Circular Knitting Machine
A circular knitting machine is commonly used for single knits. Circular knitting machines pull their yarn supply from creels located either above the machine or next to the machine.
To make it easier for technicians to service the creels, it is more usual to locate creels at the side. In this position, less lint falls into knitting elements, larger packages can be used and more feeds can be placed on the machine. Whether the creel is at the side or at the top the principles for circular knitting are the same.
Fed from above the knitting elements, yarns move from the yarn supply, or creel, through guides to stop motion controls above the machine.
Then back down through tension controls and yarn feeding devices to the knitting elements. Quality products can be produced only when stop motion and yarn feeding functions are properly set. The intricate action of knitting where needles form loops occurs in the middle of the machine between the take-up and the yarn feeding mechanism. Circular weft knitting needles are knit one after the other in a sequence for each yarn. Loops are formed horizontally by needles knitting around a cylinder knitting forming a tube. After the yarn is knit on the knitting elements, the knitted fabric is passed over a spreader mechanism through take-up rolls and is wound into a roll. This elliptical spreader distributed the take-up tension uniformly and enables the fabric to conform to a flat tube.
Knitting machines are designed so that each needle can be placed in a groove cut into the outside of a metal cylinder. The cuts or grooves are also referred to as slots or tricks. The top edge of each groove is referred to as the verge. These cylinders are very precisely manufactured so that the diameter measured at any place is equal. Machines are classified by the number of cuts per linear inch. This is referred to as the cut or gauge of the machine.
For example, an 8-gauge machine has 8 cuts per inch. The total number of cuts around the circumference of the cylinder would indicate the number of needles in the cylinder—the more needles, the wider the fabric.
Parts of a Latched Needle
At the top of each needle is a hook. Below this is a latch attached with a rivet. The bottom edge or cup of the latch is curved to fit over and completely close the hook.
At the bottom of the needle is a butt that plays a part in controlling how needles activate, up or down.
A needle with a latch is very efficient. When the latch needles are used to create weft knits, the knitting cycle can be completed without any auxiliary attachments.
How a Latched Needle Works
- Rest – At rest or running position, a knit loop rests above or on the latch.
- Clearing – As the needle moves up, the old loop, already formed, drops below and clears the latch.
- Yarn Receiving – As the needle moves down, it receives the new yarn to begin forming a new stitch.
- Cast Off or Knockover – The latch is knocked over by the old loop and this old loop is cast off.
- Stitch Formation – The needle moves further down to fully form and complete the new stitch.
The amount of yarn used to form a new stitch determines the stitch length. This is important because stitch length affects the weight, width, and aesthetics of the fabric. On modern-day knitting machines, needles make millions of loops or stitches a day. Needles may need to be replaced due to wear but they usually last up to six months depending on the construction, yarn type, fiber type, and speed.
Next, let’s take a look at what causes the needles to move up and down. The butt of the needle guides the needle through a path formed by cams. Each cam is designed to allow the needle to run straight or to move up and down.
Here’s how the needle travels through various stages:
- Rest Cam – At the rest or running position, the needle runs straight over the rest cam.
- Clearing Cam – When it hits the clearing cam, it rides up at a steep angle which forces the needle to rise and clear the old loop.
- Stitch Cam – The needle drops when it contacts the stitch cam.
- As it continues on its path, it catches the new yarn. It continues further down, pulling the new yarn far enough for a new loop or stitch to form as the old loop is cast off.
- Upthrow Cam – The upthrow cam returns the needle to its resting position so it can begin the cycle again.
The machine has a cylinder that contains vertical grooves and slots to hold the needles that move.
Between each pair of needles is a sinker. Sinkers control the movement of fabric as the machine knits. A sinker is a thin steel element with a distinctive shape. A sinker has a hold, a throat, a nose, and a butt with a place to insert a cam.
As the needle goes up, the sinker moves in to catch the fabric in its throat. Since the fabric can’t go up with the rising needle, the old loop now clears the latch. When the hook catches a feeder yarn as the needle moves down, the sinker moves back out of the way and knockover, cast off, and stitch forming take place. As the new stitch is formed, the fabric rests on top of the nose.
Sinker moves to catch fabric in its throat
Old loop clears the latch
Needle moves down while sinker moves out of the way
New stitch is formed and fabric rests on top of the nose
Knit, Tuck, & Float Stitches
There are only three types of loops or stitches possible in weft knitting: knit, tuck, and float.
If every needle is fed a yarn and goes through the basic knitting cycle, the product is referred to as a single jersey. All loops are knitted and all loops look precisely alike. The length of yarn in that loop is called the stitch length. Each loop has legs and a crown. Referred to as a jersey stitch, stitches arranged in this pattern have a distinctly different look and feel from the face to the back.
On the technical face side, the stitches have an overall vertical appearance. You see primarily legs rather than the crown. On the technical back side, the stitches have an overall horizontal appearance. On this side, you see mainly crowns.
Another type of stitch is referred to as the tuck stitch because one yarn is tucked behind the other and hides.
The pattern on the left shows the technical face for a tuck stitch. Follow the green shaded course of yarn across the pattern and it looks like a loop has been tucked behind another.
The pattern on the right shows the technical back for a tuck stitch. From the back, the tuck is more visible to the eye.
How is a tuck stitch made?
During the tuck cycle, at feed one, the needle moves up from its rest position and the old stitch that has been formed is held and not allowed to clear the latch yet the needle moves up far enough to grab a second yarn which is put into a tuck position, both yarns are then kept at the rest position. The knit cycle occurs with the next feed of yarn. At this time both yarns are cleared. The new yarn is fed and pulled through both the held and tuck loops forming a tuck stitch. The stress caused by holding one elongated stitch for an extra course causes more length shrinkage but less in width than a regular knitted stitch. The tuck loop makes the fabric wider and thicker and slightly less extensible.
What is the structural effect of tucks on knits?
Tuck stitches can give the fabric a cellular appearance. Some people refer to this as mesh. Tucks are the basis for pique, typically used for golf and tennis shirts that need to breathe and retain their shape but have some stretch. A wide range of pique constructions can be made, depending on the use and frequency of tuck stitches.
The third kind of stitch is the float or missed stitch.
The drawing on the left is a technical face. On the face in the middle course of yarn and middle of the wale, it looks like the machine has hidden the colored yarn in the back. It is not captured or knit with any other stitch. This is the float or missed stitch.
On the technical back, you can see how the missed stitch floats.
To produce the float stitch, on feed one yarn is laid to rest behind the hook of the needle. The needle remains in the rest position, it is not activated in the float cycle. In the knit cycle, it is activated. When a subsequent yarn is knit at the next feed, the missed yarn floats to the technical back of the fabric.
Loops can be made to float over a series of wales. To make the structure secure, some float yarns can be tied to the ground with a jersey or tuck stitch. Float loops make the fabric more narrow and less extensible because the floated yarn is in a straight configuration. Why would you produce a structure with loops that float? Floats are useful for pattern effects where some colors appear on the front and others are hidden on the back. This checkerboard pattern uses gray and white yarns. When the gray yarn knits form a gray square on the front, the white yarn floats to the back. If you turn the fabric over and inspect the back, the colors appear reversed. A second use is to create surface effects or change the performance of the fabric. You can make loops float on one side of the fabric, then nap them to produce a fleece. If not napped, these floats could be used for aesthetics or function.
All three of these stitch formations can be produced on warp or weft machines or used for single or double knits.
TERMS TO KNOW (click to flip)
A knit fabric made on one set of needles.view in glossary
The basic unit of construction of knitted fabric, consisting of the loop of yarn formed by the needle. The stitch…view in glossary
A knitting stitch that produces tuck or open effects by having certain needles hold more than one loop at a…view in glossary
A yarn placed behind a knitting needle so that it does not knit or tuck. Also known as a miss…view in glossary