Nips are everywhere. We can hardly manufacture or convert webs without them. They are found on calenders, coaters, laminators, printers, winders and other machinery. They are used on film, metal, nonwovens, paper and other materials. Nips are so important to control closely that variations of nip pressure are almost synonymous with variations of product. Yet, unfortunately, many nip controls are done quite poorly. Check yours against these dozen design guidelines and judge for yourself.

1. Nip load should be calibrated (not merely calculated) so that a nip setting of zero means zero force between rolls. Anything else is needlessly confusing to operators.
2. Nip load should be calibrated (not merely calculated) so that readouts are in PLI or lbs/in. of web width (kN/m in metric units). Total force does not allow us to readily translate experience between varying web widths. Pressure (PSI) to the cylinders is the worst of all choices. It does not allow us to translate experience anywhere. These units are needlessly confusing to operators, process developers and engineers alike.
3. Nip controls should have a fixed pressure on one side of the cylinder to zero the system, as given in Rule No. 1, and a control side on the other side of the cylinder. The fixed side is buried in the cabinets and is for service people only. The word "fixed" does not preclude varying this side by cam or computer to compensate for geometry changes. Giving operators control of both sides of a cylinder is needlessly confusing.
4. The minimum control load for good performance, pneumatic or hydraulic, is around 10 psi. Control load is not necessarily the load applied to the cylinders. A smaller pressure indicates poor design such as oversized cylinders or attempting to lift too-heavy rollers rather than move them sideways.
5. Pneumatics should not be used in applications that are prone to vibration or for those that require very precise control.
6. The total mechanical friction and control hysteresis should not exceed 10 percent of the minimum control load. The most common source of friction is the seals on the cylinder. Slides can be a problem and chains almost certainly are.
7. Biasing of a nip, front to back, must be done with great care as it can cause more problems than it cures. To make the system user-friendly, the average should be set on one knob and the bias on the other. Unfortunately, most systems have a front and a back setting instead of an average and bias.
8. Nip-load control selection should have four positions: open, close, stop (return to center switch) and load.
9. Nip rollers must not deflect so that nip pressure varies excessively between the middle and the edges.
10. Nip roller surfaces must be round, such that the nip pressure footprint varies excessively across the width or with rotation.
11. The nip-pressure footprint should be measured via nip-impression paper on regular intervals during the expected life of the roller surface. This and the previous guideline speak primarily to maintenance while the rest deal with design.
12. Nips are very dangerous. Safety includes guarding, E-stop quick opens, lock-outs for service, warnings and, most importantly, sensitivity training.

Web handlers everywhere know how vital tension control is to process health. We measure, calibrate and control tension. Nip is no less important. Yet, we have neglected nips so that few installations measure or calibrate, without which we can't even talk about control.

We need nothing less than a revolution in the way we do our nip-control business. Everyone must do their part. We have no less than a million nips out there. Many are performing poorly now and more are coming every day.