What is a master drive point? (also known as pacer, lead section)

The master drive point is the single drive point intended to control the overall speed of a web line. The master drive point (also called the pacer or lead section) sets the speed for the entire line through acceleration, steady speeds, and deceleration. If the master drive point slips, the web speed and tension control will be uncontrolled.

Each additional drive point beyond unwinding, master, and winding non-master are considered tensioning drive points or followers and create additional tension zones. Tensioning drive points will run near the master speed, but trim their speed or torque, as needed, to increase or decrease tension.

How is the master drive point selected?

Guideline to selecting a master drive point:

  1. No slip – The master drive point should have sufficient web-to-roller traction to ensure that you always know the relationship between the master’s speed and the web speed. (However, see my notes on measuring web speed.)
  2. Don’t bypass the master – The master is much good if you don’t use it.
  3. Select a master near your most speed-sensitive process – In coating lines, the coating backup roller or the closest driven roller to the coating process is usually the master. In extrusion processes, the extrusion nip rollers or casting drum are almost always the master. In both cases, speed variations directly translate into web or coating thickness variations. The master is the one driven point in a web line that isn’t trimming its speed to control tension.
  4. Make the highest inertia process element the master – High inertia components do not easily change their speed, so make them the master and control everything else relative to the ‘biggest animal’ in the room. In a tentering process, the tenter is always the master. If you have a high inertia driven roller, such as a roller of large diameter, thick steel walls, or filled with oil or water, consider making that roller the master. (Note: Be careful with large diameter master drive points, as larger diameter rollers will more easily lubricate, losing traction and failing guideline #1 – don’t slip.

When are more than two tension zones recommended?

As I said above, the two-zone web line should be the starting point for any web process. But when is 3 or 4 or 13 tension zones the right answer?

The two tension zone line is designed to compensate for the diameter changes and process needs of unwinding and winding, but in web lines, we usually do more than just rewind, we have processes with their own tensioning needs.

Adding a fourth drive point creates a third tension zone potentially independent of unwinding and rewinding. As more processes and rollers are in the web path from unwinding to winding, more tension zones and drive points may prove beneficial.

How many tension zones does a process need?

 Here’s how to decide on more than two tension zones:

  1. Divide your process into steps, including unwinding and winding as process steps.
  2. For each process step, define the tension requirements of that step, including the nominal tension need (high, med, low) and the sensitivity to tension variations (high or low).
  3. Look for compatibility between consecutive process steps. If two or more steps are compatible in nominal tension and sensitivity to variations, group them together as a single zone.
  4. Add drive points as needed where high inertia components will take too much from web tension to be web driven. Consider both single component inertia and the inertia from a group of elements, such as a large number of idler roller in a drying or curing oven.
  5. Select a master (based on guidelines above).
  6. Select tension control method for each zone.