way the knitting area will be varied as desired.
The main characteristic of variable needle-bed machines is the possibility of varying the number of operating needles and, by using special thread cutting and retaining tools, of manufacturing an open cloth whose width can be pre-set according to the desired size; this entails considerable yarn saving.
In practice, these machines create knit products similar to those manufactured on flat bed machines, with a definitely higher output, thanks to a greater number of feed systems compared to the flat bed machines.
From a technical point of view, these machines are offered with 40 or 42-inch diameters for the manufacturing of 277 and 291-cm open cloths, respectively; the available gauges range from E 5 to 18, and the number of stripe pattern motions varies from 4 to 5.
The base number of feed systems is 18 or 24, which can be differently combined in knitting and transfer feed systems: for example 18 knitting feed systems, and a fixed double transfer feed system and extra feed systems replacing the knitting feed systems with the transfer feed systems, or 12 knitting feed systems and 6 transfer feed systems or even 24 feed systems for both knitting and transferring tasks.
The early models of these machines incorporated rotating cam frame and fixed needle-beds. On more recent machine models, however, thanks to the electronic programming systems, the cam frame rotation has been eliminated since machine control is possible without obliging all the feed systems to pass through the head. In the latest versions, the cylinder and dial turn while the cam frames stand still. Stepper motors drive the lowering cams; they adjust the length of the knit stitch which can vary for each course.
Modern machines can grant up to 60 different positions of the lowering cams with an accuracy of 0.1 mm.
To make patterns similar to those obtained from flat bed knitting machines, and typical of outwear knitted goods (cable, pointed patterns, etc.) these machines must be able to transfer the knit stitches as broadly as possible.
Recent models are equipped with double-direction transfer systems, i.e. from the cylinder to the dial and vice-versa. These machines also allow a quick change-over of all the operating systems, thanks to which the knitting systems can be alternated with the transfer systems. It is also possible to rack the needles of the dial in relation to the needles of the cylinder; some machines even can carry out differentiated transfer techniques.
As far as the needle selection is concerned, some machines allow the 3-way working technique (i.e. knit, tuck and miss stitches) only on the cylinder while more sophisticated models allow the 3-way working technique on both needle-beds, thus increasing the machine versatility. On the latest machines, electronic needle-by-needle Jacquard selection is carried out by means of piezoelectric actuators.
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