Pneumatic accumulators are designed to take up slack and maintain web tension if a drive stops. The need may come up during splices, cut-overs or due to equipment jams. Because the accumulator is part of the tension control system for the web handling line, it must be calibrated.
The first step is to cancel the tare weight of the moving roller assembly. This tare weight is a bulk property of the accumulator and is balanced with fixed air pressure. With web removed from the accumulator, adjust the pneumatic air pressure to balance the tare of the moving roller assembly. When properly balanced, it should be possible to raise or lower the accumulator by hand. Please be aware of the hazards associated with a sudden movement of the accumulator when pressure is increased or decreased. Record the pressure needed to tare the accumulator.
Calibrating the pressure difference needed to establish tension is more complicated. Thread a non-stretching rope or cable through the accumulator following the web path. Tie the rope down on the side toward the line pacer. A weight corresponding to the design tension needs to be tied to the other side of the accumulator. Be aware that the accumulator and the weight will travel during calibration - love your toes.
As shown in the illustration above, the accumulator acts as a block and tackle pulley system with a mechanical advantage on the moving side. Very little web tension will raise or lower the accumulator, even though it has a considerable mass.
Determine the pneumatic pressure change required to balance the weight. You can expect that the pressure for full tension may be less than 10% of the pressure required for taring the accumulator. You may find a different pressure is required at the full vs. empty position (non-linear). You may find quite a bit of difference in pressure required to fill and empty the accumulator (hysteresis due to stiction of the cylinders). No one said this would be easy! Find the best value for the pressure difference needed to balance the weight corresponding to the design tension of the line.
This pressure difference must be scaled into web handling units of tension. That is N/m or PLI. This value will be needed in the controller programming or on a chart understandable by operators if manual regulator knobs are used.