SURFACE STRUCTURE

A surface finishing can be described as “friendly to the yarn” when:
- the surface does not damage the yarn
- the coefficient of friction is reproducible;
- spin finishes are not removed
These demands are satisfied by a surface with a homogeneous conformation, consisting of rounded grains without sharp edges.

The surface of the thread guide is determined by:
- the material composition;
- the manufacturing process;
- the surface treatment;
- the surface density.

a) FRICTION COEFFICIENT

The friction coefficient is calculated by measuring the two yarn tensions before and after the thread guide.
The frictional behaviour between yarn and thread guide is influenced by various factors such as type of yarn, off-take speed, pretension, wrap angle, yarn oil and the humidity and temperature of the immediate environment.
Low Friction materials allow the best roughness to be achieved with a reduced friction coefficient.
The equipment used to measure the friction coefficient is very simple and is shown in the figure below. The yarn runs with constant speed. Its input tension is determined by a tension device; the yarn runs over the thread guide and its wrap angle is measured. Two tension meters measure the tensions T1 and T2. The friction coefficient is calculated by Eytelwein’s law:

Where:
T2 Input tension (T2 = 0,1 Denier + (Denier / Filaments))
T1 Output tension
α Wrap angle
f Friction coefficient

b) ROUGHNESS AND FINISH

This feature is very important to avoid broken filaments and to reduce the “snow” effect. Materials should thus be used with a suitable grade of roughness (the best is not necessarily the lowest).
The “Ra” value is an excellent measurement for defining the surface finishing: in fact, as its value decreases, the brightness degree increases.
The term Low Friction does not define the surface roughness, which may be between 0.2 and 0.6 _m, but refers to a surface quality which allows the yarn to glide over the ceramic surface.