Proper web handling is to provide good web-tension isolation between the unwind, process and the winding tension. Pull rolls are critical components in web handling system
Function of pull rolls
Pull Rolls are incorporated in web lines for many reasons including (link):
- conveying the web at a set speed
- controlling the web tension as it travels between sections
- changing the web tension between sections
- isolating one equipment section from another
- feeding web to/removing web from a process point/operation
Types of pull rolls
There are three types of pull rolls used for tension isolation (link). There is another pull roll, Omega-wrapped pull roll, which is not commonly used for tension isolation (reference).
- Nip type. A rubber-covered roll is loaded against a driven steel roll to isolate up-stream and downstream tension, as long as sufficient nip loading is provided. A general rule of thumb for the amount of nip required is three times the web tension. If at all possible, design a process that doesn’t require nips. For wrinkle-sensitive webs, tensioning nips should be replaced with S-wrap or vacuum pull rollers (Reference)
- “S Wrap” type. It consists of two or more driven rolls, usually rubber covered, with sufficient web wrap and web-to-roll coefficient of friction to isolate the tension differential
- Vacuum or suction roll. It is used when only one side of the web can be contacted. Vacuum rolls the web into intimate contact with the roll, and it increases friction forces that help prevent web slippage. Vacuum roll is a good option if nipping or contacting both sides of the web is not possible or desired
Calculation of tension
F₁/F₂=eᶬᶱ
Where:
F₁ = the force (tension) on the web prior to the pull roll
F₂ = the force (tension) on the web after the pull roll
ɱ = the coefficient of friction between the pull roll and the web
θ = the wrap angle on the pull roll(s)
Driving the Pull Rolls
One of the following two ways to drive a pull roll typically, depending on process requirements.
Speed regulated drive
- motor’s speed is set to rotate the pull roll at a set surface speed
- typically the case when the pull roll is being used to establish the speed of the process or to feed a section
Tension regulated drive
- a force transducer roll is located in the web path to measure the web’s tension and automatically adjusts the motor’s speed/pull rolls’ speed to maintain the tension at a set point/level