Yarn Friction

Friction is a very important factor in all phases of knitting. In case of yarn friction applies to resistance developed by yarn sliding over another yarn or over metallic or ceramic bodies. There are two types of friction 1) static 2) kinetic. Resistance offered by the yarn in contact with guides bars or eyelets. When the machine is started is an example of static friction while movement of yarn through the guides or needles when the machine is running is kinetic friction. Static friction is generally greater than kinetic friction. Frictional resistance or coefficient of friction depends on many factors such as:
i.Surface smoothness
ii.Yarn color
iii.Yarn twist
iv.Package hardness
v.Moisture content in the yarn
vi.Yarn lubricants used
vii.Yarn tension etc

Surface Smoothness: It is generally assumed that smoother the surface lesser is the coefficient of friction. However it has been found in the case of synthetic yarns that the smooth filaments of these materials stick to smooth surface and produce a large area of contact and therefore large frictional drag. But the mat surface presents less area of contact reducing the coefficient of friction. Of course the mat surface should be only microscopically visible.

Yarn Color: All factors remaining same when color is changed it results in change of dimensions. Darker shades give more friction increased tension and hence lower stitch length. Dyed yarn has usually a higher coefficient of friction via 0.30.

Yarn Twist: Increased twist in yarn reduces friction. Low twist yarns spread out and have increased area of contact.

Package Hardness: Package hardness affects withdrawal tension. Package hardness is related to yarn friction. A higher friction yarn produces a harder package due to higher winding tension and this will knit a tighter structure.

Moisture Content: Moisture improves pliability of yarn and makes it more plastic so that more regular and uniform loops are formed. Moisture enables knitting with ease. Moisture content in the yarn should be 10 to 40 per cent. Small changes in moisture do not change friction as such. Wide differences in moisture however do cause differences in dimensions due to changes in frictions.

Yarn Lubricants: As mentioned earlier yarns are lubricated to reduce friction before supplying it to the machine. A suitable selected lubricant for a given fiber and guide surface may cut the coefficients of friction to half its value. What is equally important is that yarns with different initial coefficient of friction can be brought to a comfortable low friction level after lubrication thus avoiding variations in quality. The quality of lubricant and the amount of lubricant to be put on yarn is also very important. Between 1/2 and 2 per cent lubricant gives good results. The conventional method of applying lubricant is to place a paraffin wax dist between the metallic discs used as tensioner as shown in figure. This method is however inefficient as grooving of the wax causes variations in wax application disc. A more efficient method is the application of wax emulsion as shown in figure.

Yarn Tension: When the yarn is knitting on the machine kinetic friction is developed. The measure of yarn friction is calculated by measuring the input and output tensions and using the Amontons equation To =Ti.eµƟ where to is the output tension Ti is the input tension e=2.718 µ is coefficient of friction and Ɵ is the angle of warp in radians shown in figure. Output tension thus depends on the input tension coefficient of friction and the angle of contact. It has been estimated that during the formation of any one loop the yarn must pass over as many as a dozen guide surfaces and due to the friction involved the tension in the yarn increases. The increased tension causes the loop length to change and thus fabric quality is changed. It has been found that if the coefficient of yarn friction is less than 0.2 faults in knitting usually associated with friction are mostly eliminated. Spun yarns like wool when not waxed will have values of µ=0.4.Such high values cause excessive yarn failure.
          If for example coefficient of friction =0.50 and the angle =90º =Π/2 then,
                                                                             To= Ti.eµƟ  0.5 ×1.57
                                                                                 = Ti × 2.718
                                                                                = 2Ti (approx)
          Thus the output tension becomes double the input tension under such conditions. If the yarn is warping partly round a number of guides then the tension is built up progressively. A 3-g input tension may rise up to 300-g output tension up to knitting point. It’s therefore necessary to test the yarn friction regularly before knitting. A number of instruments are now available to test the yarn friction. The use of positive fee reduces to some extent the detrimental effect of high yarn friction knittability. As a rough guide both for staple and filament yarns the optimum yarn input tension is found to be about 10% of the tex number of the yarn. However it is also recommended that the input tension does not drop below 1.5g otherwise difficulties arise in setting such low tensions. The coefficient of kinetic friction increases with the yarn speed. Thus a 15 denier nylon yarn passing through chromium plated guides has coefficient of friction = 0.35 at about 100 meters per minute.