M36 and M50 Proximity Sensors: 20 mm and 30 mm Sensing Distance

Most plants never need these. The ones that do need them badly, and usually find out after an M30 has been replaced three times because the real problem was that the gap could never be held.

M36 and M50 are large barrel inductive proximity switches. M36 senses at 20 mm, M50 at 30 mm. They exist for one reason: standoff.

When you actually need them

Four situations come up repeatedly:

• The target cannot be brought closer. A large rotating drum, a flywheel, a kiln shell, anything where the mechanical design fixes the gap and it is more than an M30 can reach.
• There is material in the gap. Scale, dust buildup, product carryover, a wear plate. The sensor has to see through the standoff plus whatever accumulates in it.
• Thermal growth moves the target. On a hot line the gap at operating temperature is not the gap you set cold. If the swing is several millimetres, you need a sensing distance that covers the whole range.
• The sensor must be protected. Setting it well back behind a guard or a non metallic wear plate costs you sensing distance, and a large barrel pays for it.

If none of those apply, fit M30 and save the money and the panel space.

What the size costs you

Be clear eyed about the trade. A large barrel needs:

• A bigger clear zone. Unshielded parts need a clear annular area around the face of roughly three barrel diameters. On M50 that is 150 mm of clear space, which on a crowded frame is a design constraint, not a detail.
• More spacing between sensors. At least twice the barrel diameter centre to centre, so 100 mm on M50.
• A heavier bracket. The sensor has mass and the mounting has to hold a gap within a millimetre or two. A bracket that was adequate for M12 will resonate under an M50.
• A larger hole. Retrofitting into an existing plate usually means a new plate.

AC two wire on large barrels

Our M36 and M50 parts are in the A20C AC two wire range, 200 mA, available normally open or normally closed. That matches where large barrel sensors are usually found: older heavy plant with relay logic and no 24 V DC supply at the sensing point.

Two wire AC switches in series with the load, so the usual cautions apply. There is a small leakage current when off and a voltage drop when on. Feeding a modern PLC input card directly from an AC two wire sensor is where people get into trouble, because the leakage can be enough to hold a high impedance input on. Switch a contactor or an interposing relay, and take the PLC signal from that.

If you do have 24 V DC at the point of use, say so when you enquire. A DC three wire part in a large barrel is a better match for a modern input card, and we can discuss what is available.

Mounting and commissioning

Set the gap at 70 to 80 percent of rated, so 14 to 16 mm on M36 and 21 to 24 mm on M50. Then verify it hot. On the applications where large barrels are specified, the cold gap and the running gap are routinely different by several millimetres, and the only way to know is to measure at temperature.

Correct for target material as usual. A large barrel does not exempt you from the reduction factor, it just gives you enough margin to absorb it.

Ordering

A20C-3620-M-2 is M36, 20 mm, normally open, AC two wire at 200 mA. A20C-5030-M-2 is the M50 at 30 mm. Substitute B for normally closed. The ordering code guide explains each position.

These are not fast moving stock items in the way M18 is, so talk to us about lead time before you design the bracket. If the application is marginal we will usually tell you to fit M30 instead, because a part we hold is better for you than a part we build.