A nip is the pressure between two rollers that are forced together. A nip impression is a measure of the
uniformity of that pressure. The pressure profile is sometimes referred to as a footprint. It can be measured
in many ways. The simplest is to make sure that there is pressure everywhere across the width. This can
be done by shining a light through the nip. Light escaping through a gap between the rollers tells us that
there is no nip there. However, even if there is contact everywhere, the pressure may still be quite uneven.
A better approach is to measure the width of the footprint. If the width is uneven, the pressure profile must
also be uneven. However, measuring nip widths accurately can be difficult if the width is narrow. Also, as
we will see, nip width variations are not a very sensitive measure of nip uniformity. The very best measure
of the nip is to measure the pressure profile directly. Both nip widths and nip pressures can be measured
with paper thin products often referred to as nip impression paper, even though some are these products are
made of plastic rather than paper. Nip impression paper behaves like carbon or carbonless paper which
turns color when a sufficient pressure has been applied such as by typing or writing with a pen. Nip
impression paper is an indispensable tool for machine PM or process troubleshooting.