Easy Roller Gap Measurement Ensures Calendering Success

Many roll-to-roll finishing processes typically use a calender, or series of hard pressure rollers, to deliver smooth, high-quality products of plastic, textile, or paper (Fig 1). Ensuring a consistent material thickness, however, depends on the ability to monitor, and maintain, a precise gap between rollers. This application note describes a quick and easy means for roller gap measurement.


Establishing a roller gap and checking for parallelism across roller width can be a slow and cumbersome procedure. Manual measurement requires multiple sized feeler gauges which can introduce errors. Misinterpreting feeler gauge drag, for instance, can lead to an incorrect gap reading. Accidentally tilting the feeler gauge can also result in a gap measurement error.

Maintaining a precise roller gap is also cumbersome as operating conditions may exacerbate or add to measurement errors. For example, excessive bearing play can lead to out-of-round conditions (Fig 2). This run out must be corrected – often requiring bearing replacement – and the gap measured all over again.


MTI’s capacitive gap measuring probes eliminate the need for mechanical feeler gauges. Users quickly measure the gap between rollers electronically – as well as roller parallelism, roundness, and bearing run out – simply by positioning the probes at several lateral locations across the rollers.

This gap monitoring system includes two components: a Digital Accumeasure amplifier (Fig 3) and gap monitoring wand (Fig 4). The wand contains two opposing capacitance probes, one per roller. Each probe/roller sets up a classic two-plate capacitive gap sensor (Fig 5).

To work, the amplifier injects a current into each probe, and then measures the impedance of the respective capacitive gaps. The impedance measured is directly proportional to the probe/roller gap by the formula: Gap = (area of the probe X dielectric constant of air) / (capacitance of the gap).

Summing the two probe/roller gaps, and adding the respective probe thicknesses, gives the gap between rollers. If roller grounding is poor or non-existent, a wand equipped with MTI’s Push-Pull probes can be used to make the measurements (Fig 6).

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  • Non-contact measurement eliminates mechanical error
  • Gap measurements can be electronically stored for future reference (Fig 7)
  • The capacitance amplifier can be networked for process control and active gap control
  • Real-time monitoring spots excessive bearing play

Image showing roller gaps

Figure 2) The ability to monitor gap parallelism in real time can prevent unintentional out-of-round situations.

Accumeasure D Series Amplifier with up to four measurement channels

Figure 3) MTI’s Accumeasure™ D Series amplifier provides up to four independent measurement channels in a rugged, compact amplifier package. Features include multiple unit synchronization, range extension, sub-nanometer resolution, and 0.01% FSR linearity. With Digital Accumeasure software, data can be displayed on a laptop or saved to CSV file for further data reduction.

Gap Measurement Wand with two opposing capacitive gap measurement probes.

Figure 4) Gap monitoring wand. The tip contains two opposing capacitive gap measuring probes.

Illustration of gap probe between rollers showing prove faces and contact to rollers.

Figure 5) Gap monitoring wand inserted between rollers: capacitance probes are tangent to both roller faces. An adjustable plastic roller stop aids with depth of insertion.

Push-pull probes with two sensing elements in one probe eliminate need to electrically ground the rotor

Figure 6) MTI’s Push-Pull probes feature two sensing elements built into one probe body eliminate the need to electrically ground the rotor.

Graph showing Gap Measurement changes during the adjustment of rollers

Figure 7) Plot of a changing gap as would be encountered when adjusting a roller gap.

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