Web press tension control can be a real source of grief. Thankfully pressmen have a few principles that can apply to almost any web offset printing press. Though there are exceptions the rule, I would like to share those principles that may help you to get control of web tension on your press.
How Web Press Tension Works
Though a web offset printing press prints on one continuous roll of paper, each part of the press is not running at the same speed. In order to maintain tension, each component has nip points that drive the web at different speeds in order to control web tension. These differences are marginal, but crucial for each part of the machine to be able to perform its function. The number is carefully calculated according to a reference.
The speed of each part of the web press is referenced to the printing units. These are the most critical "nip points" on the press and everything else must work around them. Much care is taken so that the blankets are set to the perfect amount over the bearer height and maintained, governing the speed at which the paper is delivered from the printing unit. Any miscalculation results in a myriad of print problems, well beyond the scope of web tension.
Another point to keep in mind before calculating the proper speed of each press component is the type of paper being used. While it is possible to approximate how much tension will be required throughout the web press, each paper has its own needs when it comes to tension. Paper is made of various grades and thicknesses. Don't expect all of them to be the same. When you learn these numbers, record them and save them for future reference.
How Web Press Tension Works
The general principle is this: The rollstand and infeed run slightly slower than the print units, while all components of the press after the print units progressively run faster as the paper gets to the web press folder. Here is a web press broken down in its parts.
1. The paster or zero-speed splicer. The roll itself should have minimum tension exerted on it. Some tension is of course necessary to keep control of the web as it passes through the festoons or idler rollers. However it has little to do with the overall scheme of runnability through the web press.
2. The infeed. Now tension becomes critical. The purpose of the infeed is to bring tension variations down to a very small window. This is critical for the print units. This nip point literally pulls the paper back from the print units by running slightly slower. How slow depends on the paper type and web offset printing process.
3. The printing units. As stated, cylinders must be packed perfectly and tension controlled so that doubling or slurring are kept to a minimum.
4. The heatset oven. If you have one, this ads a new dimension to tension control. Air bars make the web literally float around them causing a ripple shape as it travels through the dryer. Though tension is not adjusted at this point, air flow greatly effects tension.
5. Chill rollers. There is always a nip point at the chill rollers. This component must literally drive the web slightly faster than the units are printing. It is critical for keeping the paper taut and controlled. Without this increase in speed, the web would ride up the cylinders, and wander through the heatset oven.
6. Superstructure nips. In order to accommodate various page counts, many web presses have various angle bars that allow slitters to cut the web and overlap the resulting ribbons one on top of the other. Here is where I depart from conventional wisdom. The nips right after the angle bars should run slightly slower to allow the paper to slide around the angle bars. I have never had good control unless it is done this way.
7. Folder. The RTF nip must graduate to a slightly higher speed while the draw nips after the former board must slightly add to this. One indication as to whether you are drawing enough paper is to look at your pin holes created on your pin and tuck cylinder. If the holes are tearing, you don't have enough draw. Here are some tips about web tension to keep in mind for your web press.
- Adjustments should be minimal and gradual. This is what offset printing is all about, right?
- Too much tension will cause web breaks while too little will cause web wander. Find the balance.
- Tension should always be run at the minimum required for the best control.
I will be writing about more tension issues that concern a web press in the future. Please subscribe below for any future updates.