Before going into the details, one should understand -- what is frictioning?
Friction means, in general term, a sort of rupture/rub effect. This process is generally adopted on grey cotton fabric, printed cotton saris, dyed poplins and other material to make them stiff and look bright with reflection.
How to get the friction effect in a calender machine with the help of mechanism called friction arrangement?
If one observes an old calender machine [friction calender - seven bowl/five bowl/three bowl] one finds on the opposite end of drive side a set of big gears with different diameters and with different numbers of gear teeth.
Yes, they are friction gears. On the cast iron bowl shaft there is a big gear with specific number of teeth. Again on the chilled cast iron bowl shaft one finds another gear wheel with less number of teeth.
And in between there is another gear, which helps in smooth rotation of both the vital gears [it is known as idle gear or idler].
What happens when the machine is on and all the gears are engaged?
Because of the proportion of teeth [different count of teeth on chilled cast iron bowl and cast iron bowl] both the bowls rotate with different speeds. Obviously, the chilled cast iron bowl rotates faster than the bottom cast iron bowl and the cotton bowl in between is a free bowl and rotates on its own accord. In this process of varied speeds there is a friction effect on the fabric. Due to friction, extra rubbing, the fabric gets crushed and attains extra glaze. Friction is a ratio so there is no unit to specify. Hence it is described in terms of ratio.
To calculate the friction ratio or to design the gears for a specific ratio – What are the factors to be considered?
First of all one should know the circumference of chilled cast iron bowl and cast iron bowl. Suppose, for calculation purpose, circumference of chilled bowl is "X" and the circumference of cast iron bowl is "Y".
Say, X is 38 inches and Y is 56 inches.
Let T1 be no of teeth on gear of cast iron bowl.
Let T2 be no of teeth on chilled cast iron bowl.
The teeth on the gears were counted and found to be 76 on T1.
Similarly T2 is having 33 teeth.
So, T1 ÷ T2 = R [ratio].
Hence, T1 = R T2
ie, [X] × [T1 ÷ T2]
----------------------- = R
Accordingly, R = T1 X ÷ T2 Y
ie, R = 76 X ÷ 33 Y
So, finally, [76 × 38] ÷ [33 × 56] = R
So, the ratio of friction is = 1.56:1
It means, when the chilled cast iron bowl is rotating 1.56 times then the cast iron bowl is rotating only once.
Similarly, one can design the ratio of friction of his choice for desired finish by designing gears of different sizes and teeth.
Note: The life of cotton bowl reduces due to frictioning by about 30%. While doing frictioning on fabric, care should be taken to see that the inputs in the fabric, viz, starch, gums and other chemicals do not stick to the cotton bowl. If not, the fabric will tear at regular intervals. To avoid this problem, run the machine blank for at least 30 minutes before passing the fabric and during the blank run the metal roll temperature is at least 70°C. Engage the friction gears only when fabric is fed into the machine. If the cotton bowl is not having even surface with proper chamber, even then the fabric may tend to get curled from sides thus damaging the fabric.
For further information:
D V Prabhakar
Calender machines specialist