Stitch Formation Motions

Circular knitting machines, both the single or double-bed types according to our initial classification, can incorporate various stitch formation motions depending on the machine's technical features.

The Main Components of a Single-bed Machine

Single-bed circular knitting machines are equipped with only one series of needles sliding in the grooves of a circular needle-bed. The needles are usually latch needles but some manufacturers have designed machines equipped with compound needles.

The cams, which drive the movement of the needle forming the stitch, are placed outside the needle-bed; each feed system is provided with its own cam group.

All the cams are fixed to a bearing structure called "cam frame". On single-bed machines, the cam frame is stationary, while the needle-bed revolves.

Outside the cams, on each feed system, there are special micrometric screws, which adjust the stroke of the lowering cams and determine accurately the length of the yarn fed.

Mayer And Cie
Picture 84 -Knitting head of a single-bed circular knitting machine

In their simplest structure, the cams are screwed to the cam frame and command a single movement of the needle: for example, when for a certain feed system we only have one group of lowering and rising cams, the selection possibilities will be very restricted. In fact, in this feed system, the needles must knit or remain idle (this is the typical situation of jersey knitting machines). In this case, to modify the pattern it is necessary to change the cam. These technical limits have been overcome by increasing the number of needle butts and the corresponding cam tracks necessary to drive the needle.

Now machine manufacturers are able to offer modern single-bed machines with up to 5 selection tracks.

Picture 85 - Single-bed machine with 2 open sections exposing the 4 cam tracks.

Single-bed machines must also incorporate sinkers to carry out the knitting cycle: the sinkers hold the fabric already formed while the needles rise for the next stitch formation cycle. The sinkers also support the fabric when the previous course is knocked-over. Sinkers are driven by special cams whose shape depends on the type of the sinker itself.

Mayer Sinker Plush Knitting
Picture 86 - Sinker cams of a single-bed fleece knitting machine The Main Components of a Double-bed Machine

Double-bed circular knitting machine are equipped with two series of needles: one series of needles fits in the circular needle-bed, called "cylinder", while the other series is accommodated inside radial grooves positioned at 90° with respect to the cylinder, on a special circular plate called "dial". Double-bed circular knitting machines usually incorporate latch needles, but some manufacturers also offer machines equipped with compound needles.

The cams that command the various needles are fastened to two cam frames, one around the cylinder and the other above the dial.

Picture 87 - Cams e cam frames for dial and cylinder Two technical options are available for this type of machine:

Revolving cam frames and stationary needle-beds (for continuous fabric manufacturing machines)

Stationary cam frames and revolving needle-beds (for cloth manufacturing machines)

Picture 87 - Cams e cam frames for dial and cylinder Two technical options are available for this type of machine:

Revolving cam frames and stationary needle-beds (for continuous fabric manufacturing machines)

Stationary cam frames and revolving needle-beds (for cloth manufacturing machines)

Picture 88 - The knitting head of a double-bed machine

Also manufacturers of double-bed machines provide a wider range of knitting possibilities by adding extra cam tracks. Today manufacturers offer rib knitting machines, i.e. machines with the needles of the dial staggered with respect to the needles of the cylinder, featuring up to four tracks on the cylinder and two tracks on the dial.

Picture 89 - Cam tracks for double-bed machines: 2 tracks for the dial and 4 tracks for the cylinder

Interlock knitting machines make up a very important category of circular double-bed machines. The long and short needles of an interlock knitting machine are placed opposite to one another on dial and cylinder; the needles work alternatively in two consecutive feed systems. Also important are double cylinder machines with double latch needles driven by sliders. On this type of double-bed machines, all the knitting components that operate during the stitch formation cycle are controlled by cams usually sliding inside different tracks.

No sinkers are needed on double needle-bed machines since during the stitch formation cycle, the fabric formed by the rising needles of one needle-bed is held by the needles of the opposite needle-bed.

The Technical Evolution

The rush towards higher productivity has led manufacturers to greatly increase the number of feed systems above all on single-bed jersey knitting machines.

The number of feed systems can be increased either by extending the range of diameters available (manufacturers now offer machines with 60-inch diameter), or by reducing the size of feed systems.

Today, 3 to 4 feed systems per inch for single bed machines and 2 to 2.4 feed systems per inch for rib knitting machines are quite common features. Obviously, the feed systems for knitting more complicated patterns are bigger.

In any case, it is worth mentioning that one of the main problems in the manufacturing of circular knitting machines is the spiral shape of the fabric, and this spiral is emphasised in proportion to the increase in the number of feed systems.

In order to increase the output rates of circular knitting machines, it seems therefore more reasonable to limit the number of feed systems and study new technical solutions for increasing the machine throughput speed.

For this reason, many machine manufacturers have decided to incorporate curvilinear cams sliding inside closed tracks, as a result allowing a constant control of the needle butt during the entire knitting cycle. This method grants higher speeds and safer working conditions. Furthermore, to reduce cycle times, many manufacturers have provided their machines with special systems ensuring an easier adjustment of cams according to the work to be carried out by the needle. The cams can be adjusted with an external screw without knocking down the machine section (a cam or a group of cams that can be opened at the same time). In this way, it is possible to set the needles easily and quickly on knit-stitch, tuck-stitch or miss-stitch positions.

Picture 90 - Cam adjusting system with external screw
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