M18 Proximity Sensors: Thread Pitch, Dimensions, Sensing Range and Mounting

M18 is the size most Indian plants actually run on. It is large enough to give useful sensing distance, small enough to fit where it is needed, and the bracket for it already exists on most machines. If you are designing a sensor in or replacing one, these are the numbers that matter.

The thread pitch catches people out

Proximity sensors in M18 use a fine thread, M18 x 1, not the coarse M18 x 1.5 you would use for a general fastener. The fine pitch gives finer adjustment of the sensing gap, which is the whole point of a threaded barrel with locknuts either side.

The practical consequence: an M18 x 1.5 nut will not run on an M18 x 1 sensor, and a bracket tapped M18 x 1.5 will not take one. If you are drilling a new bracket, make it a clearance hole and clamp with the locknuts rather than tapping it. That also lets you adjust the gap, which a tapped hole does not.

When ordering replacement locknuts, specify M18 x 1. Ordinary hardware shops will hand you coarse by default.

Sensing range, and the number you should actually design to

Our D30C range offers M18 in two sensing distances, 5 mm and 8 mm. The 5 mm version is shielded and can be mounted flush in metal. The 8 mm version senses further and needs a clear zone around the face.

Three numbers get quoted for sensing distance and they are not the same:

• Rated sensing distance is the nominal figure, measured against a standard mild steel target under standard conditions.
• Effective sensing distance allows for manufacturing tolerance.
• Assured operating distance allows for tolerance, temperature and supply voltage variation together. This is the number you design to.

As a working rule, set the mounting gap at around 70 to 80 percent of the rated distance. An M18 with 5 mm rated should see the target at 3.5 to 4 mm, not at 4.8 mm. A sensor set at the edge of its range works on the bench and becomes intermittent in the hottest month of the year.

Then correct for the target. Stainless steel, aluminium, brass and copper are all detected at less than the rated distance. Our reduction factor table has the arithmetic, and it is the single most common reason an M18 that should work does not.

Mounting clearances

These matter as much as the sensing distance, and they differ between shielded and unshielded.

A shielded sensor has a metal sleeve concentrating the field forward. It can be embedded flush in a steel bracket with the face level with the surrounding metal.

An unshielded sensor lets the field spread sideways. Mount it flush in steel and it will detect the bracket and stay on permanently, which gets diagnosed as a faulty sensor roughly half the time. It needs a clear zone around the face and must stand proud of the mounting surface.

Two more clearances apply to both: keep a gap between adjacent sensors so their fields do not interact and cause chattering, and keep metal out of the area facing the sensor other than the target itself. Our note on shielded versus unshielded mounting gives the spacing rules.

Tightening

A proximity sensor barrel is a thin-walled tube with electronics potted inside it. It is not a bolt. Over-tightening the locknuts crushes the barrel and damages the sensor, and it is a surprisingly common cause of failures that look electrical.

Use a locknut on each side of the bracket, tighten by hand plus a short turn with a spanner, and do not use an impact driver. If the sensor needs to resist vibration, use a thread locking compound rather than more torque.

Cable, or connector

M18 sensors are supplied either with a fixed cable or with an M12 connector on a short pigtail. The connector version is worth specifying where the sensor is removed regularly for changeovers, because a cable pulled through a machine frame every week eventually fails at the gland. Where the sensor is fitted once and left, fixed cable in the exact length you need removes a junction box.

We can supply either, with the cable length specified rather than standard. On a machine build that is usually worth asking about.

Ordering an M18

The code is readable once you know the pattern. D30C-1805-PM-3S is an M18 body, 5 mm sensing, PNP, normally open, three-wire with short-circuit protection. Swap the digits for 1808 and you have the 8 mm version. Our part number guide decodes the whole range.

If you need a dimensional drawing for a design, ask and we will send it. Tell us the target material, the gap you have to work with, whether the sensor can stand proud of the bracket, and the output your input card expects, and we will confirm the variant and the mounting arrangement before you commit the design.