Gap Sensors and Feeler Gauges Information

Gap sensors and feeler gages detect or measure the gap or clearance between two components or surfaces such as turbine blades tip clearance, nip or roller gap, lubricant gaps, weld seams, adhesive joints, extruder die gap, spark plug electrodes, generator or motor air gap, mold spacing, and workpiece-fixture gap. Gap sensing products use mechanical feel (feeler gages), air pressure, optical, capacitance, or other electronic technologies to sense gaps or clearance.

Gap sensors are subtly different than proximity, presence, or position sensors. Gap sensors, electronic feeler gages, and other gap instruments may utilize proximity or position sensing elements within the gap sensing product. While proximity sensors detect the presence of a solid object, gap detection sensors detect the occurrence of a void, clearance, or gap. Further, while position sensors detect the position of an object from the surface of the position sensor, gap sensors or feeler gauges provide a measure of the size or width of the gap.

 Laser gap sensor


Gap detector / gap detection sensor—Gap detectors and gap detection sensors indicate the presence of a gap, clearance, spacing, void, break, or tear within a part or between two surfaces or parts. They provide only the gap's occurrence, not a direct reading or measurement of the gap. Some gap sensors can be set to switch or trigger for gaps below a specific size.


Fixed feeler gauges—Fixed feeler gauge and fixed step gages are mechanical gaging components used to measure gap, clearance, or spacing size.


Gap sensor / gap switch—Gap sensors provide a dimensional measurement of gaps or clearances. Gap sensors do not typically have a visual display. Air, pneumatic, and electronic feeler gauges can be considered gap sensors or gap sensing instruments depending on their display capability. Gap switches, gap sensing switches, and gap sensor switches provide an output to trigger activate or deactivate factory machinery.


Gap monitor / instrument—Gap monitors detect, measure, and track gaps within products, part clearance, roller nip alignment or gaps, mold spacing, and breaks or gaps in webs. Gap instruments, gap sensing systems and gap monitors typically have a visual display indicating the gap size. Gap monitors and instrument may also provide an output for machine control. Some gap monitors have control function and set points to trigger or activate or stop equipment when a gap or void is detected or the size exceeds allowable limits.


Thin/narrow gap sensor—Thin or narrow gap sensors area used to measure very small widths within a product or between the faces of components. Thin or narrow gap examples include the joint spacing between two surfaces before welding, brazing, or adhesive bonding; turbine blade to outer casing gap measurement; rotor to stator gap measurement; slot die coater gaps; disc brake rotor to brake pad gap; workpiece to fixture gaps and spark plug gaps. Feeler gages, optical or imaging gap gages, capacitance gap sensors, eddy current gap sensors, and air or pneumatic gap sensors are technologies used to measure or detect fine gaps.


Web/wide gap sensor—Wide gap sensors detect or measure large breaks, voids or gaps in web, sheet, or panel applications. Transmitted or reflected light is often used to detect large gaps in webs or spacing between panels.


Non-contact gap sensor—Gap sensing and feeler gauges can utilize contact or non-contact technologies. If the product is conductive, then a capacitance gap sensor may be used. If the products are non-conductive, a collapsible contact feeler gauge consisting of two thin metal strips—which compresses when inserted into the gap—may be used. The capacitance change between the two strips is converted into gap thickness. Optical, eddy current, and microwave gap sensors can have noncontact gap sensing capabilities. Gaps occurring in webs moving at high speed are detected using noncontact sensing technologies such as light transmission.




Several different technologies are utilized to provide gap sensors, gap sensing systems, and gap monitors.

 Inductive displacement gap sensor

Air pressure / pneumatic—Air pressure or pneumatic gap sensor use the change in air pressure to sense gaps. When a gap sensor is push closely against a part, a nozzle is closed, which generates air pressure. The air pressure moves a diaphragm, which triggers a switch. Some air gap sensors have two legs and web material or product can pass through the legs. When a gap occurs in the web or product, air flows across it, triggering or detecting the presence of a gap.

Capacitance—Capacitance gap gages measure by utilizing the change in capacitance that occurs when the spacing between two conductors changes.

Eddy current—Eddy current sensors induced current loops and feedback from these induced eddy current loops are sensed with a pickup coil to determine gap size.

Fixed—Fixed feeler gages, step gages, and gap gages use the fit of precision shims or spacers to measure gap. The gage is inserted into the gap or between the surfaces. The feel gage should just fit snug into the gap without forcing (overtightness) or looseness.


Mechanical—Large gaps can be measured with an internal or ID mechanical gage. Mechanical gap gages are usually called internal gages or inner diameter gages. Roller gap gages and gap micrometers are examples of mechanical gap gages. Fixed gap or feeler gauge could be considered mechanical gage without moving parts.

Optical/imaging—Several different types of gap sensors or gap sensing systems are based on optical, imaging and laser technologies. Imaging gap sensors capture an image and measure the gap visually. Profile or scanner instruments project a laser and then use reflected light to detect and/or measure the gaps. Transmissive light sources such as laser or LED sources are used for fast moving web applications, when a gap occur, light passes through the gap and is detected.




AGL—Understanding nip dynamics

Sensorland—Gap sensor technology


Image credits:

Festo Corporation | KEYENCE


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